Distribution of the Polychaete assemblages on the continental shelf of the Northern Gulf of California, Eastern Pacific

This study was aimed at analyzing the effect of environmental factors on the composition and distribution of polychaetes in the continental shelf of the Northern Gulf of California. Twelve stations at depths from 30 to 106 m were selected as representative of the region. A total of 2510 polychaetes belonging to 33 families and 152 species were identified. The Spionidae (15 species), Lumbrineridae (11 species), Onuphidae and Cirratulidae (10 species each) were the families with the highest number of species, although only the spionids (96.29 ind./0.1 m(2)) were found in considerable densities. Distribution of the assemblages clearly reflected the influence of sediment type. The polychaete fauna was divided in two main groups: those distributed in sandy sediments and those found in sediments with higher mud content. The sandy assemblages were represented by the Phyllodoce sp. 1 group, distributed in the outer continental shelf to the south of the study area; the Aglaophamus verrilli, Chone sp. 1 and Spiophanes duplex group, which was found on the inner and middle shelf; and by Prionospio (P.) ehlersi, Chaetozone setosa, Monticellina dorsobrachialis and Leiocapitella glabra, a group located in the deeper zones of the northern coasts. The assemblages associated with muddy-sand bottoms were mainly characterized by Paranaitis sp. 1, found on the eastern coasts, and by the fauna distributed in the western region, where Scoloplos texana was the most common species.