Some Characteristics of the Dominant Intertidal Molluscs from Rocky Shores in Pernambuco, Brazil

The distribution and morphological features of the dominant intertidal molluscan fauna on a system of sandstone reefs at Recife, Brazil, are studied. A total of 18 species is described, eight of which are common surface-dwelling forms. In general, the members of the assemblage are characterized by small size and relatively weak sculpturing of the shell, perhaps in response to the intense scouring by sand, especially on the inner reef. Of the eight species of surface dwellers, four are restricted to eastern South America, two extend into the southern Caribbean, and two range fully into the Caribbean. In contrast to this, five of the six species from beneath boulders are known to be wide-ranging tropical West Atlantic forms. It is suggested that biogeographic provinces or subdivisions thereof should not be defined by the fauna as a whole, but rather should be defined in terms of one or a set of particular habitats.