Shell fights in the hermit crabPagurus geminus: Effect of cheliped use and shell rapping

In shell fights of the hermit crab,Pagurus geminus, frequently it is observed that large crabs (attackers) grasp the thoracic appendage of small crabs (defenders) with the major cheliped and pull the smaller crabs out of their shells. If this is a standard occurrence and result, then the interaction should not be called a “negotiation” (Hazlett 1978). The role of cheliped use by the attckers in the eviction of defenders was therefore studied using crabs with tubes on their chelipeds, and the effect of shell rapping, which is thought to be necessary for eviction, was studied using crabs without shells. The experimental crabs evicted the defenders but fighting was significantly prolonged. Therefore, the negotiation model cannot be rejected. Specific aspects of shell fights in hermit crabs are discussed.     

Shell wars II: the influence of relative size on decisions made during hermit crab shell fights

The value of contested resources (shells) in hermit crab fights depends on the sizes of the crabs relative to the sizes of the resources. Thus when relative contestant size is the main experimental variable, motivational factors associated with shell size will also be an experimental problem. Two experiments are described that together overcome this problem. Relative crab size influences all stages of shell fights including pre-fight display, escalation, eviction and examination of the opponent's shell by the victor both before and after eviction of the loser. Shell fights occur more often between disparately-sized animals than between those similar in size. This apparent contradiction of recent theory (Maynard-Smith & Parker 1976) is probably due to the high cost of being without a shell and the small chance that an escalated fight will result. Relative crab size influences the time taken in resource assessments and thus the effectiveness of these assessments is also probably influenced. Causal factors influencing each of the major decisions in shell fights are described and although these fights are more complex than most they are in general agreement with theory on animal contests.     

Relative Growth and Sexual Maturity of the Hermit Crab Paguristes erythrops (Anomura, Diogenidae) from South Atlantic

Some morphometric relationships were studied in the hermit crab Paguristes erythrops to describe its relative growth and the size at which sexual maturity is reached, as well as the shell influence on this process. Individuals were collected monthly from January to December/1999, by SCUBA diving and 12 different characters were measured on males and females. Sexual size dimorphism was observed in length and width of left cheliped. Size at sexual maturity was estimated between 2.8 and 3.5 mm shield length based on the left cheliped relations of the males and on the endopod and exopod of the second pleopod of the females. Differences in the allometry of individuals occupying different shell species were observed. The present work demonstrated the importance of characterizing the type of growth of some parts (pleopods) not yet used in other studies on relative growth of hermit crabs that may correspond to indicators of the size at sexual maturity.     

Initiation and Resolution of Fights between Swimming Crabs (Liocarcinus depurator)

Fights between male swimming crabs (Liocarcinus depurator) were studied in the laboratory and in the field. These crabs fight readily in the laboratory, interactions being initiated equally often by the larger and the smaller of the two opponents, but usually being won by the larger. Many different combinations of cheliped and swimming leg postures are seen during fights, which fell into 4 categories: 13% involved only stationary display, 38% were resolved by a single approach and retreat by displaying crabs, 9% were resolved by multiple approaches and retreats and 39% involved physical contact between the crabs. Fight length was variable, and depended both on the absolute size and on the relative size of the participants. During the course of the fights, few behavioural differences were observed between the eventual winners and losers. Fights were less common in the field, but the same rules determined their initiation, content and outcome. The results are discussed in the context of game theory.     

