| Summary: | The fauna of the Argentine (SW Atlantic Ocean) shelf allows to distinguish two biogeographic provinces: the Argentine and the Magellanic that are part of the Subtropical and Subantarctic Regions. The former extends from 30° S-32° S to 41° S-44° S and from the coast to the 82-95 m isobath, between 35° S-39° S, to 70 m depth in the north Patagonia. The latter extends from the Valdés Peninsula to the southern extreme and from 43° S northward, east area of the Argentine Province. The different physiographic characteristics allow to distinguish movable sandy bottoms in the Argentine Province and in the Magellanic gravel bottoms where large algae grow. The climatic differences explain the prevalence of northern winds in the first, where warm and temperate-cold coastal waters alternate and western strong winds in the second with net predominance of subantarctic waters from the Malvinas Current. The factors mentioned explain the differences in the faunal composition of both Provinces; the Argentina is characterized by a marked heterogeneity of its components and the Magellanic by its homogeneity and own taxa. The detailed analysis of the faunal composition of both Provinces performed using benthonic organisms (echinoderms, crustaceans and molluscs) and nektonic (fishes) resulted in subdivisions that correspond to the Uruguayan District, down to 38° S-39° S and the Rionegrin, south of said latitude in the Argentine Province and the Chubutian and Southpatagonic, north and south of 47° S in the Magellanic. The hydrographic regime allowed to establish the Santacrucean and Fuegian Subdistricts within the Southpatagonic District, which supports the warm drift theory in the Uruguayan and Rionegrin Districts of the Argentine Province and in the Chubutian of the Magellanic. The feature that characterizes the Southpatagonic District is the absence of a warm water tongue. Published
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