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.     

Semilunar courtship rhythm of the fiddler crab<Emphasis Type="Italic"> Uca lactea</Emphasis> in a habitat with great tidal variation

Semilunar courtship rhythm is a widely distributed phenomenon among fiddler crabs in the genus Uca (Decapoda, Ocypodidae). Typically, synchronous courtship has been reported to peak near spring tides. To determine whether a region of large tidal variation shifts reproductive activity, we measured the frequency of specific courtship behaviors including claw-waving and semidome building for U. lactea males on Kanghwa Island, Korea. We found that synchronized courtship for U. lactea peaked near neap tides, whereas near the spring tides, seawater flooded the habitat and males predominantly fed on the mudflat. Although active females, which hold their burrows and usually feed on the mudflat, are abundant near to spring tides, males rarely claw-waved to attract females. This pattern is atypical for the species because other populations of U. lactea on Japan and Taiwan are synchronous around spring tides. We suggest that males invest most of their time in feeding during spring tides because foraging is limited during neap tides. During neap tides, males feed infrequently and thus expend stored energy on courtship signals. We conclude that patterns of reproductive synchrony may be dependent on food availability in periodically changing environments.     

Stereotypy and variation in the claw waving display of the fiddler crab <Emphasis Type="Italic">Uca tangeri</Emphasis>

We measured temporal and spatial components of the waving display in a Uca tangeri population to look for inter-individual differences in male waving structure that may convey information about individual identity. We found evidence that the spatial components of wave structure, especially “Maximum amplitude” are responsible for most of the between-male variation of the display. This variation could reflect differences in individuals’ condition and/or could be used by conspecifics to discriminate amongst familiar and unfamiliar individuals.     

Adult fiddler crabs Uca pugnax (Smith) enhance sediment-associated cues for molting of conspecific megalopae

Adult-associated chemical cues can stimulate settlement and metamorphosis of invertebrate larvae into habitats with an enhanced likelihood of juvenile and adult survival. For example, sediments from adult fiddler crab habitat stimulate fiddler crab megalopae to metamorphose (molt) sooner than sediments without adult cues. A similar stimulation of molting occurs after exposure to waterborne chemical cues from adult habitats and to exudates and extracts of adult crabs. We tested whether sediments from habitats without adult Uca pugnax (Smith), which do not stimulate molting of their megalopae, could become stimulatory through brief exposure to adult crabs. Sediments were collected from tidal flats at several distances (∼ 1 m, ∼ 50 m, and ∼ 5.4 km) from adult habitats, and incubated for 24 h with or without adult crabs. Molting rates of laboratory-reared megalopae exposed for 48 h to adult-conditioned sediments were compared to those for untreated controls. Sediments collected in or within 1 m of adult habitat elicited the highest molting rates, and natural sediments from 50 m and 5.4 km had little or no effect on molting. However, incubating sediments collected away from adult habitat with adult crabs produced a higher molting response, and the magnitude of the enhancement increased with distance from adult habitat. Results suggest that the chemical cues that adult crabs release are retained by sediments and consequently stimulate molting of megalopae, regardless of the nature of the sediments themselves. Lack of chemical cues may retard colonization of newly created or heavily disturbed habitats that are otherwise suitable settlement and adult habitat.     

Impact of fiddler crab foraging and tidal inundation on an intertidal sandflat: season-dependent effects in one tidal cycle

Intertidal sandflats inhabited by fiddler crabs are ideal systems in which to study the effects of physical and biological processes. This study addressed two questions: (1) Do fiddler crab feeding and tidal inundation have measurable effects on the sandflat over one tidal cycle? (2) Does the sandflat change over the course of a year? In field exclusion/inclusion experiments, fiddler crabs reduced sediment organic content by 40%, Chlorophyll a levels by 20% and meiofaunal density by 60% in one tidal cycle. Effects were most pronounced in the spring when organic content and meiofaunal densities were maximal. Effects of foraging were not erased by the tide and accumulated over time. The sandflat had highest levels of all variables in spring and minimal levels during summer and fall. Crabs graze the sandflat to minimum levels in the spring. Due to crab foraging, the flat is barren during the summer and fall, and recovers during the winter when crabs are minimally active.     