Hair-trigger autotomy in porcelain crabs is a highly effective escape strategy

The benefits of autotomy, the voluntary shedding of limbs, havebeen adequately demonstrated in vertebrates but are poorlystudied in invertebrates. We provide strong experimental evidencefor an antipredatory benefit to autotomy in two porcelain crabs(Petrolisthes cinctipes and P. manimaculis). Since the outcomeof autotomy depends critically on the species of predator andprey involved, we first surveyed field populations of porcelaincrabs to identify ecologically relevant predators to use insubsequent experiments. We then examined the escape tacticsof the porcelain crabs in response to the four potential predatorswe identified, all larger predatory crabs. Cheliped autotomywas induced by three of the predator species (Cancer antennarius,Hemigrapsus nudus, Pachygrapsus crassipes); the fourth didnot attack porcelain crabs. Autotomy occurred in response to34% of all attacks, and in 67% of attacks in which the preywas held at some point by only the cheliped. Autotomy was ahighly effective escape mechanism against these predators;58 of 59 crabs that autotomized escaped, usually while thepredator stopped pursuit to eat the shed cheliped. Relianceon autotomy as a primary mechanism for escaping predators maybe particularly common in small crabs that cannot adequatelydefend themselves by other means and in suspension-feedingcrabs that do not need their chelipeds for feeding.     

Evaluation of the sample size used for the rapid bioassessment of rivers using macroinvertebrates

Experiments were designed to investigate selective predation by medium (40–55 mm carapace width: CW) and large (55–70 mm CW) Carcinus maenas when feeding on four bivalves of contrasting shell morphology. Size-selection was examined by presenting individual crabs with a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas and Cerastoderma edule. Medium-sized crabs preferred mussels 5–15 mm shell length (maximum shell dimension: SL) and cockles 5–10 mm SL, whereas large crabs preferred mussels 15–25 mm and cockles 10–20 mm SL. Crabs generally showed no preference for any particular size of either oyster species. Species-selection was examined by presenting individual crabs with paired combinations of the four bivalves in various proportions. When offered mussels and oysters simultaneously, both size categories of crabs consistently selected mussels, and food choice was independent of prey relative abundance. By contrast, C. maenas selected mussels and cockles as expected by the frequency in which each size category of crab encountered the preferred size ranges of prey. Crab preference clearly paralleled the rank order of prey profitability, which in turn was mainly determined by prey biomass, suggesting that active selection takes place at some point of the predation cycle. Experiments with epoxy resin models showed that initial reluctance of crabs to attack oysters was not associated with the ultimate energy reward. Moreover, they suggest that foraging decisions are partly based on evaluations of overall prey shape and volume, and that the minimum dimension of the shell constitutes an important feature which crabs recognise and associate with prey value.     

Predation by blue crabs, Callinectes sapidus, on rapa whelks, Rapana venosa: possible natural controls for an invasive species?

Blue crabs Callinectes sapidus are voracious predators in Chesapeake Bay and other estuarine habitats. The rapa whelk Rapana venosa is native to Asian waters but was discovered in Chesapeake Bay in 1998. This predatory gastropod grows to large terminal sizes (in excess of 150 mm shell length (SL)) and has a thick shell that may contribute to an ontogenetic predation refuge. However, juvenile rapa whelks in Chesapeake Bay may be vulnerable to predation by the blue crab given probable habitat overlap, relative lack of whelk shell architectural defenses, and the relatively large size of potential crab predators. Feeding experiments using three size classes of blue crab predators in relation to a size range of rapa whelks of two different ages (Age 1 and Age 2) were conducted. Blue crabs of all sizes tested consumed Age 1 rapa whelks; 58% of all Age 1 whelks offered were eaten. Age 2 rapa whelks were consumed by medium (67% of whelks offered were eaten) and large (70% of whelks offered were eaten) blue crabs but not by small crabs. The attack methods of medium and large crabs changed with whelk age and related shell weight. Age 1 whelks were typically crushed by blue crabs while Age 2 whelk shells were chipped or left intact by predators removing prey. Rapa whelks less than approximately 35 mm SL are vulnerable to predation by all sizes of blue crabs tested. Rapa whelk critical size may be greater than 55 mm SL in the presence of large blue crabs indicating that a size refugia from crab predation may not be achieved by rapa whelks in Chesapeake Bay until at least Age 2 or Age 3. Predation by blue crabs on young rapa whelks may offer a natural control strategy for rapa whelks in Chesapeake Bay and other estuarine habitats along the North American Atlantic coast.     

