Hermit crabs in a mangrove swamp: clustering dynamics in Clibanariu slaevimanus

Abstract

Some dynamic patterns of clustering in the mangrove-dwelling hermit crab Clibanariuslaevimanuswerestudied during a semi-lunar tide cycle in Kenya. Clusters composed of hundreds of quiescent individualsoccurred at every low tide, developing either on the mangrove prop roots or in the open (but always withina 4-m belt from the mangrove fringe). They disbanded at ebb tide, when most animals migrated in linestowards feeding grounds. There they grazed on vegetable debris during high water. When the tidereceded, they came back home in crowded groups. The position of aggregating sites remained constantthroughout the tides, as well as their number and size, and the shape of clusters was stable with time.Individual hermits often changed clusters (only rarely they were found scattered), but transfers onlyoccurred between aggregations where shell size range matched the shell worn by the moving crab.Dispersal was slower than predicted by random walk models. Questions were aimed on the proximate andultimate factors underlying this spatial strategy which contrasts the nomadic behaviour shown by thesympatric C. longitarsus.