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.     

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.     

Crab: snail size-structured interactions and salt marsh predation gradients

We studied size-structured predator-prey interactions between blue crabs (Callinectes sapidus) and marsh periwinkles (Littoraria irrorata) with a combination of field studies, laboratory experiments and individual-based modeling. Size distributions of Littoraria differed among years at the same sites in a salt marsh and could largely be explained by dominance of strong cohorts in the population. At a given site, abundance increased with elevation above tidal datum. Size-selective predation by blue crabs does not appear to be an important regulator of snail size distributions but may have a major effect on local abundance. Laboratory studies indicated that predator-prey interactions between Callinectes and Littoraria are strongly size-dependent. Crabs were generally effective at feeding on periwinkles at size ratios greater than approximately 6 (crab width: snail length). At lower size ratios crabs were far less effective at manipulating the snails, which often survived but with damaged shells. An individual-based model which incorporated information about incidence of snail shell scarring (resulting from non-lethal interactions) and snail density, predicted reduced predation rates and smaller average crab size with distance from the low tide refugium for crabs.     

Shell occupation as a limiting factor for Pagurus brevidactylus (Stimpson, 1859) in the Marine State Park of Laje de Santos,Brazil

Hermit crabs are soft-abdomen decapods. They depend upon gastropod shells for protection, with the latter influencing growth rate, mortality and reproduction. The present work aims to provide information about the biology and reproduction of Pagurus brevidactylus in a conservation area in Brazilian southeastern littoral. The material was sampled through active searching by SCUBA diving and refuge traps. We analyzed population structure, fecundity, reproductive output (RO), reproductive periodicity and shell utilization for P. brevidactylus. In total, 54 males, 18 non-ovigerous females, 34 ovigerous females and 10 juveniles were sampled. The females had a fecundity of 69.5 ± 65 eggs, which was lower than that compared to other populations from different localities. The RO ranged from 1.48% to 63.04%, with a mean of 26.75 ± 18.23%. There was a positive relationship between egg number and female size. The animals had continuous reproduction, but they reached smaller maximum sizes when compared with other populations. The shell occupation pattern was also different from other localities, the shells of Phrontis alba being the most occupied; this shell has a smaller size when compared to others that P. brevidactylus occupies in other localities. Therefore, P. alba could have had limited the growth and reproduction of P. brevidactylus.     

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]。     

Shell preference and utilization patterns in littoral hermit crabs of the bay of Panama

Shell utilization patterns of three sympatric hermit crab species from the Bay of Panama are examined. Shell preferences, as shown by laboratory choice experiments and the selective use of empty shells experimentally added to hermit crab populations, are shown to be important determinants of shell utilization under natural conditions.Factors which influence the types and sizes of shells occupied by hermit crabs in separate populations include: (1) the presence and relative abundance of different gastropod species; (2) the specific shell preferences of different hermit crab species; and (3) the presence and relative abundance of sympatric hermit crab competitors for the limited supply of empty shells. Since the size and type of shell occupied by a hermit crab influences its growth rate and reproductive output, these factors appear to have a direct effect on hermit crab fitness and the demographic structure of separate hermit crab populations.     

GEOGRAPHIC VARIATION IN REPRODUCTIVE BEHAVIOR AND SIZE STRUCTURE OF THE AUSTRALIAN CYCAD MACROZAMIA COMMUNIS (ZAMIACEAE)

Macrozamia communis is a common cycad that occurs in eastern New South Wales, Australia, where it forms an understory in various forest or woodland communities. Leaf number of reproductive individuals as well as of nonreproductive ones, taken as an indication of size class, varies clinally from north to south, with reproductive individuals in southern populations having mean leaf numbers about 12 times greater than those at the northern end of the range of this species. Female cone size and mature ovule weight appear to vary also along this gradient. Using leaf number as a predictor of sexual maturity, from 37 to 100% of the individuals present in a population were judged to be mature. The percent of mature individuals participating in the 1987 coning episode ranged from none in three southern populations to 68% elsewhere. In 9 of 10 samples more males than females participated in the 1987 coning episode, with a collective sex ratio slightly male biased for all populations sampled. In three population samples average cone numbers for male and female plants were equal. In two samples males produced more cones per plant than females. Leaf numbers of males and females in individual populations were equal. Few individuals producing cones in 1987 showed evidence of recent prior cone production, suggesting that successive cone production by both sexes is separated by an interval of several years. The periodicity with which fire is used as a fire suppression method may affect the frequency of cone production in some populations, perhaps preventing it entirely. The apparent absence of recruitment in some populations may result from long-term seed collection by humans for horticultural purposes as well as the very poor seed dispersal that appears to characterize this cycad throughout its range. The weevil Tranes internatus was observed occupying the internal tissues of female cones; this insect may be involved in the pollination of M. communis.     