Resource assessment in hermit crabs: the worth of their own shell

Animals gather information about the quality of a resource throughits assessment and behave accordingly as a result of adaptivemotivational changes. In the hermit crab Pagurus longicarpus,we investigated whether an individual was affected in its motivationto acquire a new shell by the quality of the domicile shell(own resource value [ORV]), of the offered shell (external resourcevalue [ERV]), or of both and asked whether its motivation wasaltered by the information gathered during shell investigation.We analyzed the behavior of hermit crabs inhabiting shells ofdiffering qualities and compared their willingness to acquirean offered shell—optimal, smaller than optimal, or largerthan optimal—by measuring the latency to approach it,the number of shell investigation, and its total duration. Crabsin smaller shells (SSs) approached more quick and often theoffered shell, whereas crabs in larger shells investigated theoffered shell more thoroughly. The readiness of crabs to approachthe offered shell and the extent of its investigation were independentof the ERV but were exclusively affected by the ORV, whereasthe number and duration of shell investigation did not changewith time as investigation proceeded, except for crabs in SSs.These results suggest that P. longicarpus' motivation to acquirea new shell is exclusively influenced by the value of the shellit inhabits rather than by the quality of the shell it is offeredand that this species does not gather—or does not use—informationabout ERV during investigation.     

Effects of chemical context on shell investigation behavior in hermit crabs

Specific chemicals in the environment evoke significant changes in the behavior of many aquatic organisms. We studied in the laboratory whether satiated individuals of the hermit crab, Pagurus longicarpus Say 1817, adjust their investigatory behavior towards an empty, optimal gastropod shell according to differences of chemical context. We also explored to what extent shell investigation by a crab in the same hunger state was affected by occupying an inadequately sized shell. Our results confirmed in part previous findings that crabs can discriminate the odor of freshly dead snails from the odor of freshly dead conspecifics. In the presence of the former odor, crabs inhabiting shells of inadequate size were more responsive and active than those in better-fitting shells. To the contrary, regardless of the quality of the inhabited shell, P. longicarpus remained practically motionless when presented with the odor of freshly dead conspecifics, possibly because the risks of incurring in predators would outweigh the benefits of acquiring a new shell. Unexpectedly, we found that crabs in both types of shell quality exhibited nearly the same behavior in control water, while crabs in adequate shells were more responsive in the presence of food odor. Individuals appeared insensitive to the odor of live snails; indeed, only one hermit crab species has been seen removing living snails from their shells. An intriguing result was that water conditioned by the odors of live conspecifics exerted a strong effect on all the individuals by inducing an intense shell investigation. Our study underlines the central role exerted by chemical detection in hermit crabs' behavior and demonstrates the existence of a complex interplay among chemical context, the physiological state of the animal, and the ecological pressures of the habitat.     

Does the handedness of the pebble crab Eriphia smithii influence its attack success on two dextral snail species?

The predaceous crab Eriphia smithii (Xanthidae) has one larger claw with molar teeth on either the right or the left cheliped, which it uses to crush the shell of prey. Whether the handedness of crabs affected successful predation on two snail species, Nerita albicilla (Neritidae) and Planaxis sulcatus (Planaxidae) was experimentally investigated. The fate of snails of each species was analysed by multiple logistic regression with three explanatory variables: handedness, shell-size index and individuality of crabs. No effect of handedness was detected in attacks on N. albicilla , probably as a result of the spherical and more symmetrical shell morphology of this species. In contrast, right-handedness contributed to greater attack success on P. sulcatus , which has more conical shells. Further investigation of how snail shells were broken revealed that left-handed crabs had more difficulty breaking the aperture of larger P. sulcatus , which was thought to cause the difference in attack success between right- and left-handed crabs. The advantages conferred by handedness are discussed.     

