Sand Fiddler Crab Uca Pugilator Research Articles

Individual movement rates are sufficient to determine and maintain dynamic spatial positioning within Uca pugilator herds

Spatial location within aggregations (i.e., periphery, central) is of biological significance to gregarious animals. Because these positions are a potential consequence of consistent individual behavioral differences, or personality, a better understanding of potential mechanisms concerning personality is central to predicting an individuals’ location. To determine the effects of individual personality on the dynamic spatial positioning of Uca pugilator while herding, field data collection and agent-based modeling were employed. Individuals were assayed to establish their personalities and returned to the field for observation as a means of identifying location preference within selfish herds. There was a significant difference between the extreme personalities and the proportion of time spent on the edge of the herd. The active individuals were at the periphery ~50 % more of the time than less active individuals. An individual-based model qualitatively replicated these field results by applying the mechanism of activity level as an indicator of individual personality. This suggests that differences in personality-dependent movement are sufficient to explain the spatial positioning of individuals within selfish herds. This study enhances our understanding of the possible mechanisms that govern group movement, and has implications for modeling population dynamics that can be influenced by individual personality. Animal personality influences the level of exploratory activity of individual animals, with some individuals being consistently more active and others consistently less active, and may therefore be used to mechanistically predict the location of individual animals within groups or herds. Here, we investigate the relationship between individual personality and location on the edge vs. in the center of foraging herds of the sand fiddler crab Uca pugilator. We combine observational testing of activity level in lab, observations of spatial positioning within herds in the field, and agent-based computer simulation modeling to demonstrate that differences in exploratory movement can sufficiently explain the spatial positioning of individuals within herds. Thus, the spatial positioning of individual animals within herds can be explained without relying on choices by individual animals.

Size-dependent temperature and desiccation constraints on performance capacity: Implications for sexual selection in a fiddler crab

Reproductively active males of the sand fiddler crab Uca pugilator are routinely exposed to high body temperatures and desiccation while defending breeding-burrows from other males or displaying to attract females. Such males are generally larger than individuals in other parts of the marsh, in part, because large males win more contests and have greater energy stores. We propose an additional explanation: that higher thermal inertia and lower mass-specific water loss rates may make large males less susceptible to thermal and hydration stresses than small males. We experimentally investigated the effects of variation in body temperature (Tb) and desiccation on physiological performance and behavior of male sand fiddler crabs as a function of body size. Treadmill endurance and sprint speed increased to a maximum before decreasing rapidly with further increases in Tb. Moderate dehydration (7–10% body water loss) resulted in significantly reduced locomotor performance, with small crabs exhibiting the greatest reductions in both body water and performance capacity. Fiddler crabs appear to be sensitive to the potentially conflicting demands of body temperature and hydration state. In a laboratory thermal gradient, crabs on wet sand chose Tb's associated with high locomotor performance capacity. In contrast, crabs on dry sand selected lower Tb's that presumably minimized water loss, with small crabs choosing the lowest Tb's. In addition to providing empirical support for a general model of ectotherm locomotion, our results are consistent with the idea that larger individuals are better able to tolerate harsh environmental conditions above-ground, increasing the proportion of time they can spend engaging in fitness-enhancing reproductive activities rather than sheltering in a burrow.

Food availability in beach and marsh habitats and the size of the fiddler crab claw, a sexually selected weapon and signal

Males of the sand fiddler crab Uca pugilator possess a greatly enlarged claw that is used as a weapon in ritualized contests for control of breeding burrows and is waved to attract females to breeding burrows. Approximately 5400 crabs were collected along the Atlantic coast of North America at 14 localities, all of which had both beach and salt marsh habitats. Five measurements were made on each claw. Principal components analysis was used to generate a single measure of claw size from the seven correlated measures and scores of the claw. Carapace width was measured to index body size. Claw size was greater in beach than marsh habitats, controlling for body size. However, body size did not differ by habitat type. Claw size was also greater in laboratory-reared males receiving more food, suggesting that differential access to food could influence habitat-associated differences in claw size. Chlorophyll a concentration and total organic content, reflecting, respectively, the abundance of benthic algae and other food, were greater in beach than marsh habitats. Moreover, feeding opportunities were greater in the wetter beach habitat because crabs there, but not in marsh habitat, can feed at breeding burrows. Adult fiddler crabs continue to molt and grow in both body and claw size as they age. Energetic investment in the claw relative to the body is plastic. It appears that the availability of food can affect the amount of energy invested in the claw.