Melatonin: neuritogenesis and neuroprotective effects in crustacean x-organ cells

Melatonin has both neuritogenic and neuroprotective effects in mammalian cell lines such as neuroblastoma cells. The mechanisms of action include receptor-coupled processes, direct binding and modulation of calmodulin and protein kinase C, and direct scavenging of free radicals. While melatonin is produced in invertebrates and has influences on their physiology and behavior, little is known about its mechanisms of action. We studied the influence of melatonin on neuritogenesis in well-differentiated, extensively-arborized crustacean x-organ neurosecretory neurons. Melatonin significantly increased neurite area in the first 24 h of culture. The more physiological concentrations, 1 nM and 1 pM, increased area at 48 h also, whereas the pharmacological 1 μM concentration appeared to have desensitizing effects by this time. Luzindole, a vertebrate melatonin receptor antagonist, had surprising and significant agonist-like effects in these invertebrate cells. Melatonin receptors have not yet been studied in invertebrates. However, the presence of membrane-bound receptors in this population of crustacean neurons is indicated by this study. Melatonin also has significant neuroprotective effects, reversing the inhibition of neuritogenesis by 200 and 500 μM hydrogen peroxide. Because this is at least in part a direct action not requiring a receptor, melatonin's protection from oxidative stress is not surprisingly phylogenetically-conserved.     

Developmental shift in the selective tidal-stream transport behavior of larvae of the fiddler crab Uca minax (LeConte)

Following hatching, larvae of the fiddler crab Uca minax (La Conte) are exported from the adult habitat in estuaries to coastal and shelf waters where they undergo development prior to re-entering estuaries as postlarvae (megalopae). Studies of the spatial distribution of both newly hatched zoeae (Stage I) and megalopae indicate they undergo rhythmic vertical migrations associated with the tides for dispersal and unidirectional transport (selective tidal-stream transport) both within estuaries and between estuaries and the nearshore coastal ocean. We tested the hypothesis that U. minax zoeae possess a circatidal rhythm in vertical migration that facilitates offshore transport in ebb tidal flows, while postlarvae (megalopae) return to estuaries using a similar flood-phased endogenous rhythm. We also determined if the expression of the rhythm was influenced by the salinity conditions zoeae and megalopae experience as they transition between low-salinity regions of estuaries and high-salinity coastal waters. Stage I zoeae were collected by holding ovigerous female crabs in the lab until hatching. Megalopae were collected from the plankton and identified to species using molecular techniques (PCR-RFLP). Under constant laboratory conditions, both zoeae and megalopae exhibited endogenous circatidal rhythms in swimming that matched the principal harmonic constituent of the local tides (12.39 ± 0.07 h; X¯ ± SE). Upward swimming in Stage I zoeae occurred 2.5-4 h after high tide near the time of expected maximum ebb currents in the field. Rhythmic swimming of megalopae occurred slightly earlier in the tide (2.5 ± 0.09 h after high tide; X¯ ± SE) but was not entirely synchronized with flood currents, as expected. Salinity conditions had no apparent effect on the expression or pattern of the rhythms. Results indicate that this circatidal rhythm forms the behavioral basis of selective tidal-stream transport (STST) in early stage U. minax zoeae, but does not undergo a sufficient phase shift to account for vertical distribution patterns exhibited by megalopae in the field.     

