Testing vacancy chain predictions in Pagurus longicarpus hermit crabs: Does ecological gain and behavioral motivation match environmental context?

Both ecological and social contexts are recognized as key variables in hermit crab shell switching behavior, but the impact of site variability on social shell choice has not yet been explored. Pagurus longicarpus marine hermit crabs are common inhabitants of highly variable intertidal habitats where they compete for gastropod shell resources. The goal of this study is two-fold: (1) to assess natural resource (shell) quality and use in the field; (2) to determine how differences in resource quality and ecological context affect social behavioral decisions in the lab. We used vacancy chain theory to analyze the impacts, benefits and consequences of shell choice on the population. Specifically, we quantified movement of shells through a group and described hermit crab shell decision patterns from two different intertidal populations: Beverly (rocky shore) and Nahant, MA (mudflat). Shell damage and shell fit are both known to negatively impact hermit crab fitness and survival; we found that Nahant crabs inhabited poorer quality shells compared to Beverly crabs. Similarly, Nahant crabs were more crowded and spent more time investigating shells. Despite the clear motivational and behavioral differences in populations, vacancy chain lengths were extremely similar between Nahant and Beverly populations, and chains always terminated when the vacant shell was highly damaged, suggesting that chain length is mediated most by shell quality. These results suggest that relative (rather than absolute) differences in resource quality may dictate the dynamics of social vacancy chain outcomes in this species.

► Hermit crabs compete for gastropod shell shelters.
► Snail-inhabited, crab-inhabited, and empty shells vary with habitat.
► Social shell decision patterns also vary with habitat.
► Despite clear habitat differences, shell switching patterns were similar.
► Relative (not absolute) differences in shell quality may dictate switch dynamics.