Population structure of the Atlantic sand fiddler crab Uca pugilator along the eastern coast of US revealed by molecular data

Abstract The Atlantic sand fiddler crab Uca pugilator is an extremely abundant crab found along the eastern coast of the United States. Fiddler crabs have a life cycle with an obligatory planktonic larval phase of 30 - 90 days, which might be expected to lead to widespread larval dispersal and consequent genetic homogeneity over considerable distances. However, a large amount of morphological and behavioral variation is found between northern and southern populations along the eastern coast. This study was undertaken to determine the population genetic structure of U. pugilator and to determine whether these differences may have a genetic basis. The population structure of the fiddler crab was analyzed using 472 individuals collected from 12 sites along the eastern coast. PCR-based single stand conformation polymorphism (SSCP) was used to investigate between-site variation in the mitochondrial 16S rRNA gene of these individuals. Analysis of genetic variation indicated frequent gene flow between nearby localities' but much reduced levels between populations separated by larger geographic distances. Thus, despite the potential for high dispersal by planktonic larvae, population differentiation and isolation by distance is evident between northern and southern populations of U. pugilator. A high amount of genetic differentiation (FST = 0.3468) was found between northern and southern regions suggesting that the morphological and behavioral differences between these two regions have a genetic basis and may represent subspecies.

Costs of bearing a sexually selected ornamental weapon in a fiddler crab

1 Sexually selected structures with dual function of combat and display are likely to be honest signals of male quality to opponents and mates, but should be costly to produce and maintain. 2 Male fiddler crabs use a single greatly enlarged claw as both a weapon in agonistic contests with other males and an ornament to attract females for mating. Given the extreme size of this structure (up to half the total body mass), there is surprisingly little evidence for costs as predicted by theory. 3 We experimentally investigated several potential costs of the large claw to male sand fiddler crabs Uca pugilator. Mass-specific metabolic rates were significantly higher and treadmill endurance capacity significantly lower for males bearing an intact major claw compared with those without. 4 In contrast, presence of the claw did not affect maximal sprint speeds, suggesting that the massive structure does not compromise the ability of male crabs to evade predators. These counterintuitive results conform to recent theoretical models of energy costs of locomotion. 5 Our study provides empirical support for a key assumption of sexual selection theory – energetic and endurance-related locomotor costs incurred while bearing this ornamental weapon act in opposition to sexual selection favouring larger claws.

Predator-driven fragmentation of fiddler crab droves into selfish miniherds of biased composition

An explanation for animal groups is the selfish herd, characterized by aggregation as each member tries to shield itself from a predator by moving into a tight gap between other members. We test the hypotheses that: (1) droves, the large feeding groups of fiddler crabs, are selfish herds; (2) the miniherds that form when droves fragment on approach of a large predator are selfish herds; (3) selfish herds form when refugia are unlikely to be reached before an approaching predator arrives; and (4) the composition of selfish miniherds is biased toward individuals most vulnerable to predation. The study was conducted in South Carolina (USA) by videotaping the movements of sand fiddler crabs Uca pugilator when approached by a human predator. In both droves and miniherds, interindividual distance decreases with predator approach, consistent with behavior in a selfish herd. However, two other expectations for selfish herds—herd cohesion and sacrificing distance from the predator in order to get closer to other herders—are only met in miniherds. Crabs farther from refugia are more likely to form and remain in miniherds, indicating that selfish herding is only favored when refugia cannot be quickly reached. The composition of the smallest miniherds, consisting of 2–18 crabs, is biased toward females and small males. These individuals may be more vulnerable to predation because they lack the enlarged claw of large males that deters some predators. The small miniherds are relatively homogeneous with respect to the size and sex of their members, which may enhance cohesion and effectiveness as selfish herds. Miniherds will be effective selfish groups when predator attack has a significant vertical component and when the strike distance is large relative to both the size of the prey and the distance between group members. Droves are not selfish herds but permit crabs to flee feeding grounds as members of selfish miniherds.

