Binary individual recognition in hermit crabs

One mechanism that permits the maintenance of dominance hierarchies is individual recognition, defined as the ability of an animal to recognize a conspecific on the basis of one or more identifying cues, and to associate it with experiences of victories or defeats that the animal has gained from preceding encounters with that particular individual. We examined whether the long-clawed hermit crab, Pagurus longicarpus, could differentiate between unfamiliar and familiar opponents. The experimental protocol was designed to control in pairs of interacting individuals several factors together, such as status and relative size of the opponent, as well as species, quality, and fit of the inhabited shell. The hermit crabs were more reactive and their agonistic level was higher in unfamiliar than in familiar pairs; in addition, betas were more prone to initiate an interaction with unfamiliar than with familiar alphas. The alternative explanation—that the ability to discriminate between familiar and novel shells can explain our results per se— was tested following, in part, Jackson and Elwoods (1989) protocol for Pagurus bernhardus and was, at least for this species, rejected. This study did not determine whether a true individual recognition occurs, but demonstrated that P. longicarpus categorizes the individuals into two heterogeneous subgroups, thus being capable of a binary discrimination among opponents.Communicated by T. Czeschlik     

Toxicity of Brachuryan crabs in India

Crabs are the most abundant macrofauna in the coastal environment, and may belong to many different species and even families. Some of these crabs contain specialized protein substances that act as toxins. Toxins, especially proteins that are produced by living cells or organisms, are poisonous substances. Toxins can be divided according to their pharmacological effects (neurotoxins, myotoxins, hepatotoxins, nephrotoxins, hematological toxins, and locally acting toxins) and their sources, such as snakes, scorpions, mussels, spiders, marine creatures, or sea anemones. Crab toxins are heat resistant and are found in the crabs’ viscera. Hence, they cannot be destroyed by cooking, and there is also no known treatment available for these toxins. The only alternative is to educate fishermen, holiday makers, and seafood consumers to recognize these poisonous species and avoid them. Several toxins identified from crabs are neurotoxins, related to tetradotoxin (TTX), which is found in pufferfish. Other toxins include saxitoxin (STX), neurosaxitoxin (NeoSTX), and some unidentified toxins. Some poisonous crabs possess a mixture of these toxins. This article describes the toxins and Brachuryan crabs producing toxicity.     

Shell-size selection by intertidal sympatric hermit crabs

This study evaluated selection for shell size by three species of tropical intertidal hermit crabs, Clibanarius antillensis, C. sclopetarius, and C. vittatus, from species of shells which are frequently used in nature. Crab size and weight were strongly and significantly related to all measured parameters of the selected shells. The strength of these relationships (r² values) depended neither on the crab nor on the shell variables taken into account. The relationships between crab size and the dimensions of the selected shells showed higher r² values than the corresponding relationships with the shells that the crabs had occupied when they were collected (0.482–0.903 in comparison to 0.091–0.652, respectively), indicating that the crabs were occupying sub-optimal shells in nature. Negative allometry was frequently found in the relationships between crab and shell variables, indicating that large crabs select and use proportionally lighter shells than do small crabs. This negative allometry was stronger for the shells used in nature (except for C. antillensis), i.e. larger crabs tended to select heavier shells in the laboratory than in nature. Different allometric relationships were also recorded among the dimensions of shells used in nature and those selected by the hermit crabs in free-access experiments: as shell length increased, the selected shells were heavier and had larger apertures than the shells used in nature. The relationships between crab size and the length and weight of the selected shells did not depend on the species of crab or species of shell, but only on crab size. Therefore, analyses using these variables can be performed without taking the species of crab or shell into account, i.e. data from different crab or shell species can be pooled. The influence of crab and/or shell species was recorded only in the models fitted for aperture length and width, variables which were more related to shell architecture than did shell length or weight. In contrast, if crab weight is used as an independent variable, different crab or shell species can be analyzed together independently of the particular shell parameter. This indicates that crab weight may be less susceptible than crab shield length to shell morphological constraints. Finally, the results indicate that the preferred size of a given shell type chosen by a given hermit crab will depend more on crab size or weight, than on the crab or shell species under consideration, i.e. crab shell-size relationships are not species specific.Communicated by P.W. Sammarco, Chauvin     

