Performance Implications of Divergent Shell Size Preferences and Exoskeleton Mass of Two Closely Related Hermit Crabs

Hermit crabs use gastropod shells for protection from abiotic stressors and predators. However, two sympatric species of hermit crab, Pagurus hirsutiusculus and Pagurus granosimanus, have divergent shell size preferences. Differences in shell size use were evident in the field: for a given body mass, P. granosimanus used shells that were 136%-300% larger than P. hirsutiusculus. The present study examined the possible morphological adaptations associated with the shell size preferences of P. hirsutiusculus and P. granosimanus as well as the costs and benefits associated with the preference of P. hirsutiusculus for shells that are too small to enclose and protect the entire body of the crab. When exposed to desiccation conditions commonly encountered during low tide emersion, P. hirsutiusculus using large shells survived much longer than individuals using small shells. And in motility trials, P. hirsutiusculus moved significantly faster when using a small shell than when using a large shell. It was therefore hypothesized that P. hirsutiusculus might produce a heavier exoskeleton than P. granosimanus to compensate for the reduced protection obtained by P. hirsutiusculus from small shells. Our findings support this hypothesis: relative to body mass, the carapace was 15%-90% heavier in P. hirsutiusculus than in P. granosimanus, a difference further confirmed by the claw mass, which was 59%-81% heavier in P. hirsutiusculus than in P. granosimanus. The use of larger shells in P. granosimanus provides enhanced protection, allowing for the production of a lighter exoskeleton, but likely imposes reduced motility and increased energetic cost. Pagurus hirsutiusculus, on the other hand, uses smaller, lighter shells that impose lesser energetic costs and allow greater motility but leave the animal more vulnerable to stressors; P. hirsutiusculus compensates for this increased vulnerability by producing a heavier carapace and claws. The differences in shell size and exoskeleton mass suggest ecological implications for these species, particularly with regard to microhabitat use.