Life-History Variation Among Three Temperate Hermit Crabs: The Importance of Size in Reproductive Strategies

The supply and quality of empty gastropod shells may play important roles in the ecology and evolution of hermit crabs. We compare the life histories of three subtidal hermit crabs in Nantucket Harbor, Massachusetts: Pagurus annulipes, P. longicarpus, and P. pollicaris. Specifically, we examine seasonal patterns of reproduction in females, male and female size structure, reproductive effort, and temporal patterns of larval abundance. We also compare shell size among the three species. Life-history features vary with size among the three species. The smallest species (P. annulipes) reproduce soon after metamorphosis and have a high reproductive effort. The two larger species (P. longicarpus and P. pollicaris) delayed reproduction to an intermediate size, and have lower reproductive efforts than P. annulipes. There is no effect of body size on reproduction in P. annulipes, but there is a strong positive effect in P. longicarpus and P. pollicaris. Seasonal patterns of early stage larvae correlated with seasonal patterns of ovigery in all three species, with highest larval densities sampled in P. annulipes and P. longicarpus. Size differences among species were related to patterns of shell usage. Male and female P. annulipes were always found in large shells relative to body size. In comparison, male and female P. longicarpus and P. pollicaris were found in small shells compared to body size. We suggest that early maturity and high reproductive effort have evolved in response to a high risk of mortality associated with small shells. Delayed maturity and low reproductive effort are favored in species that reach a size refuge from shell-crushing predators. Effects of shell limitation are more likely to be common in large species and may also be an important selective in shaping hermit crab life histories.