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

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

Effects of size upon salinity tolerance and volume regulation in the hermit crab Pagurus bernhardus

Whereas small Pagurus bernhardus (L.) occur in large numbers in the intertidal areas of rocky shores, large hermit crabs living in whelk shells are most uncommon, although they are easily collected by sublittoral dredging. Experiments were carried out to assess the salinity tolerance, and the ability to regulate body volume in a dilute medium, of both small and large hermit crabs. Large, offshore hermit crabs are significantly less tolerant of 60% sea water (100%=34 S) than small, littoral specimens. Small littoral crabs increase in weight by about 15% in the first hour in 60% sea water, but increased urine output prevents swelling thereafter; indeed, there is considerable loss in weight, presumably caused by solute loss. Large, offshore, crabs increase in weight by about 15% in the first hour, thus exhibiting a higher permeability to water than small crabs, and continue to increase in weight, since urine output is not sufficient to cope with the water load. Dissections suggest that insufficient release of urine causes the low urine output in large hermit crabs, rather than inadequate production of primary urine. Calculations indicate that the nephropores of large hermit crabs are too small to allow a sufficiently high urine output to remove a large water load, unless greater pressures are developed inside the excretory systems of large hermit crabs than occur in those of small littoral individuals. In view of the soft, flexible nature of the walls of the excretory system and abdomen of P. bernhardus, it seems unlikely that high pressures could be maintained within the excretory system. It is concluded that the stenohalinity of large, offshore hermit crabs may exclude them from the littoral zone, or may be the result of exclusion by other factors, since there will be no selective advantage for large hermit crabs in maintaining the degree of euryhalinity possessed by the small littoral crabs, if the large crabs are confined to the sublittoral zone.     

Obligate commensalism of <Emphasis Type="Italic">Curvemysella paula</Emphasis> (Bivalvia: Galeommatidae) with hermit crabs

Curvemysella paula is a markedly crescent-shaped bivalve that lives inside snail shells occupied by hermit crabs. Here, we describe the unique symbiotic life, growth pattern, and reproductive biology of this bivalve, based on specimens collected from the shallow, muddy bottom of the Seto Inland Sea, Japan. C. paula was found attached to columellae in the siphonal canal, mainly of nassariid snail shells occupied by two types of hermit crabs: Diogenes edwardsii (Diogenidae) and Spiropagurus spiriger (Paguridae). The crescent-shaped shell of C. paula is an adaptation to symbiotic life in the narrow interspace between the snail shell and the hermit-crab abdomen. C. paula is a protandric hermaphrodite. In our samples, each host snail shell harbored one (or rarely a few) large female and several males. All the female bivalves settled on the host shells with their anterior end facing outward and benefited from currents created by the hermit crab when feeding. In the muddy bottom, snail shells are a limited resource for both the hermit crabs and symbiotic bivalves. The bivalves benefit from the mobility of the hermit crabs, which prevent the shells from becoming buried in the mud. C. paula represents the only example of obligate commensalism with hermit crabs found in Bivalvia.     

Analysis of lipid components for determining the condition of anchovy larvae,Engraulis mordax

It has been presumed that intertidal spawning by Limulus polyphemus minimizes the loss of egges to subtidal predators; however, this strategy involves considerable risks. Massive beach strandings of adults accompany seasonal spawning migrations of crabs along Cape May in Delaware Bay, (USA). At least 190000 horseshoe crabs, approximating 10% of the adult population, died from beach stranding along the New Jersey shore of Delaware Bay during the 1986 (May to June) spawning season. Abnormalities of the telson (which is used in righting behavior) were significantly more common among stranded crabs than among individuals actively spawning on the intertidal beach. The number of stranded crabs per day was not correlated with tidal height or environmental variables (wind speed, wave height) which characterized the conditions at spawning. A complex suite of factors, including the size of the available spawning population, tidal and weather conditions, and beach slope, influence the number stranded during the breeding season. Horseshoe crab stranding results in a large loss of gravid females from the population, and may represent a major input of organic matter to intertidal sandy beaches in certain regions of Delaware Bay.     

Shell choice in<Emphasis Type="Italic"> Pagurus longicarpus</Emphasis> hermit crabs: does predation threat influence shell selection behavior?

