The molluscan macrofauna from 13 oceanic sheltered intertidal unconsolidated environments and its relationship with abiotic factors were studied in order to establish the degree of species richness and to understand the role environment plays in structuring such assemblages. Four distinct intertidal habitat types were recognized based on molluscan assemblage descriptors (diversity, richness and density) and abiotic characteristics. The mean grain size (in phy units) and the beach slope showed a negative relationship with the diversity, richness and density. Coarser sediments were favorable to molluscan fauna in the study areas, contrasting the well-known negative effect of this type of sand on fauna in typical oceanic beaches. The low-tide terraces, typical from tide-dominated areas, and the presence of physical (rocky fragments) and biogenic (gravel) structures, were also associated to the higher values of richness. The high richness in the study area as a whole seemed to be a direct consequence of its environmental heterogeneity, once it was composed by quite distinct habitat types.
Intertidal; molluscan assemblage; beach structure; environmental heterogeneity