Seasonal fluctuations in the abundance and diversity of eighteen species including four subspecies of euthecosomatous pteropods have been observed from 235 surface plankton samples at five ocean weather stations in the western North Atlantic. Four faunal categories have been recognized.
Category I consists of one arctic species, Limacina helicina, which appeared only rarely at Station Bravo in night tows during the winter. Category II is represented by one subarctic species, Limacina retroversa, whose distribution extends southward into the Transitional Zone
during the winter. Category III is composed of ten subtropical species including two subspecies (typified by Limacina inflata and Styliola subula) that live year-round in the Sargasso Sea in subsurface waters during daylight hours and ascend to the surface at night. The seven
subtropical warm-tolerant species and two subspecies of Category IV were more abundant in the northwestern Sargasso Sea (Bermuda) than in the north central Sargasso Sea (Station Echo) and in both areas they were primarily found in surface waters during the warmer months.
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