What can we learn from confusing Olivella columellaris and O. Semistriata (Olivellidae, Gastropoda), two key species in panamic sandy beach ecosystems?

Abstract

Olivella columellaris (Sowerby 1825) and O. semistriata (Gray 1839) are suspension-feeding, swash-surfing snails on tropical sandy beaches of the east Pacific. While they often are the numerically dominant macrofaunal element in their habitats, their biology is poorly understood; the two species actually have been confused in all of the few publications that address their ecology. Frequent misidentifications in publications and collections contributed also to an overestimation of the geographic overlap of the two species. To provide a sound taxonomic basis for further functional, ecological, and evolutionary investigations, we evaluated the validity of diagnostic traits in wild populations and museum collections, and defined workable identification criteria. Morphometric analysis demonstrated that shell growth is allometric in O. columellaris but isometric in O. semistriata, suggesting that the species follow distinct developmental programs. The taxonomic confusion is aggravated by the existence of populations of dwarfish O. semistriat, which originally had been described as a separate species, O. attenuata (Reeve 1851). At our Costa Rican study sites, the occurrence of such dwarfish populations correlates with low wave energies but not with predation pressure and anthropogenic disturbances, indicating significant ecological plasticity in the development of O. semistriata.

Full text

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Biota Neotrop., vol. 12, no. 2
What can we learn from confusing Olivella columellaris and O. semistriata
(Olivellidae, Gastropoda), two key species in panamic sandy beach ecosystems?
Alison I. Troost1, Samantha D. Rupert1, Ariel Z. Cyrus1, Frank V. Paladino1,2,
Benjamin F. Dattilo3 & Winfried S. Peters1,2,4
1Department of Biology, Indiana/Purdue University Fort Wayne, 2101 East Coliseum Boulevard,
Fort Wayne, IN 46805‑1499, USA
2Goldring Marine Biology Station, Playa Grande, Santa Cruz, Guanacaste, Costa Rica
3Department of Geosciences, Indiana/Purdue University Fort Wayne, 2101 East Coliseum Boulevard,
Fort Wayne, IN 46805‑1499, USA
4Corresponding author: Winfried S. Peters, e‑mail: peters@ipfw.edu
TROOST, A.I., RUPERT, S.D., CYRUS, A.Z., PALADINO, F.V., DATTILO, B.F. & PETERS, W.S. What
can we learn from confusing Olivella columellaris and O. semistriata (Olivellidae, Gastropoda), two key
species in panamic sandy beach ecosystems? Biota Neotrop. 12(2): http://www.biotaneotropica.org.br/v12n2/
en/abstract?article+bn02112022012
Abstract: Olivella columellaris (Sowerby 1825) and O. semistriata (Gray 1839) are suspension‑feeding,
swash‑surfing snails on tropical sandy beaches of the east Pacific. While they often are the numerically dominant
macrofaunal element in their habitats, their biology is poorly understood; the two species actually have been
confused in all of the few publications that address their ecology. Frequent misidentifications in publications
and collections contributed also to an overestimation of the geographic overlap of the two species. To provide
a sound taxonomic basis for further functional, ecological, and evolutionary investigations, we evaluated the
validity of diagnostic traits in wild populations and museum collections, and defined workable identification
criteria. Morphometric analysis demonstrated that shell growth is allometric in O. columellaris but isometric in
O. semistriata, suggesting that the species follow distinct developmental programs. The taxonomic confusion is
aggravated by the existence of populations of dwarfish O. semistriata, which originally had been described as
a separate species, O. attenuata (Reeve 1851). At our Costa Rican study sites, the occurrence of such dwarfish
populations correlates with low wave energies but not with predation pressure and anthropogenic disturbances,
indicating significant ecological plasticity in the development of O. semistriata.
Keywords: Olivella, Pachyoliva, panamic faunal province, sandy beach intertidal, shell growth (allometry),
suspension feeder.
TROOST, A.I., RUPERT, S.D., CYRUS, A.Z., PALADINO, F.V., DATTILO, B.F. & PETERS, W.S. ¿Qué
podemos aprender de la confusión de la Olivella columellaris y la O. semistriata (Olivellidae, Gastropoda),
dos especies con un papel clave en los ecosistemas de las playas arenosas panamicas? Biota Neotrop. 12(2):
http://www.biotaneotropica.org.br/v12n2/pt/abstract?article+bn02112022012
Resumen: La Olivella columellaris (Sowerby 1825) y la O. semistriata (Gray 1839) son caracoles filtradores que
navegan en la zona de vaivén de las playas arenosas tropicales del Pacífico oriental. Si bien son frecuentemente
el elemento macrofáunico dominante en su habitat, su biología está insuficientemente entendida; de hecho, las
dos especies han sido confundidas en las pocas publicaciones que han tratado de su ecología. La identificación
equivocada tanto en las publicaciones como en las colecciones ha contribuido también a sobrestimar el solapamiento
geográfico de las dos especies. Para proporcionar una base taxonómica segura para futuras investigaciones
funcionales, evolutivas y ecológicas, evaluamos la validez de los rasgos diagnósticos en poblaciones silvestres
y en colecciones museísticas, y definimos criterios de identificación para ser usados. El análisis morfométrico
mostró que el crecimiento de la concha es alométrico en la O. columellaris pero isométrico en la O. semistriata,
lo que sugiere que las dos especies siguen programas de desarrollo diferentes. La confusión taxonómica se ha
visto agravada por la existencia de poblaciones de O. semistriata enanas, que fueron originalmente descritas
como una especie separada: O. attenuata (Reeve 1850). En nuestro sitio de estudio en Costa Rica, la ocurrencia
de tales poblaciones enanas se correlaciona con olas de baja energía, y no con la presión de depredación ni con
disturbios antropogénicos, lo que indica una plasticidad ecológica considerable en el desarrollo de la O. semistriata.
Palabras clave: Olivella, Pachyoliva, provincia fáunica panamica, zona intermareal de playa arenosa, crecimiento
de concha (alometría), animales filtradores.

