Fishes in Marine Caves

Abstract

Fishes in marine caves have attracted limited attention by the scientific community in comparison to subterranean fishes which have lost eyes and pigmentation. They constitute a largely unexplored component of marine fish diversity, except for the relatively well-studied marine caves of the Mediterranean Sea. These habitats are characterized by steep environmental gradients of decreasing light and decreasing water exchange. The fishes recorded so far in marine caves are not exclusive residents of this habitat and they are also present at least in the other mesolithial habitats. In the Mediterranean marine caves, 132 fishes have been recorded to date, representing about 17% of the total Mediterranean fish species richness. Most of these species are reported from the anterior cave zones where some light still exists, while a smaller number of species are known from the totally dark zones. Among them, 27.3% are accidental visitors, 53.8% are the regular mesolithial visitors and switchers between mesolithion and open water, 5.3% are permanent residents of the mesolithion, but also occur in other habitats, and 13.6% are exclusive permanent residents of mesolithion. Some mesolithial exclusive permanent residents recorded in marine caves share similar morphology, probably as adaptations to these habitats.

Full text

Citation: Kovaˇci´c, M.; Gerovasileiou, V.;
Patzner, R.A. Fishes in Marine Caves.
Fishes 2024,9, 243. https://doi.org/
10.3390/fishes9060243
Academic Editors: Bror Jonsson,
Maria Elina Bichuette and Yahui Zhao
Received: 10 May 2024
Revised: 12 June 2024
Accepted: 17 June 2024
Published: 20 June 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
fishes
Review
Fishes in Marine Caves
Marcelo Kovaˇci´c 1, * , Vasilis Gerovasileiou 2,3 and Robert A. Patzner 4
1Prirodoslovni Muzej Rijeka, Lorenzov Prolaz 1, HR–51000 Rijeka, Croatia
2Department of Environment, Faculty of Environment, Ionian University, 29100 Zakynthos, Greece;
vgerovas@ionio.gr
3
Hellenic Centre of Marine Research (HCMR), Institute of Marine Biology, Biotechnology and Aquaculture (IMBBC),
71500 Heraklion, Greece
4
Department of Environment & Biodiversity, University of Salzburg, Hellbrunnerstrasse 34, 5020 Salzburg, Austria;
ra.patzner@gmail.com
*Correspondence: marcelo@prirodoslovni.com; Tel.: +385-51553669
Abstract: Fishes in marine caves have attracted limited attention by the scientific community in
comparison to subterranean fishes which have lost eyes and pigmentation. They constitute a largely
unexplored component of marine fish diversity, except for the relatively well-studied marine caves
of the Mediterranean Sea. These habitats are characterized by steep environmental gradients of
decreasing light and decreasing water exchange. The fishes recorded so far in marine caves are
not exclusive residents of this habitat and they are also present at least in the other mesolithial
habitats. In the Mediterranean marine caves, 132 fishes have been recorded to date, representing
about 17% of the total Mediterranean fish species richness. Most of these species are reported from
the anterior cave zones where some light still exists, while a smaller number of species are known
from the totally dark zones. Among them, 27.3% are accidental visitors, 53.8% are the regular
mesolithial visitors and switchers between mesolithion and open water, 5.3% are permanent residents
of the mesolithion, but also occur in other habitats, and 13.6% are exclusive permanent residents of
mesolithion. Some mesolithial exclusive permanent residents recorded in marine caves share similar
morphology, probably as adaptations to these habitats.
Keywords: fishes; marine caves; mesolithion; ecology; adaptation; diversity; distribution
Key Contribution: Marine caves are part of mesolithial habitats, together with pseudocaves and
cryptobenthic habitats. Fishes recorded in marine caves are not exclusive cave residents. Five different
morphological types of fish can be identified in relation to their adaptation to mesolithial habitats.
1. Introduction
The term “cave fish” is usually applied to “real” cave-exclusive species, also named
subterranean or hypogean fishes [
1
]. As residents of subterranean habitats, these fishes
are characterized as stygobitic species, considering that they inhabit aquatic subterranean
environments [
2
] (Figure 1). Stygobitic and troglobitic animals (i.e., the respective term for
terrestrial animals) have evolved unusual and highly specialized morphological and life history
traits for subterranean life (troglomorphy), being no longer able to survive above ground [
3
].
Troglobites and stygobites usually develop regressive features like the loss or reduction in
size of the eyes and in pigmentation, but they also develop constructive traits, such as longer
appendages and enhancements of nonvisual sensory systems [
3
]. Subterranean or hypogean
fishes are restricted to underground freshwaters [
4
] and anchialine systems, with only fourteen
species reported from anchialine systems in the last available summary [5].
Subterranean ecosystems include a variety of habitats ranging from aerobic caves and
endolithic systems to underground streams and pools, groundwater ecosystems, anchialine
systems and sea caves [
6
]. Marine caves (also known as sea caves) are globally distributed
Fishes 2024,9, 243. https://doi.org/10.3390/fishes9060243 https://www.mdpi.com/journal/fishes

Fishes 2024,9, 243 2 of 34
in coastal headlands, rocky reefs and coral reefs [
7
] and constitute a characteristic feature of
coastal areas with extensive rock outcrops such as the Mediterranean basin [
8
,
9
] (Figure 2).
Fishes 2024, 9, x FOR PEER REVIEW 2 of 37
Figure 1. Garra longipinnis cave form, Hotta Cave, Oman. Photo by H.R. Esmaili.
Subterranean ecosystems include a variety of habitats ranging from aerobic caves
and endolithic systems to underground streams and pools, groundwater ecosystems,
anchialine systems and sea caves [6]. Marine caves (also known as sea caves) are globally
distributed in coastal headlands, rocky reefs and coral reefs [7] and constitute a
characteristic feature of coastal areas with extensive rock outcrops such as the
Mediterranean basin [8,9] (Figure 2).
The study of the marine cave environment became possible only after the
development of autonomous diving (SCUBA), which allowed not only marine cave
exploration, but also direct observation and sampling by marine scientists [9]. The
scientific study of marine caves started in the north Mediterranean Sea during the mid-
twentieth century [10]. Marine caves are today acknowledged for their rich biodiversity
[11]. The presence of fishes in marine caves was reported in the first exploratory surveys
in these habitats [12,13]. Abel [12] published the first study on Mediterranean marine cave
fishes. However, it soon became evident that marine caves do not host genuine
stygobionts and that fishes, even if they are permanent residents of caves, show no
adaptations similar to those of stygobitic species [13–15]. Proudlove [16], in his review of
subterranean fishes, assigned sea cave fishes to the group of non-stygobitic fishes of caves
and other subterranean habitats. Despite the existence of about seven decades of marine
cave research, the fish fauna occurring in marine caves, including permanent residents,
has been rarely investigated, with most available studies restricted to the Mediterranean
Sea [17–19]. Therefore, this biotic component of the marine cave environment has never
been extensively reviewed.
Figure 1. Garra longipinnis cave form, Hotta Cave, Oman. Photo by H.R. Esmaili.
The study of the marine cave environment became possible only after the development
of autonomous diving (SCUBA), which allowed not only marine cave exploration, but
also direct observation and sampling by marine scientists [
9
]. The scientific study of
marine caves started in the north Mediterranean Sea during the mid-twentieth century [
10
].
Marine caves are today acknowledged for their rich biodiversity [
11
]. The presence of
fishes in marine caves was reported in the first exploratory surveys in these habitats [
12
,
13
].
Abel [
12
] published the first study on Mediterranean marine cave fishes. However, it
soon became evident that marine caves do not host genuine stygobionts and that fishes,
even if they are permanent residents of caves, show no adaptations similar to those of
stygobitic species [
13
–
15
]. Proudlove [
16
], in his review of subterranean fishes, assigned sea
cave fishes to the group of non-stygobitic fishes of caves and other subterranean habitats.
Despite the existence of about seven decades of marine cave research, the fish fauna
occurring in marine caves, including permanent residents, has been rarely investigated,
with most available studies restricted to the Mediterranean Sea [
17
–
19
]. Therefore, this
biotic component of the marine cave environment has never been extensively reviewed.
Fishes 2024, 9, x FOR PEER REVIEW 3 of 37
Figure 2. The marine cave Lučice, Adriatic Sea: (A) cave entrance, (B) and (C) interior. Photos by
(A) M. Kovačić, (B) and (C) D. Laslo.
The aim of this work is to provide an overview of the existing knowledge on fishes
in marine caves of the world and more specifically (a) to discuss their ecological
relationships to caves; (b) to describe the biodiversity and distribution of fishes in marine
caves and (c) to compare fish characteristics in marine caves with those known for other
biota in aquatic subterranean ecosystems (freshwater and anchialine caves and open
marine habitats). Ultimately, due to the limited available knowledge on the subject, this
review highlights the main gaps of knowledge regarding fish fauna in marine caves and
suggests future research directions. The applied taxonomy and nomenclature match that
of Fricke et al. [20].
2. Marine Caves
The term “cave” is commonly applied to natural openings, usually in rocks, which
are large enough for human entry [21,22]. This definition is clearly anthropocentric,
implying also that a cavity is connected to the surface through entrances and can be
distinguished from surface landforms by shape. Caves are geological formations whose
long dimension (length or depth) is greater than the cross-sectional dimensions at the
entrance [21]. Caves are formed by different processes in various rock types and in
unconsolidated sediments. The great majority of natural caves, including the largest ones,
are solution (or karst) caves, which have been created principally by the dissolution of
bedrock (limestone and gypsum) by water circulating through initial openings such as
fissures and pores [21]. Other caves, distinguished as to origin, are volcanic, glacier,
crevice, littoral, pipe and erosion caves [21,23].
Marine caves are located either directly on the coast or can be wholly submerged
beneath the sea floor and contain marine waters that freely exchange with the sea [23,24]
Figure 2. The marine cave Luˇcice, Adriatic Sea: (A) cave entrance, (B) and (C) interior. Photos by
(A) M. Kovaˇci´c, (B) and (C) D. Laslo.

Fishes 2024,9, 243 3 of 34
The aim of this work is to provide an overview of the existing knowledge on fishes in
marine caves of the world and more specifically (a) to discuss their ecological relationships
to caves; (b) to describe the biodiversity and distribution of fishes in marine caves and (c) to
compare fish characteristics in marine caves with those known for other biota in aquatic
subterranean ecosystems (freshwater and anchialine caves and open marine habitats).
Ultimately, due to the limited available knowledge on the subject, this review highlights the
main gaps of knowledge regarding fish fauna in marine caves and suggests future research
directions. The applied taxonomy and nomenclature match that of Fricke et al. [20].
2. Marine Caves
The term “cave” is commonly applied to natural openings, usually in rocks, which are
large enough for human entry [
21
,
22
]. This definition is clearly anthropocentric, implying
also that a cavity is connected to the surface through entrances and can be distinguished from
surface landforms by shape. Caves are geological formations whose long dimension (length or
depth) is greater than the cross-sectional dimensions at the entrance [
21
]. Caves are formed by
different processes in various rock types and in unconsolidated sediments. The great majority
of natural caves, including the largest ones, are solution (or karst) caves, which have been
created principally by the dissolution of bedrock (limestone and gypsum) by water circulating
through initial openings such as fissures and pores [
21
]. Other caves, distinguished as to origin,
are volcanic, glacier, crevice, littoral, pipe and erosion caves [21,23].
Marine caves are located either directly on the coast or can be wholly submerged
beneath the sea floor and contain marine waters that freely exchange with the sea [
23
,
24
]
(Figures 3and 4). These caves are usually of littoral origin. Littoral caves, commonly
known as sea caves, are found throughout the world, being formed either actively along
present coastlines, or as relict sea caves on former coastlines [
7
,
25
]. The main driving force
for the development of littoral caves is wave action. Erosion is ongoing wherever waves
batter rocky coasts, but rock is removed at greater rates along sea cliffs which contain zones
of weakness [
25
]. Flank margin caves are also formed at sea level but have a different
origin [
26
]. They are formed by the corrosion at the mixing zone along the halocline in
very young carbonates [
8
]. Blue holes constitute another kind of coastal cave, with several
hypotheses about their formation [
26
]. Marine caves are not only formed at sea level but
can also commonly form part of the submerged karst where original solution (or karst)
continental caves were submerged due to a rise in sea level [8].
Fishes 2024, 9, x FOR PEER REVIEW 4 of 37
(Figures 3 and 4). These caves are usually of littoral origin. Littoral caves, commonly
known as sea caves, are found throughout the world, being formed either actively along
present coastlines, or as relict sea caves on former coastlines [7,25]. The main driving force
for the development of littoral caves is wave action. Erosion is ongoing wherever waves
batter rocky coasts, but rock is removed at greater rates along sea cliffs which contain
zones of weakness [25]. Flank margin caves are also formed at sea level but have a
different origin [26]. They are formed by the corrosion at the mixing zone along the
halocline in very young carbonates [8]. Blue holes constitute another kind of coastal cave,
with several hypotheses about their formation [26]. Marine caves are not only formed at
sea level but can also commonly form part of the submerged karst where original solution
(or karst) continental caves were submerged due to a rise in sea level [8].
Figure 3. (A) Littoral or semi-submerged cave, (B) cave with internal air dome and (C) submerged
blind-ended cave. Based on Riedl [13] and Gerovasileiou and Bianchi [9]. Drawing by M. Kovačić.
Figure 4. (A) Littoral or semi-submerged and (B) submerged cave entrance in Plave grote, Adriatic
Sea. Photos by M. Babić.
Anchialine caves differ from sea caves in terms of marine influence and connection
to the atmosphere [27,28]. The influence of sea water in caves can extend beyond the
coastline. Anchialine caves contain water bodies which have a subterranean connection
to the sea and little or no direct connection to the atmosphere [28]. They can be found up
to several kilometres inland from the coast and typically contain surface layers of
freshwater or brackish water, separated by a thermochemocline from underlying fully
marine waters, low in dissolved oxygen, which have a long residence time of months to
years [24]. Therefore, the most striking abiotic ecological drivers of anchialine ecosystems
are salinity gradients influenced by tides and currents, generating water stratification
[28,29]. The main biological characteristic of these habitats is the presence of a specialized
anchialine fauna, consisting of stygobitic species of marine origin [28,29]. Anchialine caves
usually occur in karstified limestone or in lava fields [29]. The delimitation of anchialine
and marine environments in caves is not always clear. Blue holes can be found in the ocean
or inland, with the former being open directly into the marine environment and usually
containing marine water with tidal flow [30]. Inland blue holes are isolated by present
Figure 3. (A) Littoral or semi-submerged cave, (B) cave with internal air dome and (C) submerged
blind-ended cave. Based on Riedl [13] and Gerovasileiou and Bianchi [9]. Drawing by M. Kovaˇci´c.
Anchialine caves differ from sea caves in terms of marine influence and connection to
the atmosphere [
27
,
28
]. The influence of sea water in caves can extend beyond the coastline.
Anchialine caves contain water bodies which have a subterranean connection to the sea
and little or no direct connection to the atmosphere [
28
]. They can be found up to several
kilometres inland from the coast and typically contain surface layers of freshwater or
brackish water, separated by a thermochemocline from underlying fully marine waters, low
in dissolved oxygen, which have a long residence time of months to years [
24
]. Therefore,
the most striking abiotic ecological drivers of anchialine ecosystems are salinity gradients
influenced by tides and currents, generating water stratification [
28
,
29
]. The main biological

