Variation in Diacria trispinosa (De Blainville, 1821) and Clio pyramidata Linnaeus, 1767 (Pteropoda, Mollusca) of the Amsterdam Mid North Atlantic Plankton Expedition 1980

Two thecosomatous pteropods, Diacria trispinosa and Clio pyramidata, caught during an expedition in the Mid North Atlantic Ocean in 1980 have been studied. The morphological variation of the northern and southern formae of both species is reported. The distributions of the formae are given, transitional zones are described and their abundance throughout the area is estimated. Variation in both species shows a rather smooth cline while the transition of northern into southern formae starts between 40° and 45°N which coincides with the transition of temperate into subtropical waters. The presence of very broad specimens of Clio pyramidata forma lanceolata between 40° and 45°N suggests the inflow of West Atlantic tropical water populations at these latitudes, carried in by the Gulfstream and the North Atlantic Current.