The spatial arrangement of the continents and oceans, combined with the influence of temperature and latitudinal gradients, local circulation patterns, and water properties divide the oceans into a series of provinces, or biogeographic regions with characteristic assemblages. Boundaries between provinces can be water mass borders, major thermal or salinity discontinuities (Levinton, 1995).
Using zoogeographic provinces defined by Palacio (1980), The Paulista Province is considered a broad transition zone between warm and cold water species and a north-south gradient defining the gradual disappearance of the tropical fauna and the beginning of the Patagonian species. Several authors agree with this broad transition zone (Dana, 1853 apud Palacio, op. cit., Carcelles, 1944; Boltovskoy, 1964; Stuardo, 1964; Coelho & Santos, 1980; Tommasi, 1985).
Some authors claim that Cabo Frio is the limit between the Tropical Province and the Temperate or Patagonian Province (Ekman, 1953; Briggs, 1974); others think that it is a transition zone that starts at the Cabo Frio region, acting as an important ecological "filter" for the temperate species (Vannucci, 1964; Absalão, 1989) and continues to southern Brazil. However, these authors did not recognize Cabo Frio as a provincial zone, only as a transition region (Melo, 1985; Absalão, 1989).
The aim of this work was to compare the species distribution of gastropods at various sites along the Caribbean and Brazilian coast. Studies of this nature can be a valuable tool for a better understanding of the southwestern Atlantic marine biodiversity and biogeography.