behavioural evidence for body colour signaling in the fiddler crab uca perplexa (brachyura:...
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Journal of Experimental Marine Biolog
Behavioural evidence for body colour signaling in the fiddler crab
Uca perplexa (Brachyura: Ocypodidae)
Satoshi Takeda *
Marine Biological Station, Tohoku University, Asamushi, Aomori 039-3501, Japan
Received 8 April 2005; received in revised form 18 August 2005; accepted 19 September 2005
Abstract
In the reproductive season, mature females of the fiddler crab Uca perplexa leave their burrows and wander about their habitat
for mating. To clarify whether the fiddler crabs respond to colour or luminosity, I examined the behavioural responses of the males
to the wandering females before and after the females were painted white, red, black or blue. The behaviours of the males were
categorized into three types: lateral–circular wave and lateral–straight wave for courting, and repelling. Before painting, almost all
of the males courted the females. After painting, significantly fewer males courted the red-, black- and blue-painted females than
courted the white-painted females. These results mean that the fiddler crabs can discriminate colours or luminosity. The role of
body colour as a visual signal in crab society is discussed.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Body colour; Colour discrimination; Fiddler crab; Uca perplexa; Courting display
1. Introduction
The vision of brachyurids depends on the environ-
ment to which they are adapted (Zeil et al., 1986).
For instance, crabs inhabiting bare flat terrain, gener-
ally ocypodid crabs, have eyes with vertical resolving
power at the top of closely spaced long eyestalks
(Zeil et al., 1986; Zeil and Al-Mutairi, 1996). In
contrast, crabs inhabiting flat terrain with vegetation,
generally grapsid crabs, have stereoptic eyes at the
top of widely spaced short eyestalks (Zeil et al.,
1986), and can distinguish shapes (Cannicci et al.,
2002).
Semi-terrestrial crabs have various body colours
(e.g., Crane, 1975; Detto et al., 2004). Their body
colour changes with growth and body size (e.g., Detto
0022-0981/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jembe.2005.09.021
* Tel.: +81 17 752 3388; fax: +81 71 752 2765.
E-mail address: [email protected].
et al., 2004), and in the reproductive season (e.g.,
Crane, 1975; Detto et al., 2004). The males of some
species change their body colour a few minutes after
losing a fight or being captured (Crane, 1975; Zucker,
1981; Zeil and Hofmann, 2001). In addition, the spec-
ular reflections on the cuticle of the body become
polarized after wetting (Zeil and Hofmann, 2001). It
has been shown that ocypodid crabs (genus Uca) have
two kinds of visual pigments — a precondition for
vision (Harch et al., 2002) — and can distinguish
colours (Hyatt, 1975).
Although vision and body colour as a potential
signal have been studied separately, the role of body
colour as a signal in social communication has never
been studied. The objective of this study was to see
whether semi-terrestrial crabs use their body colour as a
signal.
Fiddler crabs are suitable for examining this ques-
tion because they have various body colours and well-
y and Ecology 330 (2006) 521–527
S. Takeda / Journal of Experimental Marine Biology and Ecology 330 (2006) 521–527522
developed social behaviours (Crane, 1975; Zucker and
Denny, 1979). A characteristic feature of the fiddler
crab is hypertrophy of one of the male chelipeds,
resulting in a striking asymmetry (Crane, 1975; Rosen-
berg, 2001). The hypertrophic, or major, cheliped is
used directly in agonistic behaviour and indirectly as a
visual signal, for example as a distinctive indicator of
the male sex during the reproductive season (Salmon
and Stout, 1962; Crane, 1975). The male communicates
to nearby individuals by waving its major cheliped,
which it does in courting females or in signalling
aggression (Crane, 1975). Each species has a peculiar
rhythm and motion of the major cheliped and other
thoracic legs, which allow many species of fiddler
crab to coexist sympatrically (Altevogt, 1969; Crane,
1975).
Fiddler crabs with eyes at the top of long vertical
stalks perceive movement but not shape (Zeil et al.,
1986; Hemmi and Zeil, 2003). Therefore, particular
movements of the body or major cheliped characterized
by coloration and luminosity will be recognized as
signals.