MORPHOLOGY AND BEHAVIOR OF CRABS AND GASTROPODS FROM LAKE TANGANYIKA,AFRICA: IMPLICATIONS FOR LACUSTRINE PREDATOR-PREY COEVOLUTION

The shells of most lacustrine gastropods are typically small, weakly calcified, and modestly ornamented to unornamented. Similarly, most lacustrine crabs are usually small detritivores with weak chelae. A number of invertebrate taxa in Lake Tanganyika, however, deviate from these generalities. This study explores a predator-prey coevolution model as an explanation for the large, heavily calcified, and ornate gastropods and the robust, durophagous crabs of Lake Tanganyika. The endemic thiarid and viviparid gastropods from Lake Tanganyika have significantly thicker shells and higher frequencies of terminal apertural lip thickening than closely related cosmopolitan taxa from outside the lake. Tanganyikan gastropods also display considerably higher incidence of shell repair, following nonlethal shell damage, than cosmopolitan taxa of the same families. There is a strong positive correlation between gastropod apertural lip thickness and shell repair frequency among all the gastropod species analyzed. The endemic Tanganyikan potamonautid crab Platytelphusa armata (a molluscivore) possesses larger, more robust crushing chelae than other African potamonautid or potamonid crabs. In contrast with the cosmopolitan African crabs, the Tanganyikan crabs display molariform, rather than serrate dentition on their crushing chelipeds. In shell-crushing experiments, the Tanganyikan gastropod shells were an order of magnitude stronger than typical lacustrine gastropod shells, many well within the range of tropical marine gastropod shell strengths. Predation experiments with the endemic gastropods Spekia, Neothauma, Lavigeria spp., Paramelania spp. and the crab Platytelphusa armata showed that increased size, apertural lip thickness or shell sculpture reduced the successful predation rate of P. armata. Crabs with large chelae have a greater ratio of successful: unsuccessful attacks than crabs with small chelae. Among cases of successful predation, crabs with large chelae employed predation methods that required less time and energy (such as crushing the shell in the cheliped) than the methods employed by crabs with small chelae (such as peeling the shell from the aperture or the spire). The morphological, shell-crushing, and aquarium experiment data, considered in concert, provide strong support for the idea that the endemic gastropods and crabs of Lake Tanganyika have coevolved over the past 7 million years.     

The enigma of reversed asymmetry in lithodid crabs: absence of evidence for heritability or induction of morphological handedness in Lopholithodes foraminatus

SUMMARY Mutations or environmental factors that result in reversal of conspicuous left–right asymmetries provide an opportunity to study developmental mechanisms. They may also provide insight into evolutionary changes in asymmetry states within and between species. King crabs (family Lithodidae) have a larger right claw and females typically exhibit a dextrally offset abdomen. Nevertheless, I observed a high incidence of left handedness in laboratory reared box crabs ( Lopholithodes foraminatus ) and captured the first known egg-bearing female lithodid to exhibit reversed asymmetry. This provided a unique opportunity to characterize the reversed phenotype and to compare the incidence of reversed asymmetry in the offspring of normal and reversed females. Asymmetry of the chelae became apparent in the first postzoeal stage (glaucothoe) and handedness was maintained through subsequent instars. Females with larger left claws developed reversed abdominal asymmetry by the fourth crab stage. No reversed asymmetry was observed in the mandibles of zoea larvae or juveniles of either handedness. The incidence of reversed asymmetry in glaucothoe reared from one reversed and three normal females was high (between 20% and 30%), and independent of maternity ( P =0.67). Removal of the right cheliped of fourth stage zoeae, and the major cheliped of glaucothoe, did not reverse the direction of asymmetry. Elevated larval rearing temperature also did not affect the frequency of reversed individuals. This lack of evidence for either heritability or induction of handedness is enigmatic. Further investigation of reversed asymmetry in lithodid crabs may provide valuable insights into the development and evolution of bilateral asymmetries.     

Analysis of waving and sound‐production display in the ghost crab,Ocypode stimpsoni

The ghost crab Ocypode stimpsoni displays waving and sound production. Sounds are produced by thumping the sand substratum with the major cheliped, and two types of sounds can be discriminated; one with a low frequency of about 12 Hz, called rapping, and another with a higher frequency (about double), called quivering. In our observations, a sequence of waving and sound emission would sometimes terminate abruptly, or appear as independent components but the component order never changed. The most frequently observed patterns were “waving with rapping and quivering”;, “waving with quivering”; and “quivering only”; quivering sounds being involved in more than 80% displays. Quivering sometimes occurred immediately after crabs emerged from the burrow, or when they returned to the entrance after discarding an excavated sand mass. The occurrence frequency of waving and sounds, the wave amplitude, and the frequency of the sound increased when other crabs approached.     

