Reproductive costs of the symbiotic hydroid Hydractinia symbiolongicarpus (Buss and Yund) to its host hermit crab Pagurus longicarpus (Say)

Studies on the interaction between the hermit crab Pagurus longicarpus and its symbiotic hydroid Hydractinia symbiolongicarpus have focused on positive effects of hydroids on their host hermit crabs (e.g., protection from predators). Yet, these benefits may be balanced with reproductive costs, which are rarely studied. Results from field observations, laboratory trials, and a mesocosm experiment indicate that female hermit crabs in hydroid-colonized shells exhibit depressed ovigery, smaller clutch sizes, and increased clutch failure relative to females in bare shells. Frequent switching between bare and hydroid-colonized shells may alleviate negative effects when the density of hydroids in the environment is low, but at high densities Hydractinia may significantly impact hermit crab reproduction.     

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

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

A forced association between the slippersnail Crepidula convexa and the hermit crab Pagurus longicarpus?—possible influence from a third party

The suspension-feeding slippersnail Crepidula convexa is commonly associated with hermit crabs (Pagurus longicarpus) living in periwinkle shells (Littorina littorea) at our study site in Nahant, MA, USA. In 15 field surveys conducted at Nahant in 2000, 2001 and 2003, we found that (1) more than 61.8% of individuals of C. convexa resided on shells occupied by hermit crabs, as opposed to the shells of live periwinkles, empty periwinkle shells or other solid substrates; (2) an average of 8.3% of hermit crabs carried at least one individual of C. convexa; and (3) 39.1-75.0% of hermit crabs carrying C. convexa were carrying “large” individuals (snails with wet weight >10% of the weight of the periwinkle shells they occupied). However, it is unlikely that individuals of C. convexa seek out shells occupied by hermit crabs to colonize, and they showed no preference for empty periwinkle shells over other solid substrates in the laboratory. Moreover, in the laboratory the hermit crabs preferentially occupied intact shells bearing individuals of C. convexa only when the alternatives were shells that had been drilled by naticid snails. Thus, neither party preferentially associates with the other: rather, extensive predation by naticid snails on periwinkles at Nahant appears to limit the availability of suitable shells for the hermit crabs, forcing them to inhabit shells bearing “large” individuals of C. convexa. Individuals of C. convexa may benefit from this inadvertent association with hermit crabs: by facilitating snail dispersal, transport by hermit crabs should reduce the potential for inbreeding, an important consideration for a species that lacks free-living larvae in its life history.     

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.     

The SW Atlantic burrowing crab Chasmagnathus granulatus Dana affects the distribution and survival of the fiddler crab Uca uruguayensis Nobili

In this paper, we address the question of whether the presence of the burrowing crab Chasmagnathus granulatus affects the habitat use of the fiddler crab Uca uruguayensis. Field samples showed that the species have a disjoint spatial distribution. Male fiddler crab density decreased in zones with C. granulatus, however, female density increased. Male fiddler crabs avoided feeding on sediment affected by C. granulatus and were more preyed. Predation was higher during the fiddler crab reproductive season and, probably due to predation risk, males showed lower reproductive display in shared zones. Field experiments shows that when C. granulatus were excluded, densities of U. uruguayensis increased mainly due to an increase in density of males. Habitat differentiation of these species may be because C. granulatus affects U. uruguayensis in several ways, including direct predation, disturbance and behavioural changes associated to predation risk. Males and females are affected differentially probably because of the extreme sexual dimorphism of this crab species. Coloration on enlarged claw and waving activities are all factors that increase predation risk for male and the presence of only one feeding claw may increase sediment-mediated effects.     

Shell shape affects movement patterns and microhabitat distribution in the hermit crabs Calcinus elegans, C. laevimanus and C. latens

This study examined the influence of shell shape on the distribution and movement patterns of three species of Hawaiian hermit crabs: Calcinus elegans, C. laevimanus, and C. latens. Field surveys showed strong differences in shell use depending on habitat. Individuals of C.elegans and C. latens were more frequently in unusual shapes of shells (the cowrie Cypraea caputserpentis and the variable worm shell Serpulorbis variabilis) when in tide pools and in more standard gastropod shells, such as the dog whelk Nassarius papillosus, when found in the subtidal. In addition, for both C.elegans and C. latens in tide pools, most crabs in unusual shaped shells were out on top of rocks, whereas most crabs in shells that were standard shapes were under rocks.In the laboratory, individuals of C.elegans and C. laevimanus in unusual shells initiated more shell exchanges and when given empty shells crabs readily occupied the standard shaped shells, but crabs did not move into the unusual shaped shells. Mark-recapture experiments in the field showed that C. elegans in standard shaped shells moved out of tide pools and stayed longer when placed on subtidal coral heads, whereas crabs in unusual shaped shells stayed in tide pools and did not stay on subtidal coral heads (in part due to predation). Laboratory tests showed that C. elegans in unusual shaped shells were more readily dislodged by surge than crabs in standard shaped shells. Thus, the difference in movement patterns in preferred vs. unpreferred shell shapes is an important factor influencing the microhabitat distribution of these hermit crabs.     