Morphological and behavioural adaptations to the rocky substrate by the fiddler crab Uca panamensis (Stimpson, 1859): preference for feeding substratum and feeding mechanism

The fiddler crab Uca panamensis (Stimpson, 1959) inhabits rocky shores. We examined its preference for feeding substratum—sand or rock—and its manner of feeding. The crab made its burrow in the sand among rocks but preferred to feed on rocks. The feeding time decreased as the distance between the burrow and the rock increased. We consider this to be a result of exclusive interaction among the crabs because they defended their feeding area on the rocks against others.The crab wetted a small area of rock with water held in the branchial chambers before and during feeding. It pinched up the wetted surface in the minor chelipeds, which have bundles of setae on the posterior tips of the dactyl and pollex, and put the material into its buccal cavity. It never expelled sand pellets while feeding on rock, which indicates that it swallowed the food particles directly, without sorting. The bundles of setae retained water by capillary attraction, which suggests that they capture the suspended fine food particles scraped from the rock. The wetting action may prevent the fine materials from dispersing. We consider that morphological alteration of the minor chelipeds, the application of water from the branchial chambers, and direct swallowing permit the fiddler crab to feed on fine materials attached to rocks.     

Intraspecific shape variation in horseshoe crabs: The importance of sexual and natural selection for local adaptation

A morphometric analysis of the body shape of three species of horseshoe crabs was undertaken in order to infer the importance of natural and sexual selection. It was expected that natural selection would be most intense, leading to highest regional differentiation, in the American species Limulus polyphemus, which has the largest climatic differences between different populations. Local adaptation driven by sexual selection was expected in males but not females because horseshoe crab mating behaviour leads to competition between males, but not between females. Three hundred fifty-nine horseshoe crabs from nine populations, representing three species, were analyzed using a digitizer to position sixty morphometric landmarks in a three-dimensional space. Discriminant analysis revealed strong regional structuring within a species, which suggests strong philopatry, and showed the existence of geographically-based intraspecific variation. An admixture analysis showed regional intraspecific differentiation for males and females of L. polyphemus and males of the Asian horseshoe crab Carcinoscorpius rotundicauda, but not for females of C. rotundicauda and another Asian horseshoe crab, Tachypleus gigas. Differences in shape variation between sexes were tested with F-tests, which showed lower intrapopulation morphometric variation in males than females. These results indicate a lower degree of local adaptation on body shape in C. rotundicauda and T. gigas than in L. polyphemus and a lower degree of local adaptation in females than in males.     

Metabolic responses of sand fiddler crabs, Uca pugilator, in northwest Florida to seasonal temperature change

Metabolic responses of sand fiddler crab, Uca pugilator, populations in northwest Florida are greatly influenced by seasonal temperature fluctuations. Crabs acclimated at 20 °C and immediately transferred to either 14 or 26 °C produced an acute metabolic response with respective temperature quotient (Q₁₀) values of 3.46 and 3.91. Crabs acclimated at 10 and 20 °C exhibited a Q₁₀ of 2.62 indicating a partial compensation response. A brumation (reverse) response (Q₁₀ value of 20.11) was observed for acclimated crabs between 5 and 10 °C. Brumation is advantageous during winter when food supplies are scarce and crabs must survive extensive periods of inactivity.     

The effects of shell size and coil orientation on reproduction in female hermit crabs, Clibanarius vittatus

We investigated the effects of shell coil orientation and shell size on reproduction in field populations of the hermit crab, Clibanarius vittatus. Females were collected in the intertidal in Beaufort, NC. Shell parameters were measured and size (cephalothorax length) and reproductive status were determined for 70 females occupying Busycon shells. Crabs were categorized as berried (eggs on the pleopods), mature ovaries, or non-reproductive (no eggs). For berried females, the number of eggs was recorded. By offering a separate group of females access to empty shells, it was possible to calculate optimal shell size and the deficit in shell size for field-collected animals.Females that were berried were in shells closer to the optimal shell size than females with mature ovaries, both for shell weight and shell volume. And females with mature ovaries were in shells that were closer to the optimal size than females that were non-reproductive. For both categories of females without eggs on the pleopods, the majority of females were in shells that were too big (in weight and internal volume). While the percentage of berried females did not differ between dextral (Busycon carica) and sinistral (Busycon sinistrum) shells, the non-reproductive females had a much smaller deficit in volume in sinistral shells compared to dextral shells. For berried females, there was no relationship between the magnitude of their shell deficit and the number of eggs carried. Our results suggest that reproduction is inhibited when females occupy shells sufficiently greater than the optimal shell size.     

