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 R.V. "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 carried out by conductivity, temperature and depth casts and/or expendable bathythermographs. 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 6 m2 Isaac–Kidd midwater trawl and Bongo nets yielded a total catch of 909 specimens of early life stages of cephalopods, representing at least 13 families and 20 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 at 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 boundary.