Unusual allometry for sexual size dimorphism in a cichlid where males are extremely larger than females

When males are the larger sex, a positive allometric relationship between male and female sizes is often found across populations of a single species (i.e. Rensch’s rule). This pattern is typically explained by a sexual selection pressure on males. Here, we report that the allometric relationship was negative across populations of a shell-brooding cichlid fish Lamprologus callipterus, although males are extremely larger than females. Male L. callipterus collect and defend empty snail shells in each of which a female breeds. We found that, across six populations, male and female sizes are positively correlated with not only sexual and fecundity selection indices, but also with shell sizes. Given their different reproductive behaviours, these correlations mean that males are required to be more powerful, and thus larger, to transport larger shells, while female bodies are reduced to the shell size to enable them to enter the shells. Among the three size selections (sexual selection, fecundity selection and shell size), shell size explained the allometry, suggesting that females are more strongly subject to size selection associated with shell size availability than males. However, the allometry was violated when considering an additional population where size-selection regimes of males differed from that of other populations. Therefore, sexual size allometry will be violated by body size divergence induced by multiple selection regimes.     

Factors Influencing Shell Investigation in the Hermit Crab,Pagurus bernhardus

Laboratory experiments in which littoral specimens of Pagurus bernhardus were offered shells with blocked apertures showed that animals occupying poor quality shells approached and contacted new shells more quickly, spent longer investigating them, and spent a greater proportion of that time in active exploration of them than did animals in better shells. Animals spent less time investigating shells smaller than the optimal size than they did good quality shells, even though none of the shells could be occupied. Crabs spent less time investigating a new shell when a visible stimulus (a stone, a shell or another crab) was present. Shell assessment was thus influenced by the quality of the crab's own shell, the quality of the offered shell and the presence of other features in the animal's environment.     

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.     

Shell occupancy of the intertidal hermit crab Clibanarius erythropus (Decapoda,Diogenidae) on São Miguel (Azores)

The intertidal hermit crab Clibanarius erythropus was collected at three sites on São Miguel (Azores) during low spring tides. Shells occupied were identified and measured. Crab sizes ranged from 1.78 to 13.67 mm (cephalothoracic shield length), with an average size of 4.40 ± 1.44 mm. Of the 19 different shells utilised, the most frequent were Littorina striata (23.8%), Nassarius incrassatus (22.5%) and Mitra sp. (22.0%). At Fenais da Luz, L. striata was most frequently occupied, while at Água de Alto it was N. incrassatus and, at Caloura, Mitra sp. shells were most frequently used. Shell selection appears to be determined by respective sizes of hermit crab and shell species. Small size-class crabs occupy more shell species than larger crabs. The smallest crab was found at Fenais da Luz occupying a small Bittium sp., whereas the largest crab was found at Caloura inhabiting Stramonita haemastoma.     

Morphological convergence during pregnancy among predator and nonpredator populations of the livebearing fish Brachyrhaphis rhabdophora (Teleostei: Poeciliidae)

Predation can drive morphological divergence in prey populations, although examples of divergent selection are typically limited to nonreproductive individuals. In livebearing females, shape often changes drastically during pregnancy, reducing speed and mobility and enhancing susceptibility to predation. In the present study, we document morphological divergence among populations of nonreproductive female livebearing fish (Brachyrhaphis rhabdophora) in predator and nonpredator environments. We then test the hypothesis that shape differences among nonreproductive females are maintained among reproductive females between predator and nonpredator environments. Nonreproductive females in predator environments had larger caudal regions and more fusiform bodies than females in nonpredator environments; traits that are associated with burst speed in fish. Shape differences were maintained in reproductive females, although the magnitude of this difference declined relative to nonreproductive females, suggesting morphological convergence during pregnancy. Phenotypic change vector analysis revealed that females in predator environments became more similar to females in nonpredator environments in the transition from nonreproductive to reproductive. Furthermore, the level of reproductive allocation affected shape similarly between predator environments. These results suggest a life‐history constraint on morphology, in which predator‐driven morphological divergence among nonreproductive B. rhabdophora is not maintained at the same level during pregnancy. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 386–392.     

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.     

Changes in the selection differential exerted on a marine snail during the ontogeny of a predatory shore crab

Empirical estimates of selection gradients caused by predators are common, yet no one has quantified how these estimates vary with predator ontogeny. We used logistic regression to investigate how selection on gastropod shell thickness changed with predator size. Only small and medium purple shore crabs (Hemigrapsus nudus) exerted a linear selection gradient for increased shell‐thickness within a single population of the intertidal snail (Littorina subrotundata). The shape of the fitness function for shell thickness was confirmed to be linear for small and medium crabs but was humped for large male crabs, suggesting no directional selection. A second experiment using two prey species to amplify shell thickness differences established that the selection differential on adult snails decreased linearly as crab size increased. We observed differences in size distribution and sex ratios among three natural shore crab populations that may cause spatial and temporal variation in predator‐mediated selection on local snail populations.     

