Download Behavior of four species of fiddler crabs, genus Uca, in southeast

Hydrobiologia 523: 47–58, 2004.
Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands.
47
Behavior of four species of fiddler crabs, genus Uca, in southeast Sulawesi,
Indonesia
Judith S. Weis1,* & Peddrick Weis2
1
Department of Biological Sciences, Rutgers University, Newark, NJ 07102, USA
Department of Radiology, UMDNJ, New Jersey Medical School, Newark, NJ 07103, USA
*(Author for correspondence: Tel.: þ1-973-353-5387, Fax: þ1-973-353-5518, E-mail: [email protected])
2
Received 5 November 2002; in revised form 24 February 2004; accepted 24 February 2004
Key words: fiddler crabs, feeding, habitat, activity, behavior, Uca
Abstract
We studied the behaviors of four species of sympatric fiddler crabs on Kaledupa Island, Indonesia. Species
differences in activity level, grooming, burrowing and feeding were related to their habitat and food. Uca
chlorophthalmus, living in muddy mangrove areas, were inactive and spent most of the time feeding in place.
Females fed 50% faster than males and spent more time feeding. U. vocans was the dominant species at the
beach in silty sand and was very active. Its feeding rate was about twice that of the former species, females fed
more rapidly than males, and many crabs of both sexes fed in droves at the water’s edge during ebb tides.
During ebb tides, they spent most of their time feeding, while at flood tide they engaged in a greater variety of
activities, including burrow maintenance. They frequently walked while feeding and interacted aggressively.
U. tetragonon lived in a pebbly band along one edge of the beach, by a quay. Their feeding rate was
comparable in both sexes and slower than that of U. vocans; they fed largely on filamentous algae growing on
the quay, which provides better food, and fed faster during flood tide than ebb tide. They spent more time in
waving and other sex-related activities, and were seldom aggressive, except during the week of the full moon.
Burrowing activities included placing excavated mud balls some distance away from their burrows and rearranging them. U. dussumieri inhabited the other end of the beach in muddier substrate. They did not have
sex differences in feeding rates and their rate of scooping food into their mouths was slow, but feeding claws
made multiple pinches of the substrate, thus accumulating more material in each clawful of food.
Introduction
Fiddler crabs (Uca spp.) burrow in intertidal areas
and are important consumers of detritus, bacteria,
fungi, and benthic microalgae in coastal marsh,
mangrove, sandflat, and mudflat habitats. Their
burrowing activities aerate the soil and enhance
the growth of marsh plants such as Spartina alterniflora (Bertness, 1985). Their presence also increases the amount of meiofauna in salt marshes
(Hoffman et al., 1984). Unlike most intertidal
organisms, fiddler crabs are semiterrestrial and are
active at low tide, returning to their burrows at
high tide. Males have one greatly enlarged claw,
which can be up to 50% of its body weight (Crane,
1975), that is used for mating displays and in
combat with other males. In mating displays,
males wave their claws to attract females. The
pattern of waving is species-specific. Females, with
two feeding claws, can feed more efficiently than
males. Some species feed in flocks (droves) at the
edge of the water (Klassen & Ens, 1993) while
others do not. Foraging in U. pugilator can be
affected by the sediment water content and particle
size (Reinsel & Rittschof, 1995). The greater the
distance a crab is from its burrow, the greater is
48
Figure 1. (Map 1) Map of study sites. The arrow points to Hoga Island off the northeast coast of Kaledupa Island. The entire
Tukangbesi Archipelago lies within the Wakatobi Marine Preserve.
the risk of predation by avian predators on
U. uruguayensis (Iribarne & Martinez, 1999), and
small crabs of U. tangeri spend more time near
their burrows than larger individuals (Ens, 1993).
In many tropical environments more than one
species of Uca co-exist in similar habitats. They
may have different patterns of behavior (mating,
feeding, etc.) and microhabitat preferences, so that
their ecological niches are not identical. In this
study, we examined behaviors and habitats of four
sympatric species of Uca on the island of Kaledupa, within the Wakatobi Marine Park, SE
Sulawesi, Indonesia (Fig. 1) to see how their
behaviors might differ and how they related to their
habitat. Since many species have rhythms related
to tides and the lunar cycle, we investigated whether tidal and lunar rhythms in behavior could be
discerned.
