Shelter association between the hermit crab Sympagurus dimorphus and the zoanthid Epizoanthus paguricola in the southwestern Atlantic Ocean

Schejter, L. and Mantelatto, F.L. 2011. Shelter association between the hermit crab Sympagurus dimorphus and the zoanthid Epizoanthus paguricola in the southwestern Atlantic Ocean. —Acta Zoologica (Stockholm) 92 : 141–149. The available literature on zoanthid–hermit crab associations deals only with records of this phenomenon, providing no detailed information. We describe, for the first time, the shell‐like colonies of Epizoanthus paguricola associated with the hermit crab Sympagurus dimorphus from benthic samples taken in the Argentine Sea, between 85 and 131 m depth, and provide information about morphometric relationships between the hermits and the zoanthids. In total, 260 specimens (137 males and 123 females) of S. dimorphus were collected, 240 (92.3%) of which were living in symbiosis with E. paguricola. The remaining 20 (7.7%) were living inside gastropod shells. As the initial structure of the pseudoshell, 12 different gastropod species were found (all were almost totally covered with colonies of E. paguricola). The hermit crab lives in the spiral cavity inside the soft colony, which seemed to be slightly different depending on the initial gastropod. Aperture pseudoshell morphology did not seem to be related to the sex of the hermit crab host, although males showed larger apertures for a given colony size. This fact is probably related to a larger size of male’s cheliped (sexual dimorphic character) used like a gastropod operculum and that may serve as a template for the growing of the aperture pseudoshell edge. The number of epizoanthid polyps per colony increased in relation to the weight of the colony and to the size of the hermit crab. A process of selection of the initial shell was evident, because species of Naticidae were not the most common gastropods in this benthic community, but were those most used by hermit crabs (>60%). The puzzling association between hermit crab, shell and zoanthid presumably occurs during the hermit juvenile phase, when the crab occupies a small shell, and a zoanthid larva settles on it. Given the close relationship between S. dimorphus and E. paguricola found in this region, we support the idea that due to the low availability of adequate gastropod shells for hermit life cycle, this association allows the establishment and the continuity of the hermit crab population in the studied area.     

Land hermit crab (Coenobita clypeatus) densities and patterns of gastropod shell use on small Bahamian islands

Aim To examine patterns of abundance, density, size and shell use in land hermit crabs, Coenobita clypeatus (Herbst), occurring on three groups of small islands, and to determine how these variables change among islands. Location Small islands in the Central Exuma Cays and near Great Exuma, Bahamas. Methods Land hermit crabs were captured in baited pitfall traps and were separately attracted to baits. A mark–recapture technique was used in conjunction with some pitfall traps monitored for three consecutive days. The size of each crab and the type of adopted gastropod shell were recorded, along with physical island variables such as total island area, vegetated area, island perimeter, elevation and distance to the nearest mainland island. Results Relative abundances, densities and sizes of crabs differed significantly among the three island groups. Densities of land hermit crabs were as high as 46 m⁻² of vegetated island area. In simple and multiple linear regressions, the only variable that was a significant predictor of the abundance of hermit crabs was the perimeter to area ratio of the island. Patterns of gastropod shell use varied significantly among the island groups, and the vast majority of adopted shells originated from gastropod species that inhabit the high intertidal and supratidal shorelines of the islands. Main conclusions Although densities of land hermit crabs varied, they were relatively high on many islands, and land hermit crabs may play an important role in these insular food webs. Patterns of shell use may be strongly restricted by island geomorphology: irregular shorelines provide relatively more habitat for the gastropod species that account for the majority of adopted shells and the steep sides of the islands prevent the accumulation of marine gastropod shells. The size of adult hermit crabs appears to be limited by the relatively small gastropod shells available, while the abundance of hermit crabs may be limited by the number of shells available.     

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.     

Gastropod shells: A potentially limiting resource for hermit crabs

The availability of gastropod shells to hermit crabs in the Newport River Estuary, Beaufort, N.C. has been assessed by determining the numbers of usuable shells occurring in characteristic subtidal habitats and by measuring shell size adequacy. The proportion of useable shells occupied by hermit crabs ranged from 58–99 % and many of the shells not used by hermit crabs were judged unavailable because they were occupied by sipunculids or only uncovered by the dredge. The shell adequacy index (shell size occupied/shell size preferred) was significantly below 1.0 for the largest species (Pagurus pollicaris Say) in the one location where sufficient numbers were collected and for the next largest species (P. longicarpus Say) in three of the four locations where it was collected. The shell size adequacy index for the smallest species (P. annulipes Stimpson) did not differ significantly from 1.0 in either of the two locations in which it was found. These observations suggest that the availability of gastropod shells plays a significant rôle in limiting the abundance of at least the larger hermit crabs.     