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Z16 APO Macroscope with a DFC490 digital camera and polarization
equipment (Leica, Wetzlar, Germany).
3. Comparative ecological studies of Olivella semistriata
populations
Eight test beaches in northwest Costa Rica (Figure 2) known
from previous visits to home dense populations of O. semistriata
were selected for investigations into the dependence of maximum
body size on local ecological conditions. In Table 1, the test beaches
are listed according to their exposure to wave energy. The highest
degree of exposure was defined as that found in dissipative sandy
beaches forming part of long linear or even convex coastlines, whereas
the lowest degree was assigned to coves in the interior of complex
coast geometries providing shelter from open ocean conditions. It is
worth noting that the degree of wave exposure thus defined generally
correlated with the type of human activity: high‑exposure beaches are
frequented mostly by surfers, whereas low‑exposure sites typically
are family‑friendly bathing beaches. With respect to the extent of
human utilization, the eight test beaches ranged from pristine without
significant tourist infrastructure to strongly impacted by tourism and
other activities due to a ‘downtown location’.
Local maximum body size and predation pressure were evaluated
on the test beaches between November 20 and 27, 2010 (end of
the rainy season), during daylight hours following a standardized
procedure. Exactly two hours before low tide, we began to screen
a beach for the predatory snail Agaronia propatula (Conrad 1849;
Olividae, Gastropoda) which feeds mostly on O. semistriata
(Cyrus et al. 2012), by slowly walking in a zigzag pattern between
the uppermost waterline occasionally reached by the highest waves
Introduction
The macrofaunal communities on many dissipative sandy beaches
of the panamic faunal province (American west coast from Baja
California to north Peru) are numerically dominated by two closely
related species of intertidal snails, Olivella columellaris (Sowerby
1825) and O. semistriata (Gray 1839). These very similar species
must be assumed to be ecological key players due to their immense
densities on those beaches (Olsson 1923/1924, 1956, Aerts et al.
2004), but our knowledge of their biology is fragmentary. According
to Olsson (1956), the two species form the subgenus Pachyoliva
in the genus Olivella (Olivellidae, Caenogastropoda, Gastropoda;
while Olsson had included Olivella in the Olividae, we here follow
the more recent suggestion by Bouchet & Rocroi (2005) to separate
Olivellidae and Olividae). Olivella semistriata appears to be the
more northerly species frequently found on Central American
beaches, whereas O. columellaris seems to be common in South
America (Olsson 1956). From previous casual field observations of
O. semistriata at Playa Grande, Costa Rica, and of O. columellaris
at Colan, Peru, we conclude that both species are swash surfers that
reposition themselves in the sandy intertidal using their expanded
foot as an underwater sail for rapid locomotion. Moreover, both use
a pair of mucus nets suspended from unique lateral appendages of
the anterior foot to filter suspended particles from the backwash in
the upper beach zone. These observations are not novel, as similar
reports regarding ‘O. columellaris’ can be found in the older literature
(Seilacher 1959). However, in all of the few papers published on
the functional biology and ecology of Pachyoliva in peer‑reviewed
journals, the two species have been confused, as we will demonstrate
below. Such confusion obviously hampers our understanding of the
biology of the two species, and prevents the detection and analysis
of any behavioral, physiological, and developmental differences that
may throw light on their ecological role on panamic beaches and on
the evolution of their clade. We analyzed the classical literature and
conducted morphometric studies of populations in the field and of
museum collections, in order to identify and resolve taxonomic and
morphological issues that have caused the confusion.
Material and Methods
1. Analysis of diagnostic traits and morphometric studies
Live Olivella columellaris (Sowerby 1825), O. semistriata (Gray
1839), and O. biplicata (Sowerby 1825) were studied in their natural
habitats, mainly at Colan, Piura, Peru (04° 59’ S and 81° 05’ W), at
Playa Grande, Guanacaste, Costa Rica (10° 20’ N and 85° 51’ W),
and at Bodega Bay, California, USA (38° 19’ N and 123° 02’ W),
respectively. Shells of the former two species in the collections of
the Senckenberg Museum of Natural History in Frankfurt, Germany
(http://www.senckenberg.de), and of the Natural History Museum in
London, UK (http://www.nhm.ac.uk), were also examined. Additional
field observations from various locations along the Pacific coasts of
El Salvador, Costa Rica, and Peru are included in this report. For
morphometric analyses, we measured shell length, shell width, spire
height, and spire base width (Figure 1) of animals in the field and in
the museum collections to the nearest 0.05 mm with digital calipers.
2. Microscopy of shells
Cross sections of shells were prepared by embedding cleaned
shells in plastic cylinders with epoxy resin, slicing the cylinders at
1 mm intervals, adhering the slices to glass slides with epoxy, and
grinding them to approximately 30 µm thickness. The thin sections
were examined under glass cover‑slips with glycerine using a Leica Figure 1. Definition of the measures taken on shells of Olivella columellaris
and O. semistriata.

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Figure 2. Locations of the eight test beaches in Northwest Costa Rica. 1) Playa Junquillal; 2) Playa Avellana; 3) Playa Grande; 4) city beach of the town of
Puntarenas; 5) Playa Carrillo; 6) Playa Hermosa; 7) Playa Conchal; 8) Bahia Junquillal. For further details, see Table 1.