Materials and methods
Fiddler crabs were found on the beach by the quay
and in muddy areas behind houses slightly inland
49
in the town on Ambeua. Species were identified
using Crane (1975). The open beach area was
dominated by U. vocans, but there was a population of U. tetragonon living along the quay on one
side in an area with pebbles, and a population of
U. dussumieri living at the other end where the
substrate was muddier and shade was provided by
an overhanging roof. U. chlorophthalmus (the
smallest of the species) was the most abundant
species in the inland muddy areas, which it shared
with some U. dussumieri (the largest of the species), and other Ocypodid and Grapsid crabs.
Surficial sediment samples (single) from the sites
where the different species predominated were
collected for particle size distribution (by sieves)
and measurement of nutrients (C and N) using a
Carlo Erba N-2000 elemental analyzer. Prior to
analysis (but after weighing), carbonates were removed with HCl since sediments were partially
coral and shell fragments.
We observed the different species with binoculars, noting and describing their behaviors at ebb,
low, and flood tides. Through the month of study
in June 2002, we made observations in the mornings and afternoons, depending on the tides. In the
first week, we focused on U. chlorophthalmus, the
second week on U. vocans, the third week on
U. tetragonon and the fourth week on U. dussumieri. These studies were conducted during June
2002 throughout most of a lunar cycle. Crab species that had been the subject of focus during
earlier weeks (U. chlorophthalmus, U. vocans and
U. tetragonon) were re-examined during the fourth
week, around the full moon, to note whether
behavior patterns might be different, as lunar cycles sometimes are seen in this genus (Severinghaus
& Lin, 1990).
During low tides, meter-square and 0.25 m2
quadrat markers were used to measure the density
of burrows and the density of individual crabs
within the quadrats. Individuals were counted
within the quadrats (eight replicates) 15 min after
putting down the quadrat markers, giving the
crabs time to re-emerge from their burrows. Both
of these methods are commonly used, but do not
give very accurate population estimates. Burrow
counting tends to overestimate density, while binocular counts tend to underestimate it (Macia
et al., 2001). True counts can only be made by
excavation, which is destructive to the environ-
ment. Burrow counts were found to match the
actual counts by excavation more closely than
counting crabs on the surface (Skov & Hartnoll,
2001).
We measured feeding rates by counting the
number of scoops made in 30 s (which was doubled to get a count per minute) by randomly selected feeding individuals (>15 of each sex) on
both ebb and flood tides. Feeding rates were
compared with ANOVA.
Activity budgets were constructed by recording
the behaviors of focal individuals over a 5-min
period. Over 12 focal individuals of each sex of
each species were observed during both flood and
ebb tides. In cases where different behavior patterns were noted during ebb and flood tide, they
were analyzed separately, with an N of at least 12
for both ebb and flood tides. We scored behavior
according to the following categories: feeding,
walking, feeding while walking, standing, grooming (removing mud from appendages or carapace),
aggression and defense (agonistic behaviors),
waving, sexual interaction, mating, in burrow and
burrowing. ‘Burrowing’ was defined as digging a
burrow, moving mud balls removed from the
burrow or mud balls that it later put into the
burrow, or constructing a chimney around the
burrow. Whenever the crab was below the surface
and not visible, it was considered ‘in burrow.’ The
amount of time they spent in particular behaviors
was compared by ANOVA followed by LSD
pairwise comparisons, or when not normally distributed, by non-parametric Kruskall–Wallis
ANOVA. The average percentage of time they
spent in each activity was calculated and compared.
Results
Burrows and surface active crabs
U. chlorophthalmus
Quadrat samples had an average of 41 burrows/
m2, and contained about 10 crabs. The sex ratio of
crabs on the surface was about 1.5 males to 1
female. When we re-visited the site during the
week of the full moon, the sex ratio was closer to
1:1. Approximately equal numbers of left-handed
50
and right-handed (i.e., site of major claw) males
were observed.