Allometric constraints and variables of reproductive effort in brachyuran crabs

Allometric relationships of reproductive output were compared in 20 species from 7 families of brachyuran crabs from the east and west coasts of North America, using regression analysis of log reproductive parameters versus log body weight. Comparisons of crabs spanning 4 orders of magnitude in body weight indicated that female body size was the principal determinant of reproductive output: 95% of the variance in brood weight, 79% of the variance in the number of eggs per brood, 63% of the variance in annual brood weight, and 74% of the variance in annual fecundity were explained by body weight. Brood weight exhibited an isometric constraint to about 10% of body weight. Allometric limitations on space available for yolk accumulation in the body cavity appeared to be the main constraint on brood size. Ovum size increased significantly, but only slightly, with increasing body size. There was a significant trade-off between ovum size and the number of eggs per brood. There was no significant relationship between the number of broods per year and body size. The number of eggs per brood was significantly better than brood weight as a predictor of the number of broods produced per year by a species, indicating that demographic pressure on fecundity rather than energetic considerations is the primary selective mechanism influencing annual reproductive effort. Each of the 7 families of crabs exhibited trends toward distinct patterns for the suite of co-adapted reproductive traits. However, no interspecific reproductive patterns were apparent with respect to the variables of feeding type, salinity tolerance, habitat, and geographic range represented by the 20 species.     

Feeding and burrowing ecology of two East African mangrove crabs

The behavior and ecology of two mangrove crabs,Sesarma meinerti De Man, 1887 andCardisoma carnifex (Herbst, 1794) were investigated at the beginning of the rainy season (October–November 1988) at Mida Creek, Kenya. Both species occupy upper intertidal levels, above mean high-water neap, and completely overlap in their zonation. Each burrow lasts ca. 3 wk, with no significant difference between the species. Both are more active around dusk and dawn and also follow a similar trend in their foraging activity, but differ in that burrowing is mainly diurnal inC. carnifex and nocturnal inS. meinerti. A hierarchy of food preference, established by offeringC. carnifex andS. meinerti leaves of five mangrove species, proved similar for both, withBruguiera gymnorhiza ranking first andAvicennia marina last, but significant only forC. carnifex. A rough estimate of the amount of litter consumed by these two species and of the soil mixed up by their burrowing activity indicates that they play a role of primary importance in the ecology of East African mangroves.     

Communication and mutual resource exchange in north Florida hermit crabs

Summary The patterns of shell exchange in three species of hermit crabs which overlap in distribution and shell use were observed in the laboratory. Crabs showed no tendency to initiate more exchanges with conspecifics as compared with nonconspecific individuals and there were no specific size dominance effects. Lack of common communicatory patterns betweenClibararius vittatus andPagurus pollicaris was correlated with minimal actual exchange, whilePagurus impressus exchanged with both species and executed patterns in common with both. The pattern of shell exchanges and preferences indicated that, in some cases, both individuals may gain in interspecific exchanges.     