Pagurus longicarpus hermit crabs depend on empty gastropod shells for protection against predation. Hermit crabs avoid gastropod shells in which holes have been drilled by naticid gastropods, and hermit crabs forced to occupy drilled shells are more vulnerable to predation by green crabs, Carcinus maenas. In this study, we examined the effect of predator cues on P. longicarpus shell investigation behavior and shell choice. In paired laboratory shell choice trials, we examined hermit crab response to green crab chemical cues. We compared hermit crabs from two sites differing in the percentage of Littorina littorea shells with drill holes. The percentage of time hermit crabs spent occupying intact shells increased significantly in the presence of predator cues. The effect of predator cues on the amount of time hermit crabs spent investigating shells differed between individuals from the two sites. Predator effluent had a marginal effect on the proportion of hermit crabs initially choosing intact shells and within 15 min most hermit crabs in both treatments occupied intact shells due to shell switching. These results indicate that predation cues alter P. longicarpus shell choice behavior favoring intact shells, which provide greater protection. In summary, predation appears to be a key factor influencing hermit crab shell selection behavior.Communicated by T. Czeschlik     

Utilization of partially predated snail shells by the hermit crab Pagurus longicarpus Say, 1817

The hermit crab Pagurus longicarpus was shown to inhabit shells that were partially predated from intertidal areas of Long Island, New York. Among field collections of P. longicarpus, 2.13% of the hermit crabs (46 of 2155) were found with shells with snail tissue present. Over 90% of these partially predated snail shells were occupied by male hermit crabs. Although hermit crabs were in 8 species of snail shells, only Littorina littorea and Nassarius obsoletus were found occupied by hermit crabs and containing snail tissue. In the laboratory, we found that specimens of the spider crab Libinia emarginata were able to pull off the operculum of snails, leaving damage as found in field collections. In contrast, specimens of P. longicarpus were not able to prey on live, healthy snails. When specimens of P. longicarpus were placed in communal tanks, hermit crabs preferred partially predated snail shells to empty and original shells. However, original shells and empty shells were occupied with more frequency than partially predated shells when crabs were isolated. These findings indicate P. longicarpus actively seeks shells soon after attack and abandonment by snail predators, especially in the presence of competitors.     

Variation in zonal recovery in four Talitrus saltator populations from different coastlines: a comparison of orientation in the field and in an experimental arena

Under stressful conditions (e.g. finding themselves on dry or moisture-saturated substrates) littoral talitrids (Crustacea, Amphipoda) demonstrate zonal orientation, in which they must promptly reach the optimal zone of the beach, the wet fringe near the shoreline. A relationship might therefore exist between the use of orientation and the frequency of such stressful conditions in the natural environment. Moreover, the efficiency of orientation toward the sea could be related to the possibility of using strategies other than zonal orientation in order to avoid stress. This study analysed the actual use and efficiency of orientation under natural conditions of four Talitrus saltator (Montagu, 1808) populations from Mediterranean and northern European Atlantic coasts with different ecological features. Orientation tests were carried out on the beach with all natural cues available. Then the same individuals underwent control experiments to study their sun orientation far from the sea in an experimental arena. The following results emerge from the comparison of the circular distributions: (1) marked differences among populations in the precision of zonal recovery under natural conditions; (2) a common solar orientation capacity in the control tests far from the sea; (3) different orientation choices of the same individuals according to the test conditions, natural or controlled. The habitat diversity of the four populations (amount, distribution and kind of detritus and wrack on the beach, degree of coastal erosion, orientation of the shoreline, human use of the beach) provides an ecological interpretation for the differences in orientation observed among populations. Received: 13 October 1997 / Accepted after revision: 26 April 1998     