Fishes 2024,9, 243 4 of 34
characteristic of these habitats is the presence of a specialized anchialine fauna, consisting
of stygobitic species of marine origin [
28
,
29
]. Anchialine caves usually occur in karstified
limestone or in lava fields [
29
]. The delimitation of anchialine and marine environments
in caves is not always clear. Blue holes can be found in the ocean or inland, with the
former being open directly into the marine environment and usually containing marine
water with tidal flow [
30
]. Inland blue holes are isolated by present topography from
surface marine conditions and open directly onto the land surface or into an isolated
pond or lake; they contain tidally influenced water with variable salinity, from fresh to
marine [
30
]. Furthermore, some marine caves extend far enough inland so that water
exchange is slow enough to resemble a true anchialine habitat [
5
]. Similarly, several inland
anchialine systems have submerged entrances to the sea with significant influence by tides
and currents, and thus the anterior part of these caves functions as a marine cave [5,28].
Fishes 2024, 9, x FOR PEER REVIEW 4 of 37
(Figures 3 and 4). These caves are usually of littoral origin. Littoral caves, commonly
known as sea caves, are found throughout the world, being formed either actively along
present coastlines, or as relict sea caves on former coastlines [7,25]. The main driving force
for the development of littoral caves is wave action. Erosion is ongoing wherever waves
batter rocky coasts, but rock is removed at greater rates along sea cliffs which contain
zones of weakness [25]. Flank margin caves are also formed at sea level but have a
different origin [26]. They are formed by the corrosion at the mixing zone along the
halocline in very young carbonates [8]. Blue holes constitute another kind of coastal cave,
with several hypotheses about their formation [26]. Marine caves are not only formed at
sea level but can also commonly form part of the submerged karst where original solution
(or karst) continental caves were submerged due to a rise in sea level [8].
Figure 3. (A) Littoral or semi-submerged cave, (B) cave with internal air dome and (C) submerged
blind-ended cave. Based on Riedl [13] and Gerovasileiou and Bianchi [9]. Drawing by M. Kovačić.
Figure 4. (A) Littoral or semi-submerged and (B) submerged cave entrance in Plave grote, Adriatic
Sea. Photos by M. Babić.
Anchialine caves differ from sea caves in terms of marine influence and connection
to the atmosphere [27,28]. The influence of sea water in caves can extend beyond the
coastline. Anchialine caves contain water bodies which have a subterranean connection
to the sea and little or no direct connection to the atmosphere [28]. They can be found up
to several kilometres inland from the coast and typically contain surface layers of
freshwater or brackish water, separated by a thermochemocline from underlying fully
marine waters, low in dissolved oxygen, which have a long residence time of months to
years [24]. Therefore, the most striking abiotic ecological drivers of anchialine ecosystems
are salinity gradients influenced by tides and currents, generating water stratification
[28,29]. The main biological characteristic of these habitats is the presence of a specialized
anchialine fauna, consisting of stygobitic species of marine origin [28,29]. Anchialine caves
usually occur in karstified limestone or in lava fields [29]. The delimitation of anchialine
and marine environments in caves is not always clear. Blue holes can be found in the ocean
or inland, with the former being open directly into the marine environment and usually
containing marine water with tidal flow [30]. Inland blue holes are isolated by present
Figure 4. (A) Littoral or semi-submerged and (B) submerged cave entrance in Plave grote, Adriatic Sea.
Photos by M. Babi´c.
Marine caves are present in the continental shelf of every continent. Despite their
wide distribution in coastal headlands, rocky reefs and coral reefs of the world [
7
], studies
on marine caves exist only for the well-studied seas. For example, in the Mediterranean
Sea, while more than 3000 marine caves have been recorded so far, the actual number of
marine caves is expected to be much higher [
9
]. Most of the studied caves are usually
semi-submerged, or shallow marine caves with a maximum depth of their entrances not
exceeding 15 m. Only a small number of the studied marine caves in the Mediterranean
Sea are located at a depth range of 15–40 m, and no caves have been studied for their biota
below this depth [
9
,
11
]. The restricted depth range of the studied marine caves is caused by
the limits of employed methods (usually SCUBA diving) as well as the shallow bathymetric
distribution of most of the known marine caves. However, it could be expected that marine
caves are present in the entire range of earlier variation of the coastline, down to the present
seabed depth of 135 m, considering that the last glacial maximum occurred 26 thousand
years ago [
8
]. Deep marine caves inhabited by coelacanths Latimeria chalumnae Smith,
1939 were discovered in submarine canyons in South Africa, at a depth range of 96–133 m,
by using a research submersible [
31
]. Marine caves inhabited by coelacanths and other
fishes have also been found in deeper waters, down to 250 m in Comoros [
32
,
33
]. These
are probably the deepest records of fishes in marine caves known to date. Furthermore,
recent surveys in the deep Mediterranean Sea with Remotely Operated Vehicles (ROVs)
have shown that hard substrata in deeper waters can also have cave formations below the
continental shelf, in the upper bathyal zone at depths down to 795 m, possibly dating back
to the Messinian salinity crisis, 5.96–5.33 million years ago [34,35].
Another difference between most marine caves and anchialine or freshwater caves
which affects the synthesis and distribution of their biota concerns their dimensions. Many
continental caves, including those with freshwater or anchialine systems, are longer than

Fishes 2024,9, 243 5 of 34
100 km [
36
], while the longest known marine cave is Matainaka Cave on New Zealand’s
South Island with a total length of 1.54 km [
37
]. Most studied marine caves do not exceed
100 m in length, and the average size of most studied marine caves is usually measured in
tens of metres [25].
Considering smaller cryptic environments at the rocky bed (e.g., cavities, rock crevices
and fissures), which may have a wide range of shapes and sizes, the anthropocentric
definition of caves excludes them from the marine caves sensu stricto. The evolution
of the definition of marine caves can be followed from Riedl [
13
] to Gerovasileiou and
Bianchi [
9
]. The latter authors performed an extensive review of available knowledge and
defined “marine cave” as “a cavity of various origins, entirely or partly occupied by the
sea, accessible to humans, which has significant horizontal and volumetric development
with a possible quantitative criterion that the ratio between the numbers expressing the
total volume (in m
3
) and the entrance area (in m
2
) must be greater than 1, and that the
width of the entry must not exceed the internal average” (Figure 5A–D). This excludes
semi-closed “pseudocaves”, specifically larger crevices, gaps, overhanging rocks and coral
formations [
38
] (Figure 5E,F). Marine cave walls and ceilings consist of hard substrate, while
the substrate of the cave floor can be either soft or hard [
15
]. With regard to submersion level
and morphology, marine caves can be either submerged or semi-submerged, blind-ended
(or cul-de-sac) (Figure 5A,B,E) or with multiple openings (tunnels) [
9
,
13
] (Figure 5C,D,F).
In addition, holes or crevices within sea caves form caves within caves [15].
Fishes 2024, 9, x FOR PEER REVIEW 6 of 37
Figure 5. Submerged caves (A–D) sensu Gerovasileiou and Bianchi [9] and pseudocaves (E,F) sensu
Zander [38] of blind-ended (A,B,E) and tunnel-shaped morphology (C,D,F). Based on Riedl [13] and
Gerovasileiou and Bianchi [9]. Drawing by M. Kovačić.
The main abiotic parameters influencing the composition of biota inside marine caves
are decreasing light, decreasing hydrodynamism and substrate type [9,13]. Environmental
gradients such as the decrease in light, water movement and trophic input inside marine
caves are intense, occurring within a few metres. The same decrease in the external open
benthic environment can take place within tens or even hundreds of metres [9]. These
gradients influence species distributions inside marine caves and generate a marked
zonation of cave communities [9]. Cave entrances are usually dominated by sciaphilic
macroalgae and benthic invertebrates, while the inner semidark and dark cave
biocoenoses are animal-dominated. The dark interior of marine caves is generally
characterized by a much lower biotic cover, species richness, biomass and three-
dimensional complexity compared to the anterior cave zones [9]. Light conditions, water
exchange and biotic features inside caves largely depend on the specific cave topography
such as dimensions of the entrance, shape and length of the cave. For example, many short
sea caves may completely lack an inner dark zone but still fit the cave definition [9,13].
3. Fishes in Marine Caves
3.1. History of Research and Knowledge on Fishes in Marine Caves
The study of fishes in marine caves, sensu Gerovasileiou and Bianchi [9], started in
the second half of the twentieth century in the Mediterranean Sea and has remained
mostly geographically restricted to this sea, with the published data still being scarce and
limited elsewhere. Available data about fishes in marine caves are also restricted to those
caves occurring in the upper littoral or infralittoral zone (i.e., the bathymetric zone from
the lower limit of the tidal range down to the lowest limit where photophilic algae can
live). The pioneering study by Laborel and Vacelet [39] in the Niolon cave of France
mentioned two fish species, Apogon imberbis (Linnaeus, 1758) (Figure 6A) and Serranus
cabrilla (Linnaeus, 1758).
Figure 5. Submerged caves (A–D)sensu Gerovasileiou and Bianchi [
9
] and pseudocaves (E,F)sensu
Zander [
38
] of blind-ended (A,B,E) and tunnel-shaped morphology (C,D,F). Based on Riedl [
13
] and
Gerovasileiou and Bianchi [9]. Drawing by M. Kovaˇci´c.
The main abiotic parameters influencing the composition of biota inside marine caves
are decreasing light, decreasing hydrodynamism and substrate type [
9
,
13
]. Environmental
gradients such as the decrease in light, water movement and trophic input inside marine
caves are intense, occurring within a few metres. The same decrease in the external
open benthic environment can take place within tens or even hundreds of metres [
9
].
These gradients influence species distributions inside marine caves and generate a marked
zonation of cave communities [
9
]. Cave entrances are usually dominated by sciaphilic
macroalgae and benthic invertebrates, while the inner semidark and dark cave biocoenoses
are animal-dominated. The dark interior of marine caves is generally characterized by
a much lower biotic cover, species richness, biomass and three-dimensional complexity
compared to the anterior cave zones [
9
]. Light conditions, water exchange and biotic
features inside caves largely depend on the specific cave topography such as dimensions
of the entrance, shape and length of the cave. For example, many short sea caves may
completely lack an inner dark zone but still fit the cave definition [9,13].

Fishes 2024,9, 243 6 of 34
3. Fishes in Marine Caves
3.1. History of Research and Knowledge on Fishes in Marine Caves
The study of fishes in marine caves, sensu Gerovasileiou and Bianchi [
9
], started in the
second half of the twentieth century in the Mediterranean Sea and has remained mostly
geographically restricted to this sea, with the published data still being scarce and limited
elsewhere. Available data about fishes in marine caves are also restricted to those caves
occurring in the upper littoral or infralittoral zone (i.e., the bathymetric zone from the lower
limit of the tidal range down to the lowest limit where photophilic algae can live). The
pioneering study by Laborel and Vacelet [
39
] in the Niolon cave of France mentioned two fish
species, Apogon imberbis (Linnaeus, 1758) (Figure 6A) and Serranus cabrilla (Linnaeus, 1758).
Fishes 2024, 9, x FOR PEER REVIEW 7 of 37
Figure 6. (A) Apogon imberbis (Linnaeus, 1758); (B) Chlidichthys auratus (Lubbock, 1975); (C) Anthias
anthias (Linnaeus, 1758); (D) Sargocentron rubrum (Forsskål, 1775); (E) Myripristis kuntee
(Valenciennes, 1831); (F) Priacanthus blochii (Bleeker, 1853). Photo (A,C,D): R.A. Patzner; (B): S.V.
Bogorodsky; (E,F): K. Hagiwara.
Abel [12] published the first detailed study about marine cave fishes from a marine
cave in the Gulf of Naples (Italy), recording 32 species, and divided fishes, according to
their relationship to caves, into speleoxenous, speleophilous and speleobiont species. He
also divided caves, from the animal point of view, into the larger marine caves or “optical
caves” and the small hidden spaces of “thigmotaxic caves”. The next important step in the
study of marine caves was Riedl’s [13] influential monograph on the ecology of marine
caves, Biologie der Meereshöhlen, which summarized records of 43 fish species from
Mediterranean marine caves and included the first record of an alien species in a marine
cave habitat, Sargocentron rubrum (Forsskål, 1775) from Lebanon (Figure 6D). However,
the definition of marine caves in Riedl [13] also included other cryptic habitats at the
seabed. During the 1960s, records of fishes in marine caves were rare, e.g. [40,41]. After
these pioneering studies, Bath [42] described a new fish genus and species from a
Mediterranean marine cave, the goby Gammogobius steinitzi Bath, 1971 (Figure 7C). Zander
and Jelinek [43] studied fish fauna in the Banjole cave, in the Adriatic Sea, and its
relationship to light availability within this cave. They also described both a new genus
and species of gobies, Speleogobius trigloides Zander & Jelinek, 1976 from the Banjole cave
(Figure 7A,B). The second alien fish record in a Mediterranean marine cave, Pempheris
rhomboidea Kossmann & Räuber, 1877 (Figure 7D) (as Pempheris vanicolensis Cuvier &
Valenciennes, 1831), was recorded in 1979 in Lebanon [44]. The first studies on the ecology
of individual fish species in marine caves also started in the 1970s [45,46].
Figure 6. (A)Apogon imberbis (Linnaeus, 1758); (B)Chlidichthys auratus Lubbock, 1975; (C)Anthias anthias
(Linnaeus, 1758); (D)Sargocentron rubrum (Forsskål, 1775); (E)Myripristis kuntee Valenciennes, 1831;
(F)Priacanthus blochii Bleeker, 1853. Photo (A,C,D): R.A. Patzner; (B): S.V. Bogorodsky; (E,F): K. Hagiwara.
Abel [
12
] published the first detailed study about marine cave fishes from a marine
cave in the Gulf of Naples (Italy), recording 32 species, and divided fishes, according to their
relationship to caves, into speleoxenous, speleophilous and speleobiont species. He also
divided caves, from the animal point of view, into the larger marine caves or “optical caves”
and the small hidden spaces of “thigmotaxic caves”. The next important step in the study
of marine caves was Riedl’s [
13
] influential monograph on the ecology of marine caves,
Biologie der Meereshöhlen, which summarized records of 43 fish species from Mediterranean
marine caves and included the first record of an alien species in a marine cave habitat,
Sargocentron rubrum (Forsskål, 1775) from Lebanon (Figure 6D). However, the definition
of marine caves in Riedl [
13
] also included other cryptic habitats at the seabed. During
the 1960s, records of fishes in marine caves were rare, e.g., [
40
,
41
]. After these pioneering

Fishes 2024,9, 243 7 of 34
studies, Bath [
42
] described a new fish genus and species from a Mediterranean marine
cave, the goby Gammogobius steinitzi Bath, 1971 (Figure 7C). Zander and Jelinek [
43
] studied
fish fauna in the Banjole cave, in the Adriatic Sea, and its relationship to light availability
within this cave. They also described both a new genus and species of gobies, Speleogobius
trigloides Zander & Jelinek, 1976 from the Banjole cave (Figure 7A,B). The second alien
fish record in a Mediterranean marine cave, Pempheris rhomboidea Kossmann & Räuber,
1877 (Figure 7D) (as Pempheris vanicolensis Cuvier & Valenciennes, 1831), was recorded in
1979 in Lebanon [
44
]. The first studies on the ecology of individual fish species in marine
caves also started in the 1970s [45,46].
Fishes 2024, 9, x FOR PEER REVIEW 8 of 37
During the 1980s only a few studies about marine caves included information about
fish species, e.g. [47–50]. The first paper dedicated to the fish assemblages in marine caves,
following Abel [12] and Zander and Jelinek [43], was published by Zander [14]. He studied
fishes from the Mediterranean marine caves and associated habitats, applying the term
“mesolithial habitats” for all cave-like habitats independently of their size (i.e., including
marine caves, overhangs as well as crevice microhabitats). At the end of the twentieth
century, several papers described [51–53] or extended [54–59] the known geographic
distribution of Mediterranean fishes in marine caves. Some studies dealing with
cryptobenthic fishes also reported species from marine caves among other cryptic habitats
[60,61]. Arko Pijevac et al. [62] studied the biocoenoses of a submarine cave on the island
of Krk (Croatia, Adriatic Sea), and among more than one hundred animal species, they
also recorded 20 fish species. Bussotti et al. [63] performed the first quantitative research
of fish assemblages in marine caves, using the visual census method, and recorded 19 fish
species in shallow marine caves of the Salento Peninsula (Apulia, Italy). Bussotti et al. [64]
showed, in another quantitative study, that marine caves are the preferred habitats of the
cardinal fish A. imberbis. Harmelin et al. [65] studied a small, submerged cave in Bagaud
Island (Port-Cros National Park, France) and recorded 19 species, including 11 cave-
dwellers and eight occasional visitors. Belmonte et al. [66] studied the marine caves of
Albania for the first time and recorded six fish species. The most distant record of marine
fish inside a marine cave was that of Chelon sp. (reported as Liza sp.), found more than
1000 m away from the cave entrance in the cave system of the Grotta del Bel Torrente in
Sardinia, Italy [67]. In addition to Chelon sp., the species Conger conger (Linnaeus, 1758)
and Solea solea (Linnaeus, 1758) were recorded at 500 and 340 m from the entrance of the
same cave, respectively. In another quantitative study, Bussotti and Guidetti [68] studied
dissimilarities of fish assemblages between marine caves and rocky cliffs in Salento
Peninsula (Apulia, Italy), with 29 fish species recorded inside cave habitats and 10 of them
observed exclusively inside caves. Bussotti and Guidetti [69] similarly recorded 37 fish
species inside cave habitats.
Figure 7. (A) Speleogobius trigloides (Zander & Jelinek, 1976), male; (B) S. trigloides female; (C)
Gammogobius steinitzi (Bath, 1971); (D) Pempheris rhomboidea. Photo (A,B): R. Svensen; (C): S.
Guerrieri; (D): R.A. Patzner.
During the last decade, research on Mediterranean marine cave fishes has intensified,
providing data from previously under-studied marine areas along with quantitative data
on fish abundance and habitat preferences. Bussotti et al. [17] used visual census to study
Figure 7. (A)Speleogobius trigloides Zander & Jelinek, 1976, male; (B)S.trigloides female; (C)Gammogobius
steinitzi Bath, 1971; (D)Pempheris rhomboidea. Photo (A,B): R. Svensen; (C): S. Guerrieri; (D): R.A. Patzner.
During the 1980s only a few studies about marine caves included information about
fish species, e.g., [
47
–
50
]. The first paper dedicated to the fish assemblages in marine
caves, following Abel [
12
] and Zander and Jelinek [
43
], was published by Zander [
14
].
He studied fishes from the Mediterranean marine caves and associated habitats, apply-
ing the term “mesolithial habitats” for all cave-like habitats independently of their size
(i.e., including marine caves, overhangs as well as crevice microhabitats). At the end of
the twentieth century, several papers described [
51
–
53
] or extended [
54
–
59
] the known
geographic distribution of Mediterranean fishes in marine caves. Some studies dealing
with cryptobenthic fishes also reported species from marine caves among other cryptic
habitats [
60
,
61
]. Arko-Pijevac et al. [
62
] studied the biocoenoses of a submarine cave on the
island of Krk (Croatia, Adriatic Sea), and among more than one hundred animal species,
they also recorded 20 fish species. Bussotti et al. [
63
] performed the first quantitative
research of fish assemblages in marine caves, using the visual census method, and recorded
19 fish species in shallow marine caves of the Salento Peninsula (Apulia, Italy). Bussotti
et al. [
64
] showed, in another quantitative study, that marine caves are the preferred habi-
tats of the cardinal fish
A.imberbis
. Harmelin et al. [
65
] studied a small, submerged cave
in Bagaud Island (Port-Cros National Park, France) and recorded 19 species, including
11 cave-dwellers and eight occasional visitors. Belmonte et al. [
66
] studied the marine
caves of Albania for the first time and recorded six fish species. The most distant record
of marine fish inside a marine cave was that of Chelon sp. (reported as Liza sp.), found
more than 1000 m away from the cave entrance in the cave system of the Grotta del Bel
Torrente in Sardinia, Italy [
67
]. In addition to Chelon sp., the species Conger conger (Linnaeus,
1758) and Solea solea (Linnaeus, 1758) were recorded at 500 and 340 m from the entrance