Palp growth,regeneration, and longevity of the obligate hermit crab symbiont Dipolydora commensalis (Annelida: Spionidae)

Abstract. The polychaete Dipolydora commensalis is an obligate symbiont of hermit crabs and produces a burrow along the columella of the gastropod shells they inhabit. Adults of D. commensalis have short palps that they use to feed on particles dropped or brought in by the respiratory currents of hosts. To determine whether hermit crabs influence palp length, specimens of D. commensalis were isolated in glass capillary tubes and the growth of palps was measured over a 3‐week period. Palp length was also measured in worms isolated in gastropod shells with or without hermit crabs for 2 weeks. In addition, to determine whether adults of D. commensalis have regeneration capabilities like those of free‐living relatives, worms were cut at the fifth or 15th setiger and then monitored for 35 d. Worms extracted from shells and placed into capillary tubes had initial palp lengths of 1.0±0.4 mm (n=17); after isolation, palps were 40% longer (1.4±0.4 mm, n=17). Worms in gastropod shells with hermit crabs had an average palp length of 0.9±0.4 mm (n=31), whereas worms in shells without hermit crabs had palps that were 33% longer (1.2±0.5 mm, n=40). Adults of D. commensalis are capable of regeneration; 35 d after ablation at setigers 5 or 15, the average number of anterior setigers regenerated was 5 (n=15) and 9±1.3 (n=13), respectively. The average number of posterior setigers regenerated from the 15 setiger anterior fragments was 11±6 (n=10). The findings suggest that the palps (and sometimes anterior ends) of the worms are exposed during feeding and are cut during movement of the hermit crab. In the laboratory worms can live for >4 years, considerably longer than the functional life span of most gastropod shells inhabited by hermit crabs.     

Avoidance of drilled gastropod shells by the hermit crab Pagurus longicarpus at Nahant, Massachusetts

Most hermit crabs depend on empty gastropod shells for shelter; competition for appropriate shells is often severe. This study determined whether shells that have been drilled by naticid gastropods are suitable for occupancy by the hermit crab Pagurus longicarpus. Differences in the characteristics of empty shells and those occupied by hermit crabs were assessed at two adjacent field sites in Nahant, Massachusetts. Drilling damage was far more frequent in empty gastropod shells than in shells occupied by hermit crabs, suggesting that individuals of P. longicarpus avoid drilled shells. They did not appear to avoid shells with other forms of damage. Laboratory experiments confirmed that these hermit crabs preferentially chose intact shells over drilled shells, even when the intact shells offered were most suitable for crabs half the weight of those tested. Final shell choices were generally made within 1 h. The hermit crabs apparently discriminated between intact and drilled shells based on tactile cues, since crabs kept in the dark showed the same preference for intact shells. The hermit crabs strongly avoided, to nearly the same extent, artificially drilled shells, naturally drilled shells, and shells with holes artificially drilled on the opposite side of the shell from where they would normally be located. Possible selective forces causing P. longicarpus to show such strong behavioral avoidance of drilled shells include increased vulnerability of crabs in drilled shells to osmotic stress, predation, and eviction by conspecifics.     

Larval release behaviors of the striped hermit crab, Clibanarius vittatus (Bosc): Temporal pattern in hatching