U. tetragonon
Quadrat sampling indicated an average of 16
burrows with 8 crabs/m2 on the surface. Sex ratios
of crabs on the surface were about 2:1 males to
females. Of the males observed, 26 of 28 were
right-handed.
U. dussumieri
Quadrat samples along the edge of the beach,
indicated about 16 burrows and 12 U. dussumieri/
m2 on the surface. Males and females appeared
above ground in approximately equal numbers.
There were 14 left-handed and 2 right-handed
males seen.
% Distribution
U. vocans
Quadrat samples averaged 30 burrows/m2, with an
average of 10.3 crabs on the surface. Among crabs
visible on the surface there was a 4/1 ratio of males
to females. Nearly all males in the population were
right handed. Only one left-handed male was
found among >300 males.
75
U. chlor.
U. vocans
U. tetra.
U. dussum.
50
25
0
>2mm
>1mm
>250 µ
>125 µ
>63 µ
<63 µ
Grain Size
Figure 2. Particle size distribution of substrate for each species.
U. chlor ¼ U. chlorophthalmus, U. tetra ¼ U. tetragonon, U.
dussum ¼ U. dussumieri. For U. tetragonon, the particles within
the algal mat were analyzed.
differences in the amounts of C and N in these two
muddy areas. The U. dussumieri site had the
highest percent carbon of all sites, while the
U. chlorophthalmus site was surprisingly low in
both C and N.
Burrowing
Habitat and food
Particle size analysis of the substrate is presented
in Figure 2, and the C and N content of substrate
or food are shown in Table 1. For all species except for U. vocans, the substrate was largely fine
particles, while the U. vocans substrate had much
higher percentages of coarser particles and is largely coral and shell fragments. The U. tetragonon
territory was even coarser, but this was not their
primary food source; the algal mat upon which
they fed contained fine particles and shell fragments. The muddy substrate of U. dussumieri and
U. chlorophthalmus contains pumice, foraminifera
and plant fragments. However, there were striking
U. chlorophthalmus
No active burrowing was observed.
U. vocans
Individuals were observed digging burrows, maintaining them, and covering them up. Burrow
excavation for females involved the crab digging
with the legs on one side, turning around, and then
digging with the legs on the other side. Burrow
maintenance involved emerging from the burrow
with mud balls and depositing them on the surface.
They emerged during ebb tide while their burrows
were still under about 1 cm water, but went back
into burrows during flood tide before the water
Table 1. Analysis of nutrients in substrate for each species
U. chlorophthalmus
U. vocans
U. tetragonon
U. dussumieri
% Carbon
0.98
1.03
2.08
% Nitrogen
0.09
0.08
0.22
0.11
18.86
30.36
C/N ratio
10.8
For U. tetragonon, the algal mat was analyzed.
12.8
3.35
51
reached the burrows. During flood tide, they
excavated the burrows and removed mud balls.
Later, they picked up mud balls or newly excavated
substrate, entered their burrows and sealed them.
U. vocans
In this species, each sex fed about twice as fast as
their U. chlorophthalmus counterparts, and females
fed about 50% faster than males (Fig. 3) The sandier
environment in which they live has low nutrients
(Table 1), and the crabs have more rapid feeding
motions. Unlike U. chlorophthalmus, these crabs
frequently fed and walked at the same time, perhaps
moving on to new areas as organic matter was extracted from sediments. On occasions, males
walked, fed, and waved at the same time. During
early ebb tides, many individuals fed in droves at the
edge of the water. The feeding rates in Figure 2 are
not from droving individuals but from those higher
on the beach, but droving crabs had similar feeding
rates. Droves had approximately equal numbers of
males and females. They were constantly active,
crowding each other moving to newly exposed sediments. Areas in which the sediments were slightly
elevated forming ‘peninsulas’ projecting into the
water, were the sites of highest droving activity;
here, the crabs crowded toward the peninsular tip
but stayed at the edge of the water.