Environmental regulation of foraging in the sand fiddler crab Uca pugilator (Bosc 1802)

Sand fiddler crabs Uca pugilator (Bosc 1802) live in burrows in the high intertidal to supratidal zones of sandflats and salt marshes. Large groups of crabs regularly leave their burrows to forage on exposed sediments in the low intertidal zone. They are known to forage in response to chemical cues. We hypothesized that environmental factors that vary predictably in field sediment play a role in determining where fiddler crabs forage. To test this hypothesis, we determined the effects of four physical sediment characteristics on chemically mediated feeding behavior in U. pugilator: (1) sediment organic content; (2) the salinity of interstitial water; (3) sediment grain size; and (4) sediment water content. Our results indicate that sediment organic content and sediment water content are the most important factors that determine where fiddler crabs forage. Optimal sediment is a mixture of sizes dominated by 0.125 and 0.250 mm sands. This condition is constant across the intertidal zone. Salinity, the most variable of the environmental factors, has no effect upon feeding responses. Optimal conditions for feeding are found in sediment with organic content at or above 1.0% that is fully saturated with water. Complete saturation allows the entire dactyl to penetrate the sediment. Consequently, dactyl chemoreceptors are maximally stimulated. Optimal conditions are found in a band several meters wide immediately above the water. The band moves with the tide. Physical factors (sediment water content), rather than biological factors (food content), are responsible for temporal patchiness of the foraging environment of fiddler crabs.

Structural and functional polymorphism of the hemocyanin O 2 transport system of the sand fiddler crab Uca pugilator

Uca pugilator hemocyanin is an exceptionally heterogeneous aggregate of different polypcptide chains. In a sample of < 100 individuals as many as nine different monomers were resolved. As reported by earlier workers, this protein is also exceptionally polymorphic. Only two of the nine chains in the present sample were invariant. Seven chains varied both qualitatively and quantitatively. Five of the variable chains never reached the highest quantities attained by the other four. Phenotypes lacking one of the two variable chains that did reach maximal quantities, and/or one of the chains that reached intermediate quantities, were chosen for measurements of respiratory properties. The results strongly suggest that the more major of these two chains has a large influence on O 2 affinity and a small influence on its pH dependence. Therefore, the structural polymorphism is accompanied by functional differences as well.

Phenotypic variation and lability of the subunit composition of the hemocyanin of Uca pugilator

The monomeric subunit composition of the hemocyanin of the sand fiddler crab Uca pugilator was examined in six samples totalling 342 individuals and representing latitudinal and salinity gradients. A total of 12 different polypeptide chains were separated electrophoretically, of which as few as four and as many as nine were present in an individual. The small quantities of several chains suggest a mixed population of the predominantly 2×6 aggregates in the blood. There was no effect of sex and no clear effect of salinity. Samples of populations found near the northern and southern limits of the species, however, were very different from each other and also from samples of populations in the middle of the geographic range. Specifically, samples from the geographic limits exhibit far less variation than those from the middle of the species range. Only one group of three minor chains clearly varies as if they might be encoded at the same locus. Moreover, the condition of most of the monomers, including those three as well as several of the major chains, is not genetically fixed in the adult stage.

Direction orientation of the sand fiddler crab Ucapugilator(BOSC) in water: Onshore escape response

When dislocated offshore at their home beach, the sand fiddler crab Uca pugilator displayed an escape response, in which they moved directionally in the onshore direction. This response probably functions for predator avoidance of an area containing predators such as blue crabs. Directional orientation using celestial cues, beach slope, waves, and landmarks were tested. Male crabs correctly oriented in the onshore direction of the home beach in the presence of only celestial cues but were disoriented under cloudy skies. When exposed to slope, crabs moved in the upslope direction with the threshold slope being 3°. In a wave tank, crabs moved in the direction of wave travel. Crabs avoided a simulated landmark which was interpreted as avoidance of an approaching predator. Thus, celestial cues, beach slope, and waves can guide the onshore escape response of this fiddler crab species.

Control of chemically stimulated feeding behavior in sand fiddler crabsUca pugilator: Evidence for hemolymph feeding inhibitory factor

Uca pugilator, the sand fiddler crab, is a deposit-feeder. It feeds on exposed sand flats as the tide recedes. Feeding movements are evoked by stimulation of chemoreceptors on the dactyls. Previous studies have demonstrated that a proteinaceous factor associated with the sinus gland region inhibits chemically-stimulated feeding behavior. Here, that work is extended to show the existence of a similar inhibitory factor present in the hemolymph of fed crabs in the laboratory and in the hemolymph of crabs returning from feeding in the field. The factor is not detected in fasted laboratory crabs or in field crabs walking from burrow areas to feeding sites. Injection of glucose results in inhibition of feeding activity in intact crabs. Injection of glucose, but not galactose, stimulates feeding in eyestalk-ablated crabs. We suggest that neural responsiveness is stimulated by glucose, and that elevated glucose in intact crabs results in release of feeding inhibitory factor from the sinus gland. Release of feeding inhibitory factor into the blood enables crabs to return to burrow areas without stopping when they encounter food-laden sediments.