The fiddler crabs have two kinds of mating system
(e.g., Crane, 1975). In one, the male approaches a
nearby female, and they mate on the ground surface
near the entrance to the female’s burrow. In the other, a
wandering female is enticed to enter a male’s burrow,
then they mate in the burrow. In general, the males of
fiddler crabs which mate in their burrows perform
complex waving displays (Crane, 1975). The male
changes the form of his display according to the dis-
tance to a female, and performs his most active display
when a preferred female walks near the entrance of the
burrow (Crane, 1975).
In this study, I examine the role of body colour in
social communication by observing behavioural
responses of males to females painted different colours.
Differing responses to different colours would indicate
the ability to discriminate colour.
2. Materials and methods
2.1. Study site
I studied a population of Uca perplexa on the
sandy shore of Hanejiuchimi Bay, Okinawa Island,
Japan (1288 2V 11W E; 268 37V 44W N) (Takeda,
2003). U. perplexa inhabits sandy and muddy–
sandy shores between low and high water levels of
the neap tide. The soldier crab Mictyris brevidactylus
and the fiddler crab Uca vocans inhabit the sandy
shore below the low water level of the neap tide. M.
brevidactylus is dark blue in the nonreproductive
season, and cobalt blue in the reproductive season,
winter. U. vocans is grey in the nonreproductive
season, and whitish in the reproductive season, sum-
mer. The fiddler crab Uca dussumieri inhabits the
muddy shore of the mangrove forest above the high
water level of the neap tide, and U. crassipes inhabits
mud in the mangrove forest. U. dussumieri is bluish
black and U. crassipes is vermilion. Sometimes the
grapsid crab Helice leachi, which is light brown,
similar to the ground colour, wanders about the hab-
itat of U. perplexa.
In other U. perplexa habitats, the small ocypodid
crabs Scopimera globosa and Tmethypocoelis cerato-
phora dwell (Nakasone, 1977). Both are yellowish
brown, similar to sand.
2.2. Fiddler crabs
The females and males of the fiddler crab U. per-
plexa dig individual burrows close to each other (Naka-
sone and Murai, 1998). On Okinawa Island, the crabs
reproduce from April to September (Nakasone and
Okadome, 1981). The crabs copulate in the male’s
burrow or on the surface of the sand near the burrow
entrance (Nakasone and Murai, 1998). When the male
is larger than the female, they perform either mating
practice. In contrast, when the male is smaller than the
female, they do only surface mating.
Females ready to spawn leave their burrows spon-
taneously or by interaction with nearby males (Naka-
sone and Murai, 1998). Males wave their major
cheliped to attract the females to mate. The waving
form depends on the distance to the female (personal
observation). When the distance is 30 to 50 cm, the
males face the females, flex the major cheliped acute-
ly, and raise and lower it. When the distance is about
30 cm, the males extend the major cheliped at a right
angle, and raise and lower it. When the distance is
shorter than 30 cm, the males laterally extend the
major cheliped (lateral–straight wave). Some subse-
quently raise it, then flex it and bring it back down
to the starting position (lateral–circular wave). The
major cheliped is raised higher in the lateral–circular
wave than in the lateral–straight wave (see Crane,
1975).
The males repel the females very rarely. When a
female approaches a burrow, the owner male slightly
raises and lowers his major cheliped against the female.
The male enters his burrow, or stays outside. When the
male is larger than the female, the male occasionally
flicks the female with his major cheliped.
S. Takeda / Journal of Experimental Marine Biology and Ecology 330 (2006) 521–527 523
2.3. Methods
I fenced a quadratic area (60�60 cm) with trans-
parent acrylic plates (9 cm height) on the surface of the
sand over some large males with their own burrows.
The fence was anchored by PVC poles at each corner.
Any gap between the bottom margin of the plates and
the surface of the sand was filled with sand. Crabs
without a burrow were removed from the quadrat.
Empty burrows with no owner and any depressions
were filled with sand, and the surface of the sand in
the quadrat was flattened. I measured the diameter of
each burrow to estimate the carapace width of the
owner male, because a close relationship between the
diameter (X, mm) and the carapace width (Y, mm) was
recognized (Y=1.133X +1.451, r =0.939, n =58).