Fishes 2024,9, 243 8 of 34
of the same cave, respectively. In another quantitative study, Bussotti and Guidetti [
68
]
studied dissimilarities of fish assemblages between marine caves and rocky cliffs in Salento
Peninsula (Apulia, Italy), with 29 fish species recorded inside cave habitats and 10 of them
observed exclusively inside caves. Bussotti and Guidetti [
69
] similarly recorded 37 fish
species inside cave habitats.
During the last decade, research on Mediterranean marine cave fishes has intensified,
providing data from previously under-studied marine areas along with quantitative data
on fish abundance and habitat preferences. Bussotti et al. [
17
] used visual census to study
fish fauna in 14 marine caves of Italian marine protected areas and recorded 38 species.
Later, Bussotti et al. [
70
] studied 16 marine caves along the coasts of Spain, France and Italy
with visual census and recorded 33 fish species while also highlighting the potential role
of A.imberbis (Figure 6A) for cave ecosystem functioning in the western Mediterranean.
In the eastern Mediterranean, Gerovasileiou et al. [
71
] combined new data from marine
caves of the Aegean Sea with literature data from the entire eastern Mediterranean basin
and reported 37 fish species. New data on fishes in marine caves were also published
from Croatia [
72
], Greece [
19
,
73
], Italy [
74
], Cyprus [
75
], Montenegro [
76
], Turkey [
77
–
79
],
Israel [
80
] and Malta [
35
]. Most of the above-mentioned studies provided information
about fishes as part of general ecological and biodiversity cave assessments, while two
studies focused specifically on rarely reported cryptobenthic fishes [
19
,
73
], and one study
focused on fishes in six caves of the Aegean coasts of Turkey recording 32 species [
77
].
More recently, Kovaˇci´c et al. [
18
] studied “cave within cave” fishes in a marine cave of
Croatia using the methods for cryptobenthic fishes (Figure 8), recording 18 species inside
the marine caves. Fifteen of these species were sampled by using the square and anaesthetic
method, with nine species found in intracave cavities, including five species which occurred
exclusively in this “cave within cave” highly cryptic habitat. The last available census
reported a total of 112 fish species from Mediterranean marine caves [
9
]. Among these,
18 species are introduced, mainly originating from the Indo-Pacific Ocean [81,82].
The lack of studies on fishes in marine caves sensu stricto and their geographic limit
to the Mediterranean Sea was also emphasized by the short review on fishes in sea caves
in Proudlove’s [
1
] capital work on subterranean fishes. To the best of our knowledge,
published records of fishes from marine caves sensu stricto outside the Mediterranean
Sea, excluding anchialine systems, are limited from all other areas, e.g., the Pacific Ocean,
specifically Sulawesi Island [
83
], Japan [
84
–
86
], Hong Kong [
87
], Hawaiian Islands [
88
],
Palau [
89
], Australia [
90
–
92
], Vietnam [
93
], Indonesia [
94
] and Micronesia [
86
]; the Red
Sea and Indian Ocean, specifically the northwest Indian Ocean and the Red Sea [
38
,
95
,
96
],
Comoros Islands [
32
,
33
] and South Africa [
31
]; and the Atlantic Ocean, specifically Azores [
97
];
Bermuda [98] and the Black Sea [99,100].
The studies of fishes in marine caves sensu stricto outside the Mediterranean Sea are
mostly restricted to taxonomy, including new species descriptions from marine caves
(Apogonidae: [
94
]; Bythitidae: [
83
,
84
,
88
,
101
]; Gobiidae: [
85
,
92
,
96
]; Protoanguilidae: [
89
]
(Figure 9A); Pseudochromidae: [
95
] (Figure 6B); Serranidae, Anthiinae: [
86
,
91
]), as well as
new records of fish species [
99
] and studies on a single species [
31
,
32
]. Species of the genus
Lucifuga Poey, 1858 were also recorded from marine blue holes, even though euryhaline
species of this genus are regularly found in inland caves and anchialine systems [
101
].
Some species in marine caves were recognized as new but still remain undescribed [
102
].
Only a few studies specifically focus on fishes recorded within a targeted fish census in
marine caves [
87
,
98
] or report on the fish fauna as the part of the cave biota in ecological
and biodiversity assessments [
90
,
93
,
97
,
100
,
103
]. Depczynski and Bellwood [
104
] studied
cryptobenthic reef fish communities in Australia in four distinct microhabitats, includ-
ing marine caves. They recorded 26 cryptobenthic fish species in marine caves, mostly
Gobiidae
and Apogonidae, but also species of several other fish families, with four species
exclusively found in marine caves. However, their definition of marine caves also included
pseudocaves, and it is not clear if any marine caves sensu stricto were actually studied. The
study of Zander [
38
] on the Red Sea coast of Egypt also covered fishes from pseudocaves

Fishes 2024,9, 243 9 of 34
and not marine caves sensu stricto. Fishes have been also recorded in deep-water caves at
the continental shelf break and below in the Southwestern Indian Ocean [31–33].
Fishes 2024, 9, x FOR PEER REVIEW 10 of 37
Figure 8. Square collecting of cryptobenthic fish on (A) the wall and (B) the ceiling in the marine
cave. Photos by Z. Valić.
The lack of studies on fishes in marine caves sensu stricto and their geographic limit
to the Mediterranean Sea was also emphasized by the short review on fishes in sea caves
in Proudlove’s [1] capital work on subterranean fishes. To the best of our knowledge,
published records of fishes from marine caves sensu stricto outside the Mediterranean Sea,
excluding anchialine systems, are limited from all other areas, e.g., the Pacific Ocean,
specifically Sulawesi Island [83], Japan [84–86], Hong Kong [84], Hawaiian Islands [85],
Palau [86], Australia [87–89], Vietnam [90], Indonesia [91] and Micronesia [92]; the Red
Sea and Indian Ocean, specifically the northwest Indian Ocean and the Red Sea [38,93,94],
Comoros Islands [32,33] and South Africa [31]; and the Atlantic Ocean, specifically Azores
[95]; Bermuda [96] and the Black Sea [97,98].
The studies of fishes in marine caves sensu stricto outside the Mediterranean Sea are
mostly restricted to taxonomy, including new species descriptions from marine caves
(Apogonidae: [91]; Bythitidae: [83,85,99,100]; Gobiidae: [89,94,101]; Protoanguilidae: [86]
(Figure 9A); Pseudochromidae: [93] (Figure 6B); Serranidae, Anthiinae: [88,92]), as well as
new records of fish species [97] and studies on a single species [31,32]. Species of the genus
Lucifuga Poey, 1858 were also recorded from marine blue holes, even though euryhaline
species of this genus are regularly found in inland caves and anchialine systems [100].
Figure 8. Square collecting of cryptobenthic fish on (A) the wall and (B) the ceiling in the marine cave.
Photos by Z. Vali´c.
Fishes 2024, 9, x FOR PEER REVIEW 11 of 37
Some species in marine caves were recognized as new but still remain undescribed [102].
Only a few studies specifically focus on fishes recorded within a targeted fish census in
marine caves [84,96] or report on the fish fauna as the part of the cave biota in ecological
and biodiversity assessments [87,90,95,98,103]. Depczynski and Bellwood [104] studied
cryptobenthic reef fish communities in Australia in four distinct microhabitats, including
marine caves. They recorded 26 cryptobenthic fish species in marine caves, mostly
Gobiidae and Apogonidae, but also species of several other fish families, with four species
exclusively found in marine caves. However, their definition of marine caves also
included pseudocaves, and it is not clear if any marine caves sensu stricto were actually
studied. The study of Zander [38] on the Red Sea coast of Egypt also covered fishes from
pseudocaves and not marine caves sensu stricto. Fishes have been also recorded in deep-
water caves at the continental shelf break and below in the Southwestern Indian Ocean
[31–33].
Figure 9. (A) Protanguilla palau Johnson, Ida & Sakaue 2012, new species, genus and family described
from marine cave, photo by J. Sakue. (B) Grammonus ater (Risso, 1810), photo by R.A. Patzner. (C)
Gaidropsarus mediterraneus (Linnaeus, 1758), photo by M. Kovačić. (D) Ophidion barbatum, photo by
S. Guerrieri.
3.2. Fishes in Marine Caves as a Part of Mesolithial Habitats
3.2.1. Ecological Classifications of Fishes in Marine Caves
The Schiner–Racovitza system [105,106] divides subterranean diversity into
troglobites, troglophiles and trogloxenes. Trajano [106] redefined the Schiner–Racovitza
categories: troglobites or troglobionts correspond to exclusive subterranean source
populations; sink populations may be found in surface habitats; troglophiles include
source populations both in hypogean and epigean habitats, with individuals regularly
commuting between these habitats, promoting the introgression of genes selected under
epigean regimes into subterranean populations (and vice-versa); trogloxenes are instances
of source populations in epigean habitats, but using subterranean resources (the so-called
obligatory trogloxenes are dependent on both surface and subterranean resources).
Abel [12] divided fishes in marine caves according to their relationship with caves to
speleoxenous, speleophilous and speleobiont species, a division superficially similar to
the Schiner–Racovitza system. According to Abel [12], the cave-avoiding species
(speleoxenous fish) are opposed to obligate cave settlers (speleobionts), while the fishes
occasionally found in cave systems (speleophilous fishes) can be further divided into
those that temporarily visit caves as shelters and those that occasionally colonize caves.
However, Riedl [13], who studied the entire invertebrate fauna in addition to fishes in
marine caves, concluded that the inhabitants of marine caves are not an independent
fauna, but speleophilic forms whose populations are connected via the open sea, and
classified fishes only into speleophilic, indifferent and speleophobic fishes. Indeed, no
Figure 9. (A)Protanguilla palau Johnson, Ida & Sakaue 2012, new species, genus and family described
from marine cave, photo by J. Sakue. (B)Grammonus ater (Risso, 1810), photo by R.A. Patzner.
(C)Gaidropsarus mediterraneus (Linnaeus, 1758), photo by M. Kovaˇci´c. (D)Ophidion barbatum, photo by
S. Guerrieri.

Fishes 2024,9, 243 10 of 34
3.2. Fishes in Marine Caves as a Part of Mesolithial Habitats
3.2.1. Ecological Classifications of Fishes in Marine Caves
The Schiner–Racovitza system [
105
,
106
] divides subterranean diversity into troglobites,
troglophiles and trogloxenes. Trajano [
106
] redefined the Schiner–Racovitza categories:
troglobites or troglobionts correspond to exclusive subterranean source populations; sink
populations may be found in surface habitats; troglophiles include source populations
both in hypogean and epigean habitats, with individuals regularly commuting between
these habitats, promoting the introgression of genes selected under epigean regimes into
subterranean populations (and vice-versa); trogloxenes are instances of source populations
in epigean habitats, but using subterranean resources (the so-called obligatory trogloxenes
are dependent on both surface and subterranean resources).
Abel [
12
] divided fishes in marine caves according to their relationship with caves
to speleoxenous, speleophilous and speleobiont species, a division superficially simi-
lar to the Schiner–Racovitza system. According to Abel [
12
], the cave-avoiding species
(speleoxenous fish) are opposed to obligate cave settlers (speleobionts), while the fishes
occasionally found in cave systems (speleophilous fishes) can be further divided into
those that temporarily visit caves as shelters and those that occasionally colonize caves.
However, Riedl [
13
], who studied the entire invertebrate fauna in addition to fishes in
marine caves, concluded that the inhabitants of marine caves are not an independent fauna,
but speleophilic forms whose populations are connected via the open sea, and classified
fishes only into speleophilic, indifferent and speleophobic fishes. Indeed, no genuine
speleobiontic fishes have been found to date in marine caves, and permanent cave-dwellers
do not show any adaptation similar to those found in the stygobitic fishes [
13
–
16
]. For
example, the three fish species originally designated as obligate cave settlers by Abel [
12
],
Anthias anthias
(Linnaeus, 1758) (Figure 6C)
,Microlipophrys nigriceps (Vinciguerra, 1883)
(Figure 10A,C) and
Tripterygion minor
Kolombatovi´c, 1892 (Figure 10B), can also be found
in other marine benthic habitats with dim light and decreasing currents [
107
]. Zander [
14
]
noticed that the same fish species, that he designated as speleophilic, occurred in differ-
ent habitats such as marine caves (sensu Gerovasileiou and Bianchi [
9
]), shaded spaces
below overhangs or in the crevicular microhabitats. Therefore, he proposed the term
“mesolithion” (meaning between stones) for this community, deriving from the Greek
words “meso” (
µ´
εσ
o), meaning “middle” or “between”, and “lithos” (
λ´
ιθ
o
ς
), which means
“stone” or “rock”. Zander also distinguished speleophilous fish, which are photophobous
or heliophobous versus euryphotic fishes [
14
,
15
,
43
]. He also gave examples of the ecologi-
cal classification of fishes in marine caves of the Red Sea, specifically speleoxenous fish, like
Pseudanthias squamipinnis
(Peters, 1885), which may be found in cavities and also in open
water, while speleophilous fish use the cave as a resting or hiding place as with most day
active species (e.g., Pseudochromis fridmani Klausewitz, 1968, serranids or chaetodontids).
As Zander’s [
38
] study was performed in pseudocaves, his use of Abel’s speleo-terms is
not really suitable for pseudocaves and for the mesolithion as a whole. In addition to the
absence of genuine speleobionts restricted to marine caves, the existence of the exclusive
permanent fish residents of the mesolithial habitats in general should be clarified and
described more fully.
Apart from the Mediterranean Sea, there exist very few examples of the classification
of fishes according to their relationship with marine caves. For instance, Micael et al. [
97
]
distinguished residents, seasonal and occasional species among eleven fish species recorded
in a subtidal tunnel of São Miguel Island in Azores, Atlantic Ocean.
Understanding the position of benthic fishes in relation to the bottom is also necessary
for studying and explaining the habits of mesolithial fishes (Figure 11). Cryptobenthic
fish live in the small hidden spaces underneath the bottom surface; epibenthic fish lie on
the bottom surface with physical contact with the substrate; hyperbenthic fish swim or
hover above the bottom, more or less within 1 m from the surface [
108
,
109
] (Figure 11). The
position to the bottom is characteristic of any individual in a particular moment, but it can
be generalized for species when they exclusively or predominantly occur in one position or