Body and resource size at the land–sea interface

Body size in animals varies with many parameters, amongst them taxonomic affiliation, lifestyle and ambient environment oxygen levels. Size has considerable implication to possibilities for animals; for example, parasites need to be small and top predators large. Body size and resource requirements (shell size) were investigated across the land–sea interface in hermit crabs (Crustacea: Malacostraca: Decapoda) and snails (Mollusca: Gastropoda: Prosobranchia). These are two of the few taxa to occur in the sea, on the shore and on land as residents. Both taxa are also appropriate for such an analysis as they are abundant, speciose, cohabit the same environments and are linked—gastropod shells are a critical resource to hermit crabs. Both the maximum and mean sizes of hermit crab species showed parabolic relationships with shore height, decreasing from the sublittoral and supralittoral to the eulittoral. Average maximum size of gastropods exhibited a similar intertidal minimum although variability was high. It is suggested that this pattern is robust: not only did two distantly related taxa show the same pattern, but neither region nor site contributed significantly to total variability. The mass of resources (gastropod shells) used by hermit crabs, however, showed a converse pattern. The smallest shells (relative to hermit crab body size) were used in the sublittoral and supralittoral. Response to environmental stress and predation pressure are offered as two alternate theories to explain the observed body dwarfism and resource gigantism in the intertidal zone.Communicated by J.P. Thorpe, Port Erin     

Compression of home ranges in ghost crabs on sandy beaches impacted by vehicle traffic

Animal movement is a pivotal element of many ecological processes, and on ocean-exposed sandy shores, ghost crabs (genus Ocypode) undertake extensive nocturnal forays on the beach surface. Because crab populations are also threatened by vehicle traffic, indicators that can detect sublethal effects before population declines are manifest are important. To this end, we tested on a barrier island in Eastern Australia whether movement patterns of crabs respond predictably to disturbance by vehicles; this was done by tracking (using the spool-and-line technique) crabs at night in beach sections open and closed to traffic. Beach traffic not only halved population densities of crabs on the unvegetated beach seawards of the dunes, but it also fundamentally changed crab behaviour and movement: individuals from beach areas rutted by tyre tracks travelled shorter distances in a more erratic zigzag pattern, and they had significantly compressed home ranges. Such behavioural changes linked to human pressures could be well suited as an early warning signal for wider negative ecological impacts (as demonstrated by reduced abundances). They also emphasize the need to incorporate sublethal effects into the assessment and management of ecological changes resulting from beach recreation.     

Influence of occupant microhabitat on the composition of encrusting communities on gastropod shells

The exploitation of microhabitats is widely considered to increase biodiversity in marine ecosystems. Although intertidal hermit crabs and gastropods may inhabit the same shell type and shore level, their microhabitat may differ depending on the state of the tide. On the south coast of Wales the hermit crab Pagurus bernhardus mainly inhabits the shells of Nucella lapillus (84%). Hermit crab shells had a significantly different encrusting community compared with live N. lapillus shells. At low tide the live gastropods were found on exposed rock surfaces whereas hermit crabs were restricted to tidal pools. Communities encrusting live gastropod shells were characterised by lower species richness and abundance compared with shells inhabited by hermit crabs (12 species found in total). A greater abundance and richness of epibionts was recorded from both shell types during the summer compared with winter. Differences in community composition between shell occupant types were attributed to microhabitats used by gastropods and hermit crabs and the associated desiccation pressures, rather than competitive interactions or shell characteristics. This contradicts earlier studies of subtidal shells where biological processes were considered more important than physical factors in controlling species abundance and richness patterns. The use of rockpool microhabitats by hermit crabs increases the biodiversity of rocky shores, as some species commonly found on hermit-crab-inhabited shells were rare in other local habitats.Communicated by J.P. Thorpe, Port Erin     

Resource degradation: a subtle effect of bottom fishing

Populations of hermit crabs are critically limited by the availability of suitable gastropod shells that they utilise to reduce their risk of predation and environmental stress. Common whelks are the main source of shells for large hermit crabs in the northern Atlantic but are vulnerable to direct and indirect effects of fishing activity. This study examined the potential consequences of degrading shell resources for common hermit crabs. Laboratory trials demonstrated that hermit crabs avoid low-quality damaged shells throughout their life history. This laboratory preference was corroborated by direct field observations of shells preferentially occupied by hermit crabs, compared with shells available for occupation. In the field, 8 times as many empty shells had holes compared to shells occupied by hermit crabs. In the North Sea, the abundance and biomass of live whelks and hermit crabs collected at sites where they co-occurred were significantly related. However, whelks occurred at far fewer sites overall and were more patchily distributed at high abundance than hermit crabs, which were more widespread. At a subset of sites, whelks of the same body-mass range occurred in the Irish and North Sea. However, at these sites, hermit crabs sampled from the North Sea had a significantly lower biomass. This suggests that the shells available for occupation at the North Sea sites would not support crabs of a body mass comparable to that found in the Irish Sea. Using published data, we calculated that in some of the intensively fished areas of the North Sea, 24% of the available shell resource will be damaged each year. The reduction in shell quality in the North Sea may impose a physical constraint on the upper size limit currently attainable by hermit crabs and hence may have implications for population viability.Communicated by J. P. Thorpe, Port Erin     

Shell acquisition by hermit crabs: which tactic is more efficient?