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.     

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.     

The influence of some symbionts on the shell-selection behaviour of the hermit crabs, Pagurus pollicarus and Pagurus longicarpus

The influence of some symbionts on the shell-selection by the hermit crabs Pagurus pollicarus and P. longicarpus was examined by placing individual hermit crabs with two similar shells in a choice situation and recording the shell occupied after 12 hr. One shell contained a symbiont species and the other did not. The results indicated that organisms normally found on or in empty shells influence the shell-section by these species of hermit crab. P. pollicarus preferred shells occupied by the sea anemone Calliactis tricolor or by the hydroid Hydractina echinata as opposed to bare shells. P. longicarpus also preferred shells with H. echinata. Both crab species rejected shells with the barnacle Balanus amphitrite. Shells containing the molluscs Crepidula fornicata or C. plana were rejected by the smaller hermit crab P. longicarpus. These molluscs appeared to exert no influence on P. pollicarpus unless they were large or abundant, at which point their weight or occlusion of available space possibly has negative effects on the crab.     

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.     

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.     

Hermit crab biocoenoses: a worldwide review of the diversity and natural history of hermit crab associates

The symbiotic associates of hermit crabs (excluding parasites and flora) are reviewed worldwide. The review includes species found on the shells occupied by hermit crabs (epibiotic species), species boring into these shells (endolithic species), species living within the lumen of the shell (either free-living or attached to the shell), species attached to the hermit crabs themselves, and hypersymbionts. In total over 550 invertebrates, from 16 phyla are found associated with over 180 species of hermit crabs. Among these associates, 114 appear to be obligate commensals of hermit crabs, 215 are facultative commensals, and 232 are incidental associates. The taxa exhibiting the highest number of associates are arthropods (126), polychaetes (105), and cnidarians (100). The communities of species associated with Dardanus arrosor, Paguristes eremita, Pagurus bernhardus, Pagurus cuanensis, and Pagurus longicarpus are the best studied and harbor the most diverse assemblages of species. While trends in biodiversity of hermit crab assemblages do not follow predicted patterns (e.g., hermit crabs within the Indo-West Pacific do not harbor more species than those from temperate regions), this is suggested to reflect a lack of sampling rather than a true representation of the number of associates. Hermit crabs date to at least the Cretaceous and provided a niche for a number of groups (e.g., hydractinians, bryozoans, polydorids), which were already associates of living gastropods. Apparently hermit crab shells initially supplied a substrate for settlement and then these symbiotic relationships were reinforced by enhanced feeding of symbionts through the activity of the hosts. Through their use and recycling of gastropods shells, hermit crabs are important allogenic ecosystem engineers in marine habitats from the intertidal to the deep sea. Hermit crabs benefit from some symbionts, particularly cnidarians and bryozoans, through extension of shell apertures (alleviating need to switch into new shells) and by providing protection from predators. However, hermit crabs are also negatively impacted (e.g., decreased reproductive success, increased predation) by some symbionts and a review of egg predators is provided. Thus, the symbiotic relationships between hermit crabs and many associates are difficult to characterize and often exhibit temporal changes depending on environmental and biological factors. Research on the biology of these symbionts and the costs/benefits of their associations with hermit crabs are analyzed. While some associates (e.g., Hydractinia spp.) have been studied in considerable detail, for most associations little is known in terms of the impacts of symbionts on hosts, and future experimental studies on the multitude of relationships are suggested.     

Predation on symbiont sea anemones by their host hermit crab Dardanus pedunculatus

Feeding by host hermit crabs Dardanus pedunculatus on their symbiotic sea anemones Calliactis polypus was investigated using animals collected at Shirahama, Wakayama Prefecture, Japan. In the first experiment, changes in the number of sea anemones on hermit crab shells were recorded in single‐and double‐crab trials without food and single‐crab trials with food. The number of sea anemones significantly decreased under starved conditions. The extent of this decrease per single hermit crab was higher in the double‐crab trials than in the single‐crab trials. Direct observations and video recordings showed that hermit crabs occasionally removed sea anemones from their own shells, and also from partners’ shells in the double‐crab trials, and consumed them. In the second experiment, fed and unfed hermit crabs with or without sea anemones were examined for body weight changes. Fed hermit crabs gained weight whereas unfed hermit crabs lost it. The degree of weight loss in unfed hermit crabs was significantly higher in those without sea anemones, which indicates some value of the latter as food. We offer some speculations on the course of development of this symbiosis, with predation on sea anemones having played an important initial role.     