Males in the quadrat courted females outside it, and
males outside it courted females inside it, which indi-
cates that the crabs could communicate with each other
across the plates.
Subsequently, I captured a large female which
walked about on the sand surface while avoiding the
waving males. Such a female was considered to be
suitable for the test of the response to several males,
because she did not enter burrows easily (Murai, per-
sonal communication). I measured her carapace width,
then put her in a conical white vessel (about 5 cm
diameter) in a corner of the quadrat. About 5 min
later, when most crabs had resumed their surface activ-
ities, I carefully opened the vessel by pulling on a fine
string. Usually the female began walking slowly about
the quadrat. I observed the males’ responses for at least
30 min, and recorded the behaviour pattern when a
male was excited by the female (the female approached
the male or his burrow within 10 cm).
After observation, I recaptured the female and
painted her whole body white, red, blue or black
(Paint-marker PX-30, Mitsubishi, Japan). The luminos-
ities of the red and blue were 39.7, that of the white was
100, and that of the black was 0, as measured using
Adobe Photoshop ver. 3.0 J for Macintosh, although
visual luminosity differs between human and crab due
to the different spectral sensitivities of the human and
crab photoreceptors (e.g., Kelber et al., 2003). The
painted female was released again in the quadrat as
before. Then I recorded the behaviour of the males
towards her. After observation, the painted female
was released in another habitat. The quadrat was set
up again on the sand surface more than 5 m away from
the previous site for the next observation. The field
experiment was conducted between 22 May and 21
June 2000 and between 11 May and 15 June 2001.
The colour of the carapace of the wandering females
varies (Crane, 1975). In the reproductive season, the
pattern follows two types: a simple colour pattern and a
lateral-striped pattern, like that of mature males. In
general, the lateral-striped pattern consists of light
brown and some black strips. The simple pattern is an
ashy red-brown, brown, yellow, white, blue or purple.
Usually, the distal tips of the chelipeds are cobalt blue,
and the proximal part of the merus of the walking legs
is red. These colours become dark when the females are
captured.
In the nonreproductive season, the colour of the
carapace of the females is yellowish brown, similar to
the sand colour.
2.4. Statistical analysis
The Kruskal–Wallis test was used to determine
differences in the carapace width of the females
painted in four colours, the density of males in the
quadrat, and burrow diameter of the males tested with
different coloured females among the four groups. The
G-test was used to determine differences in the beha-
vioural responses to the unpainted females between
males that were smaller and larger than the presented
female, and in the proportion of males that repelled
the females after they were painted among the four
colour groups of smaller or larger males. Fisher’s
exact probability test was used to determine differ-
ences in the proportion of males that courted or re-
pelled the females after painting between the lateral–
circular wavers and lateral–straight wavers that
courted the females before painting in each group,
and between each pair of the four groups of smaller
or larger males. It was also used to determine differ-
ences in the number of quadrats in which the males
did or did not repel the females after painting between
each pair of the four groups. Differences at p b0.05
were taken to be significant.
3. Results
3.1. Female and male sizes
The carapace width did not differ among the four
colour groups of painted females (Kruskal–Wallis test:
df =3, H =1.473, p =0.6884) (Table 1). This means that
the quality of the females did not differ among the four
groups, because larger females have better reproductive
success.
The density in the quadrat did not differ among the
four groups of males tested with the painted females
Table 1
Carapace width of females painted in four colours, density of males in the quadrat, and burrow diameter of the males presented with the different
coloured females
Colour White Red Blue Black Total
Female carapace width
Mean (mm) 12.92 12.56 12.75 12.98 12.80
SD 0.75 0.94 1.15 0.71 0.91
Range (mm) 11.60–14.35 11.05–14.10 10.80–14.80 11.85–14.20 10.80–14.80
N 16 17 18 16 67
Male burrow density
Mean 3.69 3.35 3.06 3.81 3.46
SD 1.25 1.22 1.16 1.11 1.20
Range 1–6 1–5 1–5 2–5 1–6
N 16 17 18 16 67
Male burrow diameter
Mean (mm) 10.89 10.95 11.06 10.92 10.95
SD 0.95 1.00 1.13 0.94 1.00
Range (mm) 9.10–13.15 8.90–14.05 8.85–15.60 9.10–13.20 8.85–15.60
N 59 57 55 61 232
S. Takeda / Journal of Experimental Marine Biology and Ecology 330 (2006) 521–527524
(Kruskal–Wallis test: df =3, H =4.009, p =0.2605)
(Table 1). The burrow diameter also did not differ
among the four groups of males (Kruskal–Wallis test:
df =3, H =0.409, p =0.9384) (Table 1).