Sex-specific differences and the role of predation in the interaction between the hermit crab, Pagurus longicarpus, and its epibiont, Hydractinia symbiolongicarpus

The interaction between the hermit crab, Pagurus longicarpus, and the shell epibiont, Hydractinia symbiolongicarpus, varies from mutualism to parasitism based on the environmental context. We tested the hypothesis that this interaction also varies as a function of hermit crab sex. Given that recent work showed a negative effect of Hydractinia on female reproduction, we predicted a greater frequency of males in Hydractinia shells in the field and a stronger preference by males than females for shells with Hydractinia. Field collections documented a significantly greater proportion of males than females occupying shells with Hydractinia, and a significantly greater proportion of ovigerous females than non-ovigerous females in shells with Hydractinia. In laboratory shell-switching experiments, a greater proportion of males than females chose to enter shells with Hydractinia, but there was no difference in the proportions of males and females that vacated shells with Hydractinia.We examined whether the presence of Hydractinia influenced predation rates. Blue crabs fed on more than twice as many hermit crabs in shells with Hydractinia as compared to bare shells, but there was no significant difference for stone crabs. Laboratory experiments showed that the force required to crush shells was significantly greater for shells without Hydractinia. Thus, the lower occupancy and preference exhibited by females than males for shells with Hydractinia appears to result both from the decreased reproduction shown in past studies and an increase in predation risk.     

The size and shape of shells used by hermit crabs: A multivariate analysis of Clibanarius erythropus

Shell attributes such as weight and shape affect the reproduction, growth, predator avoidance and behaviour of several hermit crab species. Although the importance of these attributes has been extensively investigated, it is still difficult to assess the relative role of size and shape. Multivariate techniques allow concise and efficient quantitative analysis of these multidimensional properties, and this paper aims to understand their role in determining patterns of hermit crab shell use. To this end, a multivariate approach based on a combination of size-unconstrained (shape) PCA and RDA ordination was used to model the biometrics of southern Mediterranean Clibanarius erythropus populations and their shells. Patterns of shell utilization and morphological gradients demonstrate that size is more important than shape, probably due to the limited availability of empty shells in the environment. The shape (e.g. the degree of shell elongation) and weight of inhabited shells vary considerably in both female and male crabs. However, these variations are clearly accounted for by crab biometrics in males only. On the basis of statistical evidence and findings from past studies, it is hypothesized that larger males of adequate size and strength have access to the larger, heavier and relatively more available shells of the globose Osilinus turbinatus, which cannot be used by average-sized males or by females investing energy in egg production. This greater availability allows larger males to select more suitable shapes.     

Chromatic Patterns of the Hermit Crab Calcinus tubularis Related to the Occupied Shell

Crustacea are known to develop different chromatic patterns due to many factors. Regarding decapods, chromatism was mainly studied in crabs, while very little is known about chromatic patterns in hermit crabs. Calcinus tubularis is a typical infralittoral rocky bottom hermit crab, studied for different aspects of its biology except chromatic variations. This paper aims at describing the different colour morphologies of C. tubularis, discussing hypothesis of why they develop, and testing if in nature the crab prefers a shell with a chromatic pattern similar to that of its body. One hundred and forty crabs were observed and filmed in the laboratory. They were subdivided into two groups, according to their chromatic pattern: 1) light and 2) dark crabs; the shells they occupied were also subdivided into the two groups of 1) light and 2) dark shells on the basis of the epibionts encrusting them. Observations of 129 crabs suggest that the colour depends neither on depth nor on size, intermoult period, diet, reproductive period but it might be connected to genetic factors and might help crab to camouflage. Camouflage is suggested by the fact that 79.3% of the total examined specimens occupy shells with a chromatic pattern resembling that of the crabs themselves. This phenomenon is significantly more recurrent in females than in males and could help the crabs to be cryptic, first with the occupied shell and secondly with the habitat (rocks encrusted by photophylous algae).     

Why small hermit crabs have large shells

Frequent shell exchanges among hermit crabs imply the enigmatic circumstance that large crabs frequently obtain large shells from smaller crabs. This seeming anomaly is explored as a key to the shell resource system. It is hypothesized to reflect how, where, and how often shells become available to the crabs. Shells become available infrequently, as snails die, and are available to the crabs for only a brief time before they become inaccessible. The standing crop of empty shells is almost always low and is irrelevant to rates of shell turnover in the crab population. Crabs are most likely to encounter shells of the wrong size, and the chance of encountering a shell of the desired size decreases as a crab grows. Snails and crabs are usually found on different portions of the shore; thus, crabs must make “foraging trips” for shells. Under this regime of shell supply, a crab will get a suitable shell the fastest when it accepts any fresh shell that is larger than its initial shell. It can then trade with other crabs to improve its shell fit. This behavior will make small crabs into a regular source of large shells for large crabs, and a shell exchange ritual will be strongly favored because both participants will benefit. Shells are an unusual resource because they are the object of both competitive and mutualistic interactions. This ambiguous quality is revealed in the intraspecific and interspecific responses of crabs to each other and to shells.     

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.     

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.