Differential seed and seedling predation by crabs: impacts on tropical coastal forest composition

Recently, the importance of seed predation by crabs on mangrove species distributions and densities has been established by several studies. In a tropical coastal terrestrial forest in Costa Rica, we investigated the relative importance of predation by land crabs, Gecarcinus quadratus, and hermit crabs, Coenobita compressus, on measured forest composition through a series of seed removal and seedling establishment experiments. We also used natural light-gaps and adjacent non-gap sites to test how canopy cover affects crab predation (seed removal) and seedling establishment. We found fewer tree species (S=18) and lower densities (seedlings, saplings, and adults) in the coastal zone within 100 m of coastline, than in the inland zone (S=59). Land crab densities were higher in the coastal zone (3.03±1.44 crabs m^–2) than in the inland zone (0.76±0.78 crabs m^–2), and hermit crabs were not present in the inland zone. Seed removal and seedling mortality also were higher in the coastal zone than in the inland zone, and in the open controls than in the crab exclosures. Mortality of seeds and seedlings was two to six times higher in the controls than exclosures for four of the five experiments. Crabs preferred seeds and younger seedlings over older seedlings but showed no species preferences in the seed (Anacardium excelsum, Enterolobium cyclocarpum, and Terminalia oblonga) and seedling (Pachira quinata and E. cyclocarpum) stages. We conclude that the observed differences in tree densities were caused by differential crab predation pressure along the coastal gradient, while the differences in species composition were due to predator escape (satiation) by seed quantity. Canopy cover did not affect seed removal rates, but did affect seedling survival with higher mortality in the non-gap versus gap environments. In summary, crab predation of seeds and seedlings, and secondarily canopy cover, are important factors affecting tree establishment in terrestrial coastal forests.     

Effects of substrate on interactions between juvenile sea scallops (Placopecten magellanicus Gmelin) and predatory sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say)

We investigated the effect of substrate (glass bottom, sand, granule, pebble) on predation of juvenile sea scallops (Placopecten magellanicus) by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) at two prey sizes (11-15 mm and 24-28 mm shell height), and two prey densities (10 and 30 scallops per aquarium) in laboratory experiments. Specifically, we quantified predation rate and underlying behaviours (proportion of time a predator spent searching for and handling prey, encounter rate between predators and prey, and various outcomes of encounters). We detected a significant gradual effect of particle size of natural substrates on sea star predation: specifically, predation rate on and encounter rate with small scallops tended to decrease with increasing particle size (being highest for sand, intermediate for granule, and lowest for pebble). Substrate type did not significantly affect predation rates or behaviours of sea stars preying on large scallops or of rock crabs preying on either scallop size classes. Other factors, such as prey size and density, were important in the scallop-sea star and scallop-rock crab systems. For example, predation rate by sea stars and crabs and certain sea star behaviours (e.g. probability of consuming scallops upon capture) were significantly higher with small scallops than with large scallops. As well, in interactions between small scallops and sea stars, predation rate and encounter rate increased with prey density, and the proportion of time sea stars spent searching was higher at low prey density than high prey density. Thus, substrate type may be a minor factor determining predation risk of seeded scallops during enhancement operations; prey size and prey density may play a more important role. However, substrate type still needs to be considered when choosing a site for scallop enhancement, as it may affect other scallop behaviours (such as movement).     

Predator-prey dynamics between recently established stone crabs (Menippe spp.) and oyster prey (Crassostrea virginica)

Range expansion and population establishment of individual species can have significant impacts on previously established food webs and predator-prey dynamics. The stone crab (Menippe spp.) is found throughout southwestern North Atlantic waters, from North Carolina through the Gulf of Mexico and the Central American Caribbean, including the Greater Antilles. Recent observations suggest that stone crabs have become better established on certain oyster reefs in North Carolina than in the early 1900s when they we first observed in NC. To assess the predatory impact of stone crabs on oysters, we (1) quantified stone crab densities on subtidal oyster reefs in Pamlico Sound, NC using scuba surveys, and (2) conducted laboratory predation experiments to assess the functional response of stone crabs to varying densities of oysters. We then (3) analyzed previously unpublished functional response data on another important oyster predator, the mud crab Panopeus herbstii. Finally, we (4) compared and contrasted potential predatory impacts of stone, mud and blue crabs (Callinectes sapidus). The functional response data and analyses for both stone crabs and mud crabs were consistent with a type II functional response. Mud crabs, on a m² basis, inflicted the highest proportional mortality on oysters over a 24 hour period, followed by stone and then blue crabs. Proportional mortality did not vary significantly with oyster size; however, relatively small and large oysters were consumed disproportionately less than medium-sized oysters, likely due to the mechanical inability of stone crabs to handle small oysters, and the inability to crush large oysters. Although stone crabs appear to be established in Pamlico Sound at densities equivalent to densities in other systems such as the U.S. Florida Panhandle, their predatory activities on oysters are not expected to have as significant a negative impact on oyster populations compared to other resident predators such as mud crabs.     

