Hermit crabs select shells based on size, weight, species of gastropod, and shell condition. However, in many environments, empty, undamaged gastropod shells are in short supply and this shortage may influence hermit crab shell selection behavior. In this study, the effects of isolation, competition, and predation on shell selection behaviors were investigated using Pagurus granosimanus (Stimpson), a hermit crab from San Juan Island, WA. The hermit crabs were provided with a choice between undamaged, slightly damaged, or very damaged Nucella lamellosa (Gmelin) gastropod shells and were subjected to either isolation (control treatment), water infused with the chemical cues of conspecific hermit crabs (competition treatment) or water infused with both predator (Cancer magister, Dana), and dead conspecific chemical cues (predation treatment). In all 3 experimental treatments, after a trial duration of 20 min, hermit crabs preferentially selected the undamaged shell. However, the frequency of shell switches varied significantly between the three experimental conditions. Hermit crabs in the control treatment switched between shells most frequently and hermit crabs in the competition group switched least frequently. Additionally, larger hermit crabs switched shells more frequently than smaller hermit crabs in all experimental conditions. In settings where competition is the primary environmental stress, such as in the San Juan Islands, the reduced frequency of shell switching may have a long-term effect on hermit crab defense against predation, in turn reducing the reproductive success and population size of hermit crabs.
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