Effects of shell status and social context on the agonistic behavior of the tropical hermit crab,Clibanarius signatus

In the hermit crabClibanarius signatus, the influence of both shell status and social context on agonistic behavior was examined. The experimental hermits were of the same size and sex, but differed in the status of the inhabited shell, which was either optimal, sub-optimal or damaged. The evidence from this experiment suggests that the agonistic behavior ofC. signatus is influenced to a great extent by asymmetries in the resource value; attacks are mainly performed by crabs which inhabit shells in a relatively poor condition and are mostly directed towards opponents with better-fitting shells. However, this effect differs according to the social context, being more pronounced when the potential gain is greater. Shell fights are decidedly more complex than most animal combats; more work is certainly needed to understand the mechanisms underlying resource assessment and decisions made during hermit crabs' interactions.     

Ecological relations between hermit crabs and their shell-supplying gastropods: Constrained consumers

Hermit crabs are critically dependent upon gastropod shells for their survival and reproductive fitness. While anecdotal reports have suggested that hermit crabs may be capable of removing live gastropods from their shells to access the essential shell resource, no systematic experiments have been conducted to investigate this possibility. This paper reports experiments on both marine (Pagurus bernhardus) and terrestrial (Coenobita compressus) hermit crabs in which crabs were paired in the laboratory with the gastropods whose shells they inhabit in the field. Pairings included both shelled and naked crabs and spanned the full range of the gastropod life cycle. Neither marine nor terrestrial hermit crabs were successful at removing live gastropods from their shells. Furthermore, only a small fraction of the crabs (5.7%) were capable of accessing shells in which the gastropod had been killed in advance, with its body left intact inside the shell. Finally, although hermit crabs readily entered empty shells positioned on the surface, few crabs (14.3%) were able to access empty shells that were buried just centimeters beneath them. These results suggest that hermit crabs are constrained consumers, with the shells they seek only being accessible during a narrow time window, which begins following natural gastropod death and bodily decomposition and which typically ends when the gastropod's remnant shell has been buried by tidal forces. Further experiments are needed on more species of hermit crabs as well as fine-grained measurements of (i) the mechanical force required to pull a gastropod body from its shell and (ii) the maximum corresponding force that can be generated by different hermit crab species' chelipeds.     

Sexual selection in hermit crabs: a review and outlines of future research

The information currently available on sexual selection in hermit crabs is reviewed to identify the role of males and females before, during and after mating. According to this information, possible mechanisms of male–male competition, female choice and/or sexual conflict are suggested. Important male components that may affect mating success include dragging the female shell, rotations of the female's shell and male cheliped palpations, and male size and/or shell characteristics (species and size). Possible female determinants of male mating/fertilization success include size (as an indicator of egg production capacity), signalling of sexual receptivity to males, delay from mate guarding to copulation and mating duration. Avenues for deeper exploration in males include the role of the number and morphometry of male sexual tubes during sperm transfer, and whether ejaculate size and sperm number can be adjusted with variable situations of sperm competition intensity and risk. In females it would be interesting to investigate the chemical and behavioural mechanisms affecting spermatophore breakage for sperm release and the variable duration from sperm transfer to spawning. Given these possibilities, and that sperm is externally deposited on the female's body but inside her shell (except for those species that do not use shells, e.g. Birgus , or species where shells are rather small and do not cover the body totally, e.g. Parapagurus ), we conclude that hermit crabs are unique subjects for separating male and female effects, particularly with respect to the applicability of current ideas in sexual selection such as female choice and sexual conflict. Some practical ideas are provided to disentangle both hypotheses using these animals.     