Eyestalk factor modulation of chemically stimulated feeding in sand fiddler crabs Uca pugilator (Bosc 1802)

Abstract Uca pugilator (Bosc 1802) is a deposit-feeding semiterrestrial crab. Feeding is mediated by dactyl chemoreceptors and stimulated by hexose sugars. Eyestalk ablation increasessensitivity of crabs to hexose stimulants. Here, the potential for endocrine control of feeding responses to external chemical stimuli was examined. Injection of extracts from the sinus gland region of the eyestalk into ablated crabs reduced chemically stimulated feeding responses to levels found in intact crabs. Feeding inhibitory factor was found in the sinus gland region of shrimp and crabs. The active factor is 5000–10000 Da in molecular size, and is sensitive to heat and trypsin. These results suggest that the sinus gland contains a proteinaceous factor that can control feeding.

Experimental studies of particle ingestion by the sand fiddler crab Uca pugilator (Bosc)

Particle ingestion by the sand fiddler crab Uca pugilator (Bosc) was investigated using clean sand sediment inoculated with glass and plastic beads. For all bead types, the number ingested was positively correlated with the initial sediment bead concentration. Ingestion efficiency, however, was inversely correlated with bead diameter, weight, and initial sediment bead concentration. The latter result suggests that the particle sorting mechanism can be saturated at high substratum particle concentrations. Crab size was directly related to the efficiency at which large beads were ingested, but had no effect on the ingestion of small beads. Ciliates, unbound bacteria, and bacteria attached to sand grains were ingested with high efficiency, but the crabs did not preferentially ingest live over inert particles. Other results suggest that maximization of food intake by sand fiddlers occurs at the habitat patch level rather than at the level of particle ingestion. Supporting this view is the observation that inert particles which were ingested with high efficiencies did not stimulate crab feeding. Sediment harvesting rates were inversely related to substratum organic and silt concentrations suggesting that sand fiddlers can control ingestion rates by regulating sediment harvesting rates

Deposit-feeding by the ghost crab Ocypode quadrata (Fabricius)

Observations using a videotape system on a secluded, Georgia, barrier beach revealed that the ghost crab Ocypode quadrata (Fabricius) exhibits diurnal deposit-feeding behavior similar to that manifested by the sand fiddler crab Uca pugilator (Bosc). The only other ghost crabs reported to engage in deposit-feeding, Ocypode ceratophthalma and guadichaudii, use their chelae in a different manner than O. quadrata or Uca pugilator. Ocypode quadrata individuals collect more substratum per feeding motion and produce larger feeding pellets, aggregations of harvested but uningested substratum, than do sand fiddlers. The difference in feeding pellet size appears to be related to differences in buccal cavity size. Differences in Chl a concentrations between feeding pellets and unforaged substratum showed that O. quadrata is less efficient than Uca pugilator at extracting algae from the harvested substratum. However, ghost crabs can extract up to 70% of the available algae. Findings of this study suggest that deposit-feeding is an important adjunct to predatory behavior.

Experimental studies on the foraging behavior of the sand fiddler crab Uca pugilator (Bosc, 1802)

The purpose of this study was to test the hypothesis that foraging sand fiddler crabs. Uca pugilator (Bosc), move through the habitat in response to low substratum food levels even though these movements may take the crabs considerable distances from the safety of the burrow area. Chl a and ATP concentrations were used as measures of food density in foraged and unforaged substratum. Field and laboratory feeding experiments showed that crab foraging intensity in a habitat patch was directly correlated with food density in the patch either in the presence or absence of alternative food patches. Other experiments showed that sand fiddlers can respond to differences in food level on a scale of millimeters and do this by probing the substratum with minor chelae. Food levels in aggregations of non-ingested particles harvested by sand fiddlers, feeding pellets, correspond to low foraging intensities predicted from foraging experiments and crabs exhibit low foraging intensities on substratum patches derived from feeding pellets. Substratum food levels in two distinct areas corresponded to high predicted foraging intensities and there was no consistent trend in the level of food in the burrow vs. the nonburrow microhabitats. These results suggest that the movements of foraging sand fiddlers are to some extent controlled by the reduction in substratum food levels due to feeding during a single foraging episode. Sand fiddlers can extract over 70% of the food from harvested substratum over a broad range of substratum food densities but harvest only 42% of the available substratum.