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Table 1. Characteristics of the Costa Rican beaches on which Olivella semistriata populations were studied; compare map in Figure 2. Abbreviation in the last column: A.p., density of the predatory snail Agaronia
propatula in individuals per 100 m of beach.
Location Exposure Beach morphology Utilization A.p.
Exposed beaches:
(1) Playa Junquillal
10° 10’ N and 85° 49’ W
Straight beach (1.9 km) in a linear coastline, fully
exposed to oceanic waves
Moderately flat dissipative beach; intertidal plain
flooded by the highest waves at low tide >35 m
wide
Turtle breeding site; remote location but
popular with surfers and other tourists
0.6
(2) Playa Avellana
10° 14’ N and 85° 50’ W
Straight beach (1.4 km) in a long linear coastline,
fully exposed to oceanic waves
Flat dissipative beach; intertidal plain flooded
partly by the highest waves at low tide >70 m wide
Remote location but very popular with
surfers and other tourists
3.4
(3) Playa Grande
10° 20’ N and 85° 51’ W
Slightly curved beach (4.7 km); some sheltering
effects by protruding coastline in the N and S, but
mostly exposed to oceanic waves
Flat dissipative beach; intertidal plain that at low
tide is flooded by the highest waves is >50 m wide
Turtle breeding site in national park with
restricted access, but very popular with
surfers
21.0
Intermediate beaches:
(4) City beach of Puntarenas
9° 58’ N and 84° 50’ W
Part of a spit protruding into the Golfo de Nicoya;
exposure to wave action limited
Moderately steep dissipative beach; backwash
zone suitable for suspension feeding of
O. semistriata is hardly ever >10 m
Adjacent to the downtown area of
Puntarenas; extensively utilized by tourists
and the local public, by far the most polluted
site studied
14.7
(5) Playa Carrillo
9° 52’ N and 85° 30’ W
Semicircular bay of 1.8 km diameter, opens to
the open ocean
Moderately flat dissipative beach; width of
intertidal plain flooded by largest waves at low
tide <20 m wide
Despite vicinity to the touristic city of
Samara, relatively moderate utilization by
tourists
0.9
Sheltered beaches:
(6) Playa Hermosa
10° 35’ N and 85° 41’ W
Cove of 1.3 km diameter at the opening of the
Bahia Culebra with several islands just off the
shore
Moderately steep reflective beach; in the lower
part, current ripples form at retreating tide
Small bay in ‘downtown location’, utilized
by tourists, the local public, and local small‑
scale industries
10.7
(7) Playa Conchal
10° 24’ N and 85° 49’ W
Sheltered southern edge of the Bahia Brasilito,
protected by the protruding Punta Sabana
Sandy beach with interspersed rocks and pebble
fields. In the lower part, current ripples form at
retreating tide
Unattractive for surfing and bathing, but
frequented at low tide by off‑road vehicles
and horse‑back riders; launching spot for
jet skis
4.3
(8) Bahia Junquillal
10° 58’ N and 85° 41’ W
Sheltered bay of 1.6 km diameter in the interior
of the Golfo de Santa Elena
Steep reflective beach; backwash zone suitable for
suspension feeding of O. semistriata never >5 m
Remote and pristine cove; limited access and
little infrastructure, very low touristic impact
0.0

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and a line where the water was about ankle‑deep (10 to 20 cm)
during the phases of wave retreat. After two hours (that is, at low
tide), the counting of Agaronia was stopped. The estimated density
of Agaronia was expressed as the number of individuals per 100 m of
beach. Obviously, this number provided a relatively crude minimum
estimate of the true density of Agaronia in the intertidal zone since
only actively hunting individuals that were visible on the surface were
counted. In the following two hours, the same stretch of beach was
screened for particularly large specimens of O. semistriata. Their shell
lengths were measured to the nearest 0.05 mm with digital calipers;
the animals were put back to their original location immediately.
Up to 80 and not less than 52 individual measurements were taken
on each beach; these particularly large animals represented a tiny
minority of the total population present at each site. The 25 largest
animals found on a given beach were selected from each dataset and
their size spectrum served as an estimate of the maximum size which
O. semistriata reached at that location. To transform shell length
measurements into biomass, shell length as well as the weight of
the live animals (measured to the nearest 0.01 g) were determined
for a representative sample of 266 individuals from Playa Grande
that was selected to cover all size classes. The relation between
shell length and body mass was expressed as the geometric mean
functional relationship (GMFR), a type II correlation model (Laws
& Archie 1981).
Results and Discussion
1. The subgenus Pachyoliva
In the most recent revision of extant and extinct members of the
genus Olivella, Olsson (1956) established a number of subgenera,
mainly based on shell characteristics. However, the most obvious
morphological characters that identify the subgenus Pachyoliva
(comprising O. columellaris and O. semistriata) are found in
the anterior soft body. Olivella generally lacks eyes and cephalic
tentacles, and the most anterior portion of the foot (propodium) is
set off against the main part (metapodium) by a shallow groove. The
tips of the crescent‑shaped propodium protrude slightly from the
lateral edges of the foot in most species (Figure 3a shows O. biplicata
as a typical example). In contrast, the lateral tips of the propodium
are thickened and elongated in O. semistriata and O. columellaris
(Figures 3b, c). In live animals, these enlarged propodial tips are
quite conspicuous and occasionally have been confused with cephalic
tentacles, for instance by Gray (1839) in the original description of
O. semistriata. This author included the species in Oliva rather than
in Agaronia, precisely because Oliva possesses cephalic tentacles
whereas Agaronia does not (Gray 1839 p. 129). In fact, however,
the elongated propodial tips are unknown from other Olivellidae
and Olividae, and thus represent an autapomorphy of Pachyoliva. A
smaller, less conspicuous appendage protrudes from each side of the
e f
ab c d
Figure 3. Morphology of Olivella species. a) O. biplicata (from Bodega Bay, California, USA) as an example of the typical morphology of the genus. Cephalic
tentacles carrying eyes are absent; the left and right halves of the propodium (asterisks) are visually separated from the metapodium by transverse dark lines, and
the lateral tips of the propodium (arrowheads) extend slightly beyond the lateral edges of the foot. These lateral propodial tips are greatly enlarged in the two
members of the subgenus Pachyoliva: b) O. columellaris from Colan, Peru, and c) O. semistriata from Playa Grande, Costa Rica. d) A smaller lateral appendage
is present on each side of the anterior metapodium, highlighted here by the arrow in a ventral view of an O. semistriata that has assumed the surf posture. e)
Food acquisition by O. semistriata in the backwash; direction of water flow from right to left. The snail has burrowed into the sand; only the propodium and
most anterior metapodium extend above the surface. The lateral propodial appendages arch outwards and support translucent mucus nets that balloon in the
flow; at this late stage in the filtering cycle, the nets have accumulated suspended detritus and are clearly visible. A characteristic trident flow mark has formed
downstream of the animal. f) Head‑view of a filtering O. columellaris; direction of water flow from bottom to top. The mucus nets are suspended between the
propodial (arrowhead) and the greatly expanded metapodial (arrow) appendages, but are not yet visible in this early stage of the filtering cycle. Scale bar in
a) applies to all photographs.