红眼寄居蟹在实验室和野外条件下对贝壳的利用

占据适宜的贝壳对于寄居蟹的发育、繁殖和存活至关重要。尽管很多研究探讨了蟹类对贝壳的选择,但寄居蟹选择多大的贝壳仍不清楚。在实验条件下,本文作者用巴西Anchieta岛上野生红眼寄居蟹( Pagurusbrevidactylus)最常利用的黑衣蟹守螺( Cerithium atratum)和节桑椹螺( Morula nodulosa)进行了贝壳选择实验,通过回归分析确定目标贝类及其大小。观察到寄居蟹对一种独特贝类的选择具有性别意义,从而验证了野外观察结果。雄性明显地倾向选择黑衣蟹守螺的贝壳,而产卵和非产卵的雌性个体对腹足类贝壳的选择差异不显著。尽管两性之间对适宜贝壳的选择存在差异,贝壳适宜度指数(SAI)表明,种群占据那些足够大的贝壳(SAI =1·20±0·23)。红眼寄居蟹对贝壳利用的这种模式可能是为了避免与体型相似同域物种的竞争,从而在后来的生长中减少频繁地更换贝壳。根据目前的数据可以得到以下结论:红眼寄居蟹对贝壳的选择不仅取决于贝壳的参数,而且还与寄居蟹的个体和性选择有关[动物学报51 (5) : 813 -820 , 2005]。     

Interference and exploitation components in interespecific competition between sympatric intertidal hermit crabs

This study was designed to evaluate the effect of interference and exploitation competition in shell partitioning between two hermit crab species (Pagurus criniticornis and Clibanarius antillensis). Field samples revealed that shells of the gastropod Cerithium atratum were the main resource used by both hermit crab species and that Pagurus used eroded or damaged shells in higher frequency than Clibanarius. The exploitative ability of each species was compared between species in the laboratory using dead gastropod (Cerithium) baits to simulate predation events and signalize newly available shells to hermit crabs. Pagurus reached the baits more rapidly than Clibanarius, but this higher exploitative ability did not explain shell utilization patterns in nature. Another experiment evaluated the dominance hierarchy between these two hermit crab species and revealed that Clibanarius was able to outcompete Pagurus for higher quality shells in agonistic encounters. This higher interference competitive ability of Clibanarius in relation to Pagurus may explain field observations. Nevertheless, Pagurus may be responsible to enhance shell availability to other hermit crab species that have lower ability to find and use newly available shells. Differently, the poorer condition of shells used by Pagurus, the higher ability of this species to attend gastropod predation events and its higher consumption rate by shell-breaking crabs (Menippe nodifrons) may increase its predation risks, thus revealing the disadvantages of such an exploitative competitive strategy for hermit crabs.     

Shell use by juvenile fiddler crabs Uca pugnax and U. pugilator

The present study documents for the first time shell use by juvenile fiddler crabs in the salt marsh. Twenty visits were made to six salt marsh sites at Tybee Island, Georgia between 2007 and 2009. One hundred empty Littorina irrorata shells were collected at each site on each field trip. Juvenile carapace width was measured, crabs sexed, and species identification completed using RFLP analysis. Shell use of up to 79% was observed. Two species of fiddler crabs were found in empty shells, Uca pugnax and U. pugilator. U. pugnax was the dominant species at all sites representing 62-84% of the juvenile fiddler crab population. Juvenile sex ratios were female-biased (1.7:1) at all six sites. Juvenile size did not vary significantly between species but males of both species were significantly larger than females. Size frequency distribution of carapace width revealed that shell use varied with size and sex. In the 3 to 4 mm size class, juvenile females outnumbered juvenile males in empty L. irrorata shells while in the 5 to 6 mm size class and greater, juvenile males outnumbered juvenile females in shells. Significantly more juvenile fiddler crabs were found in empty shells during flood than ebb tide at 3 of the sites. This discovery illuminates the resourcefulness of juvenile fiddler crabs and provides another mechanism that might enhance survival.