U. tetragonon
These crabs moved mud balls out of burrows and
placed them in piles or walls about 10–12 cm
away. They sometimes then re-arranged the mud
balls. They emerged during ebb tide after U. vocans, and returned to their burrows at flood tide
well before the tide came in, so they had less time
overall to be active.
U. dussumieri
Burrowing activities were noted in females but not
males. They resembled those of the other species.
One female was observed excavating mud balls
and adding them to a chimney around her burrow.
This was the only chimney observed among any of
the populations studied.
U. tetragonon
This species fed at a slower rate than U. vocans and
both sexes fed at about the same rate (Fig. 3).
However, they fed faster during flood than ebb
tide. Most of their feeding was done on the sloping
wall of the jetty, where there was a mat of fila-
Feeding
U. chlorophthalmus
These crabs spent most of their time feeding while
standing in place. As seen in Figure 3, females fed
about 50% faster than males.
FEEDING RATES
200
Scoops/min +/- SD
d
Male
150
Female
c
bc
100
abc bc
a
a
ab
ab
ab
50
0
U. chlor
U. vocans
U. tetrag
(ebb)
U. tetrag
U. duss
(flood)
Species
Figure 3. Feeding rates (scoops per minute) of both sexes of the four species of Uca. For U. tetragonon, separate feeding rates for ebb
and flood tide are shown. Letters over the bars indicate groups that are not significantly different from one another by Bonferroni
pairwise comparisons.
52
mentous algae. This more nutritious food has
higher levels of C and N (Table 1) and they fed at
a slower rate than U. vocans.
U. dussumieri
The feeding rates for this species did not differ
significantly between males and females or at different phases of the tidal cycle. Their rate of
delivering material to their mouths was slow, but
they took an average of about 6 pinches of substrate before bringing their claw up to their
mouths. The accumulation of material with each
pinch enables them to take a greater amount to
their mouths with each feeding motion. The substrate where they lived was muddier than the main
part of the beach and had more fine particles and
more carbon in it (Table 1, Fig. 2).
Agonistic (aggressive and defensive) interactions
U. chlorophthalmus
Males frequently waved at each other when no
females were in the vicinity. Elevated aggression
was seen when, after waving at each other, two
males dashed forward towards each other. We also
observed fights including manus touching and
pushing until one of the males finally retreated.
Aggressive interactions were also seen between two
females. On a few occasions, as a large U. dussumieri male walked along the marsh surface, all
U. chlorophthalmus in the area dashed into their
burrows as he approached.
U. vocans
Males were aggressive toward other males, waving
at them, chasing them and fighting. Waving was
more likely to be directed at other males than at
females, as it was seen when no females were
nearby, and was often followed by further
aggression. Males with ‘display white’ coloration
on their carapace (Crane, 1975) were more likely
to be aggressive. Fights consisted of manus pushing and lasted about 30 s. Females generally
chased away approaching males, and on one occasion we saw a female tap her feet on the substrate to ward off a female of another species.
Foot-tapping may be a defensive activity (Crane,
1975). Males were not aggressive toward females.
U. tetragonon
When we first watched this species we saw almost
no aggressive interactions. However, during the
week of the full moon, rates of aggression were
comparable to those of other species. Males chased
other males away from females’ burrows, and females chased both males and females away by
dashing at them or warned them by tapping claws
on the substrate.
U. dussumieri
Aggressive interactions of large males were seen at
the beach and at the mangrove area. Males waved
at each other and occasionally left their major
chela extended for a few seconds before flexing it.
In fights, they pushed each other and locked claws.
Females sometimes stretched out their walking
legs and ‘pranced’ or strutted as a negative signal
when males approached. Two females fought over
a burrow by pushing with their walking legs.
Grooming
U. chlorophthalmus frequently groomed themselves
just after emerging from their burrows. Males used
the minor chela to wipe the major chela, mostly on
the pollex and dactyl but also on the joints. They
also wiped their eyestalks with their minor chela.