The proportion of behaviour responses to the un-
painted females before painting differed between the
males that were smaller and larger than the female (G-
test: df =2, G2=10.491, p =0.0053) (Table 2). That is,
when the males courted the females, two-thirds of the
smaller males performed the lateral–circular wave, but
fewer than half of the larger males did so (G-test: df =1,
G2=10.401, p =0.0013) (Table 2). After painting, some
males of both sizes changed their waving behaviour
(Table 2). These results indicate that the males changed
their waving form according to the situation of the
females.
Table 2
Numbers of larger (L) and smaller (S) males that changed their pattern of b
Behaviour White Red
(NaturalYAltered) L S L S
CircularYCircular 9 4 4 2
CircularYStraight 14 1 3 5
CircularYRepulsion 1 3 4 4
(Total) (24) (8) (11) (11)
StraightYCircular 3 0 0 0
StraightYStraight 14 5 17 0
StraightYRepulsion 2 2 14 3
(Total) (19) (7) (31) (3)
RepulsionYStraight 1 0 0 0
RepulsionYRepulsion 0 0 1 0
(Total) (1) (0) (1) (0)
Circular; lateral–circular wave, Straight; lateral–straight wave.
3.2. Individual test
The males that repelled females before they were
painted were excluded from the statistical analysis,
because the mature males were considered to be
attracted by the females before painting. The propor-
tion of males that repelled females after painting did
not differ between the lateral–circular and lateral–
straight wavers in each group of the larger or smaller
males tested with the females painted in different
colours (Fisher’s exact probability test: p N0.05).
When the lateral–circular and lateral–straight wavers
that courted females before painting were pooled, the
proportion of larger or smaller males that repelled
females after painting differed among the four colour
groups (G-test: larger; df =3, G2=26.208, p b0.0001;
ehaviour towards the females after painting
Blue Black Total
L S L S L S
10 0 9 2 32 8
3 0 4 2 24 8
6 5 10 9 21 21
(19) (5) (23) (13) (77) (37)
7 1 1 0 11 1
9 0 6 1 46 6
9 2 13 2 38 9
(25) (3) (20) (3) (95) (16)
0 0 0 0 1 0
2 1 1 1 4 2
(2) (1) (1) (1) (5) (2)
Fig. 1. Proportion of larger (A) and smaller (B) males (relative to the presented females) that courted the females before painting and then repelled
them after painting, and proportion of quadrats (C) including males that repelled females after painting. Treatments with different letters are
significantly different (Fisher’s exact probability test, p b0.05).
S. Takeda / Journal of Experimental Marine Biology and Ecology 330 (2006) 521–527 525
smaller; df =3, G2=8.140, p =0.0432). This result
means that the male fiddler crabs discriminated the
colours or luminosities.
To clarify the function of body colour as a signal in
crab society, I compared the proportion of males that
courted or repelled females after painting between each
pair of the four colour groups (Fig. 1A). The larger
males formed two clusters: white; and red, black and
blue (Fig. 1A), and the smaller males showed a signif-
icant difference between white and blue (Fig. 1B).
3.3. Subgroup test
To exclude the possibility that the males’ responses
affected each other (pseudo-replication), I tested the
number of quadrats in which the males did or did not
repel the females using Fisher’s exact probability test.
The four groups formed two clusters: white; and red,
black and blue (Fig. 1C). This result supports the
contention that male fiddler crabs discriminate colours
or luminosity.
4. Discussion
The differences between the behavioural responses
of males of the fiddler crab U. perplexa to conspecific
females before and after painting of the females indicate
that they can discriminate colour or luminosity. Colour
or luminosity functions as a visual signal, promoting
the modification of attitude and motion.
