Burrows with Chimneys of the Fiddler Crab Uca thayeri: Construction, Occurrence, and Function

Abstract

Building of soil structures is observed in a variety of semi-terrestrial crustaceans. In fiddler crabs (Genus Uca), this behavior occurs in several species, some of which build structures that are largely ornamental and others construct barriers that are apparently for defense. Although there is a relative abundance of studies on this type of behavior in Uca, the relationship between the social context and the occurrence of these structures remains poorly studied. Thus, this study attempted to analyze in detail the construction, occurrence, and function of mud chimneys built by the fiddler crab Uca thayeri; these sedimentary structures are possibly associated with burrow defense. Field investigations and laboratory experiments were conducted. Both sexes were often found in burrows with chimneys; however, laboratory experiments showed that only females actively built and maintained chimneys, with some difference in the morphology of these structures between sexes. The social context had little influence on the construction of chimneys, which showed that the stimulus for constructing chimneys could be endogenous. Our results suggest that burrows with chimney of U. thayeri may have functions other than defense, and may act in regulating the internal conditions of the burrow, as observed in other crustaceans with such building behavior. http://zoolstud.sinica.edu.tw/Journals/51.5/598.pdf.

Full text

Burrows with Chimneys of the Fiddler Crab Uca thayeri: Construction,
Occurrence, and Function
João B. L. Gusmão-Junior1, Glauco B. O. Machado2, and Tânia M. Costa3,*
1Centro de Estudos do Mar, Universidade Federal do Paraná, Pontal do Paraná, Paraná 83255-971, Brazil.
E-mail:gusmao.jb@gmail.com
2Departamento de Biologia Animal, Universidade Estadual de Campinas, Campinas, São Paulo 13083-970, Brazil.
E-mail:gbomachado@gmail.com
3Campus Experimental do Litoral Paulista, Universidade Estadual Paulista, São Vicente, São Paulo 11330-900, Brazil
(Accepted January 20, 2012)
João B. L. Gusmão-Junior, Glauco B. O. Machado, and Tânia M. Costa (2012) Burrows with chimneys
of the fiddler crab Uca thayeri: construction, occurrence, and function. Zoological Studies 51(5): 598-605.
Building of soil structures is observed in a variety of semi-terrestrial crustaceans. In ddler crabs (Genus Uca),
this behavior occurs in several species, some of which build structures that are largely ornamental and others
construct barriers that are apparently for defense. Although there is a relative abundance of studies on this type
of behavior in Uca, the relationship between the social context and the occurrence of these structures remains
poorly studied. Thus, this study attempted to analyze in detail the construction, occurrence, and function of mud
chimneys built by the ddler crab Uca thayeri ; these sedimentary structures are possibly associated with burrow
defense. Field investigations and laboratory experiments were conducted. Both sexes were often found in
burrows with chimneys; however, laboratory experiments showed that only females actively built and maintained
chimneys, with some difference in the morphology of these structures between sexes. The social context had
little inuence on the construction of chimneys, which showed that the stimulus for constructing chimneys could
be endogenous. Our results suggest that burrows with chimney of U. thayeri may have functions other than
defense, and may act in regulating the internal conditions of the burrow, as observed in other crustaceans with
such building behavior. http://zoolstud.sinica.edu.tw/Journals/51.5/598.pdf
Key words: Construction behavior, Biogenic structure, Crab behavior.
*To whom correspondence and reprint requests should be addressed. Tel: 55-13-35697100. Fax: 55-13-35697106.
E-mail:costatm@clp.unesp.br
Several species of semi-terrestrial
crustaceans have a characteristic behavior of
burrow building, which can be described as the
stacking of sedimentary material in the vicinity of
the burrow opening. Among taxonomic groups
in which this behavior occurs, crayfish from
the superfamily Astacoidea and crabs from the
superfamily Ocypodoidea stand out (McManus
1960, Crane 1975, Takeda et al. 1996).