To clean their legs, crabs flexed and rubbed them
together, presumably because they could not reach
these surfaces with their minor chela. Grooming
was observed less often in U. vocans and U. tetragonon, presumably because they live in sandier
substrate with little sticky mud. In U. dussumieri,
females groomed more often than males, although
this was not statistically significant (p ¼ 0.1).
Waving and sexual interactions
U. chlorophthalmus
The wave is a slow extension and flexion of the
major chela. Some males fed while waving. The
average number of waves in a bout was 13.5 ± 7.9
waves at a rate of 0.83 waves per second (n ¼ 10).
Overall, little sexual interaction was noted, and
one surface mating was observed. One particular
male that could be readily identified because of a
deformed claw was observed for several days in a
row. He waved at females far more than the other
males. On two occasions he approached a female
53
and then stood on top of her. This did not result in
mating, however. Another interaction was seen
when a male waved, approached a female who
went into a burrow. He waved again, groomed his
legs (rubbed them together, which may have produced substrate vibrations) and went halfway into
that burrow but then shortly re-emerged and left.
U. vocans
Sexual interactions were rare. Males waved infrequently and often fed while waving. More waving was directed aggressively toward other
males than toward females. The wave was vertical and out on tiptoe, with no movement of
the minor chela. On four occasions we noted a
male standing motionless on tiptoe with his major
chela outstretched for a long period of time, in one
case for over 5 min. On two of these occasions the
male was standing on a pile of mud balls so he was
more visible, and sometimes he waved a few times
(over 5–10 s) during this period. Other sexual
interactions included males pushing females by
backing against them, in some cases into burrows.
Females generally avoided males. We saw one
mating, but not the preceding behavior. In this
pair, the female was on top of a much smaller
male.
U. tetragonon
This species spent significantly more time waving than all the others (F ¼ 5.78, p ¼ 0.002, followed by LSD pairwise comparisons). Waving
was seen frequently and was directed towards females. The waves were upward sweeps of the claw
at a 45° angle, at a rate of about one per second.
The wave increased in height but not frequency as
a female approached the waving male. A sequence
of behaviors was observed leading to mating: the
female tried to avoid the male, he pushed her into
a burrow and followed her in. He then left the
burrow and she came partially out. He then
stroked her and then they mated at the edge of the
burrow for about 1 min. Then he left and she
groomed. In another case, a female entered a
burrow with a mud ball and was followed by a
male. They remained in the burrow for over a
minute.
On three occasions, we saw crabs with two
small chelae, presumably females, doing two-han-
ded waves, standing on tiptoe when the claws went
up, for periods of a few seconds up to about 1 min.
The significance of this behavior is unknown.
U. dussumieri
Sexual interactions were frequent, but we saw no
matings. The wave consisted of about three jerks
upward, followed by a single downstroke. Males
waved at females, approached and stroked them.
They also straddled females and stroked them. On
one occasion, a male climbed on top of a female
and held on while she walked around and eventually moved in such a way as to make him fall off.
He later went into her burrow, but she pushed him
back out. Unreceptive females walked in a
‘prancing,’ legs up, position, similar to agonistic
displays described for several species (Crane, 1975).
Activity budgets
The activity budgets for all species are in Figure 4.
Since each species was studied mostly during
1 week, these budgets may not be truly representative of their behavior overall.
U. chlorophthalmus
These crabs spent the greatest amount of their time
feeding. Females spent more time feeding than
males, and fed faster than males. The data indicate
that the males spend more of their time in their
burrows, but this may be an artifact of the study
design. All focal crabs were ones that were on the
surface at the beginning of the 5-min period, and
the sex ratio suggests that there may be more females that were below the surface in their burrows
during observation periods, unless the sex ratio of
the population is skewed. It is likely that the females spend more time in burrows and come up to
the surface primarily to feed, their most common
activity. These crabs were very inactive. The time
this species spent walking (while feeding or not
feeding) was significantly less than all the other
species (Kruskall–Wallis non-parametric ANOVA
39.36, p ¼ 0.0001, comparison of mean ranks
p ¼ 0.05).