Ovigerous hermit crabs, Clibanarius vittatus (Bosc), were examined in the laboratory to (1) determine if the time of larval release is a synchronous event, (2) determine the influence of a damaged gastropod shell during the egg hatching process, and (3) describe larval release behaviors. Ovigerous hermit crabs from natural light:dark (LD) and tidal cycles were moved to constant conditions 2-3 days prior to the predicted time of larval release. Larval release was synchronous, occurring near the time of expected sunset. Females with early-stage embryos placed under constant conditions displayed a free-running circadian rhythm, suggesting that the rhythm is under endogenous control. Hermit crabs with early-stage embryos that were placed under a shifted LD cycle (advanced 12 h relative to the ambient photoperiod) before being placed under constant conditions advanced their larval release rhythm by 12 h, indicating the rhythm can be entrained by the LD cycle. Hermit crabs with an intact shell released larvae in bursts at sunset over several consecutive nights (period = 24.2 h). In contrast, hermit crabs with damaged shells released larvae at different times over the course of a single day. Ovigerous females with intact shells exhibit several stereotypical hatching behaviors. The female stands on her walking legs and thrusts her abdomen, moving the shell in an oscillating motion. This movement may assist in breaking the outer membrane of the egg case. The female generates a water current inside the shell with her scaphognathite and mouthparts, which transports the newly hatched larvae out of the shell. Females in damaged shells did not display these behaviors; instead, larval release was a prolonged event with little movement of the female, and often the newly hatched larvae were not viable. These results indicate that an intact shell plays an important role in the hatching process for this hermit crab.     

What makes a good home for hermits? Assessing gastropod shell density and relative strength

The survival and reproductive success of hermit crabs is intrinsically linked to the quality of their domicile shells. Because damaged or eroded shells can result in greater predation, evaluating shell structure may aid our understanding of population dynamics. We assessed the structural attributes of Cerithium atratum shells through assessments of (a) density using a novel approach involving computed tomography and (b) tolerance to compressive force. Our goal was to investigate factors that may influence decision making in hermit crabs, specifically those that balance the degree of protection afforded by a shell (i.e. density and strength) with the energetic costs of carrying such resources. We compared the density and relative strength (i.e. using compression tests) of shells inhabited by live gastropods, hermit crabs (Pagurus criniticornis) and those found empty in the environment. Results failed to show any relationship between density and shell size, but there was a notable effect of shell density among treatment groups (gastropod/empty/hermit crab). There was also a predictable effect of shell size on maximum compressive force, which was consistent among occupants. Our results suggest that hermit crabs integrate multiple sources of information, selecting homes that while less dense (i.e. reducing the energy costs of carrying these resources), still offer sufficient resistance to compressive forces (e.g. such as those inflicted by shell-breaking predators). Lastly, we show that shell size generally reflects shell strength, thus explaining the motivation of hermit crabs to search for and indeed fight over the larger homes.     

Biological features of a puzzling symbiotic association between the hermit crab Dardanus insignis and the porcellanid crab Porcellana Sayana (Crustacea)

The symbiotic lifestyle is widespread among porcellanid crabs, which maintain ecological and co-evolutionary associations with annelid polychaetes, poriferans, cnidarians, echinoderms, gastropod mollusks, and other crustaceans such as shrimps and hermit crabs, among others. We investigated the ecological association between the hermit crab Dardanus insignis and the porcellanid Porcellana sayana, in southeastern Brazil. Porcellanid crabs, hermit crabs, and available shells were collected monthly from July 2001 to June 2003, with a shrimp boat equipped with two double-rig trawl nets. The majority of P. sayana specimens were collected in shells occupied by D. insignis (96.6%); a few were found in empty shells (3.4%). The catch of both symbionts and hosts increased with increasing depth, with the highest occurrence at 35 m. The P. sayana crabs of various sizes could be found solitary or forming aggregations of up to 14 individuals per host, showing no sex or size segregation. In spite of the high diversity of shell species occupied by the hermit crabs and also available in the field, only a few of them were also utilized by P. sayana. The majority (93%) of shells utilized by P. sayana also hosted other symbiont species, constituting the basis of extensive symbiotic complexes. Thus, the ecological relationship between D. insignis and P. sayana may be classified as a non-obligate and non-specific symbiosis that may also involve other facultative organisms such as sea anemones.     

Are the preference and selection patterns of hermit crabs for gastropod shells species- or site-specific?