The genus Uca Leach 1814 (fiddler crabs)
contains a large number of species with such
construction behavior (Crane 1975). However,
the morphology and possible functions of such
structures built by these species vary, but there are
3 basic types: chimneys which are a sedimentary
structure similar to a wall surrounding the entire
edge of the burrow opening (Crane 1975, Slatyer
et al. 2008); hoods which are a sedimentary
structure of semicircular shape, forming a covering
that protects part of the burrow opening (Zucker
1974, Crane 1975, Christy et al. 2001, Kim et al.
2004, Christy 2007, Mokhlesi et al. 2011), and
pillars which are sedimentary structures piled next
to the burrow opening (Crane 1975, Christy 1988,
Mokhlesi et al. 2011).
In some species, only males construct hoods,
Zoological Studies 51(5): 598-605 (2012)
598

which may act as an ornamental structure
(Yamaguchi 1971, Christy et al. 2001, Kim et al.
2004) or be associated with territoriality (Zucker
1974, Christy 1988). In other species, as is the
case of U. thayeri Rathbun 1900, apparently only
females build chimneys that possibly protect the
burrow from intruders (Crane 1975, Salmon 1987).
There are also species in which both sexes build
structures, as in the case of U. arcuata De Haan
1835 and U. capricornis Crane 1975, the chimneys
of which also have a defensive function (Wada and
Murata 2000, Slatyer et al. 2008).
Most studies evaluating the roles of chimneys
did not consider how the social context could
influence the construction of these structures.
Chimneys are thought to act as barriers that
give an advantage to the resident crab, and con-
sequently it is expected that the investment in the
construction of these structures increases when
potential intruders are present.
This study attempted to investigate the
construction, occurrence, and function of chimneys
built by the fiddler crab U. thayeri through field
investigations and laboratory experiments. The
field investigations were performed to verify the
occupants of burrows with chimneys, and to
assess temporal variations in the occurrence and
size of the structures. Laboratory experiments
were conducted to examine how the chimneys are
constructed and to test if the presence of potential
conspecific intruders affects the construction of
these structures.
MATERIALS AND METHODS
Study species
The species Uca thayeri occurs along the
east coast of the Americas and in the Caribbean
islands, and is mainly distributed between latitudes
of 32°N and 28°S, where it is commonly found
in muddy and shaded areas on the outer edge
of mangrove forests (Crane 1975, Melo 1996).
This species shows the behavior of building a
sedimentary structure in the form of a chimney
surrounding its burrow; these chimneys are
apparently only built by females (Crane 1975,
Salmon 1987).
Field sampling
Study site
The study site consisted of a mangrove
forest in the municipality of Praia Grande, on the
coast south of São Paulo, Brazil (23°59'15"S,
46°24'23"W). This forest is characterized by the
presence of Rhizophora mangle on its margin next
to the water line, and Avicennia schaueriana in
its inner portion. Two sampling areas separated
by about 80 m were established, each with
dimensions of approximately 50 × 18 m, located
10 m from the water line (tide height = 0.0 m).
The sediments predominantly consisted of silt and
clay (with an average grain size of 5.3 ± 1.4 ɸ),
an organic matter content of 16.5% ± 8.8%, and a
moisture content of 24.7%.
Sampling techniques
Samples were obtained monthly from Nov.
2008 to Oct. 2009 using the quadrat technique. A
quadrat size of 75 × 75 cm was determined in a
previous pilot study, according to the method of
Wiegert (Krebs 1989).
Each month, 9 quadrats were randomly
sampled in each sampling area. The number of
burrows with and without associated sedimentary
structures and the dimensions of the structures,
namely the width (W) and height (H) (Fig. 1),
were recorded. The crabs were captured by
excavation of each burrow using a garden shovel.
This method was selected due to the fact that
capturing the animals after they had come to the
surface could take a long time and might introduce
some deviations in the results because females
(mostly ovigerous ones) may be less active and
conspicuous than males. Sex and carapace width
(CW) of the crabs were recorded, and they were
Fig. 1. Measurements of sedimentary structures of U. thayeri.