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anterior metapodium (Figure 3d, f). These metapodial appendages,
which have not been described previously, appear to be absent in
species with typical foot morphology such as O. biplicata. Pachyoliva
snails use their foot appendages to deploy mucus nets for suspension
feeding in the backwash of sandy beaches: one semispherical net
is suspended on each side between the large propodial and small
metapodial appendage (Figure 3e, f). As noted by Seilacher (1959),
these nets are hard to see at the beginning of a filtering cycle, before
they have become loaded with plankton and detritus (Figure 3f).
According to modern identification keys (Olsson 1956, Burch
& Burch 1963, Keen 1971), the two species can be distinguished
using two structural characters of the shells (shell coloration, a
feature focused on by many classical authors, is variable and of little
diagnostic value). First, while shells of both species exhibit callus on
the inner lip (parietal callus) that extends beyond the posterior end of
the aperture (spire callus), callus development supposedly is stronger
in O. columellaris. Second, only O. semistriata is thought to possess
fine, longitudinal striae that cover the upper half of the body whorl,
as the species name indicates. In addition, shells of O. columellaris
frequently were described as stockier than those of O. semistriata;
especially the spire of the latter was claimed to be higher and more
sharply pointed. Finally, it is worth noting that in the modern keys,
similar sizes (14–15 mm length) are given for mature shells of both
species.
2. Critical evaluation of diagnostic shell characters and
their application
To evaluate the usefulness of the above diagnostic criteria in field
studies, we examined populations of O. columellaris in Peru and of
O. semistriata in Costa Rica, as well as the extensive collection of
the Senckenberg Museum which houses, among others, the shells
collected by the authors of the first ecological studies of Pachyoliva
(Schuster 1952, Schuster‑Dieterichs 1956, Seilacher 1959).
Callus – In ‘typical’ specimens of O. columellaris resembling
those shown in identification guides (Figure 4a), the parietal and
spire callus is more strongly developed than in ‘typical’ O. semistriata
(Figure 4b). Strong spire callus formation in O. columellaris causes
a characteristic kink in the outline of the shell at the suture above
the body whorl (highlighted in Figure 4a), which, according to the
original species description by Sowerby (1825, Appendix p. 34),
“[...] gives to this shell a very extraordinary appearance, and
forms the characteristic feature of the species.” However, in every
O. columellaris population that we have seen in the field, specimens
without strongly developed spire callus (Figure 4c) actually were
more abundant than ‘typical’ shells. Analysis of a representative
sample (n = 315) of O. columellaris shells from our study site at
Colan, Peru, indicated that the ‘typical’ morphology does not develop
before the animals grow from 10 to 13 mm shell length (Figure 4d).
ab c
d e
Figure 4. Callus formation in Pachyoliva shells. a) ‘Typical’ O. columellaris shell from Colan, Peru, showing a kink in the outline of the shell above the
aperture (highlighted by white line). b) ‘Typical’ O. semistriata shell from Playa Grande, Costa Rica. The outline of the shell above the aperture is straight
(highlighted by white line). Note the notch in the anterior inner lip (arrow). c) ‘Atypical’ specimen of O. columellaris from the same population as the shell
in a); no kink in the outline of the shell is visible. d) Proportion of O. columellaris shells with kinked outline in size classes defined by shell length (1 mm
class width), determined in the population of Colan, Peru. Numbers at the bases of the bars indicate the number of animals examined in that size class. e) Two
O. semistriata from Playa de Cuco, El Salvador, demonstrating the variability of callus formation in this species. Scale bar in a) applies to all photographs.

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The situation is further complicated by the significant variability
of callus formation in O. semistriata. If the extent of spire callus
were the only diagnostic criterion available, the two shells shown in
Figure 4e probably would be classified as O. columellaris (left) and
O. semistriata (right). In fact, the two shells come from the same
O. semistriata population in east El Salvador, and both show the
striae that are characteristic of this species (see below). We have found
individuals with similarly strongly developed callus as the one on the
left in numerous O. semistriata populations in Central America, but
this always was a minority phenotype.
Striae – In the words of Keen (1971 p. 631), a “[...] faint series of
vertical striae at the upper margin of the body whorl is distinctive.”
in O. semistriata. These structures were explicitly highlighted in the
species description (Gray 1839 p. 130), and the name semistriata
refers to them. The striae are unmistakable in clean and dry shells
from which the light reflects (Figure 5a), but may be overlooked in
the field especially under poor light. They are spaced at 230–300 µm,
which corresponds to the geometry of the internal crenulation and
terminations of prominent laminae within the outermost shell layer
(Figure 5b, c). While the striae reflect a distinct process of routine
shell accretion in O. semistriata, they do not resemble and must not
be confused with the major growth lines that may form in response
to disruptions of growth, either from trauma or during senescence,
in both Pachyoliva species. Small O. semistriata shells lack striae; at
our study site Playa Grande in Costa Rica, striae gradually occurred
in the size classes from 7 to 12 mm shell length (Figure 5d; size
of the representative sample examined, n = 267) and were never
lacking in larger animals. In contrast, we failed to find even a single
semi‑striated shell among the northern Peruvian populations that we
had identified as O. columellaris due to the presence of individuals
showing the ‘typical’ callus (this identification was in agreement with
classical studies undertaken at the same localities; Olsson 1923/1924).
Intriguingly, Olsson (1956) did not even mention the characteristic
striae in his description of O. semistriata. The O. semistriata shell
ab c
d
Figure 5. Striae on the shell of Olivella semistriata. a) Light reflections from an O. semistriata shell reveal the structural difference between the striated upper
(posterior) part and the smooth lower (anterior) part of the body whorl. b) Cross‑section of an O. semistriata shell, taken from the uppermost quarter of the
body whorl. The striae on the surface correspond to terminations of individual laminae in the outermost shell layer. c) No striae are present in the shell of
O. columellaris. d) Proportion of O. semistriata shells with visible striae in size classes defined by shell length (1 mm class width), determined in the population
of Playa Grande, Costa Rica. Numbers of individuals examined in each size class are given at the bases of the bars.