U. vocans
The activity budgets were different during ebb and
flood tides, so they are presented separately in
Figure 4. At early ebb, crabs mostly fed, while
54
Figure 4. Activity budgets of both sexes (m and f) of the four species of Uca. For U. vocans, activity budgets are shown separately for
ebb and flood tides. The horizontal line in the ‘feeding’ section separates the times at which they are walking and feeding (indicated
with a ‘w’ from times when they are feeding while in place).
later in the tidal cycle, they (primarily males) devoted more time to other activities. For the activity
budget of this and the following two species, the
category ‘feeding and walking’ is combined with
‘feeding,’ but within that category, the amount of
time spent walking is indicated with a ‘W’ in the
figure. Of the total feeding time about 1/3 was
while walking. The most important activities that
increased at flood tide are burrowing and in-burrow, probably in preparation for retiring into the
burrow at high tide.
U. tetragonon
In addition to their relatively slow feeding rate,
these crabs spent a relatively small percentage of
their time feeding (Fig. 4). ANOVA of the feeding
time of all species indicated significant differences
(F ¼ 8.03, p < 0.0001) and LSD pairwise comparisons of means demonstrated that U. tetragonon spent significantly less time feeding than all the
other species. Of the total feeding time, about 25%
was while walking. They had more sexual interactions than the other species, and spent greater
proportions of time standing and walking. ANOVA for waving behavior had F ¼ 5.78, p ¼ 0.002,
with LSD pairwise comparisons showing that
tetragonon males waved more than all the other
species. Males waved more at females and less at
other males. They seldom interacted aggressively.
ANOVA for agonistic behaviors had F ¼ 2.09,
p ¼ 0.05, with LSD pairwise comparisons showing
tetragonon males and females and chlorophthalmus
females spending less time in these behaviors than
all other crabs. Also, they were far less likely than
neighboring U. vocans to be frightened by people
walking on the quay or by other disturbances.
While U. vocans generally ran into their burrows,
the U. tetragonon generally remained above
ground near their burrows during such occasions.
Later, around the full moon, U. tetragonon
crabs were more aggressive, moved around more,
and became more territorial. Time spent by males
in aggression increased from <1 to 3.4% and in
females from 0 to 4.6% of their activity budget.
Other changes were an increase in walking by
males from 5 to 20.7% and in females from 16.5 to
23% of their time.
U. dussumieri
These crabs, with a relatively low feeding rate,
spent over two-thirds of their time feeding. Their
sediments had the most carbon, but since nitrogen
was comparable to the other sediments, the C/N
ratio was poor. Of the feeding time for males, 41%
was while walking, and for females, 33% was while
walking. Males moved long distances. On a few
occasions, a crab that was far away at the beginning of the observation period walked all the way
55
up the beach, and came so close that binoculars
could not be used to observe it. There were no
significant differences between activity budgets at
ebb vs. flood tides.
Discussion
These four species exhibited different patterns
of behavior. U. chlorophthalmus are ‘lethargic’
as Crane (1975) describes them, and spend most
of their time quietly feeding. In contrast, U. vocans
are very active, feeding rapidly, walking and running, easily frightened and aggressive towards
one another. U. tetragonon were more involved
in sexual activities and unaggressive, except during the week of the full moon. U. dussumieri
were not easily disturbed. These differences do
not appear to be directly related to their phylogeny,
since U. dussumieri is in the subgenus Deltuca, U.
vocans and tetragonon are in Thalassuca (which is
derived from Deltuca, both being ‘narrow fronts’),
and U. chlorophthalmus (the only ‘broad front’) is
in Amphiuca (Rosenberg, 2001). The handedness of
U. vocans and U. tetragonon has been previously
noted (Barnwell, 1982).
