Field observations on activity and clustering in two intertidal hermit crabs, Clibanarius virescens and Calcinus laevimanus (Decapoda, Anomura).

Activity and clustering in two intertidal species of hermit crabs, Clibanarius virescens and Calcinus laevimanus, were studied during a semilunar tide cycle in Somalia. Three groups were marked and examined throughout 16 low waters, and 12 samples of clustered and scattered crabs were collected from similar habitats. During high water, the animals took refuge in mass in holes of the cliff; they began to reappear (especially at night) and feed during ebb‐tide (feeding phase). When exposed to the air, the crabs entered a resting phase with some aggregating in clusters, then when the tide rose became active again before going back to their refuges. Clusters composed of up to 95 quiescent individuals (which were even more crowded during nocturnal and spring low tides) occurred at every low tide and at approximately the same place. Size and sex composition of clumps differed from scattered samples, suggesting that the clustering habit is exclusive to crabs which either need to frequently change their shells (due to their faster growth rate) or are less efficient in the competition to acquire such resources.