Zonation of Intertidal Organisms and Community Structure of Small Animals Associated with Patches of the Mussel Mytilus edulis L. along the Rocky Coast of Dinard, Brittany, France

A marked zonation pattern was recognized on the rocky intertidal along the coast of Dinard, Brittany, France. Patella spp., Gibbula spp. and balanoid barnacles showed a habitat segregation in terms of vertical distribution. The community structure of small animals associated with various sized patches of Mytilus edulis (“Mytilus islands”) was analyzed at two tidal levels and differences in the species composition were found. At both levels, the species richness and number of individuals increased with patch size, but the species density (number of species/patch area) decreased. On the other hand, the absolute density (number of individuals/patch area) was relatively constant irrespective of patch size. This was due to the great abundance of small barnacles such as Chthamalus monlagui, Balanus balanoides and Elminius modestus, which were attached to the Mytilus shells, and the small bivalve Lasaea rubra. Species diversity (H') did not show any trend with patch size, but equitability (J') decreased. The importance of micro spatial characteristics in discussion of species diversity is emphasized.