Description and comparison of major foregut ossicles in hydrothermal vent crabs

Major foregut (gastric mill) ossicles, including the dorsal median tooth, lateral teeth, accessory lateral teeth, and cardiopyloric valve, of hydrothermal vent crabs were dissected and examined during the summer of 1996 from specimens housed at the Natural History Museum of Los Angeles County. Ossicles are described for two species of hydrothermal vent crabs (family Bythograeidae Williams, 1980). The western Pacific Austinograea williamsi Hessler and Martin has an unusual dorsal median tooth. The surrounding cuticular flange is scalloped and bears spinulose setae at the tip of each of the protruding edges, a condition perhaps unique in the Brachyura. The lateral teeth are mostly unremarkable, bearing the typical large anterior denticles and deep serrations seen in other crab families, but with a higher number of serrations than is known for any species previously described. The accessory lateral teeth bear flattened, plate-like spines that are widest basally and that taper to a cylindrical tip. The basic armature of the foregut of Bythograea thermydron Williams, known only from vents in the eastern Pacific, is very similar. Scalloping of the median tooth borders is less pronounced, however, and the shape of the tooth itself and of the plate from which it arises is slightly different. The lateral teeth bear fewer and more widely spaced grooves, and the cardiopyloric valve entrance appears less setose at its extremity. Comparison with foregut ossicles in other crab families based on earlier studies, most of which have not employed SEM, reveals some similarities between bythograeids and some xanthids, but does not clarify the phylogenetic position of the bythograeids. Because of the paucity of other SEM studies of the brachyuran foregut, it is difficult to ascertain whether some of the many spine and setal types in the bythograeid foregut are unique or even unusual compared to those of other crab families. Nothing about the foregut of the vent crabs is indicative of their unusual habitat. Anecdotal observations of feeding in vent crabs indicate that they are opportunistic scavengers and omnivores, which is in keeping with the non-specialized nature of the foregut. The debate between adaptation vs phylogeny as determinants of the form of the gastric mill components is briefly discussed. Received: 10 December 1996 / Accepted: 13 November 1997     

Richness,abundance and shell use of subarctic and arctic hermit crabs

Hermit crabs are an obvious, common and abundant feature of global shallow water environments but almost nothing is known of their ecology at their extreme latitudinal ranges, the Arctic and southern South America. In this study, we report the first investigation on hermit crabs and their use of a key resource, gastropod shells, in a subpolar and a polar environment—amongst the most rapidly changing places on earth. Hypothesised low levels of richness were confirmed by surveys—only Pagurus pubescens was found in western Spitsbergen and only three pagurids were found in northern Norway. At the northern-most of their extent, hermit crabs were fairly common but Arctic abundances (1–5 m⁻²) were an order of magnitude lower than in many warm temperate or tropical localities. Along the open coast of Spitsbergen, the occurrence of P. pubescens was infrequent and very patchy, but it was more abundant in the fjords. In Isfjorden, the largest studied fjord, the population of P. pubescens was mainly represented by small individuals. Spitsbergen P. pubescens used few shell types and >87% just occurred in Margarites or Buccinum shells. The proportion of the gastropod shells, used by P. pubescens, which were damaged, was high and increased with shell size. These hermit crabs are at the edge of the range for both their species and Anomura. The extremes of their location are reflected by: They probably represent the least rich assemblage, with the lowest and most patchy typical abundances reported to date and are amongst the smallest individuals using the least diverse and most damaged gastropod shells.     

Palatability of autotrophic dinoflagellates to newly hatched larval crabs

To determine the general palatability of autotrophic dinoflagellates to newly hatched crab larvae and whether there are taxonomic, predator/prey size relationships, or toxicity components to their ability to discriminate among dinoflagellates, larvae of six species of crabs from two families were fed 16 species/strains of dinoflagellates from three orders. Dinoflagellate cell length ranged from 18 to 50 µm, and toxic and non-toxic species/strains were included. Experiments measuring incidence of prey ingestion, grazing rates on individual constituents of selected prey combinations, and development on one toxic species shown to be readily ingested were conducted between 2000 and 2002. Thirteen of sixteen dinoflagellates were palatable to larvae, with no consistent pattern of prey discrimination based on taxonomic affinity, toxicity, larval hatching season, or predator/prey size relationships. Although the three dinoflagellates not ingested were toxic, three other toxic species/strains were ingested, with accelerated mortality occurring in the one case. Ingestion of non-favored prey occurred only at very low rates when mixed with readily ingested prey, indicating selectivity. Larvae hatching in winter generally ingested dinoflagellates as readily as did zoeae hatched in spring and summer. Newly hatched larvae ingested a wide variety of dinoflagellates, while discriminating among related species. Such discrimination will not always prevent larval ingestion of prey that will result in mortality.Communicated by J.P. Grassle, New Brunswick     

How do weaponless male fiddler crabs avoid aggression?

