Faunal affinities among abyssal Atlantic basins

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Abstract
Some generalities concerning the distribution of deep-sea tunicates can now be presented, on the basis of American and French oceanographic expeditions. It has been possible to reach these conclusions because of the small number of tunicate species in relation to other taxonomic groups, the increase in species diversity at great depths, and the coherence of taxonomic research in this field. All the basins are not represented by equal numbers of samples and it is therefore necessary to establish a similarity index taking this into account. The most striking feature of tunicate distribution is the large number of species common to the northern and eastern parts of the Atlantic Ocean, while the North American, Surinam, and Brazilian Basins have only a low percentage of common species. Affinities from north to south are weak in the West Atlantic but are significant between the Labrador and Cape Basins and strong between the European and Cape Basins. Some regions appear isolated such as the very poor Norwegian Basin and the Argentine Basin, which has more Antarctic affinities. The Mid-Atlantic ridge is not to be considered as a barrier but more as a boundary, a zone of convergence where the faunae of several basins are superposed (Azores).

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