Gastropod shells are limited resources for most hermit crab species, acting as primary factors in various aspects of their biology. To investigate the efficacy of different behavioral tactics adopted for their acquisition (locomotion, attendance at shell-supplying sites, interactions with conspecifics in aggregation) we conducted observations and experiments at a salt marsh in New England (USA). Locomotion, fast and meandering, significantly increased the chances of encountering empty shells and conspecifics. However, shell exchanges were rare. Simulated gastropod predation sites quickly attracted a larger number of hermit crabs than the other shell-supplying sites tested (live and dead conspecifics, live snails) and induced the rapid occupancy of all the empty shells offered, usually by the first crabs arriving at the site. Aggregations seemed not to function as shell exchange markets, as previously suggested for several other species. In the short run, exploitation seems to be more efficient for the acquisition of new shells by Pagurus longicarpus. In the long run, it is the density of nondestructive gastropod predators that regulates the availability of new shells of good quality in the pool available to this hermit crab species.     

Ecology of subtropical hermit crabs in S.W. Madagascar: cluster structure and function

The Madagascar coast (both) has a higher density and diversity of hermit crabs than is known from any other locality in the western Indian Ocean. Of the 20 species occurring at Anakao (S.W. Madagascar), 11 aggregated into clusters, including all but one of the species above the subtidal zone. The mean number of hermit crabs and species in clusters varied with several spatial parameters and time. Over 80% of the community clustered in certain habitats in particular tidal zones, whilst as low as 3% clustered in others. The highest intensity of clustering with shore zone was coincident with peak numbers of hermit crabs. The initiation and duration of clusters of hermit crabs above the eulittoral was driven by circadian rhythms, whilst those in the eulittoral were governed by tidal state. Clusters above and below the eulittoral were longer in duration, and those in the subtidal were more temporally variable than those above it. Certain pairs of species showed positive correlations of occurrence and (more rarely) of abundance, and all the correlations (of occurrence) of one, Clibanarius eurysternus, were negative. Positive correlations of occurrence were related to the degree of shell-use commonality between species pairs. Eulittoral species clustered with other individuals of approximately similar size and exchanged shells upon cluster disintegration. There was evidence of a dehydration-reduction function to shell clustering in addition to shell-exchange facilitation. The highly variable species-specific strategies of clustering may be important in alleviating both intra- and interspecific competition in assemblages of similar and highly abundant species.     

Niche segregation in sandy beach animals: an analysis with surface-active peracarid crustaceans on the Atlantic coast of Spain

Several species of sandy beach invertebrates regularly switch between burial in the sand during the day and surface activity at night to feed on stranded organic matter. Because all species consume essentially the same type of food (i.e., wrack) deposited over a restricted area, the potential for competition exists. Conversely, spatial and temporal segregation of surface activity behaviour is predicted to allow for niche separation. Here, we tested whether such behavioural niche separation occurs in three species of sympatric crustaceans (the talitrid amphipods Talitrus saltator (Montagu 1808) and Talorchestia brito Stebbing 1891, and the oniscoidean isopod Tylos europaeus Arcangeli 1938). We also assessed whether surface activity is modulated by weather, sea conditions and moon phases. Surface activity was measured over three consecutive moon phases on an exposed beach on the Atlantic Coast of Spain using pitfall traps along three transects from the foredunes to the swash. Adults of the amphipod T. saltator and the isopod T. europaeus overlapped spatially and temporally in their surface activity for most of the night. By contrast, the activity of T. brito was strongly disjunct in time and concentrated into significantly shorter bouts around dawn or dusk. Niche differentiation was also evident along the space axis, where T. brito occurred distinctly lower on the beach. Although several environmental factors (e.g., air and sand temperature, humidity, atmospheric pressure, wind speed and wind direction) were correlated with surface activity, this was highly variable amongst species, life stages, and synodic phases, and did not obscure the fundamental mosaic of spatio-temporal heterogeneity amongst species. Nocturnal feeding on beach wrack creates the potential for competition amongst sympatric crustaceans which can, however, be mitigated by behaviour patterns that separate consumers in time and space.     

