SMALL SCALE CHANGE IN MOLLUSK DIVERSITY ALONG A DEPTH GRADIENT IN A SEAGRASS BED OFF CABO FRIO, (SOUTHEAST BRAZIL)

We know little about animal communities in seagrass beds in the southeast Atlantic. The aim of the current study was to characterize and quantify the mollusk assemblages within these unique tropical seagrass habitats at a spatial scale fine enough to relate change in assemblage structure to gradients in exposure to air/depth. The study was carried out off Cabo Frio, Southeast Brazil. A 1250 m(2) area vegetated by the seagrass Halodule wrightii was defined (50 m parallel to the shoreline x 25 m distance from the shore margin). Twenty-six transects were established and the sampling carried out in order to characterize and to quantify the associated mollusk fauna. The mollusk assemblage presented overall density, density of key species and mean richness which increased systematically along the onshore-to-offshore gradient. Furthermore over the short spatial distance of the study the mollusk assemblage changed sufficiently for ordination to detect different assemblages.