Fishes 2024,9, 243 11 of 34
microhabitat [
109
]. The relationship of benthic fish species with the bottom influences their
body form [
110
], so that the latter can be indicative of where and how the fish species lives.
Fishes 2024, 9, x FOR PEER REVIEW 12 of 37
genuine speleobiontic fishes have been found to date in marine caves, and permanent
cave-dwellers do not show any adaptation similar to those found in the stygobitic fishes
[13–16]. For example, the three fish species originally designated as obligate cave settlers
by Abel [12], Anthias anthias (Linnaeus, 1758) (Figure 6C), Microlipophrys nigriceps
(Vinciguerra, 1883) (Figure 10A,C) and Tripterygion minor Kolombatović, 1892 (Figure
10B), can also be found in other marine benthic habitats with dim light and decreasing
currents [107]. Zander [14] noticed that the same fish species, that he designated as
speleophilic, occurred in different habitats such as marine caves (sensu Gerovasileiou and
Bianchi [9]), shaded spaces below overhangs or in the crevicular microhabitats. Therefore,
he proposed the term “mesolithion” (meaning between stones) for this community,
deriving from the Greek words “meso” (μέσο), meaning “middle” or “between”, and
“lithos” (λίθος), which means “stone” or “rock”. Zander also distinguished speleophilous
fish, which are photophobous or heliophobous versus euryphotic fishes [14,15,43]. He also
gave examples of the ecological classification of fishes in marine caves of the Red Sea,
specifically speleoxenous fish, like Pseudanthias squamipinnis (Peters, 1885), which may be
found in cavities and also in open water, while speleophilous fish use the cave as a resting
or hiding place as with most day active species (e.g., Pseudochromis fridmani Klausewitz,
1968, serranids or chaetodontids). As Zander’s [38] study was performed in pseudocaves,
his use of Abel’s speleo-terms is not really suitable for pseudocaves and for the mesolithion
as a whole. In addition to the absence of genuine speleobionts restricted to marine caves,
the existence of the exclusive permanent fish residents of the mesolithial habitats in
general should be clarified and described more fully.
Figure 10. (A) Microlipophrys nigriceps (Vinciguerra, 1883) (Blenniidae, northern Mediterranean); (B)
Tripterygion minor (Kolombatović, 1892) (Tripterygiidae, northern Med.); (C) M. nigriceps (southern
Med.); (D) Tripterygion melanurus (Guichenot, 1850) (southern Med.). Photo (A,C,D): R.A. Patzner;
(B): M. Kovačić.
Apart from the Mediterranean Sea, there exist very few examples of the classification
of fishes according to their relationship with marine caves. For instance, Micael et al. [95]
distinguished residents, seasonal and occasional species among eleven fish species
recorded in a subtidal tunnel of São Miguel Island in Azores, Atlantic Ocean.
Understanding the position of benthic fishes in relation to the bottom is also
necessary for studying and explaining the habits of mesolithial fishes (Figure 11).
Cryptobenthic fish live in the small hidden spaces underneath the bottom surface;
epibenthic fish lie on the bottom surface with physical contact with the substrate;
hyperbenthic fish swim or hover above the bottom, more or less within 1 m from the
surface [108,109] (Figure 11). The position to the bottom is characteristic of any individual
Figure 10. (A)Microlipophrys nigriceps (Vinciguerra, 1883) (Blenniidae, northern Mediterranean);
(B)Tripterygion minor Kolombatovi´c, 1892 (Tripterygiidae, northern Med.); (C)M.nigriceps (southern Med.);
(D)Tripterygion melanurus Guichenot, 1850 (southern Med.). Photo (A,C,D): R.A. Patzner; (B): M. Kovaˇci´c.
Benthopelagic fishes, like Boops boops (Linnaeus, 1758), and neritic epipelagic fishes,
like Seriola dumerili (Risso, 1810), have also been recorded from the water column inside
large marine caves [
71
], though clearly as accidental visitors. In common with benthic cate-
gories, categorization of these fishes by pelagic zones should also be defined in the assign-
ment of species recorded in marine caves to different pelagic zones: littoral benthopelagic
fishes live and feed near the seabed, as well as in midwaters or near the surface [
111
];
epipelagic fishes live and feed in the open sea in the surface waters or at midwater to
depths of 200 m, while neritic fishes live in the shallow pelagic zone over the continental
shelf in nearshore marine ecosystems [111] (Figure 11).
Fishes 2024, 9, x FOR PEER REVIEW 13 of 37
in a particular moment, but it can be generalized for species when they exclusively or
predominantly occur in one position or microhabitat [109]. The relationship of benthic fish
species with the bottom influences their body form [110], so that the latter can be
indicative of where and how the fish species lives.
Benthopelagic fishes, like Boops boops (Linnaeus, 1758), and neritic epipelagic fishes,
like Seriola dumerili (Risso, 1810), have also been recorded from the water column inside
large marine caves [71], though clearly as accidental visitors. In common with benthic
categories, categorization of these fishes by pelagic zones should also be defined in the
assignment of species recorded in marine caves to different pelagic zones: littoral
benthopelagic fishes live and feed near the seabed, as well as in midwaters or near the
surface [111]; epipelagic fishes live and feed in the open sea in the surface waters or at
midwater to depths of 200 m, while neritic fishes live in the shallow pelagic zone over the
continental shelf in nearshore marine ecosystems [111] (Figure 11).
Figure 11. Habitat position of fishes occurring in marine caves. Benthic fishes: (1) cryptobenthic fish,
(2) epibenthic fish, (3) hyperbenthic fish. Pelagic fishes: (4) benthopelagic fishes, (5) neritic
epipelagic fishes. Drawing by M. Kovačić.
3.2.2. Mesolithial Fish
Marine caves sensu stricto apparently lack independent fish fauna and genuine
speleobiontic fishes, since marine caves share fish fauna at least with the other mesolithial
habitats [13–15]. The terms accidental cave visitors, regular cave visitors, switchers
between caves and open water, and finally, cave permanent residents (including both
those that occur in other habitats and those which are cave exclusive permanent residents)
should be, therefore, expanded to the entire mesolithion [14] (Table 1). Abel’s [12]
classification of accidental cave visitors, occasional cave dwellers and cave-exclusive
dwellers, with a number of subcategories, corresponds to the categories used in the
present classification (Table 1).
Table 1. Classification of fish presence in the mesolithial habitats and Abel’s (1959) corresponding
categories.
Fish Presence in Mesolithion Abel’s [12] Corresponding Categories
1. Accidental visitors A, B.I.a
2. Regular visitors and switchers between
mesolithion and open water:
Figure 11. Habitat position of fishes occurring in marine caves. Benthic fishes: (1) cryptobenthic fish,
(2) epibenthic fish, (3) hyperbenthic fish. Pelagic fishes: (4) benthopelagic fishes, (5) neritic epipelagic
fishes. Drawing by M. Kovaˇci´c.

Fishes 2024,9, 243 12 of 34
3.2.2. Mesolithial Fish
Marine caves sensu stricto apparently lack independent fish fauna and genuine speleobion-
tic fishes, since marine caves share fish fauna at least with the other mesolithial habitats [
13
–
15
].
The terms accidental cave visitors, regular cave visitors, switchers between caves and open
water, and finally, cave permanent residents (including both those that occur in other habitats
and those which are cave exclusive permanent residents) should be, therefore, expanded to the
entire mesolithion [
14
] (Table 1). Abel’s [
12
] classification of accidental cave visitors, occasional
cave dwellers and cave-exclusive dwellers, with a number of subcategories, corresponds to the
categories used in the present classification (Table 1).
Table 1. Classification of fish presence in the mesolithial habitats and Abel’s (1959) corresponding categories.
Fish Presence in Mesolithion Abel’s [12] Corresponding Categories
1. Accidental visitors A, B.I.a
2. Regular visitors and switchers between mesolithion and open water:
2.1. Visitors and switchers with no time regularity B.I.b, B.I.c, B.III.a in part, B.III.b in part
2.2. Diel switchers B.II.b, B.II.c
2.3. Seasonal switchers B.II.a
3. Permanent residents whose populations also occur in other habitats but
individuals do not migrate B.III.a in part, B.III.b in part
4. Exclusive permanent residents C
The terms “mesolithion” community and its “mesolithial” habitats are defined as
the parts of the littoral system with dim light and decreasing currents, where seden-
tary suspension feeders dominate [
14
,
15
]. The term mesolithial habitats is applied to
marine caves in a broad sense [
13
]. According to Zander [
38
], caves, pseudocaves and
cryptobenthic habitats are specific habitats of the mesolithion (Figure 12). Therefore,
mesolithial habitats include: (1) “cryptobenthic” habitats, specifically the small restricted
living spaces underneath the surface of the seabed’s substrate or biocover, with a physical
barrier to any open spaces (e.g., cavities, crevices in bedrock, interstitial spaces among
boulders, pebbles or gravels, or spaces produced by biocover), which harbour “cryptoben-
thos” such as small-sized cryptobenthic fishes [
109
]; (2) “pseudocaves”, specifically larger
crevices, gaps, overhanging rocks and coral formations, where the greatest proportion of
fish is hyperbenthic [
38
], though epibenthic and cryptobenthic fishes may also be present;
and (3) “real” marine caves, having large, mostly closed spaces where light and currents
decrease sharply [
14
,
38
] (Figure 12). Marine caves are inhabited by hyperbenthic and
epibenthic fishes [
38
], as well as by cryptobenthic fishes present in “cave within cave”
small spaces inside marine caves [
18
] (Figure 12). The term “mostly closed” means that the
marine cave opening to the outer environment is small compared to total surface area of the
cave (in contrast to pseudocaves), while the term “large size” means that marine caves have
enough space to contain large motile fauna such as hyperbenthic fishes, which cryptoben-
thic habitats do not usually have. Abel’s [
12
] term for “thigmotaxic caves” actually refers
to “cryptobenthic” habitats, which occur as crevices, holes and cavities on the sea floor, but
also as “cave within cave” inside larger “optical caves” [
12
]. The described characteristics
of the third listed type of mesolithial habitats, “real” marine caves, correspond more or less
to the definition of marine caves sensu Gerovasileiou and Bianchi [9] (Figure 12).
The two examples provided by Zander [
38
] for the ecological classification of fish
species in the Red Sea pseudocaves (Pseudanthias squamipinnis and Pseudochromis fridmani)
correspond to a mesolithial visitor and to a diel switcher between the mesolithion and the
open water, respectively (Table 1). Therefore, none of these exemplifies the permanent
mesolithial residents. On the other hand, the three examples of “speleobiont“ fishes by
Abel [
12
], A.anthias (Figure 6C), M.nigriceps (Figure 10A,C), and T.minor (Figure 10D), can
be confirmed as exclusive permanent mesolithial residents, rarely or never seen outside
these habitats (Table 1). Nevertheless, it remains unclear as to whether the dependence of

Fishes 2024,9, 243 13 of 34
A.anthias on hidden habitats continues to the deep benthic environments of circalittoral
rocky beds [107].
Fishes 2024, 9, x FOR PEER REVIEW 14 of 37
2.1. Visitors and switchers with no time
regularity
B.I.b, B.I.c, B.III.a in part, B.III.b in part
2.2. Diel switchers B.II.b, B.II.c
2.3. Seasonal switchers B.II.a
3. Permanent residents whose populations also
occur in other habitats but individuals do not
migrate
B.III.a in part
,
B.III.b in part
4. Exclusive permanent residents C
The terms “mesolithion” community and its “mesolithial” habitats are defined as the
parts of the littoral system with dim light and decreasing currents, where sedentary
suspension feeders dominate [14,15]. The term mesolithial habitats is applied to marine
caves in a broad sense [13]. According to Zander [38], caves, pseudocaves and
cryptobenthic habitats are specific habitats of the mesolithion (Figure 12). Therefore,
mesolithial habitats include: (1) “cryptobenthic” habitats, specifically the small restricted
living spaces underneath the surface of the seabed’s substrate or biocover, with a physical
barrier to any open spaces (e.g., cavities, crevices in bedrock, interstitial spaces among
boulders, pebbles or gravels, or spaces produced by biocover), which harbour
“cryptobenthos” such as small-sized cryptobenthic fishes [109]; (2) “pseudocaves”,
specifically larger crevices, gaps, overhanging rocks and coral formations, where the
greatest proportion of fish is hyperbenthic [38], though epibenthic and cryptobenthic
fishes may also be present; and (3) “real” marine caves, having large, mostly closed spaces
where light and currents decrease sharply [14,38] (Figure 12). Marine caves are inhabited
by hyperbenthic and epibenthic fishes [38], as well as by cryptobenthic fishes present in
“cave within cave” small spaces inside marine caves [18] (Figure 12). The term “mostly
closed” means that the marine cave opening to the outer environment is small compared
to total surface area of the cave (in contrast to pseudocaves), while the term “large size”
means that marine caves have enough space to contain large motile fauna such as
hyperbenthic fishes, which cryptobenthic habitats do not usually have. Abel’s [12] term
for “thigmotaxic caves” actually refers to “cryptobenthic” habitats, which occur as
crevices, holes and cavities on the sea floor, but also as “cave within cave” inside larger
“optical caves” [18]. The described characteristics of the third listed type of mesolithial
habitats, “real” marine caves, correspond more or less to the definition of marine caves
sensu Gerovasileiou and Bianchi [9] (Figure 12).
Figure 12. Mesolithial habitats. (A) Marine caves sensu Gerovasileiou and Bianchi [9]. (B)
Pseudocaves sensu Zander [38]. (C) “Cryptobenthic” habitats sensu Kovačić et al. [109]. Drawing by
M. Kovačić.
Figure 12. Mesolithial habitats. (A) Marine caves sensu Gerovasileiou and Bianchi [
9
]. (B) Pseudocaves
sensu Zander [38]. (C) “Cryptobenthic” habitats sensu Kovaˇci´c et al. [109]. Drawing by M. Kovaˇci´c.
3.2.3. Permanent Mesolithial Residents in Marine Caves
Permanent mesolithial residents includes cryptobenthic fishes as well as hyperbenthic
and epibenthic fishes using larger volumes of caves and pseudocaves [18,19,38].
Cryptobenthic fishes are well-studied in the Mediterranean Sea (Glaviˇci´c et al. [
112
] and
references therein), with the data also available for marine caves [
18
,
19
]. Available quantitative
studies have found that the gobies Odondebuenia balearica (Pellegrin & Fage, 1907), Zebrus zebrus
(Risso, 1827) (Figure 13A) and Corcyrogobius liechtensteini (Kolombatovi´c, 1891) (Figure 13B),
are among the most frequent and abundant cryptobenthic fishes of the littoral zone of
the Adriatic Sea (Glaviˇci´c et al. [
112
] and references therein) (Figure 13). In addition to
Gobiidae, other cryptobenthic permanent mesolithial residents of the Mediterranean Sea
belong to the families Blenniidae, Gobiesocidae and Tripterygiidae (Glaviˇci´c et al. [
112
]
and references therein). These small fishes are not merely permanent mesolithial residents
but are also exclusive mesolithial residents which do not inhabit other habitats and are
rarely recorded outside cryptic habitats (Glaviˇci´c et al. [
112
] and references therein). In the
“cave within cave” microhabitat of Mediterranean marine caves sensu stricto, the dominant
fish species composition of cryptobenthic fishes is slightly different, with C.liechtensteini
(Figure 13B), Marcelogobius splechtnai (Ahnelt & Patzner, 1995) (Figure 13D) and Z.zebrus
(Figure 13A) prevailing in terms of abundance and frequency of occurrence over other cave
fish species [
18
] (Figure 12). Again, all these species belong to the family Gobiidae. Due
to the limited size and mobility of these species it might be expected that the individuals
found inside caves are, after settling of larvae, the lifetime cave residents, although these
species can be also found in other mesolithial habitats as well. Gammogobius steintzi Bath,
1971 (Figure 7C) is the only species among gobies in marine caves with published records
known only from marine caves sensu stricto [
99
]. However, this species is not an exclusive
cave resident, as it has been observed outside caves in other mesolithial habitats by one of
the authors (MK) in the north and middle Adriatic Sea (samples were collected and stored
in the collection of the Natural History Museum Rijeka, Croatia).
Studies about marine cave fauna often completely miss not only the cryptobenthic fish
fauna of inner cave parts, but also the cryptobenthic fish fauna of shallow caves and cave
entrances which are characterized by more light. For example,
Gerovasileiou et al. [71]
listed 37 fish species from 66 marine caves of the eastern Mediterranean Sea based on
targeted cave surveys, information provided by recreational divers, and the published
scientific literature, and yet they only recorded one cryptobenthic fish, the gobiesocid
Lepadogaster candolii Risso, 1810. Specifically, this species was found inside the canals
of the sponge Aplysina aerophoba (Nardo, 1833) in the semidark part of a large and deep
Aegean cave [
71
]. The attention paid to cryptobenthic fish of Mediterranean marine

Fishes 2024,9, 243 14 of 34
caves has recently benefited by the works of Ragkousis et al. [
19
] in the Aegean Sea
and
Kovaˇci´c et al. [18]
in the Adriatic Sea. These works increased the knowledge on the
diversity, abundance, distribution and ecology of cryptobenthic and other small fishes of
marine caves in the Mediterranean Sea, recording nine species and 15 species, respectively.
Fishes 2024, 9, x FOR PEER REVIEW 16 of 37
Figure 13. (A) Zebrus zebrus (Risso, 1827); (B) Corcyrogobius liechtensteini (Kolombatović, 1891); (C)
Lepadogaster lepadogaster (Bonnaterre, 1788); (D) Marcelogobius splechtnai (Ahnelt & Patzner, 1995).
Photo (A,C): R.A. Patzner; (B,D): S. Le Bris.
Studies about marine cave fauna often completely miss not only the cryptobenthic
fish fauna of inner cave parts, but also the cryptobenthic fish fauna of shallow caves and
cave entrances which are characterized by more light. For example, Gerovasileiou et al.
[71] listed 37 fish species from 66 marine caves of the eastern Mediterranean Sea based on
targeted cave surveys, information provided by recreational divers, and the published
scientific literature, and yet they only recorded one cryptobenthic fish, the gobiesocid
Lepadogaster candolii Risso, 1810. Specifically, this species was found inside the canals of
the sponge Aplysina aerophoba (Nardo, 1833) in the semidark part of a large and deep
Aegean cave [71]. The attention paid to cryptobenthic fish of Mediterranean marine caves
has recently benefited by the works of Ragkousis et al. [19] in the Aegean Sea and Kovačić
et al. [18] in the Adriatic Sea. These works increased the knowledge on the diversity,
abundance, distribution and ecology of cryptobenthic and other small fishes of marine
caves in the Mediterranean Sea, recording nine species and 15 species, respectively.
Fish adaptations to small, flat or narrow spaces of the cryptobenthic component of
mesolithion include size reduction and changes in shape, mostly in gobies and clingfishes
[113,114]. The shape adaptation found by Vukić et al. [114], consists of reduced body size
with large head and large jaw width to body size ratio, stout anterior part of the body,
and robust body. These adaptations make these fish large predators with small bodies
inside the small spaces, such as the Mediterranean gobiid Z. zebrus (Figure 13A) or the
clingfish Lepadogaster lepadogaster (Bonnaterre, 1788) (Figure 13B), both also recorded in
marine caves. Another kind of cryptobenthic adaptation includes a small, elongated body
with flattened head, more suited for moving inside interstitial spaces of multiple layers of
particles, such as in the Mediterranean genus Chromogobius de Buen, 1930 (Figure 14A).
Extreme adaptations of this type are found in fish inhabiting the interstitium of gravel
beaches, such as the Western Pacific gobiid genus Luciogobius Gill, 1859 and the
Mediterranean clingfish genus Gouania Nardo, 1833 (Figure 14B). In addition to the
elongated body with flattened head, gravel-inhabiting Gouania species show adaptation
with troglomorphic elements, specifically reduced pigmentation and small eyes, although
both are still present [16,113]. The species of both genera, Chromogobius and Gouania, were
also recorded in marine caves (Table 2). All these fishes are permanent and exclusive
mesolithial residents, which do not inhabit other habitats and are never or rarely recorded
outside cryptic habitats.
Figure 13. (A)Zebrus zebrus (Risso, 1827); (B)Corcyrogobius liechtensteini (Kolombatovi´c, 1891);
(C)Lepadogaster lepadogaster (Bonnaterre, 1788); (D)Marcelogobius splechtnai (Ahnelt & Patzner, 1995).
Photo (A,C): R.A. Patzner; (B,D): S. Le Bris.
Fish adaptations to small, flat or narrow spaces of the cryptobenthic component of
mesolithion include size reduction and changes in shape, mostly in gobies and cling-
fishes [
113
,
114
]. The shape adaptation found by Vuki´c et al. [
114
], consists of reduced body
size with large head and large jaw width to body size ratio, stout anterior part of the body,
and robust body. These adaptations make these fish large predators with small bodies
inside the small spaces, such as the Mediterranean gobiid Z.zebrus (Figure 13A) or the cling-
fish Lepadogaster lepadogaster (Bonnaterre, 1788) (Figure 13B), both also recorded in marine
caves. Another kind of cryptobenthic adaptation includes a small, elongated body with
flattened head, more suited for moving inside interstitial spaces of multiple layers of parti-
cles, such as in the Mediterranean genus Chromogobius de Buen, 1930 (Figure 14A). Extreme
adaptations of this type are found in fish inhabiting the interstitium of gravel beaches, such
as the Western Pacific gobiid genus Luciogobius Gill, 1859 and the Mediterranean clingfish
genus Gouania Nardo, 1833 (Figure 14B). In addition to the elongated body with flattened
head, gravel-inhabiting Gouania species show adaptation with troglomorphic elements,
specifically reduced pigmentation and small eyes, although both are still present [
16
,
113
].
The species of both genera, Chromogobius and Gouania, were also recorded in marine caves
(Table 2). All these fishes are permanent and exclusive mesolithial residents, which do not
inhabit other habitats and are never or rarely recorded outside cryptic habitats.
Fishes 2024, 9, x FOR PEER REVIEW 17 of 37
Figure 14. (A) Chromogobius quadrivittatus (Steindachner, 1863); (B) Gouania pigra (Nardo, 1827).
Photo (A): M. Kovačić; (B): M. Wagner.
Small fishes that are permanent and exclusive residents of dimly sheltered
mesolithial habitats are not necessarily permanently utilizing “cave within cave”
microhabitats. Some species occur regularly on open inner surfaces of caves and
pseudocaves, such as the Mediterranean Tripterygion melanurus Guichenot, 1850 (Figure
10D), T. minor (Figure 10B) and M. nigriceps (Figure 10A,C) (Table 2) [45,46]. At the
microhabitat scale, they function as the epibenthic fish within larger cryptic spaces,
looking for shelters or “thigmotaxic caves” sensu Abel [12], only when scared or chased.
However, Abel [12] distinguished M. nigriceps and T. minor, observing that M. nigriceps
still has regular hiding places within the cave, while T. minor does not. The “epibenthic”
position means lying on the inner cave surface, which also includes the attached position
to vertical walls or the upside-down position on the roof of the larger space. Neither of
these three species has any types of morphological adaptation such as those described for
cryptobenthic permanent mesolithial residents. They are just small, with slender bodies
and a colouration pattern dominated by mostly and more or less uniformly red or orange
colour, which looks like the third body type adaptation to mesolithion, in addition to two
cryptobenthic body shapes. Two large genera of Indo-Pacific gobies with about a hundred
different species, Eviota Jenkins, 1903 and Trimma Jordan & Seale, 1906, include numerous
small species described from habitats matching with the definition of mesolithial habitats
and some of them even in the marine caves sensu stricto [89,94,101]. Hagiwara and
Winterbottom [101] stated that Trimma hayashii Hagiwara & Winterbottom, 2007 often
stays in an “epibenthic” upside-down position on the ceiling of the larger space, like the
Mediterranean fish described before, but Trimma flavatrum Hagiwara & Winterbottom,
2007 (Figure 15A) forms small schools hovering close to the surface in caves, always with
the ventral side facing the nearest substrate, so when schools are close to ceilings, it
resembles strange upside-down hovering and hyperbenthic behaviour. The hovering near
the cave roof behaviour was also observed in the Red Sea Trimma nubarum Winterbottom,
Bogorodsky & Alpermann, 2023 (Figure 15B) [94]. Many Trimma species live or can be
found in caves on the reef (R. Winterbottom, personal communication). None of them are
known as exclusive cave-dwellers, but they are permanent mesolithial residents, since all
also occur in other mesolithial habitats, like small shallow caves and crevices (R.
Winterbottom, personal communication). These species are very small and, as in the case
of the Mediterranean Tripterygion melanurus, T. minor and M. nigriceps, most have a
colouration pattern dominated by a red or orange colour.
Figure 14. (A)Chromogobius quadrivittatus (Steindachner, 1863); (B)Gouania pigra (Nardo, 1827). Photo
(A): M. Kovaˇci´c; (B): M. Wagner.