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shown on this author’s Plate 8 was 14.2 mm long, according to the
plate legend (Olsson 1956 p. 219), but the quality of the image is not
sufficient to determine unambiguously whether striae were present or
not. In any case, it is puzzling that Olsson ignored this defining trait,
and we assume that he mainly studied immature shells too small to
show this feature.
Large shells of O. semistriata but not of O. columellaris often
show a notch in the anterior inner lip (Figure 4b). This character is
not unambiguously expressed in small O. semistriata shells (data
not shown), and therefore merely confirms conclusions that can be
drawn from the presence or absence of striae on the posterior body
whorl. Moreover, the notch, if present, frequently is covered by the
foot tissue in live animals retracted into the shell, and therefore is not
a suitable character for the routine identification of live specimens in
ecological field studies.
From these findings we concluded that, despite all phenotypic
variability, it is possible to establish the taxonomic identity of a
Pachyoliva population based on the presence of striae in the posterior
body whorls (Figure 5) and of kinks in the shell outline caused by
spire callus (Figure 4), as long as a sufficient number of large animals
(shell length > 12 mm) are available for examination. To establish the
taxonomic validity of our conclusion, we examined the syntypes of
O. semistriata in the Natural History Museum in London (catalogue
entry: NHMUK 20050254). The seven shells are from 12.6 to 18.2
mm long, and are in full agreement with our above interpretation.
We were unable to locate the type specimen(s) of O. columellaris.
The oldest publication that we could link to specific specimens was
Reeve (1851). The five specimens from the Cuming Collection that
served as models for the O. columellaris drawing on Table 23 in that
monograph are being held in the Natural History Museum London.
The four larger shells (14.1 to 14.7 mm) show the ‘typical’ spire callus
described above, whereas the fifth measures only 10.6 mm and lacks
the strongly developed callus.
Application of the above identification criteria to published
studies leads to surprising results. We are not aware of more than
five articles addressing the ecology of Pachyolivae in peer‑reviewed
journals (Schuster 1952, Schuster‑Dieterichs 1956, Seilacher
1959, Vanagt et al. 2008a, 2008b). Ironically, the species seems
to have been misidentified in all of them. The older three of these
papers (Schuster 1952, Schuster‑Dieterichs 1956, Seilacher 1959)
reported field work from El Salvador that established basic facts
about the behavior, food acquisition, and predator‑prey relations of
‘O. columellaris’. These publications were cited in more recent review
articles (Declerck 1995, Davies & Hawkins 1998), and Seilacher’s
(1959 p. 365) beautiful and accurate drawings of a snail deploying its
mucus nets were reproduced in several books (Friedrich 1969 p. 270,
Hughes 1986 p. 33). However, photographs presented by Schuster‑
Dieterichs (1956 p. 19, 21) and Seilacher (1959 p. 359) show mature
O. semistriata, not O. columellaris, and all of the hundreds of shells
deposited by Schuster‑Dieterichs in the Senckenberg collection that
are large enough for identification are O. semistriata. Moreover,
O. semistriata is the only Pachyoliva on the El Salvadorian beaches
on which these researchers worked, as we have verified on field trips
in 2010. The incorrect identifications may have been due partly to
the fact that the characteristic striae on the O. semistriata shells had
received no mention in Olsson’s Olivella monograph (1956), to which
Seilacher (1959) refers. More recently, Vanagt et al. (2008a, 2008b)
scrutinized the burrowing performance of ‘O. semistriata’ and its
circatidal movements on beaches in Ecuador. While these authors
do not comment on taxonomy, species identification is discussed in
the doctoral dissertation (Vanagt 2007) on which the two papers are
based. The photographs given (Vanagt 2007 p. 26 and title page) show
typical O. columellaris, not O. semistriata. While the ecological and
physiological conclusions drawn in all of these papers remain largely
unaffected by the incorrect species identifications, our taxonomic
corrections are essential for an integration of these studies in the wider
contexts of biogeography, evolution, and comparative aspects of the
species’ physiology, ecology, and behavioral biology.
A certain degree of taxonomic confusion regarding Pachyoliva
exists in museum collections as well. Twenty‑four of the 25 items in
the Senckenberg collection that bear O. columellaris or O. semistriata
as species name included mature shells which could be identified
unequivocally. Of these items, 11 (46%) had been misidentified.
In contrast, 16 of the 17 identifiable entries in the collection of
the Natural History Museum were labelled correctly, which may
be due to the fact that the type specimens of O. semistriata were
available at this institution for comparison. It is worth mentioning
that in the Senckenberg collection, all of the five records of supposed
O. columellaris from Central America in fact are O. semistriata, which
reinforces the doubts expressed by Olsson (1956, p. 202) regarding
the validity of reports of O. columellaris from that region. On the
other hand, two of the three records in the Senckenberg collection
of supposed O. semistriata from Ecuador and Peru in reality are
O. columellaris. These findings, together with the misidentifications
in the published literature discussed above, suggest that reports of
O. columellaris from Central America and records of O. semistriata
from locations south of Colombia tend to be incorrect. Therefore it
appears questionable whether the overlap of the distribution ranges
of the two species is as broad as suggested by standard identification
guides (e.g. Nicaragua to northern Peru, according to Keen 1971
p. 631). It rather appears that O. semistriata is the only Pachyoliva
north of Colombia whereas O. columellaris dominates more or less
completely south of that country. This tentative conclusion can be
tested in the field using the identification criteria validated in the
present study.
3. Shell Shape and Mode of Development
Shell geometry may provide essential identification criteria
in difficult taxa, such as the Olividae, if strictly quantitative
morphometric approaches are followed (Tursch & German 1985).
To see whether identifications of Pachyoliva species based on
the shell characters discussed above correlate with shell shape as
suggested by current identification guides, we determined simple
geometric parameters (Figure 1) in several hundred individuals of
each species from our study sites in Costa Rica (O. semistriata) and
Peru (O. columellaris), and from the Senckenberg collection. We first
plotted shell width versus shell length for the two species to determine
whether shells of various populations identified as the same species
fell into a common region; in fact, this was the case (Figure 6a, b). An
overlay of the two clouds of data‑points showed that O. columellaris
shells actually were ‘fatter’ than those of O. semistriata, but only if
they exceeded about 11 mm length (Figure 6c). This fact became even
clearer in a plot of aspect ratio (shell length divided by shell width)
versus shell length (Figure 6d). A similar result was obtained when
we plotted spire height versus spire base width as a measure for the
pointedness of the shell. Large, but not small O. columellaris had a
smaller spire height to spire base width ratio than O. semistriata of
comparable size (Figure 6e).
Evidently, overall shell shape is a helpful criterion in distinguishing
O. columellaris and O. semistriata, but as in the cases of callus and
striae, the criterion is reliable only in individuals above a certain
critical size (i.e., about 12 mm shell length). Most individuals below
this size cannot be identified unequivocally. Our data provided further
information regarding the developmental mechanisms behind the
establishment of the distinct phenotypes in larger individuals. In
O. semistriata, simple linear relationships exist between shell length