Many of the behavioral differences (feeding, grooming, burrowing, activity) seem
more related to their habitats and food. U. tetragonon was in a rocky/pebbly area, but fed predominantly on the nutrient-rich filamentous
algal mat on the jetty. U. vocans was in a sandier/
siltier nutrient-poor substrate, and U. dussumieri in
a muddier part of the beach as well as in the
inland mud. Some U. vocans overlapped with
U. tetragonon at one end and U. dussumieri at the
other end of the beach. U. chlorophthalmus was
found in the inland muddy area and not at the
beach. The levels of C and N were surprisingly
low, since the mud appeared similar to the area
where dussumieri was found. However, while this
was a mangrove area, most mangroves had been
removed, so the input of new nutrients is probably
quite low. Grooming was noted more in species
living in muddy areas.
Burrowing
Burrowing was observed in all species at the beach,
but not U. chlorophthalmus in the inland mud,
where the substrate is consolidated. Also, since
they were further inland and were observed during
neap tides, the tidal water would do little damage.
Crane (1975) noted that individual U. chlorophthalmus may use the same burrow for weeks. Since
abandoned burrows can remain in consolidated
mud, this may be why this species had more burrows/m2 than all the others but an equivalent
number of crabs on the surface.
U. tetragonon arranged mud balls at some distance from the burrow. Many Uca species build
structures such as pillars or hoods from moist sand
or mud (Christy et al., 2001), some of which play
roles in attracting females (Christy et al., 2002) or
defining territory (Zucker, 1981). Mud balls may
be merely a by-product of burrow excavation
(Crane, 1975), but may also function as a signal
(Oliviera et al., 1998). Oliviera et al. (1998) and
Burford et al. (2000) indicate that for U. tangeri
males, mud balls mark territory and are attractive
to females. We do not know the significance of our
observations of crabs arranging mud balls and
placing them at a distance from burrows.
Feeding
Feeding was the most rapid in U. vocans, which
lives in the habitat with the coarsest sand and poor
food supply, low in both C and N (although the C/
N ratio of about 10 is good). This species was the
most active and exhibited droving. The paucity of
nutrients in the sand may be a reason for their high
level of activity seeking food, which uses up energy,
which in turn increases food requirements. Their
high mobility while feeding has been noted previously (Meziane et al., 2002). In contrast, U. chlorophthalmus fed at the same spot without moving
for long periods of time, despite the poor nutrient
levels in the mud. Their lack of activity conserves
energy and lowers their nutritional requirements.
Perhaps U. chlorophthalmus and U. vocans are
exhibiting two different ways of coping with
nutrient-poor food: the former expends minimal
amounts of energy, while the latter moves around
to find whatever food is available. The moving
around strategy might be too risky for a crab that is
bright red, since it would be more obvious to predators. U. tetragonon feeding on nutrient-rich algae, which had twice as much nitrogen as available
to U. vocans or U. chlorophthalmus, spent signifi-
56
cantly less of their time feeding than all of the other
species. Weissburg (1992) found that the feeding
rate of U. pugnax increased with food concentration in the sediment. This differs from our interspecific comparisons, in which the highest feeding
rate was in U. vocans, which had poor food, and the
lowest proportion of time spent feeding was in
U. tetragonon, which had the best food. This last
species fed more actively on the flood tide, as if
preparing for the upcoming period of inactivity in
the burrows. Another possibility is that the dried
algae were easier for the crabs to pinch up while
feeding.
The droving of U. vocans has previously been
noted (Murai et al., 1983). Their droves were
mostly large males who had their burrows high up
in the intertidal zone and were foraging in richer
areas. Ens et al. (1993) found droves had predominantly larger crabs. Feeding down on the
beach at the water’s edge provides better feeding
but also entails greater risk, since they are further
from their burrows. Droves of U. pugnax were also
largely made up of males, while U. pugilator
droves were largely females on some occasions and
males on other occasions (Pratt et al., 2002).
Droves of U. tangeri were also all male (Crane,
1975). The droves of U. vocans we saw had
approximately equal numbers of both sexes and a
variety of sizes.