Fishes 2024,9, 243 15 of 34
Small fishes that are permanent and exclusive residents of dimly sheltered mesolithial
habitats are not necessarily permanently utilizing “cave within cave” microhabitats. Some
species occur regularly on open inner surfaces of caves and pseudocaves, such as the
Mediterranean Tripterygion melanurus Guichenot, 1850 (Figure 10D), T.minor (Figure 10B)
and M.nigriceps (Figure 10A,C) (Table 2) [
45
,
46
]. At the microhabitat scale, they function
as the epibenthic fish within larger cryptic spaces, looking for shelters or “thigmotaxic
caves” sensu Abel [
12
], only when scared or chased. However, Abel [
12
] distinguished
M.nigriceps and T.minor, observing that M.nigriceps still has regular hiding places within
the cave, while T.minor does not. The “epibenthic” position means lying on the inner cave
surface, which also includes the attached position to vertical walls or the upside-down
position on the roof of the larger space. Neither of these three species has any types of
morphological adaptation such as those described for cryptobenthic permanent mesolithial
residents. They are just small, with slender bodies and a colouration pattern dominated by
mostly and more or less uniformly red or orange colour, which looks like the third body
type adaptation to mesolithion, in addition to two cryptobenthic body shapes. Two large
genera of Indo-Pacific gobies with about a hundred different species, Eviota Jenkins, 1903
and Trimma Jordan & Seale, 1906, include numerous small species described from habitats
matching with the definition of mesolithial habitats and some of them even in the marine
caves sensu stricto [
85
,
92
,
96
]. Hagiwara and Winterbottom [
85
] stated that Trimma hayashii
Hagiwara & Winterbottom, 2007 often stays in an “epibenthic” upside-down position on the
ceiling of the larger space, like the Mediterranean fish described before, but Trimma flavatrum
Hagiwara & Winterbottom, 2007 (Figure 15A) forms small schools hovering close to the
surface in caves, always with the ventral side facing the nearest substrate, so when schools
are close to ceilings, it resembles strange upside-down hovering and hyperbenthic be-
haviour. The hovering near the cave roof behaviour was also observed in the Red Sea
Trimma nubarum Winterbottom, Bogorodsky & Alpermann, 2023 (Figure 15B) [
96
]. Many
Trimma species live or can be found in caves on the reef (R. Winterbottom, personal com-
munication). None of them are known as exclusive cave-dwellers, but they are permanent
mesolithial residents, since all also occur in other mesolithial habitats, like small shal-
low caves and crevices (R. Winterbottom, personal communication). These species are
very small and, as in the case of the Mediterranean Tripterygion melanurus,T.minor and
M.nigriceps, most have a colouration pattern dominated by a red or orange colour.
Fishes 2024, 9, x FOR PEER REVIEW 18 of 37
Figure 15. (A) Trimma flavatrum (Hagiwara & Winterbottom, 2007), photo by K. Hagiwara. (B)
Trimma nubarum Winterbottom, (Bogorodsky & Alpermann, 2023), photo by S.V. Bogorodsky.
The bythitid Grammonus ater (Risso, 1810) (Figure 9B) is another Mediterranean cave
inhabitant with a long history of published records, but with no positive published
records outside marine caves [19]. This species has also been observed and collected from
other mesolithial habitats by one of the authors (MKcollected material has been stored in
the collection of the Natural History Museum Rijeka, Croatia) (Figure 9B). Grammonus ater,
therefore, can be considered as a permanent and exclusive mesolithial resident, which
does not inhabit other habitats and has never been recorded outside cryptic environments.
In the Mediterranean Sea this species has been found in numerous marine caves from the
Levantine Basin [115] to the Alboran Sea [116] (Table 2). Other Grammonus species were
also reported in littoral marine caves in other seas, such as West Pacific Grammonus thielei
Nielsen & Cohen, 2004 and Grammonus yunokawai Nielsen, 2006 [83,99] and Hawaiian
Grammonus nagaredai Randall & Hughes, 2008 [85]. Several species of Bythitidae, such as
the above-mentioned Grammonus spp., occur in shallow water mesolithion; some are
confined to brackish or freshwater, while others are deep-water fishes [117]. While
brackish or freshwater Bythitidae from anchialine habitats show troglomorphic
adaptation such as the loss of eyes and pigmentation, like Typhlias pearsei Hubbs, 1938
[118], the mesolithial littoral marine species have the typical adaptation of deep-water
bythitids, having brown or black pigmentation and small eyes reduced in size [119]. The
species Protanguilla palau Johnson, Ida & Sakaue 2012 (Figure 9A) has been described as a
“living fossil” from a marine cave, as it represents the only fish family restricted to marine
caves, the family Protanguillidae Johnson, Ida & Sakaue 2012. This species also has the
adaptation of dark pigmentation and small eyes [86] (Figure 9A). It is possible, although
still not recorded, that species like P. palau or Grammonus ater are nocturnal, leaving the
caves to forage on the reef at night. Several species of Anguilliformes and Ophidiiformes
reported by Hui et al. [87] in submarine and associated anchialine caves in Christmas
Island (Indian Ocean Territory, Australia), also exhibit this body shape. The dark
pigmentation and small eyes, combined with elongated snake-like body, resemble the
fourth body type of adaptation to mesolithion, in addition to two body and colouration
types of small cryptobenthic fish and one body and colouration type of small epibenthic
fish. However, permanent and exclusive residents of mesolithial habitats share this body
type with confirmed nocturnal species which spend the daytime in mesolithion but leave
these shelters at night, therefore making them diel switchers between open sea floor
spaces and mesolithial habitats, like Gaidropsarus mediterraneus (Linnaeus, 1758) (Figure
9C), and with eurybathic species recorded in marine caves, like Ophidion barbatum
(Linnaeus, 1758) (Figure 9D).
In addition to the cryptobenthic and epibenthic fishes, which could be lifelong
exclusive mesolithial residents, some Mediterranean hyperbenthic fishes are also
exclusive permanent mesolithial residents. Anthias anthias (Figure 6C) was already
considered to be an exclusive cave dweller by Abel [12] (Table 2). The species was indeed
recorded at the cave entrances as well as inside caves, in semidark zones [71], being an
exclusive permanent mesolithial resident. The rare records of A. anthias in the published
studies about Mediterranean caves could possibly be due to the deep bathymetric range
Figure 15. (A)Trimma flavatrum Hagiwara & Winterbottom, 2007, photo by K. Hagiwara. (B)Trimma nubarum
Winterbottom, Bogorodsky & Alpermann, 2023, photo by S.V. Bogorodsky.
The bythitid Grammonus ater (Risso, 1810) (Figure 9B) is another Mediterranean cave
inhabitant with a long history of published records, but with no positive published records
outside marine caves [
19
]. This species has also been observed and collected from other
mesolithial habitats by one of the authors (MKcollected material has been stored in the
collection of the Natural History Museum Rijeka, Croatia) (Figure 9B). Grammonus ater,
therefore, can be considered as a permanent and exclusive mesolithial resident, which does
not inhabit other habitats and has never been recorded outside cryptic environments. In
the Mediterranean Sea this species has been found in numerous marine caves from the
Levantine Basin [
115
] to the Alboran Sea [
116
] (Table 2). Other Grammonus species were

Fishes 2024,9, 243 16 of 34
also reported in littoral marine caves in other seas, such as West Pacific Grammonus thielei
Nielsen & Cohen, 2004 and Grammonus yunokawai Nielsen, 2006 [
83
,
84
] and Hawaiian
Grammonus nagaredai Randall & Hughes, 2008 [
88
]. Several species of Bythitidae, such
as the above-mentioned Grammonus spp., occur in shallow water mesolithion; some are
confined to brackish or freshwater, while others are deep-water fishes [
117
]. While brackish
or freshwater Bythitidae from anchialine habitats show troglomorphic adaptation such as
the loss of eyes and pigmentation, like Typhlias pearsei Hubbs, 1938 [
118
], the mesolithial
littoral marine species have the typical adaptation of deep-water bythitids, having brown
or black pigmentation and small eyes reduced in size [
119
]. The species Protanguilla palau
Johnson, Ida & Sakaue 2012 (Figure 9A) has been described as a “living fossil” from a
marine cave, as it represents the only fish family restricted to marine caves, the family
Protanguillidae Johnson, Ida & Sakaue 2012. This species also has the adaptation of dark
pigmentation and small eyes [
89
] (Figure 9A). It is possible, although still not recorded,
that species like P.palau or Grammonus ater are nocturnal, leaving the caves to forage
on the reef at night. Several species of Anguilliformes and Ophidiiformes reported by
Hui et al. [90]
in submarine and associated anchialine caves in Christmas Island (Indian
Ocean Territory, Australia), also exhibit this body shape. The dark pigmentation and small
eyes, combined with elongated snake-like body, resemble the fourth body type of adaptation
to mesolithion, in addition to two body and colouration types of small cryptobenthic fish and
one body and colouration type of small epibenthic fish. However, permanent and exclusive
residents of mesolithial habitats share this body type with confirmed nocturnal species which
spend the daytime in mesolithion but leave these shelters at night, therefore making them diel
switchers between open sea floor spaces and mesolithial habitats, like Gaidropsarus mediterraneus
(Linnaeus, 1758) (Figure 9C), and with eurybathic species recorded in marine caves, like
Ophidion barbatum (Linnaeus, 1758) (Figure 9D).
In addition to the cryptobenthic and epibenthic fishes, which could be lifelong ex-
clusive mesolithial residents, some Mediterranean hyperbenthic fishes are also exclusive
permanent mesolithial residents. Anthias anthias (Figure 6C) was already considered to
be an exclusive cave dweller by Abel [
12
] (Table 2). The species was indeed recorded
at the cave entrances as well as inside caves, in semidark zones [
71
], being an exclusive
permanent mesolithial resident. The rare records of A.anthias in the published stud-
ies about Mediterranean caves could possibly be due to the deep bathymetric range of
A.anthias, down to 200 m depth [
107
], and the relatively shallow depth of most studied
marine caves [
17
]. Anthias anthias shows the adaptation of large eyes for better vision
in dimly lit habitats and red colouration for better camouflage against potential enemies
in dim light. Based on the similar hyperbenthic fish adaptations of large eyes and red
body colouration, the native Mediterranean Callanthias ruber (Rafinesque, 1810) is another
candidate among hyperbenthic fishes of exclusive permanent mesolithial residents [
107
].
It occurs within a wide depth range (50–500 m) and has also been recorded from marine
caves [
120
]. It is also possible that this is an exclusive mesolithial species in the upper
circalittoral zone, and a species of open benthic environments in the lower circalittoral
and bathyal zones, selecting the same photic condition along its depth range; however,
there is no published evidence for this hypothesis. The adaptations of large eyes and
red colouration on a typical hyperbenthic body that is tall and laterally compressed re-
sembles the fifth type of body adaptation for life in the mesolithion. However, this body
type is not restricted to permanent and exclusive mesolithial residents since it is shared
with nocturnal species that spend the day in mesolithion and leave these shelters at night
and, therefore, are diel switchers between open sea floor spaces and mesolithial habitats,
like the Mediterranean cardinal fish A.imberbis (Figure 6A) [
64
,
70
,
121
] or the Indo-Pacific
species S.rubrum (Figure 6D) [
122
]. Similar adaptions can be observed worldwide in some
other fish families such as Apogonidae, Callanthidae (Figure 6C), Labridae, Holocentridae
(Figure 6E), Priacanthidae (Figure 6F), Pseudochromidae (Figure 6B) and Serranidae of the
subfamily Anthinae. Such species are rarely exclusive permanent mesolithial residents, like
Chlidichthys auratus Lubbock, 1975 (Figure 6B), and the most mesolithial species of these

