Coexistence of Juvenile with Adult Ocypode gaudichaudii at Culebra Beach, Panama: A Temporal-spatial Partitioning Compromise.

The temporal-spatial resource use patterns of juvenile and adult Ocypode gaudichaudii were studied by comparing the zonation patterns and activity budgets of the two life stages at Culebra Beach, Panama. Burrow distribution of the crabs during the day and at night was studied over six months. Diurnal activity budgets of 46 crabs (22 juveniles and 24 adults) were determined by observing seven predominant behaviors upon emergence from their respective burrows when the burrow zone is uncovered after the tide recedes. The behaviors comprise three foraging-related activities (i.e., deposit-feeding, scavenging, and probing for food), the maintenance of burrow, walking, staying within the burrow, and resting at the burrow entrance. Juvenile crabs occupied a higher intertidal zone than the adults and had a higher emergence rate at night. This temporal-spatial habitat partitioning could possibly reduce intraspecific competition between the two life stages, thereby enabling their coexistence in the habitat as well as increasing the survival rate of the juveniles, potentially raising the carrying capacity of the population at Culebra Beach. All 46 crabs-regardless of life stage-spent the highest mean proportion of time on foraging-related activities. Out of the three feeding-related behaviors, adults spent most time on deposit-feeding while juveniles spent most time probing. Only juveniles scavenged. In both life stages, a similar proportion of time was spent maintaining the burrow and staying within the burrow.

Unique Behavioral Strategies Adopted by Gravid Ghost Crab Ocypode gaudichaudii to Overcome Dehydration Stress while Minimizing Predation Risks.

Semi-terrestrial crabs require continual access to water to maintain life-sustaining processes such as circulation and feeding. When they emerge from their burrows during low tide to forage, they face the problem of dehydration as they leave the dampness of their burrows. While foraging above ground, water uptake is elicited through capillary action via the hydrophilic setae near the base of the crab's body. Extruded eggs that are borne on the abdominal flap of females tend to obstruct the contact of the setae with the wet sediment. The behavioral adaptations that enable the gravid female painted ghost crab, Ocypode gaudichaudii, to overcome dehydration stress and minimize predation risks at a sandy shore in Playa Venao, Panama were studied using field observations. Comparison of the morphometric measurements of setal tufts between 30 male and 30 female crabs was made to determine if there were morphological adaptations. Analysis of the water uptake behavior from video footage showed that gravid crabs spent a longer duration on water uptake than crabs that did not carry eggs. For the first time, masquerading behavior of a gravid O. gaudichaudii was observed, in which the crab minimized predatory detection by freezing its movement next to a stone enroute to the lower shore during the day. There was no sexual dimorphism in the length and width of the setal tufts between the male and female adult crabs. The results of this study provide the first evidence that the water-uptake behavior in gravid O. gaudichaudii is dependent on behavioral adaptations, as setal tuft morphology does not differ between the sexes.

Larder Hoarding Versus Immediate in situ Food Consumption in Two Fiddler Crab Species: Is it an Evolutionarily Stable Strategy?

Larder hoarding behavior in central place foragers has been widely studied in vertebrates, albeit, not as extensively as scatter hoarding. However, scant information is available for the invertebrate taxa, especially aquatic species. We investigated this phenomenon via an in situ food supplementation experiment in a community of two sympatric fiddler crabs, Austruca annulipes (n = 80; 40 males and 40 females) and Gelasimus vocans (n = 60; 30 males and 30 females), in a Singapore mangrove patch with an intermediate resource level. As the semiterrestrial intertidal crabs can only forage after emergence from their burrows during exposure period, the duration of time available for feeding is finite and constitutes an important constraint in the optimization of food intake. The activity budget (in terms of time spent on feeding activities, all above-ground non-feeding activities, and burrow-sequestration) as well as the occurrence (if any) of larder hoarding behavior in these two species after they first emerged were recorded by hourly intervals (three hours of observation) to determine the effect of time left for foraging on larder hoarding. Regardless of species, A. annulipes and G. vocans spent most of the time feeding when the tide was out, despite overall significant behavioral heterogeneity (multivariate analyses using ANOSIM) indicating that both species prioritized hunger satiation over other activities. Our results also showed that although the two sympatric crabs live in the same mangrove area with similar food resource levels, only A. annulipes larder hoarded. The propensity to larder hoard did not differ significantly between the sexes, nor among the three time periods of the feeding duration. Gelasimus vocans, one of the species of crabs known to form feeding droves, did not larder hoard at all. We propose that A. annulipes is a species that can deploy larder hoarding as a foraging strategy when it encounters valuable food resources, and such a strategy is highly advantageous for the species as it generally inhabits sandy habitats that are poor in nutrient levels. Hence, larder hoarding can be considered a mixed evolutionarily stable strategy (ESS) adopted by A. annulipes. In contrast, G. vocans that commonly inhabits muddy sediments-with a high level of food resources-did not larder hoard, even when provided with supplemented food, suggesting perhaps that its mixed ESS is droving behavior.

Plasticity of Foraging Strategies Adopted by the Painted Ghost Crab, Ocypode gaudichaudii, in Response to in situ Food Resource Manipulation Experiments.