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ab
c
d e
Figure 6. Shell morphometrics of Pachyoliva snails. a) Shell width plotted versus shell length for samples from three O. semistriata populations; our study
population at Playa Grande, Costa Rica (n = 267), the population studied by O. Schuster in El Salvador (shells housed in the Senckenberg collection; n = 201),
and a population from Panama (Senckenberg collection, catalogue number 60064; n = 16). b) Analogous data from five populations of O. columellaris; our
study population at Colan, Peru (n = 413), and four populations from Peru and Ecuador (Senckenberg collection, catalogue numbers indicated in the figure;
116245/22, n = 22; 256856/43, n = 43; 69517, n = 44; 60048, n = 3). c) Overlay of the data areas in a) and b); only the largest O. columellaris shells are wider
than O. semistriata shells of the same length. d) The plot of shell aspect ratio (shell length divided by width, calculated from data in a) and b)) versus shell
length shows that the geometry of shell growth is similar in the two species early in life, but that O. columellaris shells tend to become relatively wider when
shell length exceeds 10–11 mm. e) Large O. columellaris that have spire base widths of 4 mm or more show lower ratios of spire height to spire base width
than O. semistriata of comparable size (data from the O. semistriata population in Playa Grande, Costa Rica, n = 300, and the O. columellaris population in
Colan, Peru, n = 490).

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and width (Figure 6a) as well as between spire base width and spire
height (Figure 6e), and the shell aspect ratio was practically constant
for all shell sizes (Figure 6d). Thus, O. semistriata does not change
its proportions as it grows; growth is isometric, and there is no
geometrically defined state of maturity. In contrast, O. columellaris
changes its mode of shell growth once it reaches 10 mm shell length;
shells grow wider relative to their length (Figure 6b, d) after this stage,
and the spire becomes flatter (Figure 6e). This is an allometric growth
pattern with geometrically distinguishable immature and mature
phases. It should be noted that growth ceases in O. columellaris
shortly after the switch into the mature growth mode, as we never
found shells of more than 15 mm length (Figure 6b). In contrast,
O. semistriata grows isometrically up to 23 mm (Figure 6a), which
significantly exceeds the size given in recent identification keys
(15 mm; Burch & Burch 1963, Keen 1971).
Differences in gastropod shell shape and allometric growth can, in
many cases, be explained as a function or even a direct consequence
of differences and changes in shell growth rate (Kemp & Bertness
1984, Urdy et al. 2010a, 2010b). Our analysis of shell morphometrics
(Figure 6) allows conclusions regarding geometric aspects of growth,
but lack the temporal component required to evaluate growth rates.
Growth analyses based on repeated measurements of individual
snails over prolonged periods will be required to understand the
morphometric relationships quantitatively in terms of temporal
processes.
Allometric shell growth in marine gastropods frequently is plastic
and responsive to environmental conditions (Kemp & Bertness 1984,
Johnson & Black 1998, Yeap et al. 2001, Hollander et al. 2006).
Purpura columellaris (Thaididae), an inhabitant of hard substrates
in the intertidal of the tropical eastern Pacific, shows differences in
growth rate leading to shell morphologies so distinct that the faster
growing morph had been considered a separate species, P. pansa
(Wellington & Kuris 1983). The occurrence of the two morphs
correlated with gradients in predation risk (Wellington & Kuris 1983).
Could O. columellaris and semistriata be a similar case, and represent
two morphs of the same biological species? We think not, because
the two taxa do not overlap geographically as broadly as previously
assumed (see discussion above), because there are no intermediate
adult forms with respect to the decisive trait ‘semi‑striation’, and
because the dependency of shell geometry on body size indicates two
identifiable clusters of forms rather than a continuum (Figure 6c).
Moreover, the distribution of the two taxa is not correlated with any
obvious environmental factors that could be hypothesized to favor
one over the other. Considering all available evidence, it seems more
plausible that O. columellaris and O. semistriata are sister species that
have diversified with respect to their genetically fixed developmental
programs (isometric versus allometric growth). The occurrence of an
allometric shift in shell shape at a defined developmental stage in a
member of the Olivellidae is not entirely unexpected; similar cases
have been reported from the closely related Olividae (Tursch 1997).
4. Developmental and ecological plasticity in
O. semistriata
While working at Playa Grande (Costa Rica) where O. semistriata
of over 20 mm shell length occur regularly, we also studied a
population at Bahia Junquillal close to the Nicaraguan border (site 8
in Figure 2) which lacks individuals larger than 13 mm shell length.
Since we consistently found this discrepancy at all times of year, it
cannot be due to an annual growth cycle or a seasonal developmental
pattern in O. semistriata. Rather, the species appears to reach different
maximum body sizes at different locations. Phenotypic variability
in shell size and shape that appears adaptive due to its correlation
with biotic or abiotic environmental factors is common in marine
gastropods (Wellington & Kuris 1983, Kemp & Bertness 1984,
Johannesson 1986, Trussell 1996, Yeap et al. 2001, Hollander et al.
2006). Johnson and Black (1998, 2000, 2008) studied phenotypic
variability and growth plasticity in the polymorphic Bembicium
vittatum (Littorinidae) and reached the conclusion that “[...] the dwarf
phenotype is largely a plastic stunting.” (Johnson & Black 1998 p. 95)
in response to specific environmental conditions. If the same holds
for the ‘dwarfish’ populations of O. semistriata, their occurrence will
be correlated with some environmental factor(s).
We tested this hypothesis by comparing maximum body sizes
in O. semistriata populations at eight test sites (Figure 2). As a
group, the test beaches represented ecological gradients regarding
their exposure to wave energy, and with respect to the type and
extent of human activities (Table 1). Finally, the test beaches varied
significantly with regard to predation pressure on O. semistriata, with
estimated minimum densities of its main predator Agaronia propatula
(López et al. 1988, Rupert & Peters 2011, Cyrus et al. 2012) between
0 and >20 individuals per 100 m beach length. (Table 1).
Maximum shell length in O. semistriata, a parameter that is easily
established with minimum disturbance of the animals in their natural
environment, differed significantly between the sites (an example is
shown in Figure 7a). However, size differences between individuals
are most meaningful if expressed in terms of body mass since this
reflects the different costs of generating and maintaining a given body
size more directly. Therefore we determined the relationship of shell
length and live body mass for the O. semistriata population at our
primary study site, Playa Grande (site 3 in Figure 2 and Table 1).
Body mass scaled with shell length to the 2.67th power (Figure 7b),
close to the 3rd power that is theoretically expected for the relationship
between distance‑ and volume‑dependent parameters in bodies of
similar shape.
Since the determination of the largest size class present at a given
location rather than the determination of complete size spectra was
sufficient to test our hypothesis, we screened each test beach for
particularly large specimens following a standardized procedure
(see Methods section). We used the size distribution of the largest
25 individuals found at a site as an estimate of the maximum size
present at that location. The size spectra of these largest 25 animals
found at each site are presented as box‑plots in Figure 7c, where they
are graphed versus predation pressure as represented by the estimated
density of A. propatula. It is worth noting that the differences in
maximum body size that exist between local populations may be
quite substantial (Figure 7c). The estimated maximum body mass
in the population with the largest snails (Playa Grande, site 3) was
more than four‑fold that found in the population with the smallest
‘big’ snails (Playa Hermosa, site 6).
The maximum size of O. semistriata did not correlate with
predation pressure (Figure 7c). Neither did the extent of human
utilization of the beaches produce any obvious effects: Bahia
Junquillal (pristine, remote from settlements, no significant tourist
infrastructure; site 8) and Playa Hermosa (‘downtown location’; site
6) represented the extreme cases in terms of direct human impact
and utilization, but at these two locations the maximum sizes of
O. semistriata were the lowest observed. However, there was a clear
correlation between maximum body size reached at a location and the
exposure of that particular beach to wave energy. The three beaches
fully exposed to oceanic conditions had the largest O. semistriata
whereas the three sheltered beaches had the smallest; intermediately
exposed beaches had snails of intermediate size (Figure 7c).
Evidently, O. semistriata remains small in sheltered coves and grows
to large sizes on beaches exposed to the open ocean.
Due to the variability in local maximum size, researchers
familiar with specimens from only a few populations might easily