Experimental analyses of hermit crabs and their preferences for shells are essential to understand the intrinsic relationship of the crabs' dependence on shells, and may be useful to explain their shell use pattern in nature. The aim of this study was to evaluate the effect of crab species and site on the pattern of shell use, selection, and preference in the south-western Atlantic hermit crabs Pagurus brevidactylus and Pagurus criniticornis, comparing sympatric and allopatric populations. Differently from the traditional approach to evaluate shell preference by simply determining the shell selection pattern (i.e., the number of shells of each type selected), preference was defined (according to [Liszka, D., Underwood, A.J., 1990. An experimental design to determine preferences for gastropod shells by a hermit-crab. J. Exp. Mar. Biol. Ecol., 137(1), 47–62]) by the comparison of the number of crabs changing for a particular shell type when three options were given (Cerithium atratum, Morula nodulosa, and Tegula viridula) with the number of crabs changing for this same type when only this type was offered. The effect of crab species was tested at Cabelo Gordo Beach, where P. brevidactylus was found occupying shells of C. atratum, M. nodulosa, and T. viridula in similar frequencies, whereas P. criniticornis occupied predominantly shells of C. atratum. In laboratory experiments the selection patterns of the two hermit-crab species for these three gastropods were different, with P. criniticornis selecting mainly shells of C. atratum, and P. brevidactylus selecting more shells of M. nodulosa. The shell preference was also dependent on crab species, with P. criniticornis showing a clear preference for shells of C. atratum, whereas P. brevidactylus did not show a preference for any of the tested shells. The effect of site was tested for the two species comparing data from Cabelo Gordo to Preta (P. brevidactylus) and Araçá beaches (P. criniticornis). The pattern of shell use, selection, and preference was demonstrated to be dependent on site only for P. brevidactylus. The results also showed that the shell use pattern of P. criniticornis can be explained by its preference at both sites, whereas for P. brevidactylus it occurred only at Cabelo Gordo, where the absence of preference was correlated with the similar use of the three gastropod species studied. Finally, the results showed that the shell selection pattern cannot be considered as a measure of shell preference, since it overestimates crab selectivity.     

Shell recruitment in the Mediterranean hermit crab Clibanarius erythropus

Gastropod shells are vital for the majority of hermit crab species, being essential for their survival, growth, protection, and reproduction. Given their importance, shells are acquired and transferred between crabs through several modalities. We conducted observations and experiments at the Asinara Island (Sardinia, Italy) to investigate the efficacy of the different behavioral tactics adopted by the hermit crab Clibanarius erythropus to acquire shells, such as: (1) locomotion and activity at different tidal phases; (2) attendance at shell-supplying sites (simulated predation sites with five different odors: live and dead gastropods, live and dead crabs, predator); and (3) interactions with conspecifics in aggregations on simulated gastropod predation sites. In each tidal phase, locomotion was slow (0.7 cm min^− 1) and, as a consequence, the probability of encountering empty shells and conspecifics was low. Simulated gastropod predation sites quickly attracted a larger number of hermit crabs than the other sites tested. Aggregations seemed to function as shell exchange markets, as previously suggested for other species: the first attendant took the experimental shell and a chain of shell exchanges among conspecifics followed. Our results show that, in C. erythropus, aggregation is the most efficient tactic for the acquisition of new shells, whereas in other species, such as Pagurus longicarpus, it is associated with exploitation ability due to the intense locomotion. The interspecific plasticity in hermit crabs' behavior is confirmed.     

Symbiosis of sea anemones and hermit crabs: different resource utilization patterns in the Aegean Sea

The small-scale distribution and resource utilization patterns of hermit crabs living in symbiosis with sea anemones were investigated in the Aegean Sea. Four hermit crab species, occupying shells of nine gastropod species, were found in symbiosis with the sea anemone Calliactis parasitica. Shell resource utilization patterns varied among hermit crabs, with Dardanus species utilizing a wide variety of shells. The size structure of hermit crab populations also affected shell resource utilization, with small-sized individuals inhabiting a larger variety of shells. Sea anemone utilization patterns varied both among hermit crab species and among residence shells, with larger crabs and shells hosting an increased abundance and biomass of C. parasitica. The examined biometric relationships suggested that small-sized crabs carry, proportionally to their weight, heavier shells and increased anemone biomass than larger ones. Exceptions to the above patterns are related either to local resource availability or to other environmental factors.     