The feeding strategies of Ocypode gaudichaudii at two sandy beaches, Culebra Beach (CB) and Playa Venao (PV) in Panama, were studied via three experiments. Two separate manipulative in situ experiments were conducted to determine how the densities of food resources and the size of the supplemented food offered to the crabs can affect their diet and food handling behavior. The third experiment, a transplantation study, was also conducted to determine the plasticity of the feeding behavior of the displaced crabs. In the first experiment, freshly-emerged crabs showed different feeding modes when washed-sediment was seeded with different densities of diatoms and rove beetles, which suggests that they are optimal foragers. Crabs hoarded food in the second experiment when food augmentation was performed, in which small and large food pellets were placed around the burrows at the beginning and end of the crabs' feeding cycle. All freshly-emerged crabs from both sites foraged on the small pellets outside their burrows and did not cache food; when pellets were provided at the end of the feeding cycle, crabs from CB fed on some of the small pellets and returned to their burrows with the uneaten pellets left on the surface, whereas crabs at PV picked up all the small food pellets and transferred them into their burrows over several trips before plugging their burrow entrances. Only the crabs from PV carried the large food pellets supplemented at the start and end of the feeding cycle into their burrows. In contrast, the crabs at CB often left behind the partially-eaten pellets on the sand surface, probably due to the increased risk of predation associated with the prolonged handling time of the large food pellets. Excavation of the burrows of the crabs that hoarded food showed that all the pellets were deposited at the bend of the burrows, indicating that they were not consumed immediately. Crabs that fed in droves at PV stopped droving and foraged around their burrows after being transplanted to CB. This is the first documentation of food hoarding in a sandy beach macroinvertebrate at a resource-impoverished habitat. The plasticity of feeding strategies adopted by the painted ghost crab in response to different densities of food resources in the habitat could be an adaptation to the dynamic sandy beach environment.

Ecomorphological Adaptations of Second Maxilliped-setation: Insights from Three Species of Fiddler Crabs from Panama.

The functional morphology of maxilliped-setation in three species of fiddler crabs-Petruca panamensis, Leptuca terpsichores and L. beebei-collected from Panama were studied using a comparative ecomorphological approach. The coverage of spoon-tipped (ST) and plumose (P) setae on the inner surface of the second maxilliped were noted, and the abundance of each setal type was enumerated, with crab carapace width as the covariate. These attributes were then related to the sediment particle size characterization of their respective collection sites for an ecomorphological comparison. All three species have mixed setation, albeit in different proportions of coverage. For L. terpsichores, the ST:P coverage was consistently ca. 70%:30% whereas there was approximately equal coverage of both setal types (i.e., 50% ST:50% P) on the maxillipeds of L. beebei and P. panamensis. Analysis of Covariance (ANCOVA) results of setal counts between the sexes in each species showed that the number of ST and P setae did not differ significantly between males and females, indicating an absence of sexual dimorphism in mouthpart setae. When all three species were compared-sexes combined within each species-ANCOVA results reveal that for any crab of a given size, there were significantly more ST setae on the second maxilliped of L. terpsichores (sediment from sampling site classified as 'moderately-sorted medium sand') than L. beebei (sediment from sampling site classified as 'moderately-sorted fine sand') and P. panamensis (a rocky-habitat species); no significant difference was detected between ST abundance in the latter two species. Current results provide unequivocal quantitative evidence that the abundance of ST setae can be an indicator to differentiate habitats down to the level of different sand-grain size, i.e., medium sand versus fine sand habitats of L. terpsichores and L. beebei, respectively. Both L. beebei and P. panamensis had significantly fewer ST setae than L. terpsichores, possibly because the former two species' habitats have sediments with sand grains that are less coarse than the latter's. On rocks, P. panamensis does not produce pseudofecal pellets as they directly swallow food materials pinched from the rock surfaces without sorting, rendering the role of ST setae of secondary importance. The three species did not differ significantly in P setae abundance, suggesting that the adaptive function of this type of setae to habitat characteristics remains debatable. The significance of ST setae as an ecomorphological adaptation for efficient food extraction from sandy sediments is, thus, evident in L. terpsichores. As for other two species-L. beebei and P. panamensis-that live in environments with finer sediment grain sizes, the adaptive role of ST setae is of diminished functional importance. Fiddler crab species with mixed setal types may have greater potential to exhibit feeding plasticity should their habitats be threatened.

To hide or to feed: an evaluation of personality traits in the sand bubbler crab, Dotilla wichmanni, when responding to environmental interference.

Behaviour plays a crucial role in a species' ability to cope with environmental challenges. However, this ability may be affected by repeatable individual differences in behaviour, a pattern described as animal personality. The consideration of animal personality is therefore essential when understanding how a species copes with its environmental stressors. For sand bubbler crabs, feeding is often disrupted by environmental interference, in the forms of predatory events and human recreational activities. How these crabs deal with such disruption is, however, not well documented. Here, we characterised the foraging and risk-taking behaviours of Dotilla wichmanni when responding to induced disruption. Whether these are personality traits and if they form part of a behavioural syndrome were also examined. We quantify both behaviours by taking four measures (two per behaviour). All behavioural measures were consistently different among individuals, suggesting that D. wichmanni exhibits personality. Results further suggest that they could cope with some environmental interference, although this is limited. Crabs did not vary the time spent hiding in burrows with each repeated disruption nor did behavioural plasticity differ between individuals. Notwithstanding these, the absence of support for a foraging-risk propensity behavioural syndrome points to possible complexity in the crabs' coping ability.