Epibiosis on cerith shells in a seagrass bed: correlation of shell occupant with epizoite distribution and abundance

The cerith Cerithium atratum (Born 1778) is an abundant gastropod in the seagrass beds at Cabo Frio, Brazil. In order to estimate the ecological importance of cerith shells as a rare hard substratum in the seagrass bed, the abundances of C. atratum and of cerith shells occupied by hermit crabs were quantified. The mean densities of C. atratum and hermit crabs were 1887 and 100 individuals m⁻², respectively, and these provided 0.5 m² shell area m⁻² available for epizoite colonization. The tube-forming polychaete Hydroides plateni (Kinberg 1867) and oyster Ostrea puelchana Orbigny, 1841 were the dominant visible epizoites on inhabited cerith shells. These epizoite populations were compared in order to investigate whether the temporal and spatial patterns in the epibiotic community were related to ecological and behavioral aspects of the occupant species (cerith or hermit crab). Larger cerith shells had a greater abundance of epizoites. Each epizoite showed a preference for a different occupant of the shells (the oyster for C. atratum and the polychaete for cerith shells occupied by hermit crabs). The oyster showed a seasonal pattern in abundance on C. atratum, being more common in fall (March–April). The distribution of the epizoites on the shells depended on the shell occupant species and was probably related to their different foraging activity –C. atratum ploughs half buried through the sediment, while the hermit crab crawls on the sediment surface. In both cases, the activity of the shell occupant was considered to be beneficial to the epizoites, as empty shells and shell fragments did not support a macroepifauna. Received: 1 May 2000 / Accepted: 8 August 2000     

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

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

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

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

Dispersal,population structure and burrow shape of Ocypode cursor

The distribution of the sand crab Ocypode cursor (L.), as indicated by the number of burrows, was studied for 2 years in a 50×50 m sand beach area in northern Israel. A definite relationship was established between the distribution pattern from the seashore inwards towards the sand dunes, and the degree of sand moisture as it changed seasonally. During autumn, more crabs were found at a distance of 15 to 25 m from the sea where sand moisture was about 14%. At the beginning of winter crabs dispersed evely, disappearing with advancing winter. Crabs reappeared in spring, although in smaller numbers, dispersing in a pattern similar to that in autumn. At the beginning of summer and later on, more crabs appeared and concentrated closer to the sea (5 to 10 m). The population structure was analysed directly by measuring the crab's dimensions, and indirectly by counting burrows and measuring the diameter of their openings. Direct analysis revealed two distinct sizeage groups: smaller crabs 0.5 to 3 cm long, and larger ones over 4 cm long. The smaller burrows were inhabited by the first group and were mostly found closer to the sea; the second group was found more landwards. Three main types of burrow shapes are described.     

Genetic differentiation between morphotypes of the marine sponge Suberites ficus (Demospongiae: Hadromerida)

Sponges of three morphotypes of Suberites ficus (Johnston, 1842) were collected during February and March 1985 off the south-west of the Isle of Man, and were compared by using spicule size distributions and genetic allele frequencies of isozyme loci. The populations did not show any significant differences of spicule size or type, but could be easily differentiated into three separate species based on isozyme patterns. Samples of pale orange S. ficus growing on gastropod shells inhabited by hermit crabs (Pagurus spp.) were reproductively isolated from the redorange and the pale yellow colour morphs encrusting the bivalve Chlamys opercularis. These latter two colour morphs were genetically similar, but significant differences were observed at two of the 19 gene loci assayed. All the sponges studied were sympatric, and therefore the genetic differences, indicating reproductive isolation, are strong evidence for separate gene pools and, hence, that they are different species. The genetic identity between the two colour morphs of S. ficus on C. opercularis shells was 0.977, whilst between each of these and S. ficus on hermit crabs it was about 0.65. In all three species genetic variability was high, with mean expected and observed heterozygosity values per locus ranging from 0.17 to 0.36.     

Shell-size selection by intertidal sympatric hermit crabs

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