Epibiosis on cerith shells in a seagrass bed: correlation of shell occupant with epizoite distribution and abundance

The cerith Cerithium atratum (Boln 1778) is an abundant gastropod in the se agrass beds at Cabo Frio, Brazil. In order to estimate the ecological impor tance of cerith shells as a rare hard substratum in the seagrass bed. the a bundances of C. atratum and of cerith shells occupied by hermit crabs were quantified. The mean densities of C. atratum and hermit crabs were 1887 and 100 individuals m(-2) respectively, and these provided 0.5 m(2) shell area m(-2) available for epizoite colonization. The tube-forming polychaete Hyd roides plateni (Kinberg 1867) and oyster Ostrea puelchana Orbigny, 1841 wer e the dominant visible epizoites on inhabited cerith shells. These epizoite populations were compared in order to investigate whether the temporal and spatial patterns in the epibiotic community were related to ecological and behavioral aspects of the occupant species (cerith or hermit crab). Larger cerith shells had a greater abundance of epizoites. Each epizoite showed a preference for a different occupant of the shells (the oyster for C. atrat um and the polychaete for cerith shells occupied by hermit crabs). The oyst er showed a seasonal pattern in abundance on C. atratum, being more common in fall (March-April). The distribution of the epizoites on the shells depe nded on the shell occupant species and was probably related to their differ ent foraging activity - C. atratum ploughs half buried through the sediment , while the hermit crab crawls on the sediment surface. In both cases, the activity of the shell occupant was considered to be beneficial to the epizo ites, as empty shells and shell fragments did not support a macroepifauna.