The role of neutral and niche-based processes in hermit crab – gastropod networks

Abstract It is widely acknowledged that the vast majority of hermit crab species occupy empty gastropod shells to protect their fragile abdomen from environmental stresses and potential predators. As this association represents an obligatory condition to allow their survival, hermit crabs face fierce competition for adequate shells and often occupy suboptimal shells. Thus, shell adequacy is an important indicator of shell availability in hermit crab communities, measured using the shell adequacy index (SAI). In this study, we simulated values of SAI under three different hypotheses considering neutral and niche-based processes to investigate which processes, i.e. abundance, forbidden links, and morphological match, influence the probability of interaction (reflected in shell adequacy) between hermit crab species and empty gastropod shells. We recorded 121 interactions for five hermit crab species occupying 10 gastropod shell species. Simulated weighted mean values of SAI were consistent with forbidden links and morphological match, indicating the importance of morphological traits in this interaction. Our findings suggest that niche-based processes involving the adaptation of hermit crabs to the gastropod shells, and not abundance, are the most important drivers influencing the interaction between hermit crabs and empty gastropod shells.