Early life stages of cephalopods in the Sargasso Sea : Distribution and diversity relative to hydrographic conditions

The distribution of early life stages of cephalopods was studied during a cruise of the German research vessel RV Poseidon to the Sargasso Sea in March 1993, covering an area south-east of Bermuda from 24°N to 31°N and 61°W to 65°W. Hydrographic measurements were conducted by CTD and/ or XBT. The subtropical convergence zone was detected at a latitude of approximately 27°20’N and divided the Sargasso Sea into a northern and a southern area. Zooplankton sampling with a 6m² Isaac Kidd midwater trawl and Bongo nets yielded a total catch of 909 early life stages of cephalopods representing at least 14 families and 23 mainly oceanic species. Multivariate statistical analyses were performed in order to compare the species composition and abundance of cephalopods. Two different assemblages were clearly identified, north and south of the front. According to the position of the front an analysis of similarity (ANOSIM) was applied which confirmed the observed differences in species composition on a highly significant level. The Cranchiidae, mainly represented by the endemic species Leachia lemur, was the most abundant family especially in the northern part of the Sargasso Sea and was mainly responsible for the distinction between the cephalopod assemblages. In general, higher abundances of early life stages and a higher diversity was observed north of the subtropical convergence zone, which is assumed to form a faunal boundar