The present paper is the first of a series dealing with marine zoogeography of Brazil. The Brazilian marine fauna has been well worked only recently, and the contributions on the zoogeography of various groups are in dependence of conclusions of faunistic surveys. Since Dana (1852) to present, various authors have divided diversely the littoral in natural regions, according temperature, salinity and species ranges. In the present paper the littoral between French Guiana and North Argentina was divided in 26 strips 2º wide, each strip extending from the coast to the 200 m isobath. The temperature and salinity data (surface and bottom) collected in each strip by Oceanographie Ship "Almirante Saldanha" were gathered in order to calculate the mean temperature of extreme months and the corresponding salinities (other data were also used, when necessary). The species list of each strip was made with the previously published records and the data of research institution files. The distributional data for the hermit crab were analysed in three ways: horizontal distribution; bathymetric range and substrate preferences. It was possible to divide the study area in five provinces (Guyanian, Brazilian, St. Paul's, Uruguayan and Argentinian); the St. Paul's Province is a transition zone between Tropical and Anti-Boreal Regions.

In the rocky and gravely bottoms there are endolithic and epilithic species, and some soft bottom species. It was possible to distinguish in these bottoms the following communities: 1) supralittoral rock; 2) upper mediolittoral rock; 3) lower mediolittoral rock; 4) infralittoral rock; 5) infralittoral gravel; 6) circalittoral rock; and 7) semi-obscure caves.

The studied area is limited by São Francisco River mouth, to the South, and by Coroa das Lavadeiras, State of Rio Grande do Norte, to the North. This area presents only two seasons, summer and winter. During the summer, the freshwater inflow is smaller, the southeast winds are dominant, the temperature, the salinity and the transparence of water are high. The winter is characterized by greater freshwater inflow, east winds, smaller temperature and low salinity and transparence only in coastal and estuarine waters. Off São Francisco River mouth the water conditions are inverted and the salinity and transparence variations affect all continental shelf. In the soft bottoms are found the following communities: a) Supralittoral beach; 2) Mediolittoral beach; 3) Infralittoral beach; 4) Circalittoral mud (São Francisco mouth); 5) Circalittoral gravel mud (São Francisco mouth).