Fishes 2024,9, 243 17 of 34
families for which the data exist are switchers between marine caves and open benthic
environments or species also living at greater depths [91,94,95,120,123–125].
In addition to the five observed types of body adaptation for life in mesolithion, a
number of permanent and exclusive mesolithial fish residents have various other body
shapes and body colouration patterns. Some permanent and exclusive mesolithial resi-
dents show unique body shape and colouration, like the Mediterranean population of the
epibenthic gobiid Thorogobius ephippiatus (Lowe, 1839) (Figure 16A), which has frequently
been reported in microhabitats within larger cryptic spaces (e.g., marine caves), looking for
thigmotaxic shelters only when scared or chased (Table 2). Compared to the small earlier
mentioned cryptobenthic and epibenthic fishes, this is a larger goby with striking dark
flecked colouration. The seabed positioning and behaviour of T.ephippiatus in the Adriatic
Sea was described by Kovaˇci´c [60].
Fishes 2024, 9, x FOR PEER REVIEW 19 of 37
of A. anthias, down to 200 m depth [108], and the relatively shallow depth of most studied
marine caves [17]. Anthias anthias shows the adaptation of large eyes for better vision in
dimly lit habitats and red colouration for better camouflage against potential enemies in
dim light. Based on the similar hyperbenthic fish adaptations of large eyes and red body
colouration, the native Mediterranean Callanthias ruber (Rafinesque, 1810) is another
candidate among hyperbenthic fishes of exclusive permanent mesolithial residents [107].
It occurs within a wide depth range (50–500 m) and has also been recorded from marine
caves [120]. It is also possible that this is an exclusive mesolithial species in the upper
circalittoral zone, and a species of open benthic environments in the lower circalittoral
and bathyal zones, selecting the same photic condition along its depth range; however,
there is no published evidence for this hypothesis. The adaptations of large eyes and red
colouration on a typical hyperbenthic body that is tall and laterally compressed resembles
the fifth type of body adaptation for life in the mesolithion. However, this body type is
not restricted to permanent and exclusive mesolithial residents since it is shared with
nocturnal species that spend the day in mesolithion and leave these shelters at night and,
therefore, are diel switchers between open sea floor spaces and mesolithial habitats, like
the Mediterranean cardinal fish A. imberbis (Figure 6A) [64,70,121] or the Indo-Pacific
species S. rubrum (Figure 6D) [122]. Similar adaptions can be observed worldwide in some
other fish families such as Apogonidae, Callanthidae (Figure 6C), Labridae, Holocentridae
(Figure 6E), Priacanthidae (Figure 6F), Pseudochromidae (Figure 6B) and Serranidae of
the subfamily Anthinae. Such species are rarely exclusive permanent mesolithial
residents, like Chlidichthys auratus Lubbock, 1975 (Figure 6B), and the most mesolithial
species of these families for which the data exist are switchers between marine caves and
open benthic environments or species also living at greater depths [88,91,93,120,123–125].
In addition to the five observed types of body adaptation for life in mesolithion, a
number of permanent and exclusive mesolithial fish residents have various other body
shapes and body colouration patterns. Some permanent and exclusive mesolithial
residents show unique body shape and colouration, like the Mediterranean population of
the epibenthic gobiid Thorogobius ephippiatus (Lowe, 1839) (Figure 16A), which has
frequently been reported in microhabitats within larger cryptic spaces (e.g., marine caves),
looking for thigmotaxic shelters only when scared or chased (Table 2). Compared to the
small earlier mentioned cryptobenthic and epibenthic fishes, this is a larger goby with
striking dark flecked colouration. The seabed positioning and behaviour of T. ephippiatus
in the Adriatic Sea was described by Kovačić [60].
Figure 16. (A) Thorogobius ephippiatus (Lowe, 1839), photo by S. Guerrieri. (B) Gobius auratus Risso,
1810, photo by M. Kovačić.
Many small-sized benthic fishes can be considered as permanent, but not exclusive
residents, of dimly sheltered large mesolithial habitats of caves and pseudocaves. They
are also regularly recorded in other habitats, but due to their size, lifespan and behaviour,
it is unlikely that the particular individual observed in caves and pseudocaves migrates
to and from open benthic environments. These species show no adaptations for life in
mesolithial habitats. For example, the Mediterranean gobiid Gobius auratus Risso, 1810
Figure 16. (A)Thorogobius ephippiatus (Lowe, 1839), photo by S. Guerrieri. (B)Gobius auratus Risso,
1810, photo by M. Kovaˇci´c.
Many small-sized benthic fishes can be considered as permanent, but not exclusive
residents, of dimly sheltered large mesolithial habitats of caves and pseudocaves. They are also
regularly recorded in other habitats, but due to their size, lifespan and behaviour, it is unlikely
that the particular individual observed in caves and pseudocaves migrates to and from open
benthic environments. These species show no adaptations for life in mesolithial habitats. For
example, the Mediterranean gobiid Gobius auratus Risso, 1810 (Figure 16B) has been recorded
at cave entrances and semidark zones inside caves [
68
] (Table 2). Gobius auratus usually hovers
around a few to about 30 cm above the rocky or mixed substrate, especially juveniles
in small schools, and can be easily noticed by its intensively yellow colouration [
126
]
(Figure 16B). This goby, as well as several blenniid species [
17
,
18
,
71
] are permanent, but
not exclusive, residents of marine caves since they are usually observed in open benthic
environments, mostly as ambivalent fish [112,127].
3.2.4. Fishes as Accidental and Regular Visitors to Mesolithion and as Switchers
between Mesolithion and Open Benthos
Abel [
12
] distinguished marine fishes that are occasional cave dwellers regularly vis-
iting caves versus fishes which generally avoid caves despite having been recorded there.
The Mediterranean examples in the latter category, based on Abel’s records, were single in-
dividuals of neritic epipelagic Belone sp. juvenile and the hyperbenthic Symphodus ocellatus
(Linnaeus, 1758). Other characteristic examples of accidental cave visitors are the previously
mentioned benthopelagic B.boops and the epipelagic S.dumerili [71] (Table 2).
In contrast to accidental visitors, Abel [
12
] included, among the occasional cave
dwellers that regularly visit marine caves, fishes dwelling around adjacent rocky beds,
like Coris julis (Linnaeus, 1758), or which use caves as safe shelters (e.g., Serranus spp.),
for reproduction (e.g., Blenniidae), or for hiding during the day- or night-time, such as
Muraena helena Linnaeus, 1758 (Figure 17A) and groupers (Table 2). All these species can
be considered as common mesolithial visitors or as switchers between mesolithion and
open benthic environments. In addition to the time irregular visiting and to the diel hiding,

Fishes 2024,9, 243 18 of 34
both noticed by Abel [
12
], the seasonal utilization and seasonal switching to mesolithial
habitats of benthic fishes has also been observed in the Mediterranean Sea [
128
,
129
], and
probably occur in other temperate seas. Single fish species can utilize marine caves in many
of these ways, as diel or as seasonal switchers between mesolithion and open water or also
as common visitors to marine caves for different reasons with no time regularity.
The only Mediterranean pomacentrid species, Chromis chromis (Linnaeus, 1758)
(Figure 17B), is known to live in small shoals in midwater, above or near the sea floor, and
to nest on the sea floor, with the juveniles reported as using cavities as shelters during
the night [
130
] (Table 2). Quantitative cryptobenthic studies have revealed intensive use
of cryptic spaces independently of the time of day, by both adult and juvenile C.chromis
(Glaviˇci´c et al. [
112
] and references therein). The species is obviously a common switcher
between hidden spaces and the water column and has also been recorded in marine caves
sensu stricto [12].
Fishes 2024, 9, x FOR PEER REVIEW 20 of 37
(Figure 16B) has been recorded at cave entrances and semidark zones inside caves [68]
(Table 2). Gobius auratus usually hovers around a few to about 30 cm above the rocky or
mixed substrate, especially juveniles in small schools, and can be easily noticed by its
intensively yellow colouration [126] (Figure 16B). This goby, as well as several blenniid
species [17,18,71] are permanent, but not exclusive, residents of marine caves since they
are usually observed in open benthic environments, mostly as ambivalent fish [112,127].
3.2.4. Fishes as Accidental and Regular Visitors to Mesolithion and as Switchers between
Mesolithion and Open Benthos
Abel [12] distinguished marine fishes that are occasional cave dwellers regularly
visiting caves versus fishes which generally avoid caves despite having been recorded
there. The Mediterranean examples in the latter category, based on Abel’s records, were
single individuals of neritic epipelagic Belone sp. juvenile and the hyperbenthic Symphodus
ocellatus (Linnaeus, 1758). Other characteristic examples of accidental cave visitors are the
previously mentioned benthopelagic B. boops and the epipelagic S. dumerili [71] (Table 2).
In contrast to accidental visitors, Abel [12] included, among the occasional cave
dwellers that regularly visit marine caves, fishes dwelling around adjacent rocky beds,
like Coris julis (Linnaeus, 1758), or which use caves as safe shelters (e.g., Serranus spp.), for
reproduction (e.g., Blenniidae), or for hiding during the day- or night-time, such as
Muraena helena Linnaeus, 1758 (Figure 17A) and groupers (Table 2). All these species can
be considered as common mesolithial visitors or as switchers between mesolithion and
open benthic environments. In addition to the time irregular visiting and to the diel
hiding, both noticed by Abel [12], the seasonal utilization and seasonal switching to
mesolithial habitats of benthic fishes has also been observed in the Mediterranean Sea
[128,129], and probably occur in other temperate seas. Single fish species can utilize
marine caves in many of these ways, as diel or as seasonal switchers between mesolithion
and open water or also as common visitors to marine caves for different reasons with no
time regularity.
The only Mediterranean pomacentrid species, Chromis chromis (Linnaeus, 1758)
(Figure 17B), is known to live in small shoals in midwater, above or near the sea floor, and
to nest on the sea floor, with the juveniles reported as using cavities as shelters during the
night [130] (Table 2). Quantitative cryptobenthic studies have revealed intensive use of
cryptic spaces independently of the time of day, by both adult and juvenile C. chromis
(Glavičić et al. [112] and references therein). The species is obviously a common switcher
between hidden spaces and the water column and has also been recorded in marine caves
sensu stricto [12].
Figure 17. (A)Muraena helena Linnaeus, 1758; (B)Chromis chromis (Linnaeus, 1758); (C) and (D)
Sciaena umbra Linnaeus, 1758. Photo (A,C,D): R.A. Patzner; (B): M. Kovaˇci´c.
The cardinal fish A.imberbis (Figure 6A), despite being reported from the dark deep
zones of caves [
71
,
121
], was classified by Abel [
12
] as a switcher between marine caves and
open benthic environments, using marine caves as a daytime hiding place and moving out
from the caves at night to feed [
12
,
13
] (Table 2). The night shift of A.imberbis was recorded
by Bussotti et al. [
64
]. Bussotti et al. [
70
,
121
] confirmed the existence of nychthemeral (diel)
movements of this species to the outside of marine caves for feeding. The high densities
of A.imberbis and its frequency of occurrence show that this species is by far the most
represented fish within caves in the western Mediterranean [
64
,
70
]. Despite being the most
characteristic fish in Mediterranean marine caves, this species is not an exclusive mesolithial
resident. Sargocentron rubrum (Figure 6D) is also a diel switcher between open benthic
environments and marine caves, leaving cave shelters at night to feed [
13
,
81
,
82
]. Both
species have the adaptations of large eyes and red colouration on a typical hyperbenthic
body that is tall and laterally compressed. Some larger fishes also show a preference for
hidden spaces and occur in darker inner sections of marine caves during the day, such as the
Mediterranean moray eel (M.helena) (Figure 17A), C.conger,Phycis phycis (Linnaeus, 1766),
Sciaena umbra Linnaeus, 1758 (Figure 17C,D) or groupers (Table 2) [
12
,
17
]. Most of these
fishes are nocturnal species which leave their shelters at night and, therefore, are diel
switchers between open benthic environments and mesolithial habitats. The Indo-Pacific
sweeper Pempheris rhomboidea (Figure 7D), an alien species in Mediterranean marine caves,
is another diel switcher between open benthic environments and mesolithial habitats. It

Fishes 2024,9, 243 19 of 34
shelters in large schools inside caves and crevices during the daytime and leaves them at
night to feed [
44
,
59
,
82
]. The coelacanths in deep marine caves of the Indian Ocean occupied
caves during the day but not at night [
31
,
32
]. After sunset coelacanths left their caves and
moved or drifted one by one very slowly across the lava cliffs [32].
Similar body shape and colouration adaption to A.imberbis, that also matches to one
of the body adaptations described for the permanent residents in the mesolithion, can
be found in species of several fish families in other seas, like Apogonidae, Holocentridae
(Figure 6D,E), Priacanthidae (Figure 6F), Serranidae of subfamily Anthinae and Callanthi-
dae. However, these species are nocturnal, leaving marine caves at night for feeding in
open water and spending daytime in marine caves (Apogonidae, [
94
]; Holocentridae, [
125
]
or they live at greater depths (Anthinae, [
91
]; Priacanthidae, [
124
]). Others are daytime
feeders in open water, spending nights in marine caves and ranging from shallow to deeper
zones (Callanthidae, [
120
]). Species of the Apogonidae, Holocentridae, Priacanthidae and
Anthinae have also been recorded near and inside deep-water caves by Heemstra et al. [
33
],
showing visiting or switching behaviour even at large depths.
Two larger mesolithial habitats, marine caves and pseudocaves, host additional fishes
in winter. Kotrschal [
128
] observed that many Mediterranean Sparidae, Labridae, Ser-
ranidae and Pomacentridae turn from “facultative cave-dwellers” to “obligatory cave-
dwellers” during the winter. He also recorded dense aggregations of C.chromis (Figure 17B)
and Oblada melanura (Linnaeus, 1758) permanently remaining in caves during January and
February. These mixed aggregations in marine caves may also consist of a few individuals of
several species of the genus Diplodus Rafinesque, (Risso, 1810) as well as Symphodus roissali
(Risso, 1810) and C.julis, with an interindividual distance of approximately 10 cm, and no
aggressive behaviour or feeding having been observed (Table 2) [128,129].
Some fishes recorded both in marine caves and in open water are too small and static to
be either visitors to marine caves or switchers between marine caves and open benthos. For
the following examples of irregular, daily or seasonal utilization of cryptobenthic habitats
by small fishes, the fishes at the cave entrance zone very likely do the same, while those
that present deeper inside marine caves possibly also utilize “cave within cave” shelters
in the similar way. Cryptobenthic habitats are very common and abundant on shallow
benthic environments. Crevices in bedrock, interstitial spaces among boulders, pebbles
and gravels or spaces produced by biocover [
109
] are commonly present in almost all rocky
and/or mixed substrates. Almost every epibenthic fish and many hyperbenthic fishes of
rocky and/or mixed substrates will eventually look for shelter in cryptic habitats when
scared or chased and will therefore be mesolithial accidental visitors. Many small littoral
benthic fishes which are not strictly cryptobenthic, were found in significant numbers
during quantitative studies, either hidden or lying on the floor of Mediterranean marine
caves (Glaviˇci´c et al. [
112
] and references therein), thus being more than just mesolithial
accidental visitors. Various gobiid, blenniid and triplefin blennies were therefore classified
as epicryptobenthic [
109
] or as ambivalent fishes, considering habitat positioning being
regular mesolithial visitors or even common switchers between hidden spaces and the
open benthic environment [
112
]. In the Adriatic Sea the occurrence frequencies of epicryp-
tobenthic or ambivalent fishes did not change significantly between the time of day, and
so no specific day- or night-time hiding was found for these fishes [
112
]. In addition, the
cryptobenthic fish assemblage showed diel stability with no night-time presence in the open
benthic environments [
112
]. However, some night visual censuses recorded the absence
of blennies and tripterygiids, which were present at the same place during the daytime
(e.g., Azzurro et al., [
131
]). This would indicate that fishes hide in the mesolithial shel-
ters at night. Nieder and Zander [
132
] found nocturnal activity of Lipophrys trigloides
(Valenciennes, 1836) outside shelters, presuming that all other Mediterranean blennies
spend the night in holes. These findings indicate a possibility that some blennids and
tripterygiids are diel switchers between open benthic environments and cryptobenthic habi-
tats. Abel [
12
] also presumed that holes were nighttime hiding places for Mediterranean
blennies. Regarding seasonal utilization of cryptobenthic habitats, Kotrschal [
128
] recorded

Fishes 2024,9, 243 20 of 34
the winter absence of the Mediterranean Blennidae and Tripterygiidae on the surface of
the substrate, while winter and summer Quinaldine collecting yielded similar numbers
of blennies, tripterygiids and gobies over similar surface areas. Thus, he presumed that
in the winter, the collected fish were hiding in holes and crevices, showing a seasonal
switch to cryptobenthic habitats due to low temperatures. For instance, the Mediterranean
Tripterygion tripteronotus (Risso, 1810), which never hides in crevices during the warm
season, remains hidden during low temperatures [
128
]. Most gobies and blennies, which
are the richest families among the Mediterranean epibenthic fishes in terms of species num-
ber [
133
], are also regular periodic dwellers in cryptobenthic habitats for longer, seasonal
periods using the cryptic habitats as nests for their demersal eggs [
134
,
135
]. Abel [
12
] also
noticed this for Mediterranean blennies and tripterygiids. Most Mediterranean blennies uti-
lize the empty holes of endolithic bivalves as mesolithial nests [
135
]. Therefore, epibenthic
gobies and blennies are seasonal switchers to cryptobenthic habitats for their reproductive
needs. The seasonal presence of blennies inside holes in spring can be explained as a feature
of nest dwelling.
3.3. The Biodiversity of Fishes in Marine Caves and Their Relationship to Marine Caves
Fishes recorded in marine caves constitute only a minute fraction of the total fish
diversity in most marine areas of the world, probably due to the limited research ef-
fort and knowledge (see Sections 3.1 and 3.2). In this sense, it is astonishing that a to-
tal of 132 fish species have been recorded so far from the Mediterranean marine caves
sensu stricto [
9
,
18
,
82
,
136
] (Table 2and references therein), representing about 17% of the
total Mediterranean marine fish diversity and approximately one third of the Mediter-
ranean littoral benthic fish species [
133
,
137
] (Table 2). Only 3.0% are neritic epipelagic
fishes and 4.5% are benthopelagic (Table 2). All benthic species (except one) occur in the
infralittoral zone, with different depth tolerances ranging from strict infralittoral species
(37.1%) to species occurring in infralittoral and circalittoral zones (40.2%) and eurybathic
species, which occur from the infralittoral to the bathyal zone (14.4%). A single species
(0.8%), Callanthias ruber, is known from circalittoral to bathyal waters (Table 2). Regard-
ing the position to the sea floor, counting only the recorded benthic fishes, most species
among benthic fishes are hyperbenthic (49.2%) and epibenthic (38.5%), while 12.3% are
cryptobenthic (Table 2).
A total of 43 families are represented in Mediterranean marine caves. Most species
belong to the families Gobiidae (20 species), Blenniidae (14 species), Labridae (13 species),
Sparidae (12 species), Serranidae (9 species) and Scorpaenidae (6 species), while the remain-
ing families include less than five species each (25 families include only one species). Most
species have been reported from the entrance and semidark zones of the studied marine
caves (91 and 76 species, respectively), while only 34 species have been recorded in dark
cave zones. For 17 fish species there is no information concerning the cave zone where
they were recorded. Gobiidae is the most speciose family in the two anterior cave zones
(15 and 12 species, respectively), followed by Blenniidae at the entrance and Sparidae at
the semidark zone (13 and 10 species, respectively). Most species reported from the dark
zone of caves belong to the families Serranidae (6 species), Gobiidae and Scorpaenidae
(5 species each). While the most speciose families are the same in all cave zones, Blenniidae
disappear completely from the dark cave zone.
We classified fishes recorded in the Mediterranean marine caves as: (a) exclusive
permanent residents of mesolithial habitats (based on references in Sections 3.1 and 3.2;
Glaviˇci´c et al. [
112
] and references therein); (b) mesolithion permanent residents whose pop-
ulations also occur in other habitats, but do not migrate (based on data in
Bussotti et al. [17]
and Kovaˇci´c et al. [
18
]); (c) regular mesolithion visitors and switchers between mesolithion
and open water with temporal regularity (based on data in Kotrschal [
128
], Kotrschal
and Reynolds [
129
], Kotrschal [
135
], Glaviˇci´c et al. [
112
] and references therein). The
species with no published information regarding their relationship to the mesolithion
were assigned based on the authors’ judgement and unpublished experience. Among

