Ecology of subtropical hermit crabs in SW Madagascar: cluster structure and function

The Madagascar coast (both) has a higher density and diversity of hermit cr abs than is known from any other localiy in the western Indian Ocean. Of th e 20 species occurring at Anakao (S.W. Madagascar), I I aggregated into clu sters, including all but one of the species above the subtidal zone. The me an number of hermit crabs and species in clusters varied with several spati al parameters and time. Over 80% of the community clustered in certain habi tats in particular tidal zones, whilst as low as 3% clustered in others. Th e highest intensity of clustering with shore zone was coincident with peak numbers of of hermit crabs. The initiation and duration of clusters of herm it crabs above the eulittoral was driven by circadian rhythms, whilst those in the eulittoral were governed by tidal state. Clusters above and below t he eulittoral were longer in duration, and those in the subtidal were more temporally variable than those above it. Certain pairs of species showed po sitive correlations of occurrence and (more rarely) of abundance, and all t he correlations (of occurrence) of one, Clibanarius eurysternus, were negat ive. Positive correlations of occurrence were related to the degree of shel l-use commonality between species pairs. Eulittoral species clustered with other individuals of approximately similar size and exchanged shells upon c luster disintegration. There was evidence of a dehydration-reduction functi on to shell clustering in addition to shell-exchange facilitation. The high ly variable species-specific strategies of clustering may be important in a lleviating both intra- and interspecific competition in assemblages of simi lar and highly abundant species.