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reach invalid conclusions. For example, the maximum shell size
in O. semistriata is understated by about one third in the current
identification literature (Burch & Burch 1963 p. 5, Keen 1971 p. 632),
which suggests that the authors studied specimens exclusively from
populations in which only moderate body sizes were reached. In 1851,
small O. semistriata actually were described as a separate species,
‘O. attenuata’, by Reeve, an author who obviously was familiar
with the typical, large O. semistriata. The error was recognized first
by Weinkauff in 1878 who, intriguingly, is the only classical author
to report similar maximum shell lengths for O. semistriata as we
do (22 mm). Weinkauff found that some specimens in batches of
collected ‘O. attenuata’ showed the striae typical of O. semistriata,
and concluded that ‘O. attenuata’ “[...] certainly is nothing but a small
variety of O. semistriata.” (Weinkauff 1878 p. 145; our translation).
We infer that the specimens in which Weinkauff saw the typical
striation were the very largest ones in those batches, because this
is what we found in ‘O. attenuata’ samples in the collection of the
Natural History Museum, London. Snails classified in this collection
as ‘O. attenuata’ usually are below 9 mm shell length, but the few
larger individuals often show faint striae on the posterior body whorl.
From these facts it is evident that earlier researchers repeatedly
mistook small O. semistriata for a different taxon, indicating that they
must have encountered populations that lacked larger, semi‑striated
shells, which would have revealed their true identity. The observed
continuation of ‘dwarfish’ populations over prolonged periods
requires at least one of two possible mechanisms. First, the members
of the population may acquire reproductive maturity and produce
offspring without ever reaching the phenotypic (morphological)
maturity observed in other populations. This mechanism would
be in line with the open, isometric growth pattern in this species
(Figure 6) that may enable the animals to mature physiologically
at variable body sizes that are appropriate for different sets of
environmental conditions. Alternatively, ‘dwarfish’ populations may
be pseudopopulations, maintained solely through the continuous
recruitment of individuals from other areas. Which of the two
possibilities is correct in O. semistriata cannot be decided on the
basis of the information available at this time.
Conclusions
Considering the ecological significance of Pachyoliva species
that is suggested by their large densities, it certainly is desirable to
identify them unequivocally in the field in future studies. From our
analysis of diagnostic characters, we conclude that the three traits
of the shell commonly listed as criteria by which O. columellaris
and O. semistriata can be distinguished – development of parietal
and spire callus, presence of striae on the body whorl, and shell
shape – work well in large specimens but not in smaller animals
which contribute the majority of individuals in all of the populations
that we have studied in the field. For the field biologist attempting to
establish the identity of a Pachyoliva population, it will be essential
to examine the larger individuals of that population. Small animals
c
a
b
Figure 7. Dependence of maximum body mass reached by O. semistriata on local ecological conditions prevailing at selected test beaches (sites 1 to 8; see
Table 1 and Figure 2 for details). a) Example of size differentiation; representative shells of the largest size classes found at site 2 (Playa Avellana; left) and
site 8 (Bahia Junquillal; right). b) Empirical relationship between shell length and body mass established in the population at site 3 (Playa Grande; n = 266).
c) Box plots of the body masses of the largest 25 individuals found at each test beach (boxes represent the central quartiles, and whiskers mark the 5th and 95th
percentiles of these groups of 25). The abscissa represents the estimated density of the predatory gastropod Agaronia propatula as a measure of the predation
pressure O. semistriata is exposed to at the test beaches. Box plots are color coded to indicate high (blue), intermediate (yellow), and low (magenta) exposure
to wave energy (see Table 1 for further details).

