Expanding the “shell exchange market” hypothesis for clustering behavior in intertidal hermit crabs: Mating and tide as proximate factors

Clustering is a common behavior displayed by hermit crabs in intertidal environments. Aggregations are reported to occur during low-tide events, and the number of animals present may vary depending on the species. The “shell exchange market” hypothesis suggests that the main function of clustering is to allow animals to acquire new shells by fighting, bargaining, or by a chain reaction caused by a single empty shell. However, the adaptive significance and the factors that trigger this behavior remain unexplained. Here, the hypotheses tested were that both sex of Clibanarius symmetricus are performing this behavior, that clustered animals have poorly fitted shells compared to isolated ones, and that tidal phase is triggering the formation of clusters. Clustered and isolated animals were collected in the field, sexed, and measured for cephalothoracic shield and shell parameters. The Shell Adequacy Index (SAI) was compared among crabs of both sexes in clustered and isolated conditions. Additionally, manipulative experiments were performed in the laboratory, with different tidal-phase conditions. Both sexes were present in clusters, but clustered females showed a lower SAI than isolated females. Furthermore, a higher proportion of crabs were aggregated during ebb-tide than during flood-tide events. Thus, clustering behavior may be related not only to shell exchanges, but also to mating and protection against desiccation. Consequently, the adaptive meaning and function of aggregations in intertidal hermit crabs should be understood as an integrative behavior related to many aspects of the animals' biology.