H, height; W, width.
W
H
Gusmão-Junior et al. – Burrows with Chimneys of the Fiddler Crab Uca thayeri 599

classied as adult (CW > 10 mm) and young crabs
(≤ 10 mm CW) (based on Negreiros-Fransozo
et al. 2003). Given that some burrows were
apparently empty, we excluded burrows whose the
occupants were not captured from the analysis in
order to prevent deviations in the results.
Experiments in a controlled environment
Sampling site and experimentation
Specimens of Uca thayeri used in experi-
ments were collected in the same mangrove forest
where the field sampling was carried out. Adult
crabs (with a CW > 13 mm) and young (with a CW
of < 10 mm) with intact chelipeds and pereopods
were used.
The experiments were conducted in the
laboratory using matte plastic cylindrical terrarium
containers with a diameter of 24 cm and a height
of 26 cm, lled with a 15-cm layer of sediment (with
an organic matter content of approximately 16%,
a predominance of silt and clay, and a moisture
content of ca. 24%) which had previously been
sieved to remove the macrofauna. The tem-
perature was maintained at 24-28°C.
Experimental design
To record and analyze the construction of
the sedimentary structures, adult individuals of
U. thayeri were videotaped. Male, female, and
ovigerous female crabs were kept in individual
terrariums (n = 4), where they were lmed for 3 d,
from 08:00-18:00, totaling 30 h of lm per replicate.
After 3 d (72 h), we recorded and measured the
height and width (Fig. 1) of the structures that had
been built. The number of structures built by each
individual was not computed because each crab
build at least 1 burrow with a unique associated
sedimentary structure during the experiment.
To investigate the function of chimneys as
possible defensive structures, as proposed for
other species of fiddler crabs (Wada and Murata
2000, Slatyer et al. 2008), we tested the inuence
of the social context (the presence or absence of
potential conspecic intruders) on the construction
of these sedimentary structures. To this end,
ovigerous adult females in the intermediate stage
of egg maturation (resident), which are individuals
described as being active builders of chimneys
(Salmon 1987), were subjected to different
treatments (n = 12 replicates for each treatment):
treatment A consisted of a resident and an adult
male intruder; treatment B consisted of a resident
and an ovigerous female intruder; treatment C
consisted of a resident and a non-ovigerous
adult female intruder; treatment D consisted of a
resident and a young intruder (CW < 10 mm); and
the control consisted of only a resident ovigerous
female. After 72 h, each terrarium was analyzed
for the presence of sedimentary structures
associated with the burrows of resident ovigerous
females, and their dimensions (height and width in
mm) were measured.
Data analysis
The frequency data were compared by
applying a Chi-squared test (χ2), with α = 0.05.
Continuous data were compared by a one-way
analysis of variance (ANOVA), complemented
by post-hoc Fisher’s least significant difference,
with α = 0.05. The homoscedasticity was
verified by Cochran’s test (α = 0.05), with the
data transformed to a base 10 logarithm when
necessary.
RESULTS
Field data
Three different morphotypes of burrows
occupied by Uca thayeri were found: chimneys,
irregular deposits, and burrows without a surface
structure. Chimneys were sedimentary structures
that surrounded the entire burrow opening with
a cylindrical or conical shape and vertical walls.
Irregular deposits of sediments were small deposits
around the burrow opening that did not constitute
a defined structure. Finally, burrows without
associated structures had neither a chimney nor
sediment deposits associated with the opening.
In total, 1518 burrows occupied by U. thayeri
were recorded (377 in the summer, 331 in the fall,
391 in the winter, and 419 in the spring); among
them, 462 had the chimney morphotype, 193 had
irregular sediment deposition, and 863 had no
associated sedimentary structures (Fig. 2). We
collected 366 adult males, 386 young males, 388
adult females, 81 ovigerous females, and 297
young females.