Intraspecific competition and aggression for shells in the hermit crab Pagurus samuel1s

Many studies have investigated shell‐related behaviour in hermit crabs. Few studies, however, have focused specifically on the intraspecies aggression associated with shell competition. We examined intraspecies aggression in hermit crab (Pagurus samuelis) pairs as it relates to competition for a limiting resource, gastropod shells. Pairs of hermit crabs were observed in the laboratory in four different treatments that varied the presence or absence of shells for one or both of the crabs. Measurements of the latency to respond, the number of bouts, and the fight durations were recorded. There was a significant difference among treatments for all three measurements, and naked hermit crabs were much more aggressive than housed hermit crabs. There was no significant difference in aggression between males and females in any of the three treatments. The heightened aggression observed in naked P. samuelis is likely in service of acquiring a protective shell.     

Crab-resistant features of shells of burrowing bivalves: Decreasing vulnerability by increasing handling time

The vulnerability of burrowing bivalves to shell-breaking predation by crabs was found to be influenced strongly by shell features: size, shell thickness, degree of inflation, and the presence or absence of a gape.The relationship between the critical size of a bivalve (maximum size of vulnerability) and crab size was determined for four different morphotypes of bivalves. For the three bivalves where a “size refuge” was present, critical size increased with crab size. Nevertheless, when offered a choice crabs preferred clams well below the critical size and ate them in the order predicted by the critical-size experiment.Examination of the mechanics of shell crushing revealed how these shell features decreased vulnerability. Larger crabs could efficiently handle larger clams because both chela strength and degree of chela gape increased with crab size. Strain gauges attached to crab chelae showed that thick-shelled clams resisted a greater total number of force pulses than did thin-shelled clams of the same body weight. This may be related to the ability of thick-shelled clams to withstand greater loads than thin-shelled clams when loaded only once. This suggests that the reason for the increased resistance to crabs is prolongation of the shell-breaking time. Even though a large thick-shelled, tightly-closing, clam could eventually be opened, it will probably be rejected in favour of prey with shorter handling times.     

Seasonal change in sexual size dimorphism of the major cheliped in the hermit crab <Emphasis Type="Italic">Pagurus minutus</Emphasis>

Sexual size dimorphism is a common phenomenon in the animal kingdom, and its seasonal change has been reported in some species that possess traits dimorphic only in males and specialized for male mating success. However, few studies have examined seasonal change in sexual dimorphism of traits possessed by both sexes. Here, we examined the reproductive biology of the hermit crab Pagurus minutus, at a sandflat in the Waka River estuary, Japan, with special reference to seasonal changes in sexual dimorphism of the large claw (major cheliped) size by conducting population and precopulatory guarding-pair sampling. Previous investigation demonstrated that the major cheliped is used as a weapon, and its size, more than body size, determines the winner in male–male contests of this species. We found ovigerous females from November to April, peaking in January, when 80% of females were ovigerous. Sexual size dimorphism of the major cheliped was observed; the degree of dimorphism increased in the reproductive season, when only males possessed an enlarged major cheliped. In addition, in the reproductive season, precopulatory guarding males had a larger body and larger relative size of the major cheliped than did solitary males, although the major cheliped size in guarding males seemed to reach an upper limit. These results suggest that seasonal change in sexual dimorphism of the major cheliped size in P. minutus strongly reflects sexual selection favoring the development of this natural weaponry, and that the degree of the dimorphism might be limited through natural selection.     

A Mathematical Modelling for the Cheliped Regeneration with Handedness in Fiddler Crab

An enormously developed giant cheliped with the other small one characterizes the adult male fiddler crab. Some experiments with artificial severances of cheliped indicate that such a handedness in the cheliped size is maintained even after the regeneration of severed cheliped. Other experimental researches give some results about an unknown physiological system which controls the emergence and the regeneration of the handedness in the cheliped size. In this paper, with two hypothesized factors relevant to the regeneration of a severed cheliped, we propose a simple mathematical model to describe the experimental result about the cheliped regeneration with a handedness after the cheliped severance for the fiddler crab. Our model gives a suggestion about an underlying system for the cheliped regeneration in the fiddler crab or some other crustacean species.