The global zoogeographic distribution of the most widespread peracarid species occurring in three or more ocean basins below 2000 m is analysed. Basing on the published data we investigated 45 peracarid species, which have a most widespread distribution and most likely are cosmopolitan. Thirty−three species have a wide distribution in the Northern Hemisphere. Most species occur in the North Atlantic, however, 16 of these species occur also in the North Pacific, a more limited number of species occurs in the South Atlantic or South Pacific The Southern Ocean displays some special zoogeographic features and 22 widespread species occur there below 2000 m, including highly eurybathic ones. In total, 11 of the analysed species occur in all oceans. Eucopia australis (Lophogastrida), Munneurycope murrayi (Isopoda) and Eurythenes gryllus (Amphipoda) are the species with the widest distributions. Other peracarids occurring in all oceans are: the isopods Paramunnopsis oceanica and Eurycope sarsi , the mysid Caesaromysis hispida the lophogastrid Eucopia unguiculata, the amphipod Mesopleustes abyssorum and the tanaids Exspina typica, Paranarthura insignis and Pseudotanais nordenskioldi . No cumacean species has been reported with an ocean−wide distribution but Campylaspis glabra occurs in the Atlantic, Indian and Pacific oceans. Among plenty of rare species in each order there are only few species with wide distribution records. There is evidence from molecular genetic studies that some of the widespread peracarids represent several cryptic species, however, some, e.g. Eucopia australis , seem to be truly cosmopolitan species. Geography of sampling is biasing our view of biogeography. The history and quality of taxonomic work as well as the reliability of geographic records (quality control of large databases) limits our investigations of widespread or cosmopolitan species as much as the limited knowledge of variation within most species causes difficulties in defining morpho−species with certainty.
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