Adult ovigerous and non-ovigerous females
were more frequently found in burrows with
chimneys than in burrows without structures
(ovigerous: χ2 = 127, d.f. = 2, p < 0.001, non-
ovigerous: χ2 = 120.4, d.f. = 2, p < 0.01) (Table
600 Zoological Studies 51(5): 598-605 (2012)

1). Adult males were more abundant in burrows
with chimneys and those with irregular deposits
than in burrows without associated structures
(χ2 = 55.7, d.f. = 2, p < 0.01). Young crabs of
both sexes were more abundant in burrows
without associated structures than in burrows with
associated structures (young males: χ2 = 694.8,
d.f. = 2, p < 0.01; young females: χ2 = 532, d.f. = 2,
p < 0.01).
Considering only burrows with chimneys that
were occupied by adult crabs, the dimensions
of the chimney significantly differed among
males, non-ovigerous females, and ovigerous
females (ANOVA, height: p < 0.001, F = 21.8688,
d.f. = 2; and width: p < 0.01, F = 5.0271, d.f. = 2).
Structures associated with burrows of ovigerous
females were signicantly taller and narrower than
those observed with burrows of males and non-
ovigerous females (Fig. 3). Structures of burrows
of non-ovigerous females, despite having a lower
height than those of ovigerous females, were taller
than those occupied by males (Fig. 3).
Laboratory data
Adult males, non-ovigerous adult females,
and ovigerous females constructed sedimentary
Table 1. Number and percent of each demographic group of U. thayeri in the different types of burrows
All burrows Chimneys Irregular deposits No structures
No. Percent No. Percent No. Percent No. Percent
Adult males 366 24.1 140 30.3 68 35.2 158 18.3
Young males 386 25.4 28 6.1 30 15.5 328 38.0
Adult females 388 25.6 206 44.6 63 32.6 119 13.8
Ovigerous females 81 5.3 66 14.3 9 4.7 6 0.7
Young females 297 19.6 22 4.8 23 11.9 252 29.2
Total 1518 100 462 100 193 100 863 100
300
250
200
150
100
50
0
Frequency
Summer
Summer
Summer
Autumn
Autumn
Autumn
Winter
Winter
Winter
Spring
Spring
Spring
Adult Males
Ovigerous Females
Adult Females
Young Males
Young Females
(A) (B) (C)
Fig. 2. Frequency of each demographic group (adult males
and females, young males and females, and ovigerous
females) of U. thayeri in burrows with a chimney (A), in burrows
with sediment deposition (B), and in burrows with no associated
sedimentary structures (C) per season.
Fig. 3. Mean values (± standard error) of the height (H) and
width (W) of sedimentary structures occupied by adult crabs
(with a carapace width (CW) > 10 mm) sampled in the Portinho
mangrove, Praia Grande, São Paulo (n = 412). Different letters
indicate a signicant difference (p < 0.05).
10
20
30
40
50
60
Males Females Ovigerous Females
Height (mm)
cb
a
10
20
30
40
50
60
Males Females Ovigerous Females
Width (mm) -
b
aa
(A)
(B)
Gusmão-Junior et al. – Burrows with Chimneys of the Fiddler Crab Uca thayeri 601

structures. The construction process was chara-
cterized by 3 behavioral acts, as illustrated in
figure 4: (1) digging and depositing mud, in
which the crab digs into the substratum using
the 4 pereopods on 1 side of the body, removing
sediment, and depositing it with the aid of the
pereopods and chelipeds near the burrow opening;
(2) dragging and stacking mud, in which the crab
collects sediment from the surface surrounding
the burrow and drags it with the pereopods to the
edge of the burrow opening with the pereopods
and chelipeds; and (3) modifying the structure, in
which the crab stands on the sediment deposits
(composed of pellets of excavation and sediments
carried from the surroundings) and plasters them
on the edge of the burrow opening using its
pereopods and chelipeds. Of these 3 behavioral
acts, it was found that adult males exhibited only
‘digging and depositing mud’, while ovigerous
and non-ovigerous adult females performed
all 3 behavioral acts. Only females invested in
maintaining the built structures by reshaping their
form and adding more sediment. All ovigerous
females, 10 males (83.3%), and 10 non-ovigerous
females (83.3%) built sedimentary structures.