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are not identifiable on the basis of shell morphology and will have
to be classified through their association with larger, identifiable
specimens in the population. Presumably this will be unproblematic
in most regions, since no mixed populations of O. columellaris and
O. semistriata have been reported so far. Moreover, the geographic
distributions of the two species do not seem to overlap as broadly as
has been assumed in the past.
Although our comparative studies on eight test beaches utilized
relatively crude measures for the potential impact of human activities
and pollution, the beach character in terms of exposure to wave
energy, and predation pressure, we obtained support for the idea that
O. semistriata develops the dwarfish phenotype in response to the
physical environment, more specifically low wave exposure. The clear
distinction between genetic and environmental control mechanisms
of morphological variability has long been considered essential for
an understanding of the evolutionary significance of phenotypic
variation in gastropods (Johnson & Black 1998, Trussell & Etter 2001,
Conde‑Padin et al. 2009), but whether this distinction is meaningful
or even possible also has been doubted (Urdy et al. 2010a). At this
time, it remains unclear how much, if any, of the local variability
in O. semistriata body size is genetically fixed. However, the large
differences in body size between populations that can be easily
quantified, and the easily accessible, huge numbers of individuals in
each local population, make O. semistriata a particularly convenient
model for the study of the regulation of phenotypic variability.
Moreover, the significance of O. semistriata’s isometric development
for this regulation as well as for the evolution of phenotypic plasticity
could be addressed through studies including its allometricly growing
sister species, O. columellaris. With the present paper we have
paved the way for such comparative investigations by resolving the
taxonomic ambiguities that marred the earlier literature.
Acknowledgements
Our studies in Costa Rica were supported by an IPFW
Undergraduate Summer Research Grant to S.D.R., and by a grant
to A.Z.C. from the June Enoch Scholarship Fund through the IPFW
Honors Program (thanks to Shree Dhawale for enabling this support).
A.I.T. contributed to this project as a volunteer undergraduate
research assistant in the labs of W.S.P. and B.F.D. We thank Dawn
Stager for assistance with the preparation of shell sections, and Pilar
Santidrián Tomillo, the staff at the Goldring Marine Biology Station
at Playa Grande, and the Leatherback Trust (www.leatherback.org)
for logistic support in Costa Rica, where field work was carried out
under the permit number ACT‑OR‑D‑015 to W.S.P. from the Sistema
Nacional de Áreas de Conservación, Ministerio del Ambiente y
Energia, Costa Rica. W.S.P. studied Olivella biplicata at the Marine
Biology Laboratory (University of California, Davis) at Bodega Bay,
California, and thanks Jackie Sones and Lisa Valentine for their help.
We gratefully acknowledge support at the Senckenberg Museum by
Ronald Janssen and at the Natural History Museum London by Kathie
Way and Joan Pickering, and thank Klaus‑Jürgen Götting, James
Haddock, and Thomas Vanagt for helpful discussion. Thanks also to
Talia Bugel for the Spanish translation of the abstract.
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Received 12/02/2012
Revised 03/06/2012
Accepted 22/06/2012

Errata
TROOST, A.I., RUPERT, S.D., CYRUS, A.Z., PALADINO, F.V., DATTILO, B.F. & PETERS, W.S. What can we learn
from confusing Olivella columellaris and O. semistriata (Olivellidae, Gastropoda), two key species in panamic sandy
beach ecosystems? Biota Neotrop. 12(2): http://www.biotaneotropica.org.br/v12n2/en/abstract?article+bn02112022012
Página 101
Onde se lê:
of dwarfish O. semistriat, which
Leia-se:
of dwarfish O. semistriata, which
Página 101
Onde se lê:
O. semistriata; O. columellaris; O. columellaris; Olivella
columellaris
Leia-se:
O. semistriata; O. columellaris; O. columellaris Olivella
columellaris
Página 104
Onde se lê:
City beach of Puntarenas (W)
Leia-se:
City beach of Puntarenas
Página 105
Onde se lê:
and the most anterior portion of the foot (prododium) is
Leia-se:
and the most anterior portion of the foot (propodium) is
Página 110
Onde se lê:
becomes flatter (Figure 6f). This is an allometric growth
Leia-se:
becomes flatter (Figure 6e). This is an allometric growth
Página 110
Onde se lê:
population at Bahia Junquillal close to the Nicaraguan
border (site 2
Leia-se:
population at Bahia Junquillal close to the Nicaraguan
border (site 8
Página 110
Onde se lê:
0 and >20 per 100 m beach length
Leia-se:
0 and >20 individuals per 100 m beach length
Página 112
Onde se lê:
J. Mollus. Stud. 1-11
Leia-se:
J. Mollus. Stud. 78:235-245
Página 113
Onde se lê:
SCHUSTERDIETERICHS
Leia-se:
SCHUSTER-DIETERICHS

Errata
TROOST, A.I., RUPERT, S.D., CYRUS, A.Z., PALADINO, F.V., DATTILO, B.F. & PETERS, W.S. What can we learn
from confusing Olivella columellaris and O. semistriata (Olivellidae, Gastropoda), two key species in panamic sandy
beach ecosystems? Biota Neotrop. 12(2): http://www.biotaneotropica.org.br/v12n2/en/abstract?article+bn02112022012
Página 103
Onde se lê:
Leia-se:

a
b
c
Errata
TROOST, A.I., RUPERT, S.D., CYRUS, A.Z., PALADINO, F.V., DATTILO, B.F. & PETERS, W.S. What can we learn
from confusing Olivella columellaris and O. semistriata (Olivellidae, Gastropoda), two key species in panamic sandy
beach ecosystems? Biota Neotrop. 12(2): http://www.biotaneotropica.org.br/v12n2/en/abstract?article+bn02112022012
Página 111
Onde se lê:
Leia-se:
a
b
c