Zoogeographic relationships of Magellan Nudibranchia (Mollusca: Opisthobranchia) with particular reference to species from adjacent regions

Abstract

Extensive recent collections and taxonomical revisions provide a basis for the first zoogeographical analysis of Magellan Nudibranchia. Using 36 of 57 nominal species described in this area, the degree of endemism has been decreased from 70% to 31%. Very close faunistic relationships to northern Argentina are evident as five of six known species (83%) are shared with the Magellan Province. With a 43% overlap of Magellan species in the southern part of the Peruvian faunal Province (44 species), this area shows a transitional character between cold- and warm-temperate waters. Only three of 36 Magellan species are found in common with the high Antarctic (ca. 30 species), revealing a low faunal similarity of about 10%. High Antarctic waters are characterized by high nudibranch endemism (about 80%). Of the ten nudibranch species reported from South Georgia, two are exclusively shared with Magellan, two with high Antarctic and three with both Magellan and Antarctic waters, indicating transitional, Subantarctic conditions. One species is endemic to South Georgia, two other species are insufficiently known and may or may not be endemic. Generally, the Antarctic Convergence appears to be a distributional barrier for Magellan nudibranchs. In contrast, some Antarctic nudibranch species extend far into the Atlantic, obviously with cold Antarctic deep water currents. Coastal, cold temperate Magellan nudibranchs have relatively low affinities to the deep water fauna off Argentina, which seems to be predominantly submerged Antarctic species, but this bathymetrical zonation needs to be further investigated.