Most U. perplexa males courted unpainted conspe-
cific females larger than themselves by using the later-
al–circular wave. Fiddler crabs can see things above the
level of their eyes well (Layne et al., 1997). This fact
probably explains why the males that were smaller than
the females used a lateral–circular wave to court, be-
cause they have to raise their major chelipeds into the
female’s field of vision. In addition, these facts indicate
that the male fiddler crabs visually measured their
respective sizes, and then chose the appropriate form
of waving.
The U. perplexa males courted the unpainted
females regardless of their relative body size, although
the larger males copulate in the burrow (Nakasone and
Murai, 1998). When the red-, black- or blue-painted
females were presented, the proportion of courting
larger males was significantly lower than that when
the white-painted females were presented. The U. per-
plexa males perhaps recognized a white body colour as
indicating that reproduction is possible, because most
males courted white-painted females as frequently as
the unpainted females.
The smaller males showed a similar tendency, except
that the proportion of courting males was slightly lower
when the white-painted females were presented than
when the unpainted females were presented. On Oki-
nawa Island, another fiddler crab, U. vocans, dwells in
the same biotope (Murai et al., 1982). U. vocans, which
is slightly larger than U. perplexa, becomes whiter in
the reproductive season, and wander about the sandy
flat (Nakasone et al., 1983; Christy and Salmon, 1984).
These facts suggest that larger whitish females are
likely to be U. vocans, and so the colour signal dis-
courages the smaller males of U. perplexa from court-
ing, together with the sexual dimorphism of U.
perplexa whose males are lager than the females
(Crane, 1975).
In addition, the proportion of courting larger males
was much greater than that of smaller males when the
blue-painted females were presented. The soldier crab
S. Takeda / Journal of Experimental Marine Biology and Ecology 330 (2006) 521–527526
M. brevidactylus, with spherical blue body and slender
chelipeds, which is slightly smaller than U. perplexa,
inhabit the same sandy flat (Nakasone and Akamine,
1981; Takeda and Murai, 2004). The soldier crab walks
forwards, unlike the fiddler crabs which walk sideways
(Takeda and Murai, 2004). The sympatric existence of
the soldier crabs and the insufficient experience of the
smaller males may be relevant to the low proportion.
On the other hand, the proportions of courting males
did not differ between the smaller and larger males
when the red- or black-painted females were presented.
These colours perhaps mean that the females are not
ready to reproduce.
Murai et al. (2002) described that the males of the
fiddler crab Uca paradussumieri assess the reproduc-
tive situation of the females by using courting gestures
(tapping and stroking the carapace) and touching the
abdomen. These males perform scarcely any attractive
courting display (Crane, 1975). Instead, they approach
the females and mate near or in the females’ burrows.
In contrast, U. perplexa males attract conspecific
females into the males’ burrows by using courting
display without touch (Nakasone and Murai, 1998).
In addition, some of the males that courted the unpaint-
ed females flicked the painted females with the major
cheliped. These facts strongly suggest that the diurnal
fiddler crab U. perplexa depends on visual signals for
social communication, rather than on chemical signals.
In the New World, some species of fiddler crabs
inhabiting tropical and subtropical tidal flat are fre-
quently prey to birds (e.g., Koga et al., 1998). The
females are camouflaged against the ground in the
reproductive season (Crane, 1975). On the other hand,
the wandering females of U. perplexa have various
body colours, some with lateral stripes, as have other
fiddler crabs in the Old World (Crane, 1975). The
various body colours are considered to disrupt visual
searches by predators (e.g., Dawkins, 1971). The vivid
colour of the wandering females is probably a signal to
distinguish mature females from immature females,
other species and grapsid predator crabs, which are
coloured similar to the ground. The difference in the
strength of predation pressure probably influences the
development of body colour as a visual signal. The
various body colours may be relevant to the speciation
and coexistence of the fiddler crabs in the Old World.
Acknowledgement
I thank Dr. M. Murai, Mrs. Y. Nakano and S. Naka-
mura and other staff of Sesoko Station, Tropical Bio-
sphere Research Center, University of the Ryukyus, for
their help and facilitating my work there. I am grateful
to Dr. M. Murai for his invaluable information and
suggestions for this study. This study was partially
supported by Grant-in-Aid for Scientific Research (C)
from Japan Society for the Promotion of Science (No.
12640605). [PH]
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