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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. 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