Average heights of the sedimentary structures
built by males and non-ovigerous females were
signicantly lower (p < 0.05, d.f. = 2, F = 5.4939)
than those of ovigerous females (Fig. 5A). The
average width did not significantly differ among
males, ovigerous females, and non-ovigerous
females (Fig. 5B).
Resident ovigerous females, from both the
experimental and control groups, constructed
sedimentary structures. The average values of
the height and width of the structures built by
ovigerous females from the control group did not
significantly differ from those built by resident
ovigerous females from the other treatments (Table
2).
Fig. 4. Construction of chimneys by U. thayeri, an example of an adult female. (1) Digging and depositing mud: the crab digs into the
substrate using the 4 pereiopods on 1 side of its body (1A), removing sediment and depositing it with the aid of the pereiopods and
chelipeds near the edge of the burrow opening (1B). (2) Dragging and stacking the mud: the crab collects sediment from the surface
surrounding the burrow (2A) and drags it with its pereiopods to the edge of the burrow opening with the pereiopods and chelipeds
(2B). (3) Modifying the structure: the crab stands on the sediment deposits (3A) and modies them at the edge of the burrow using its
pereiopods and chelipeds (3B).
1. Digging and depositing mud
2. Dragging and stacking mud
3. Modifying the structure
(A)
(A)
(A)
(B)
(B)
(B)
602 Zoological Studies 51(5): 598-605 (2012)

DISCUSSION
The construction of the sedimentary struc-
tures by Uca thayeri showed a pattern that was
very similar to those described for U. arcuata
(Wada and Murata 2000), U. formosensis Rathbun
1921 (Shih et al. 2005), U. capricornis (Slatyer et
al. 2008), and U. lactea De Haan 1835 (Kim et
al. 2004), although for the latter, the similarities
were solely restricted to the behavioral acts. The
building behavior of U. thayeri differed from those
of U. lactea, U. musica Rathbun 1914, and other
builders of hoods or pillars in several respects.
The most notable was associated with the crab
sex, in which only reproductively mature males
build hoods (Crane 1975, Christy et al. 2001, Kim
et al. 2004, Christy 2007), while in U. thayeri both
sexes exhibited the construction behavior (as
found in this study). The hoods apparently act as
ornaments that assist in attracting females to the
burrow of males (Christy et al. 2001, Kim et al.
2004, Christy 2007). This function does not apply
to chimneys of U. thayeri because mating occurs
in the burrow of females (Salmon 1987), and
females, mainly ovigerous females, invest more
in building the tallest structures (as found in this
study).
Despite the high proportion of males
occupying burrows with chimneys found in the
field investigation, results of our laboratory
research suggests that only females actively
build chimneys (collecting material and shaping
the structure), while the structures built by males
are apparently the result of the deposition of
excavation pellets during construction of the
burrows. Our observations confirm what was
described by Salmon (1987) about the behavior of
this species. The behavioral differences between
males and females possibly explain why there
are morphological contrasts among the structure
of burrows occupied by ovigerous females (taller
and narrower than the others), adult females, and
males (lower and wider).
