DISTRIBUTIONAL LIMITS OF PYRGO SPECIES AT THE BIOGEOGRAPHIC BOUNDARIES OF THE ARCTIC AND THE NORTH-ATLANTIC BOREAL REGIONS

The distribution of twelve species of Pyrgo in the vicinity of the Gre enland-Scotland Ridge (GS-Ridge) are documented. Two new species are e rected, Pyrgo pyxis and Pyrgo labrum, and the taxonomy of ten species are revised. The well-known biogeographic boundary between the Arctic Region and the North Atlantic Boreal Region lies partly along the GS-R idge, The distribution of Pyrgo is restricted to one or more of five a reas of the sea floor which extend to the GS-Ridge. Two of these areas , the Arctic Deep (ARD) and the Arctic Shallow (ARS), are north of the GS-Ridge and are subdivisions of the greater Arctic Biogeographic Reg ion. South of the GS-Ridge are three areas, the Atlantic Shallow (ATS) , the Atlantic Deep (ATD), and a shallow-water area southwest of Icela nd, at the Reykjanes Ridge (RRS), all of which belong to the greater N orth Atlantic Boreal Biogeographic Region. These areas of the sea floo r are at different depth ranges and are covered with different water m asses, This may be crucial in limiting the distributions of the specie s which are used to characterize the areas. The ARD is covered with co ld Arctic Bottom Water and the ARS is mainly covered with cool Arctic Water. The ATS is covered with warm North Atlantic Water mixed with Ar ctic Waters, and the ATD is covered with somewhat cooler North Atlanti c Deep Water. The different environmental requirements of the species of Pyrgo are of potential use in paleo-environmental inferences. The R RS is suggested here for the first time as a special distributional ar ea. The hydrographic conditions on the Reykjanes Ridge may contribute to the formation of isolated populations of benthic species of the RRS , At greater depths the Reykjanes Ridge is encircled with a relatively cold bottom current, whereas shallower areas of the Reykjanes Ridge a re covered with the relatively warm waters of the Irminger Current. Mo st species of Pyrgo thrive south of the GS-Ridge, in areas ATD, ATS an d RRS, all of which are covered with relatively warm waters. Only thre e species of Pyrgo are present north of the GS-Ridge in areas ARS and ARD, which are covered with relatively cold waters. It is suggested th at the change between warm and cold water masses at the boundary betwe en areas ATS and ARS may form an ecological barrier to the evolutionar y diversification of Pyrgo.