Mimicry of females enables weaker males in many species to avoid intrasexual aggression. In fiddler crabs (Uca annulipes), males use their major claw in aggressive interactions to acquire and defend a territory. Males that have autotomised their major claw will be disadvantaged in fighting, but might use their temporary resemblance to females to avoid costly aggressive encounters with other males. We investigated whether: (1) courting males mistake clawless male fiddler crabs for females; (2) clawless males are able to acquire, defend and retain territories as successfully as intact males; and (3) clawless males are more cautious than intact males. Clawless and intact males differed in burrow acquisition methods and fighting behaviour, but were equally successful at acquiring and retaining burrows. While courting males treated clawless males as female, we found no evidence that clawless males mimic the behaviour of females, or that they exploit the advantage of their mistaken identity. Clawless males further appear to avoid male aggression by altering their territorial strategies to minimise the potential for conflict.     

Sperm competition in horseshoe crabs (Limulus polyphemus)

Male horseshoe crabs have two mating tactics. Some males come ashore attached to a female (clasping the posterior margin of the females' carapace with their modified pedipalps) and nest with her on the intertidal portion of the beach during the high tide. Other males come ashore unattached and crowd around nesting couples. Fertilization is external and unattached males that are in contact with a pair, i.e. satellite males, release sperm, so the assumption has been that they are fertilizing eggs. We conducted a paternity analysis to determine the proportion of eggs fertilized by attached and satellite males. Pairs with one satellite were observed during nesting on beaches in Florida and Delaware and their eggs were collected and reared to the late trilobite or first instar horseshoe crab stage. DNA was extracted from these offspring and from each adult (female, attached and satellite male) for use in paternity analysis. A Limulus-specific hypervariable microsatellite locus was identified and primers were constructed to amplify this locus via the polymerase chain reaction (PCR). Genotypes of putative parents and offspring were determined by resolving length variants of these PCR products on acrylamide gels. This allowed us to determine parentage of the offspring. We demonstrate that satellite males fertilized 40% of the eggs on average, attached males fertilized 51% and 4% of the eggs that were laid by the female were fathered neither by the attached male nor by the satellite (and 5% could not be determined unambiguously). There is high variability in the success of satellite males, ranging from 0 to 88%. Part of this variability can be explained by the position of the satellite relative to the attached male. We discuss the mechanics of fertilization and the possible advantages for multiple mating in this species.     

Species-specific avoidance responses by blue crabs and fish to chronic and episodic hypoxia

Although both chronic and episodic hypoxia (O₂<2 mg l^–1) alter the distribution and abundance patterns of mobile animals within estuaries, recent evidence suggests that some animals may be more likely to remain within hypoxic or anoxic water than others, due to differences in physiological tolerance and movement responses to the dynamics of hypoxia. Determining avoidance responses to hypoxia is important for identifying the species most susceptible to the direct and indirect impacts of these events. A trawl survey was used to examine the avoidance responses of blue crabs (Callinectes sapidus) and several fish [pinfish (Lagodon rhomboides), spot (Leiostomus xanthurus), Atlantic croaker (Micropogonias undulatus), bay anchovy (Anchoa mitchilli), and paralichthid flounders (Paralichthys dentatus and Paralichthys lethostigma)] to chronic hypoxia and episodic hypoxic upwelling events in the Neuse River Estuary, North Carolina, USA. Trawl collections were made in three depth strata (3.0–4.6 m, 1.7–3.0 m, and 0.9–1.7 m depth) to quantify changes in the depth-specific distribution and abundance patterns of the six most common estuarine taxa during three dissolved oxygen conditions: normoxia, chronic hypoxia, and episodic hypoxic upwelling events. Pinfish, anchovies, blue crabs, and paralichthid flounder abundance increased with increasing dissolved oxygen concentrations. The two taxa most closely associated with the bottom (blue crabs and flounder) showed the strongest avoidance response to hypoxia. All taxa showed a stronger avoidance response to chronic hypoxia as compared to episodic hypoxic upwelling events. This difference is attributed to a reduced ability to avoid the rapid intrusions of hypoxic water during episodic events, or to increased risks of injury and predation in shallow refuge habitats, which may force some individuals back into hypoxic water.Communicated by J.P. Grassle, New Brunswick     

