The hydroid fauna of the Kurile Islands numbers 150 species, nearly half the total number of species in the seas around the Soviet Union. Three genera and 16 species are endemic to the Archipelago. Most of the endemic hydroids occur in the central region of the Archipelago. Most of the endemic hydroids occur in the central region of the Archipelago. On the basis of the hydroid fauna the region is divided into four biogeographic areas, the Siretoko Area in the south west, which contains tropical and subtropical species; the south Kurilean which does not contain any tropical or subtropical species, the Ampikurilean area which contains sub-arctic and temperate hydroids but which is divided into a southern and northern section by the Central Kurilean Area which is a zone of upwelling of cold water in which hydroids normally found only below l00 m are found in shallow water.