While it is generally thought that female fiddler
crabs feed faster than males because they have two
feeding claws, this was found in only two of the
four species. Weissburg (1992) found that female
U. pugnax fed faster and had higher per chela
scoop rates and greater extraction rates than
males. Male U. panacea foraged faster and had
larger feeding claws than females, which allow
them to have equivalent energy inputs to females
(Caravello & Cameron, 1987). The percentage of
the time that these species, other than U. tetragonon, spent feeding was high compared to U. panacea (Caravello & Cameron, 1991). To our
knowledge, the consistent multiple pinches of
substrate observed in U. dussumieri has not previously been reported.
Agonistic interactions
While aggression was observed in the other species, it was rare in U. tetragonon except around the
full moon. This contrasts with Koga et al. (1995)
who reported cannibalism in this species. However, attacks were seen only 10 times during
149 days of observation at a site where there were
9.9 adult males/m2. It would be interesting to
know if those attacks coincided with the full
moon. Crane (1975) observed only one combat in
this species, suggesting that it is rare. In contrast,
she observed many combats in U. vocans, as did
we. She also describes the U. dussumieri females’
antagonistic posture with the 4th ambulatories
crooked high up against the carapace.
Waving and sexual activity
Waving was associated with aggression toward
other males as well as courtship toward females.
U. tetragonon used waving as an aggressive signal
only around the full moon. In U. pugilator, males
wave primarily in the presence of females, suggesting that it is used primarily for sexual purposes
(Pope, 2000a, b). Crane (1975) describes the male
display of standing with the claw extended, like we
saw in U. vocans and U. tetragonon, as a threat
posture. Two-handed waving by females, which we
noted in U. tetragonon, was seen in U. vocans by
Salmon (1984), who interpreted it as agonistic, and
by von Hagen (1993) in U. polita, which directed
the waving primarily at other females and at small
males.
The behavior of male U. vocans stroking females was described by Crane (1975) and Salmon
(1984). Relatively little surface mating was seen.
Koga et al. (1999) report underground mating in
the closely related U. paradussumieri. Extensive
mating may go on in burrows without our
knowledge. Koga et al. (2000) found that U. tetragonon has underground as well as surface mating. Goshima et al. (1996) found they mate on the
surface by the female’s burrow or underground in
the male’s burrow. Both U. vocans and U. dussumieri females feed on the surface during egg
incubation (Henmi, 2003). We saw no gravid females of any species.
Seasonal and lunar-related rhythms in waving
displays and combat behavior have been studied in
several species of fiddler crabs (Crane, 1975;
Christy, 1978; Christy et al., 2001) and lunar or
semilunar reproductive cycles are the rule (Morgan
& Christy, 1994, 1995). It is likely that the species
57
we studied also have seasonal and lunar rhythms
and biweekly cycles in their activities. We focused
on U. chlorophthalmus during the week preceding
the new moon, on U. vocans and U. tetragonon
during the week after the new moon, and on
U. dussumieri during the week preceding the full
moon. However, we examined them all again on
the days around the full moon and saw no differences except for increases in walking and aggression in U. tetragonon. The activity related to the
tidal cycle in U. vocans, with intensive feeding at
first, followed by a greater number of activities,
has been reported for U. beebei (Christy, 1988b)
and U. tangeri (Burford et al., 2001). The low level
of reproductive activity and lack of gravid females
suggests that we were not present for the peak
reproductive time for these species. The limited
duration (4 weeks) of this study limited our ability
to learn about lunar and seasonal differences.
More lengthy studies are needed to better understand tidal, lunar, and seasonal cycles in behavior.
Acknowledgements
This research was sponsored by Operation Wallacea. We are grateful to the Indonesia Institute of
Sciences (LIPI). We very much appreciate the data
gathering assistance of Operation Wallacea volunteers Joanna Leech, Ian Rickards and Louise
Ellender, the map of Robin Springett, and technical assistance of Hamid Amina Wahid. We
thank Theodore Proctor for the sediment analysis,
Gary Taghon for the carbon and nitrogen analyses, and Terry Glover, John Christy, and three
anonymous reviewers for their suggestions for
improving the manuscript.
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