Diets of deep-sea brachyuran crabs in the Western Mediterranean Sea

This study examines the feeding habits of Paromola cuvieri (Risso, 1816) and Geryon longipes A. Milne Edwards, 1881, the only two common deep-sea brachyuran crabs inhabiting the bathyal mud assemblages in the Catalan Sea (Western Mediterranean). Samples were obtained by bottom trawls at depths between 360 and 1871 m during 1983 to 1992. Both species had highly diverse diets, but very low feeding activity, as reflected by the high proportion of empty stomachs. Both characteristics may be important factors enabling deepsea crabs to adapt to bathyal zones, where trophic resources are scarce. The most important food items found in P. cuvieri were fish remains (teleost, sharks) and benthic decapods (Monodaeus couchii, Munida tenuimana). Scavenging activity plays an important role in this species. The diet of G. longipes included a broad range of benthic invertebrates. In the upper middle slope, the bivalve Abra longicallus, decapods (Calocaris macandreae and Monodaeus couchii), echinoderms and polychaetes were the dominant prey, with epibenthic peracarids as a secondary resource. On the lower middle slope, the incidence of decapod crustaceans (C. macandreae, Pontiphilus norvegicus) and peracarids in the diet declined. Small macrobenthic prey (glycerids, cumaceans or amphipods) were rare in the diet of both species, in accordance with the large size of the crab specimens studied. The absence of preferred prey items and the lack of food items of an optimum size on the lower slope may contribute to the progressive decline in abundance of P. cuvieri and G. longipes with increasing depth.     

Predaceous gastropods regulate new-shell supply to salt marsh hermit crabs

Marsh hermit crabsPagurus longicarpus Say directly acquire new shells as the predatory gastropodMelongena corona Gmelin consumes marsh periwinkles,Littorina irrorata Say. The influx rate of new shells into a salt marsh hermit crab population was measured by marking live periwinkles and daily recovering the shells from hermit crabs over periods of 3 to 6 d. Average rates of new shell acquisition ranged from 4.0 to 23.3 new shells per day from salt marsh areas of 4×10 m. Such consistently high rates contrast with the negligible rates generally assumed for new shell entry into hermit crab populations. The number of new shells acquired each day varied directly with the number of the predatory gastropod,M. corona, present in each study area at both natural and manipulated predator densities. Empty shells on the substrate are usually considered as the primary source of new shells to hermit crabs. However, over 500 empty shells had to be placed on the substrate in a 4×10 m area to provide a daily rate of 20 new shells to the hermit crab population.This is the first in a new contribution series from the Florida State University Marine Laboratory No. 1001     

Characteristics of feeding on dinoflagellates by newly hatched larval crabs

The zoeal larvae of brachyuran crabs must feed soon after hatching on a diet that includes large micro- and mesozooplankton in order to satisfy nutritional requirements. However, newly hatched larvae have been shown to ingest a variety of dinoflagellates, perhaps using microbial carbon sources to sustain them until they encounter more favored prey. Ingestion of dinoflagellates by larval crabs has been documented previously under conditions in which the larvae were exposed to algae provided in monoculture or in defined mixtures of cells. We report here on experiments conducted on the hatching stage of five crab species to determine if ingestion of dinoflagellates occurred when they were provided in combination with Artemia sp. nauplii or after a period of feeding on mesozooplankton. Quantitative measurements of chl a in the larval guts provided evidence of ingestion of algal cells. Active ingestion of the dinoflagellate Prorocentrum micans at specified intervals during an extended feeding period was determined on larvae of two crab species using fluorescently labeled cells provided for brief periods at prescribed time intervals. Stage 1 larvae of four of the five crab species ingested dinoflagellates when they were provided in combination with nauplii and larvae of all five species ingested cells after feeding solely on nauplii for 24 h. Ingestion of algal cells was first evident in the larval guts after 6 h of feeding at both low (200 cell ml⁻¹) and high (1,000 cells ml⁻¹) prey densities. Higher prey densities resulted in higher gut chl a. Larvae continuously exposed to dinoflagellates actively ingested cells at every 3 h interval tested over a 36 h period. Results confirm previous studies that larvae will ingest dinoflagellates even when they are encountered in a mixed prey field or when having previously fed. Ingestion of cells may occur on a continual basis over time.     