Endogenous swimming rhythms of juvenile blue crabs, Callinectes sapidus, as related to horizontal transport

The blue crab Callinectes sapidus settles and metamorphoses in areas of aquatic vegetation in estuaries. Crabs in the first-fifth instar stages (J1-5) then emigrate from these areas by walking on the bottom or pelagic dispersal throughout estuaries. The present study was designed to characterize the timing of this migration pattern relative to the light-dark and tidal cycles. Field sampling in Pamlico Sound, NC, USA indicated that J4-5 juveniles were most abundant in the water column during the night. J4-5 juveniles were collected from Pamlico Sound in an area near Oregon Inlet that has semi-diurnal tides, a Mid-Sound area where tides are weak, and on the West side where regular tides do not occur. Crabs from all three sites had a circadian rhythm in which they swam up in the water column during the time of darkness in the field. Peak swimming consistently occurred at about 0300 h, but was not related to the timing of the tidal cycle. Similar results were obtained for juvenile crabs from an adjacent estuary having semi-diurnal tides. Dispersal at night reduces predation by visual predators, and allows early juvenile blue crabs to disperse planktonically from initial settlement sites.     

Schistosoma mekongi: a prominent neutrophil chemotactic activity of egg antigen with reference to that of Schistosoma japonicum

Schistosoma mekongi causes granulomatous lesions around eggs deposited in the liver with neutrophil-rich inflammatory reactions in the early stage of the egg laying. To define the aspects of the typical pathogenesis of S. mekongi infection, we determined the difference between soluble egg antigen (SEA) from S. mekongi and S. japonicum with a focus on chemotactic factors for neutrophils or eosinophils. Mean volume and protein amount of S. mekongi eggs was 71 and 58% of those of Schistosoma japonicum eggs, respectively. Neutrophil chemotactic activity of S. mekongi SEA was about two times higher than that of S. japonicum. In contrast, eosinophil chemotactic activity of S. mekongi SEA was about half of that of S. japonicum SEA. Molecular analysis revealed that S. mekongi SEA contains higher molecular-weight components with a lower level of glycosylation, and this is likely to be related to the intense neutrophil chemotactic activity in comparison with S. japonicum SEA. The prominent chemotactic reactivity for neutrophils is likely to be involved in the typical pathogenesis of mekongi schistosomiasis.     

Post-mating behavior, intramolt growth, and onset of migration to Chesapeake Bay spawning grounds by adult female blue crabs, Callinectes sapidus Rathbun

After molting to maturity, female blue crabs must rebuild muscles atrophied to permit molting and grow larger ones commensurate with the larger exoskeleton. They also must acquire energy for oogenesis and for migration to high-salinity spawning habitat, a distance of >150 km for females mating in the Upper Chesapeake Bay. Using telemetry and mark-recapture techniques, post-copulatory females in the upper bay were shown to forage at high rates, alternating between meandering and directed movement in the area of mating for weeks to months, and to begin migrating in October. Consequently, females from the Upper Chesapeake Bay probably do not spawn until the season after mating. Their priority seems to be to acquire energy before migrating. After molting, energy was allocated first into somatic tissue and eventually into hepatopancreas and gonads. Telemetry of feeding and movement showed that habitat utilization, traveling velocities, foraging patterns, and movements were similar to those already determined for males. However, females appeared to invest proportionally more energy (calories per gram dry weight) into their somatic and reproductive tissues than did males. A newly designed transmitter that telemetered depth showed that females moved during both ebbs and floods and remained at or near the bottom of the water column.