Influence of prior experience on shell selection by the white spotwrist hermit crab Pagurus criniticornis (Crustacea: Paguridae)

Individuals of Pagurus criniticornis in a free-choice situation were experimentally tested under different laboratory conditions. In order to assess the effect of recently occupied shells on the size- and type-preference by hermit crabs, individuals were held for 30 days under one of the following two conditions: (1) excess of shells and (2) absence of shells. The crabs were then allowed to select shells from a wide array of empty gastropod shells of the two most-occupied species, as observed previously in the field: Cerithium atratum and Morula nodulosa. Preferred shell type (species) and size (shell aperture width and length) were correlated with hermit-crab size. The crabs showed a strong (100%) preference for C. atratum shells, demonstrating that recent and past experience did not influence either shell-type or shell-size preferences in this pagurid. Handling editor: K. Martens     

An analysis of competitive interactions between 3 hermit crab species

Summary Competition for empty gastropod shells in a group of three sympatric hermit crabs (Pagurus hirsutiusculus, Pagurus granosimanus, and Pagurus beringanus) was studied in the San Juan Archipelago, Washington State. Estimates of the competitive effects of each species on the others' shell supplies were derived using field data on shell utilization and the results of laboratory experiments to determine rates of acquisition and exchange of shells and preferences for different shell species. Each species experienced approximately an order of magnitude more intraspecific competition than interspecific competition for empty shells. This resulted from differences in preference for shell shapes, shell size use, and habitat use between P. hirsutiusculus and P. granosimanus, and largely from differences in habitat use between P. beringanus and the other two species. Experiments involving the release and recensusing of marked empty shells were used to estimate competitive effects more directly for the interaction between P. hirsutiusculus and P. granosimanus. Results were consistent with the estimates derived from data on resource partitioning. Possible causes of the low levels of interspecific competition are discussed, and results are compared with studies of other organisms that estimated both inter- and intra-specific competition.     

Positive abundance and negative distribution effects of a gastropod on an intertidal hermit crab

Summary Field experiments were used to determine the effect of a common intertidal snail (Nerita funiculata) on the use of space for foraging by the hermit crab Clibanarius digueti. Removals of Nerita resulted in an increased density of foraging Clibanarius, while additions of the gastropod had the opposite effect. The observed negative effect of the gastropod on individual hermit crabs appears to be food-related. Field surveys, however, suggested that the hermit crab population is limited by shell number, rather than food. Because Nerita contributes to the shell resource, its effect on the hermit crab population is positive. Nerita, therefore, has a negative effect on the distribution of foraging hermit crabs, but a positive effect on their abundance. Such decouplings of distribution and abundance effects are rare.     

Quality of shells occupied by the hermit crabPagurus geminus: How many hermit crabs are satisfied with their shells?

The rate of changing shells in the hermit crabPagurus geminus was investigated to determine how many hermit crabs are satisfied with their shells. Animal collected from the coast of Oya, Tanabe, Wakayama Prefecture, were presented with fresh gastropod shells newly made by removing the soft parts. Approximately 80% of the hermit crabs changed from their original shells acquired in the natural habitat into fresh shells experimentally given to them and, thus, were regarded as dissatisfied with the shells possessed in the natural habitat. This condition was thought to lead to the fact that hermit crabs occasionally attempted to exchange shells between 2 individuals and even to attack living snails in the natural habitat.     

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.     

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.     

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.     

Predator discrimination in the hermit crab Calcinus californiensis: tight for shell breakers,loose for shell peelers

Prey exposed to predators with different hunting and feeding modes are under different selective pressures, therefore it is expected that they should exhibit plastic and adaptive antipredator responses according to current risks. The hermit crab Calcinus californiensis faces two contrasting predators, the shell peeler Arenaeus mexicanus that hunts by active searching and the shell breaker Eriphia squamata that hunts by ambush. In order to discover whether C. californiensis displays plastic responses depending on the type of predatory challenge, we examined the shell size preference, the hiding time, and the escape velocity of hermit crabs in the presence of chemical cues from a shell peeler, a shell breaker, and a control. We also examined the role of shell fit on the escape velocity of the hermit crabs in natural tidal pools. Crabs chose shells with a loose fit (relatively large shells) in the presence of chemical cues from the shell peeler Arenaeus and shells with a tight fit when exposed to cues from the shell breaker Eriphia. The hermit crabs hid for shorter times and moved away faster from Eriphia than from Arenaeus stimulus. The use of a tight shell favours faster movement away from the shell breaker (pre‐capture strategy), but prevents the crab retracting deeper inside the shell, increasing the risk of be eaten by the shell peeler once captured. Hence, the use of loose shells that protect the crab from the shell peeler hinders fast escape. This study shows specific and plastic antipredatory responses to contrasting predators, each bringing adaptive benefits at different levels of the predator sequence.