THE DEEP-SEA ASELLOTE (ISOPODA, CRUSTACEA) FAUNA OF THE NORTHERN SEAS- SPECIES COMPOSITION, DISTRIBUTIONAL PATTERNS AND ORIGIN

The marine asellote (Isopoda, Crustacea) fauna of the Northern Seas, i .e. the Norwegian, Greenland, Iceland and North Polar Seas, contains 1 06 species. Most of them occur in shallow waters, but the number of sp ecies declines rapidly with increased depth. Half the species are ende mic and those occurring also outside the Northern Seas are mainly foun d in the North Atlantic. The endemism of species is low ( < 50%) at de pths < 750 m, but becomes high ( > 60%) at depths > 750 m. Only a sing le endemic asellote genus is recorded from the Northern Seas. The low diversity of the deep Arctic asellote isopods is explained partly by a short evolutionary time of the fauna within this environment, but in particular due to isolation of the Greenland-Iceland-Faeroe Ridge, whi ch acts as a barrier against the immigration of species into the North ern Seas and thus shapes the species composition. The hydrographic con dition above the ridge may also have restricted the migration of deep- sea species into the Northern Seas. Consequently, the Arctic deep-sea asellote fauna consists predominantly of species belonging to less pro nounced deep-sea families (e.g. Desmosomatidae and Nannoniscidae) and genera, or shallow water genera occurring in proximity to the Northern waters, while the most pronounced deep-sea families and genera are po orly represented (e.g. families Haploniscidae and Ischnomesidae) or ev en absent (e.g. genus Storthyngura). The Arctic deep-sea asellote faun a is considered to have originated mainly from the North Atlantic Ocea n.