The increased occurrence of adult females
in burrows with chimneys, especially ovigerous
females during spring and summer, apparently
Table 2. Experimental procedure: mean values (± standard error) of the resident and intruder size of each
treatment (n = 12) with respective mean values of the height and width of the sedimentary structures built by
the resident ovigerous female
Crabs (CW) Structures Built
Treatments Resident OF Intruder NSB Height Width
Control 17.58 ± 1.39 - 12 32.27 ± 2.30 60.61 ± 2.59
RE × OF 16.18 ± 1.76 15.82 ± 2.05 10 24.44 ± 2.00 53.38 ± 2.81
RE × F 17.79 ± 1.45 18.76 ± 2.21 11 35.95 ± 3.76 62.73 ± 3.63
RE × M 17.16 ± 1.36 19.02 ± 0.92 10 35.18 ± 3.37 64.27 ± 3.84
RE × Y 16.83 ± 1.48 7.6 ± 0.90 12 33.62 ± 3.49 51.83 ± 1.85
RE, resident crab; NSB, number of structures built; OF, ovigerous female; F, non-ovigerous adult female, M, adult male; Y, young crab;
CW, carapace width.
Fig. 5. Mean values (± standard error) of the height (H) and
width (W) of sedimentary structures built by males and non-
ovigerous and ovigerous females of U. thayeri in the laboratory
(n = 32). Different letters indicate a statistical difference (p <
0.05).
10
20
30
40
50
60
70
80
Males Females Ovigerous Females
Height (mm) -
b
a
b
10
20
30
40
50
60
70
80
Males Females Ovigerous Females
Width (mm) -
a
a
a
(A)
(B)
Gusmão-Junior et al. – Burrows with Chimneys of the Fiddler Crab Uca thayeri 603

confirms the observations of Crane (1975) and
Salmon (1987), who reported a close relationship
between these structures and the reproductive
period of this species. Some authors described
chimneys built by fiddler crabs as defensive
structures, so that the walls play the role of
barriers against conspecific intruders (Wada and
Murata 2000, Slatyer et al. 2008) or hide the
occupants from potential natural enemies (Shih et
al. 2005). Considering that in U. thayeri, ovigerous
females build structures that are taller than those
built by other crabs, we suggest that they do this
to enhance barriers against intruders and avoid
agonistic interactions with conspecics or at least
to have an advantage in case of a confrontation.
This possibly represents an advantage in the
energy reserves of residents that own burrows
with chimneys, given that the energy demand for
reproduction is very costly.
Laboratory results revealed that the social
context did not affect the construction of chimneys
by ovigerous females of U. thayeri. This led
us to suggest that the building behavior may
be triggered by another stimulus or even an
endogenous stimulus. Furthermore, the function
of the chimney might not only be restricted to the
defense of burrows and their occupants. In other
groups of semi-terrestrial decapods that also
build chimneys such as some species of craysh
(Cambaridae), the function of these structures is
related to maintaining the internal conditions of
the burrow, including regulating the evaporation
of the internal water content (Williams et al. 1974,
Trépanier and Dunham 1999). Chimneys built
by female U. thayeri might also have a regulatory
function (which does not exclude a possible
defensive function), acting to maintain the internal
conditions of the burrow, especially during the
generation and incubation of egg masses, which
could explain the greater investment by ovigerous
females in constructing taller structures.
Our findings about the construction of
chimneys by U. thayeri are summarized as follows:
(1) the chimneys do not serve an ornamental
function, as observed in other species; (2) both
sexes build sedimentary structures, but apparently
only females engage in developing and maintaining
the chimneys, which explains the morphological
contrast between structures associated with
the burrows of males and females (especially
ovigerous), found in both field and laboratory
tests; (3) the social context does not inuence the
construction of chimneys by ovigerous females,
which suggests that the stimulus triggering this
behavior could be endogenous; and (4) it could be
inferred that the defensive function might not be
the only role of the chimney of U. thayeri, and the
function of regulating certain internal conditions
of the burrow, as observed in other crustacean
chimney builders, could also be related to this
behavior.
Acknowledgments: We thank P.L.M. Garcia, H.E.
Checon, F. de Grande, and K.D. Colpo for their
help during eldwork and the Fundação de Amparo
à Pesquisa do Estado de São Paulo (FAPESP
#2008/11635-9; #2010/01268-9) for a scientific
undergraduate scholarship to the 1st author. We
also thank M. Thiel and an anonymous referee for
their valuable comments on an earlier version of
this manuscript.
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