Fishes 2024,9, 243 21 of 34
the 132 fish species recorded in the Mediterranean marine caves sensu stricto, 27.3% are
accidental visitors in the mesolithion, 53.8% are regular mesolithial visitors and switchers
between mesolithion and open water, 5.3% are mesolithial permanent residents whose
populations also occur in other habitats; and 13.6% are mesolithial exclusive permanent
residents (Table 2). These estimates are conservative, selecting the minimum strength
of the fish relationship to the mesolithion, where no other evidence was available. For
example, fishes of pelagic and sandy habitats were presumed to be accidental visitors in
the mesolithion, in the absence of other evidence. Therefore, it could be expected that
with increasing knowledge, the proportion of the species with stronger relationships to
mesolithial habitats would also increase. Among the 18 exclusive permanent mesolithial
resident fishes recorded in Mediterranean marine caves, one has a snake-like body, dark
pigmentation and small eyes (G.ater, Figure 9B), two have laterally compressed bodies
with large eyes and red colouration (A.anthias, Figure 6C and C.ruber), and three have
small, slender bodies and red or orange colouration pattern (M.nigriceps,T.melanurus,
T.minor) (Figure 10) (Table 2). Five species are small-sized fish with robust bodies and large
heads (Figure 13) and five have small, elongated bodies and flattened heads (Figure 14)
(Table 2). The remaining two exclusive permanent residents, Scorpaenodes arenai Torchio,
1962 and T.ephippiatus (Figure 16A), did not match any of these body types, and the data
on S.arenai are very scarce (Table 2).
Table 2. Fishes recorded in Mediterranean marine caves. The data on cave zones are from pub-
lished sources cited in this present work and from personal observations of the authors. Marine
cave zones (if known): CE—cave entrance; SD—semidark zone; D—dark zone. The data about
marine environments, depth zones and the position of benthic fishes to the bottom are based on
Froese and Pauly [
111
] and Dulˇci´c and Kovaˇci´c [
107
]. Categories for environments and depth
zones: I—infralittoral; IC—infralittoral and circalittoral; ICB—infralittoral, circalittoral and bathyal;
CB—circalittoral and bathyal; B—benthopelagic; NE—neritic epipelagic. Categories for position of
benthic fish to the sea floor: Hb—hyperbenthic; Eb—epibenthic; Cb—cryptobenthic. See Table 1for
the categories for relationship of fishes to mesolithial habitats and Section 3.3 for data sources.
Species First References in
Mediterranean
Marine Caves
Cave Zone Environment
and Depth Zone
Position to the
Sea Floor
Relationship
to Mesolithial
Habitats
CE SD D
Acantholabrus palloni (Risso, 1810) [55] + IC Hb 2
Aidablennius sphynx (Valenciennes, 1836) [80] + I Eb 2
Anthias anthias (Linnaeus, 1758) [12] + + + ICB Hb 4
Apogon imberbis (Linnaeus, 1758) [39] + + + ICB Hb 3
Apogonichthyoides pharaonis (Bellotti, 1874) [138] + IC Hb 2
Atherina boyeri Risso, 1810 [139] + + B / 1
Atherinomorus forskalii (Rüppell, 1838) [140] + B / 1
Atherinomorus lacunosus (Forster, 1801) [140] + + B / 1
Bathytoshia centroura (Mitchill, 1815) [17] ICB Eb 2
Belone belone (Linnaeus, 1761) [13] NE / 1
Boops boops (Linnaeus, 1758) [141] + + B / 1
Bothus podas (Delaroche, 1809) [142] + ICB Eb 1
Callanthias ruber (Rafinesque, 1810) [120] CB Hb 4
Cheilodipterus novemstriatus (Rüppell, 1838) [143] + + IC Hb 2
Chelon sp. [67] + + I Hb 1
Chromis chromis (Linnaeus, 1758) [12] + + I Hb 2
Chromogobius quadrivittatus (Steindachner, 1863) [13] + I Cb 4
Chromogobius zebratus (Kolombatovi´c, 1891) [80] + + I Cb 4
Conger conger (Linnaeus, 1758) [41] + + + IC Cb 2
Corcyrogobius liechtensteini (Kolombatovi´c, 1891) [52] + + + IC Cb 4
Coris julis (Linnaeus, 1758) [12] + + + IC Hb 2
Coryphoblennius galerita (Linnaeus, 1758) [80] + I Eb 2
Ctenolabrus rupestris (Linnaeus, 1758) [70] IC Hb 2
Dasyatis pastinaca (Linnaeus, 1758) [79] + IC Eb 2
Dicentrarchus labrax (Linnaeus, 1758) [144] + IC Hb 1
Diplodus annularis (Linnaeus, 1758) [62] + + IC Hb 2
Diplodus puntazzo (Walbaum, 1792) [145] + IC Hb 2
Diplodus sargus (Linnaeus, 1758) [12] + + IC Hb 2
Diplodus vulgaris (Geoffroy Saint-Hilaire, 1817) [12] + + + IC Hb 2
Enchelycore anatina (Lowe, 1838) [146] + + + IC Eb 2
Epinephelus aeneus (Geoffroy Saint-Hilaire, 1817)
[147] IC Hb 2
Epinephelus caninus (Valenciennes, 1843) [12] + ICB Hb 2
Epinephelus costae (Steindachner, 1878) [68] + + + ICB Hb 2
Epinephelus marginatus (Lowe, 1834) [12] + + + ICB Hb 2
Fistularia commersonii Rüppell, 1838 [140] + IC Hb 1

Fishes 2024,9, 243 22 of 34
Table 2. Cont.
Species First References in
Mediterranean
Marine Caves
Cave Zone Environment
and Depth Zone
Position to the
Sea Floor
Relationship to
Mesolithial
Habitats
CE SD D
Gaidropsarus mediterraneus (Linnaeus, 1758) [139] + ICB Cb 2
Gammogobius steinitzi Bath, 1971 [42] + + + IC Cb 4
Gobius auratus Risso, 1810 [148] + + I Eb 3
Gobius bucchichi Steindachner, 1870 [13] + I Eb 2
Gobius cobitis Pallas, 1814 [80] + I Eb 2
Gobius cruentatus Gmelin, 1789 [149] + I Eb 3
Gobius geniporus Valenciennes, 1837 [62] + I Eb 2
Gobius niger Linnaeus, 1758 [13] + + IC Eb 2
Gobius paganellus Linnaeus, 1758 [80] + I Eb 2
Gobius vittatus Vinciguerra, 1883 [149] + + IC Eb 2
Gouania willdenowi (Risso, 1810) [13] + I Cb 4
Grammonus ater (Risso, 1810) [13] + I Cb 4
Hemiramphus far (Fabricius 1775) [147] NE / 1
Labrus merula Linnaeus, 1758 [128] + I Hb 2
Labrus mixtus Linnaeus, 1758 [13] + + + IC Hb 2
Lepadogaster candolii Risso, 1810 [145] + I Cb 4
Lepadogaster lepadogaster (Bonnaterre, 1788) [13] + I Cb 4
Lipophrys trigloides (Valenciennes, 1836) [12] + I Eb 2
Lithognathus mormyrus (Linnaeus, 1758) [142] + IC Hb 1
Marcelogobius splechtnai (Ahnelt & Patzner, 1995)
[51] + + IC Cb 4
Microlipophrys canevae (Vinciguerra, 1880) [12] + I Eb 2
Microlipophrys dalmatinus
(Steindachner & Kolombatovi´c, 1883) [12] + IC Eb 2
Microlipophrys nigriceps (Vinciguerra, 1883) [12] + + I Cb 4
Mugil cephalus Linnaeus, 1758 [13] I Hb 1
Mullus barbatus Linnaeus, 1758 [142] + IC Hb 1
Mullus surmuletus Linnaeus, 1758 [63] + + + ICB Hb 1
Muraena helena Linnaeus, 1758 [12] + + + IC Eb 2
Mycteroperca rubra (Bloch, 1793) [79] + + IC Hb 2
Neogobius melanostomus (Pallas, 1814) [13] I Eb 1
Oblada melanura (Linnaeus, 1758) [12] + + + IC Hb 2
Odondebuenia balearica (Pellegrin & Fage, 1907) [18] + IC Cb 4
Ophidion barbatum Linnaeus, 1758 [17] ICB Cb 3
Pagrus caeruleostictus (Valenciennes, 1830) [79] + + IC Hb 1
Parablennius gattorugine (Linnaeus, 1758) [12] + + I Eb 2
Parablennius incognitus (Bath, 1968) [145] + I Eb 2
Parablennius pilicornis (Cuvier, 1829) [74] + I Eb 2
Parablennius rouxi (Cocco, 1833) [12] + + I Eb 3
Parablennius sanguinolentus (Pallas, 1814) [140] + I Eb 2
Parablennius tentacularis (Brünnich, 1768) [17] + I Eb 3
Parablennius zvonimiri (Kolombatovi´c, 1892) [12] + + I Eb 3
Parupeneus forsskali (Fourmanoir & Guézé, 1976)
[79] + + IC Eb 1
Pempheris rhomboidea Kossmann & Räuber, 1877 [44] + + + IC Hb 2
Phycis phycis (Linnaeus, 1766) [149] + + + ICB Hb 2
Plotosus lineatus (Thunberg, 1787) [150] IC Hb 2
Polyprion americanus (Bloch & Schneider, 1801) [13] ICB Hb 2
Pomatoschistus adriaticus Miller, 1973 [43] + I Eb 1
Pterois miles (Bennett, 1828) [150] + + + IC Eb 2
Salaria pavo (Risso, 1810) [12] + I Eb 2
Sargocentron rubrum (Forsskål, 1775) [13] + + + IC Hb 2
Sarpa salpa (Linnaeus, 1758) [71] + I Hb 1
Sciaena umbra Linnaeus, 1758 [12] + + + ICB Hb 2
Scorpaena maderensis Valenciennes, 1833 [17] + + ICB Eb 2
Scorpaena notata Rafinesque, 1810 [12] + + + ICB Eb 2
Scorpaena porcus Linnaeus, 1758 [12] + + + IC Eb 2
Scorpaena scrofa Linnaeus, 1758 [41] + + + IC Eb 2
Scorpaenodes arenai Torchio, 1962 [151] + IC Eb 4
Scyliorhinus sp. [41] + ICB Hb 1
Seriola dumerili (Risso, 1810) [71] + + NE / 1
Serranus cabrilla (Linnaeus, 1758) [39] + + + IC Hb 2
Serranus hepatus (Linnaeus, 1758) [62] + + + IC Hb 2
Serranus scriba (Linnaeus, 1758) [12] + + + IC Hb 2
Siganus luridus (Rüppell, 1829) [138] + + I Hb 1
Siganus rivulatus Forsskål & Niebuhr, 1775 [138] + I Hb 1
Solea solea (Linnaeus, 1758) [67] + IC Eb 1
Sparisoma cretense (Linnaeus, 1758) [71] + IC Hb 2
Sparus aurata Linnaeus, 1758 [145] + IC Hb 1
Speleogobius trigloides Zander & Jelinek, 1976 [43] + + IC Eb 3
Sphyraena pinguis Günther, 1874 [138] IC Hb 1
Sphyraena viridensis Cuvier, 1829 [68] + IC Hb 1
Spicara maena (Linnaeus, 1758) [65] + + B / 1
Spicara smaris (Linnaeus, 1758) [18] + + B / 1
Stephanolepis diaspros Fraser-Brunner, 1940 [150] IC Hb 2
Symphodus doderleini Jordan, 1890 [144] + I Hb 1
Symphodus mediterraneus (Linnaeus, 1758) [141] + + I Hb 2
Symphodus melanocercus (Risso, 1810) [145] I Hb 2
Symphodus ocellatus (Linnaeus, 1758) [12] + I Hb 2
Symphodus roissali (Risso, 1810) [129] + I Hb 2
Symphodus rostratus (Bloch, 1791) [141] + I Hb 2
Symphodus tinca (Linnaeus, 1758) [141] + + I Hb 2

Fishes 2024,9, 243 23 of 34
Table 2. Cont.
Species First References in
Mediterranean
Marine Caves
Cave Zone Environment
and Depth Zone
Position to the
Sea Floor
Relationship to
Mesolithial
Habitats
CE SD D
Thalassoma pavo (Linnaeus, 1758) [12] + + I Hb 2
Thorogobius ephippiatus (Lowe, 1839) [13] + + + IC Eb 4
Thorogobius macrolepis (Kolombatovi´c, 1891) [62] + + I Eb 2
Torpedo marmorata Risso, 1810 [152] + ICB Eb 1
Torquigener flavimaculosus Hardy & Randall, 1983 [138] + + IC Hb 1
Trachinus draco Linnaeus, 1758 [144] + IC Eb 1
Trachurus trachurus (Linnaeus, 1758) [17] NE / 1
Tripterygion delaisi Cadenat & Blache, 1970 [43] + + I Eb 2
Tripterygion melanurus Guichenot, 1850 [153] I Cb 4
Tripterygion minor Kolombatovi´c, 1892 [12] + + I Cb 4
Tripterygion tripteronotum (Risso, 1810) [12] + + I Eb 2
Trisopterus capelanus (Lacepède, 1800) [17] ICB Hb 2
Umbrina cirrosa (Linnaeus, 1758) [13] + IC Hb 2
Upeneus moluccensis Bleeker, 1855 [138] IC Hb 1
Upeneus pori Ben-Tuvia & Golani, 1989 [150] + IC Hb 1
Zebrus zebrus (Risso, 1827) [145] + I Cb 4
Zeugopterus regius (Bonnaterre, 1788) [62] + IC Eb 0
3.4. The Distribution of Fishes in Marine Caves
Available studies concerning fishes in marine caves outside the Mediterranean Sea
rarely include data on the distribution of fishes in marine caves and their relationship to the
environmental and biological gradients (see Section 3.1). Micael et al. [
97
] distinguished
a submarine tunnel in the Azores in the two entrance zones, two twilight zones and one
middle dark zone and divided fishes according to their presence along these zones in the
tunnel. Lam et al. [
87
] provided notes on the cave position for each fish species in the Conic
Island Cave of Hong Kong. Specific information about the relationship of fishes to the
biological community of the marine caves is absent even in cases where the fishes had been
listed as part of the described biological community [90,93,100].
The more detailed studies of the distribution of fishes in marine caves and their rela-
tionship to the environmental and biological gradients have been published only for the
Mediterranean. Zander and Jelinek [
43
] were the first to provide detailed information on
the distribution of fishes inside caves and their relationship to light. They distinguished five
biological zones within the cave Banjole (Croatia, Adriatic Sea) following Riedl’s [
13
] zona-
tion and recorded the presence and estimated abundance of several fish species along the
light gradient. However, the fifth “darkest” zone was rather semidark, being inhabited with
anthozoan Parazoanthus sp. and fishes such as Parablennius zvonimiri (Kolombatovi´c, 1892)
and Parablennius gattorugine (Linnaeus, 1758) [
43
]. Arko-Pijevac et al. [
62
] studied biocenoses
of a long submarine cave at the island of Krk (Croatia, Adriatic Sea) and recorded fish from
the coralligenous biocoenosis of the cave entrance, over the semidark middle part and to the
completely dark cave interior. The number of recorded fish species decreased with the distance
from the entrance [
62
]. However, in the deepest part of the cave they recorded the exclusive
mesolithial permanent residents G.steinitzi (Figure 7C) and T.ephippiatus (Figure 16A) as well
as some hyperbenthic regular visitors, namely Labrus mixtus Linnaeus, 1758, Serranus hepatus
(Linnaeus, 1758) and Serranus scriba (Linnaeus, 1758) [62].
Reviewing all published data from the Mediterranean Sea, it is clear that the numerous
fish species were recorded at the entrances and at the semidark zones of caves [
17
,
18
,
62
,
68
–
71
].
Records of fish species in the completely dark zones of posterior cave sections are quite rare
for very long or bended caves [
67
]. The only fish species recorded at the inner part of the
Y-Cave in the central Adriatic Sea (90 m long) was the speleophilic species G.ater [
72
], in
contrast to the shorter marine caves at Vrbnik (30 m long) where several fish species occur
in the innermost dark zones [62].
Larger or hyperbenthic fishes have also been reported from the dark sections of other
Mediterranean marine caves such as A.imberbis (Figure 6A), C.conger,C.chromis (Figure 17B),
Diplodus vulgaris (Geoffroy Saint-Hilaire, 1817), Epinephelus costae (Steindachner, 1878), P.phycis,
S.scriba and Serranus cabrilla (Linnaeus, 1758) [
63
,
71
]. Among introduced fishes, Enchelycore

