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During monthly intervals over a 1—year period, 12,000 empty snail shells were added to a small, isolated, rocky intertidal reef in the San Juan Islands of Washington. The shells added were species normally used by the high intertidal hermit crab, Pagurus hirsutiusculus, and were placed in locations accessible to that species. The shell additions resulted in an increase in density of P. hirsutiusculus at the experimental reef, whereas no density change occurred at a nearby control reef, indicating the importance of shells as a limiting resource. To establish the generality of shell limitation, the hermit crab populations of four unmolested rocky intertidal sites (three of which are typical hermit crab habitats) were quantitatively samples to obtain species compositions and size distributions of hermit crabs, their shells, and unoccupied shells. Shell preference experiments determined the preferred shell sizes and species for each hermit crab species. Except for small size classes, empty shells were rate at the three typical areas. In addition, hermit crab size distributions followed shell size distributions, and all but small hermit crabs of three species occupied shells smaller than the preferred size. These results support the conclusion that empty shells are a limiting resource for these hermit crabs. Since shells constitute a common, necessary resource in short supply, these hermit crabs are in competition for available shells. The fourth area, chosen for its unusual shell—availability characteristics, exhibited a different pattern of shell utilization not suggesting shell limitation. Shell occupancy at the three representative intertidal sites was examined to determine the strength of the relationship between hermit crab species composition and resource availability. Though resource partitioning was demonstrated, the presence and numbers of each hermit crab species and its preferred shell types were poorly correlated. Differences in hermit crab species composition are explained by differences in the physical habitat, and collections from other areas show that the same shell species can support different hermit crab species in different but adjacent habitat types. Thus, the mechanism allowing coexistence apparently involves both resource and habitat partitioning.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. Field studies have suggested that the intertidal hermit crabs of the San JuanIslands of Washington normally occupy snail shells smaller than preferred. In this study the effects of shell size on protection from predation and on hermit crab shell fighting were studied in the laboratory. A predator (Cancer) presented with two hermit crabs (Pagurus granosimanus), identical except in size of occupied shell, preyed upon the hermit in the smaller shell first in 15 out of 16 trials. This results suggests that large shell size confers a selective advantage on the occupying crab. Shell fights involving two hermit crabs (P. hirsutiusculus) of unequal size were observed in which replicates differed only in the shell size of the larger crab. The probability of the larger crab effecting a shell exchange through fighting was shown to increase as the size of its shell decreased. However, shell size was shown to have no effect on the level of aggressiveness as measured by four criteria. The mechanism underlying the former result thus appears to involve a continual high level of general aggressiveness together with an increased tendency associated with occupancy of an inadequate shell by the dominant crab for that crab to evoke a shell exchange during an aggressive interaction.

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