Respiratory metabolism of crabs from marine and estuarine habitats: an interspecific comparison

The gill-surface area and R-T (metabolic rate-temperature) response during aquatic and aerial respiration of three different species of crabs, the subtidal Scylla serrata, the intertidal Sesarma quadratum, and the supratidal Ocypoda platytarsis, were compared. Scylla serrata has the highest, Sesarma quadratum the second highest, and O.platytarsis the lowest gill-surface area. Oxygen-consumption values in water under different ambient partial pressures of oxygen indicate that all three species display equally efficient aquatic respiration despite variation in gill area and number. There is no coherent R-T response either inter- or intra-specifically. However, the ability to extract oxygen from the surrounding medium (respiratory efficiency) conditions the R-T response in the different species. The R-T trends in aquatic respiration of Scylla serrata and aerial respiration of O.platytarsis reveal a normal response to temperature. The proportion of aquaticoxygen uptake to total respiration in all sizes in both Scylla serrata and Sesarma quadratum remains more or less constant (between 88 and 92%), whereas in the supratidal O.platytarsis, this proportion decreases with increasing weight (from 88 to 64 %), indicating a progressive loss of gill respiration to total respiration. All three species are metabolically equally efficient; what one lacks in respiratory surface it compensates by respiratory efficiency.     

Mating success of male hermit crabs in shell generalist and shell specialist species

Summary The reproductive behavior of two species of diogenid hermit crabs was studied in Hawaii. In the shell generalist, Clibanarius zebra, male reproductive success varied little with size, although the largest males were less successful in obtaining copulations than were medium-large males. Male and female size were positively correlated, in successful pairs, thus larger males had the potential to fertilize more eggs when they were successful in obtaining a copulation. Female fecundity in C. zebra was not affected by species of gastropod shell inhabited once female size was taken into account. Male copulatory success was very strongly influenced by the species of gastropod shell inhabited. Males in Trochus or Nerita shells had greatly reduced reproductive success compared to males in Turbo or Nassarius shells. This result was due both to (1) males in Trochus especially dropping and otherwise poorly handling females during precopulatory behavior and (2) females not responding to precopulatory behavior patterns executed by males in Trochus and Nerita. Transferring males from good to bad shapes of shells and vice versa showed that male success was a function of shell type inhabited and not some correlated feature of the crabs. In the shell specialist, Calcinus seurati, which is found primarily in Nerita shells as an adult, males in Nerita shells were quite successful in obtaining copulations.     

Ecology of subtropical hermit crabs in SW Madagascar: short-range migrations

Many mobile animals migrate because of the different benefits provided by different localities in time and space. For hermit crabs, such benefits include resource (shell, water, food) acquisition and gamete release. One of the more successful crustacean land-invaders, Coenobita hermit crabs, undertake complex short-range migrations in SW Madagascar. Number of active hermit crabs was inversely related to wind strength and positively related to tidal range, emphasising that movement would conserve water. A circadian component was also recorded in the locomotory activity of Coenobita pseudorugosus and C. rugosus. Path linearity varied with many of the same parameters, but also with beach slope. Movement was primarily perpendicular to shore in small individuals, but the parallel proportion increased with hermit crab size and tidal range, probably driven mostly by shell and food searching. Despite the costs of movement and shell carriage in the terrestrial environment, C. pseudorugosus and C. rugosus were as fast as their marine counterparts. Their speeds varied principally with individual size and were approximately 20% faster without shells and about 20% slower when climbing up a 20° slope, compared to horizontal or downhill travel. Hermit crabs, which are highly numerous and speciose in SW Madagascar, do not seem to partition niches by differential movement patterns. Aside from provision of shells in middens and capturing large adults for bait or pets, human activity may have a profound effect on hermit crab movement: observations at rare uninhabited marine reserves like Nosy Ve show that considerable diurnal activity may take place despite the apparent hostility of the environment to an essentially marine animal.