Fishes 2024,9, 243 24 of 34
anatina (Lowe, 1838), P.rhomboidea,Pterois miles (Bennett, 1828) and S.rubrum were also recorded
in the dark zone of Mediterranean marine caves [82].
In addition to G.ater, several small epibenthic and cryptobenthic fishes were regularly
reported in the innermost sections of marine caves with poor or no light in the Mediterranean
Sea, all belonging to the family Gobiidae: C.liechtensteini (Figure 13B), M.splechtnai (Figure 13D),
G.steintzi (Figure 7C), T.ephippiatus (Figure 16A) and T.macrolepis [
18
,
19
,
62
,
68
,
73
]. Other
small mesolithial fishes belonging to the families Blenniidae, Gobiesocidae, Gobiidae and
Tripterygiidae have been found in marine caves, though usually in shallow depths with
more light or just at cave entrances, along with larger hyperbenthic fishes [
14
,
62
,
68
]. Among
these species, regular inhabitants of the cave interior occupy two different microhabitats:
bedrock ceilings and walls (C.liechtensteini and G.steintzi) (Figures 7C and 13B) or fine
sediment of the cave floor (M.splechtnaii) (Figure 13D) [
19
,
52
,
73
]. As mentioned before,
some of small fishes use holes and crevices in cave ceilings and walls, also known as “caves
within caves” [
12
,
18
]. Zander [
14
] reported a “cryptophilic to acrophilic shift” for some
small cryptic gobiids in marine caves which appear closer to the entrance cave zone with
increasing water depth and decreasing light. This was also observed for T.ephippiatus
(Figure 16A) by Kovaˇci´c [60] and for C.liechtensteini by Herler et al. [52].
4. Discussion
Fishes in marine caves have been rarely investigated; most of the available studies
took place in the Mediterranean Sea (see Section 3.1 for the details). In addition, marine
fishes were rarely mentioned in existing reviews about subterranean fishes [
2
,
154
] or were
presented as a short summary with only a few references [
1
]. For example, Romero [
155
]
mentioned only a single example about fishes in marine caves. Nevertheless, research
efforts devoted to marine cave fishes has increased over the last few decades (see Section 3.1
for the details).
Marine cave fishes are taxonomically and phylogenetically distinct when compared
to subterranean fishes from freshwaters and/or anchialine systems, except for anchialine
and marine species of the genera Lucifuga (family Bythitidae) and Ogilbia (family Dine-
matichthyidae). Others are, at best, related at the family level to stygobitic species, such
as those of the families Gobiidae and Anguillidae which occur in anchialine [
5
,
156
] and
freshwater subterranean habitats [
16
]. Furthermore, no shared adaptations were found
between fishes in marine caves and stygobitic species, although some reef dwellers of
the family Dinematichthyidae and some burrowing Gobiidae species are shallow marine
species but evolved troglomorphic adaptations, according to Proudlove [
16
]. Also, non-
stygobitic fishes in freshwater subterranean habitats show no adaptations such as any of
those presented in this work for the exclusive permanent residents of mesolithial habitats.
Clearly, marine caves, with their strong connections with the open marine environment
and rich sessile benthic communities, represent quite a different evolutionary challenge for
fishes, when compared to subterranean freshwaters and anchialine systems. Consequently,
this has resulted in quite different adaptation outcomes.
Many exclusive permanent residents of mesolithial habitats, which were recorded
in marine caves, as well as regular visitors and switchers to marine caves, show no obvi-
ous shared colouration or morphological characteristics and maintain a similar appear-
ance to fishes from open environments (Figure 7A,D). Some of them exhibit intraspecific
“cryptophilic to acrophilic shift” sensu Zander [
14
]. Centropyge colini Smith-Vaniz & Randall,
1974 (family Pomacanthidae) was observed to occupy reefs at depth of 100 m or more, while
also found in marine caves in depths less than 30 m (R. Pyle, personal communication).
However, five types of morphological and colouration adaptations can be noticed among
Mediterranean exclusive permanent residents of mesolithial habitats that were recorded in
marine caves (Section 3.2.3). The two morphological types adapted for life in cryptobenthic
habitats and recorded in marine caves (i.e., small body size with stout body and large head;
and small, elongated body with flattened head) are common in various marine cryptobenthic
habitats of the Mediterranean Sea (see examples in Section 3.2.3) but can also be found in other

Fishes 2024,9, 243 25 of 34
oceans and seas. Such are the gobies of genus Priolepis Valenciennes, 1837, for the first body
type, or the gobies of genus Callogobius Bleeker, 1874, for the second body type [
157
,
158
]. The
adaptation type of small, slender and mostly uniformly red or orange-coloured body exists in
fishes in marine caves which lie or hover over the cave bed, on cave walls or below ceilings,
protected primarily by their poorly visible colouration in low blue light instead of hiding in
holes and hollows for protection (Figures 10 and 15). It could be expected that this adaptation
(mainly colour)also occurs in the low-light conditions of circalittoral and mesophotic depths,
with a possible “cryptophilic to acrophilic shift” of these species with increasing depth [
14
].
However, congeneric species of marine cave residents which occur in mesophotic depths (e.g.,
some species of Trimma), are more yellow to orange than orange to red coloured, such as
Trimma citrum Winterbottom & Pyle, 2022 [
159
]. These minor colour differences are proba-
bly irrelevant to the fishes, since red colour disappears by about 10 m, followed rapidly
by orange and yellow. Thus, none of those colours are visible in the natural light of
Trimma habitats. However, small fishes from deep, low-light habitats in the Mediterranean
circalittoral and in the mesophotic zone of the western Atlantic, exhibit mostly yellow
to orange colouration with a rather spotted, striped or marbled than uniform pattern
(Figure 18) [
160
,
161
]. This colouration pattern is rarely present in low-light conditions
inside “real” marine caves in shallow waters. In contrast, another type of body shape
adapted to mesolithion, specifically dark pigmentation, small eyes and snake-like body
(Figure 9), is shared between marine cave species and deep-water species of Anguilli-
formes and Ophidiiformes, with some species occurring down to 2,500 m depth, such as
Thermichthys hollisi (Cohen, Rosenblatt & Moser, 1990) [
156
]. This species’ appearance
looks to be the genuine deep-water adaptation that is also found in a small number of
species of shallow marine caves. Furthermore, some of these fishes that are not perma-
nent residents of mesolithion but are nocturnal species spending daytime in mesolithial
habitats and leaving their shelters at night (e.g., G.mediterraneus, Figure 9C), are eury-
bathic, being present from infralittoral to bathyal depths [
107
]. Finally, large eyes and red
colouration on fishes with a typical hyperbenthic body (laterally compressed) are com-
mon adaptations among fishes recorded in marine caves, including both those that are
exclusive permanent residents of mesolithial habitats and those which are daily switchers
and feed in the open water at night (Figure 6). This body type is also common among
eurybathic species which reach deep, low-light benthic habitats, and also among species
exclusively preferring deeper benthos. However, for most of these species, it remains
unclear to what degree they use and they are dependent on marine cave habitats, espe-
cially with increasing depth (e.g., caves in mesophotic environments and if they exhibit
intraspecific “cryptophilic to acrophilic shift” sensu Zander [
14
]). For example, according to
observations, the red-coloured deep-sea species Symphysanodon octoactinus Anderson, 1970,
Symphysanodon berryi Anderson, 1970 (family Symphysanodontidae), Liopropoma spp. (fam-
ily Liopropomatidae),
Gonioplectrus hispanus
(Cuvier, 1828), and Jeboehlkia gladifer Robins
1967 (family Epinephelidae), often disappeared into caves and crevices when illuminated
by the lights of a submersible (C. Baldwin, personal communication). In any case, the
described body adaption seems to be quite successful since it is shared by numerous fish
species in mesolithion, and it is also widespread among fish species of different fish families
in dim light conditions in general (see Sections 3.2.3 and 3.2.4 for details).
There is no Mediterranean fish species that is an exclusive dweller of marine caves
and has not also been found in the other mesolithial habitats (see Sections 3.2.3 and 3.2.4 for
details). Also, in other seas, the fish species found and described at the innermost part of
the caves, like Pseudamia zonata Randall, Lachner & Fraser, 1985 (R. Winterbottom, personal
communication), were later found in other habitats as well. Pseudamia zonata in particular
is now known to hover in front of caves at night in depths of 10–30 m [162].

Fishes 2024,9, 243 26 of 34
Fishes 2024, 9, x FOR PEER REVIEW 29 of 37
exhibit intraspecific “cryptophilic to acrophilic shift” sensu Zander [107]). For example,
according to observations, the red-coloured deep-sea species Symphysanodon octoactinus
Anderson, 1970, Symphysanodon berryi Anderson, 1970 (family Symphysanodontidae),
Liopropoma spp. (family Liopropomatidae), Gonioplectrus hispanus (Cuvier, 1828), and
Jeboehlkia gladifer Robins 1967 (family Epinephelidae), often disappeared into caves and
crevices when illuminated by the lights of a submersible (C. Baldwin, personal
communication). In any case, the described body adaption seems to be quite successful
since it is shared by numerous fish species in mesolithion, and it is also widespread among
fish species of different fish families in dim light conditions in general (see Sections 3.2.3
and 3.2.4 for details).
There is no Mediterranean fish species that is an exclusive dweller of marine caves
and has not also been found in the other mesolithial habitats (see Sections 3.2.3 and 3.2.4
for details). Also, in other seas, the fish species found and described at the innermost part
of the caves, like Pseudamia zonata (Randall, Lachner & Fraser, 1985) (R. Winterbottom,
personal communication), were later found in other habitats as well. Pseudamia zonata in
particular is now known to hover in front of caves at night in depths of 10–30 m [162].
Figure 18. (A) Gobius kolombatovici (Kovačić & Miller, 2000), photo by S. Le Bris. (B) Lesueurigobius
friesii (Malm, 1874), photo by R. Svensen. (C) Vanneaugobius dollfusi (Brownell, 1978), photo by S. Le
Bris. (D) Varicus cephalocellatus (Van Tassell, Baldwin & Gilmore 2016), (E) Varicus decorum (Van
Tassell, Baldwin & Gilmore 2016), (F) Varicus veliguttatus (Van Tassell, Baldwin & Gilmore 2016),
photos (D–F) by B.B. Brown (coralreefphotos) and C. Baldwin.
5. Conclusions
Marine caves constitute part of mesolithial habitats, together with pseudocaves (e.g.,
large crevices and overhanging rocks) and cryptobenthic habitats (i.e., living spaces
Figure 18. (A)Gobius kolombatovici Kovaˇci´c & Miller, 2000, photo by S. Le Bris. (B)Lesueurigobius friesii
Malm, 1874, photo by R. Svensen. (C)Vanneaugobius dollfusi Brownell, 1978, photo by S. Le Bris. (D)
Varicus cephalocellatus Van Tassell, Baldwin & Gilmore 2016, (E)Varicus decorum Van Tassell, Baldwin
& Gilmore 2016, (F)Varicus veliguttatus Van Tassell, Baldwin & Gilmore 2016, photos (D–F) by B.B.
Brown (coralreefphotos) and C. Baldwin.
5. Conclusions
Marine caves constitute part of mesolithial habitats, together with pseudocaves (e.g., large
crevices and overhanging rocks) and cryptobenthic habitats (i.e., living spaces underneath the
bottom surface of the substrate or biocover, with a physical barrier to the open spaces). They
are characterized by dim light conditions, low hydrodynamic regime and their rocky surfaces
are usually dominated by sessile filter feeders (e.g., sponges and bryozoans).
Fishes in marine caves constitute an unexplored component of marine fish diversity,
except for the relatively well-studied Mediterranean Sea. Relevant studies outside the
Mediterranean Sea are restricted mostly to taxonomy and simple species lists. The fishes
recorded in marine caves are not exclusive residents of these habitats and are also present
in the other types of mesolithial habitats. More specifically, marine caves are inhabited
by accidental and regular mesolithial visitors, switchers between mesolithion and open
water, permanent mesolithial residents whose populations also occur in other habitats but
do not migrate, and finally, by exclusive permanent mesolithial residents. Some mesolithial
visitors show no regularity in the time of their visits, while others are daily or seasonal
switchers between mesolithion and open water. A single fish species can use marine caves
in more than one of the above ways. There is no fish species known to be exclusive marine
cave dwellers that is not also found in the other mesolithial habitats or that is not a diel
switcher occurring outside caves at night.

Fishes 2024,9, 243 27 of 34
Unlike anchialine or freshwater caves, fishes with troglomorphic adaptation of eyes
and pigmentation loss do not exist in marine caves. However, among the exclusive perma-
nent mesolithial residents, in addition to the various other body shapes and colourations
found in these fishes, we have identified five different morphological types related to the
adaptation to mesolithial habitats.
A unique phenomenon in marine caves is the presence of small fishes occupying
“cave within cave” niches (i.e., small hidden spaces open to inside larger marine cave volumes).
Due to the lack of data, the relationship of fishes to deep marine caves and eventual shifts in
the use of marine caves with increasing depth and decreasing light, is still unknown.
In the relatively well-studied Mediterranean Sea, fishes recorded in marine caves
represent a considerable part (17%) of the total marine fish species richness. Among the
132 fish species recorded in the Mediterranean marine caves, most are regular mesolithial
visitors and switchers between mesolithion and open water (53.8%), followed by accidental
visitors in the mesolithion (27.3%), exclusive permanent mesolithial residents (13.6%) and
mesolithial permanent residents whose populations also occur in other habitats (5.3%).
Most of these species were recorded at the entrance and semidark zones of marine caves
while only a small number of larger or hyperbenthic fish species, as well as of small
epibenthic and cryptobenthic fishes, occur in the innermost dark sections.
6. Future Research Directions
Many questions remain unanswered concerning the diversity, spatial and ecological
patterns of fishes in marine caves of the world’s oceans, due to the fragmented nature and
scarcity of available data on the global scale. This study constitutes a first effort to describe
general patterns and map gaps of knowledge, based on available information, with the
goal of stimulating future research regarding marine cave fishes. It is notknown whether the
patterns observed in Mediterranean marine caves in terms of species richness and ecological
habits are similar to those in under-studied marine areas. For example, is species richness in
caves of other marine areas as high as that in Mediterranean caves? What are the differences
between the different ecological cave zones and micro-habitats within non-Mediterranean
caves? Are exclusive fish residents present in non-Mediterranean caves, and if yes, do they
show adaptions similar to those living in the Mediterranean mesolithial habitats?
In situ collection of fish samples and data from marine caves relies on SCUBA diving,
which involves several challenges under low-light and space-limited conditions. Multiple
dives by skilled ichthyologists, marine biologists and natural history experts, are required
even in the same cave in order to record highly mobile taxa and cryptobenthic species,
especially those dwelling in “cave within cave” niches. However, in the absence of data,
any observation has merit, and data sharing between scientists and citizens in different
online platforms and biodiversity databases about fishes and marine cave environments is
highly encouraged [23,163].
Even in the relatively well-studied Mediterranean marine caves, there are still impor-
tant gaps in knowledge from under-studied areas (e.g., North African coasts), depth zones
(e.g., mesophotic and deep-sea caves), ecological groups (e.g., cryptobenthic taxa) or about
ecosystem structure, functioning and dynamics [
9
]. Biology and ecology of individual
fishes in mesolithial habitats is largely unknown, with existing studies devoted to a single
species, namely the cardinal fish A.imberbis [
70
]. The role of highly mobile larger fauna
such as fishes in the trophic structure and trophodynamics of cave communities and their
role in the energy flow in the caves was investigated only recently and, again, only for
A.imberbis [
121
]. The potential effects of non-indigenous fishes on the marine cave biota and
the functioning of cave ecosystems also remain unknown [
82
]. Only a few quantitative studies
on fish assemblages have been published so far, and their relationship with environmental
gradients are still poorly understood. In addition, anatomical, physiological and behavioural
adaptations of mesolithial exclusive permanent residents have not yet been studied.

Fishes 2024,9, 243 28 of 34
The increasing attention to marine cave environments, as shown by the increasing
number of studies about marine cave fishes since the beginning of this century, gives rise
to hope that these questions will be investigated in the near future.
Author Contributions: Conceptualization, M.K., V.G. and R.A.P.; methodology, M.K., V.G. and
R.A.P.; software, M.K.; validation, M.K., V.G. and R.A.P.; formal analysis, M.K., V.G. and R.A.P.;
investigation, M.K., V.G. and R.A.P.; resources, M.K., V.G. and R.A.P.; data curation, M.K., V.G. and
R.A.P.; writing—original draft preparation, M.K., V.G. and R.A.P.; writing—review and editing,
M.K., V.G. and R.A.P.; visualization, M.K. and R.A.P.; supervision, M.K., V.G. and R.A.P.; project
administration, M.K.; funding acquisition, M.K. All authors have read and agreed to the published
version of the manuscript.
Funding: MK was supported by a grant of the Croatian Science Foundation under the project
IP-2022-10-7542.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: All fish species mentioned in this study will be uploaded to the World Reg-
ister of marine Cave Species (WoRCS, http://www.marinespecies.org/worcs (accessed on 16 June 2024)),
a thematic species database of WoRMS which aims at creating a comprehensive taxonomic and
ecological database of species from marine caves worldwide.
Acknowledgments: We thank Marinko Babi´c, Carole Baldwin, Sergey V. Bogorodsky, Barry B. Brown
(coralreefphotos), Hamid Reza Esmaili, Daniel Golani, Kiyoshi Hagiwara, Douglass F. Hoese, Helen
K. Larson, Richard Pyle, D. Ross Robertson, Luke Tornabene, Richard Winterbottom and Fatah Zarei
for providing information about fishes in marine caves. We also are grateful for the provision of
photographs to Marinko Babi´c, Carole Baldwin, Sergey V. Bogorodsky, Barry B. Brown (coralreef-
photos), Hamid Reza Esmaili, Stefano Guerrieri, Kiyoshi Hagiwara, David G. Johnson, Dani Laslo,
Sylvain Le Bris, Jiro Sakue, Rudolf Svensen, Maximillian Wagner and Zoran Vali´c. We would also like
to thank Margaret Eleftheriou for reviewing the English language of the text and two anonymous
reviewers for their constructive comments to improve the manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.
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