Abstract
Megabenthic invertebrate assemblages from soft sediments on the <br /><br />mainland shelf and San Pedro and Santa Monica Basin slopes and floor were <br /><br />identified based on 1203 otter trawl collections made between 1971 and 1985 at <br /><br />depths ranging from 10 to 915 m. Major changes in species composition and <br /><br />abundances occur at mid-slope (~300 m) and at the basin sills (~715 m). The <br /><br />mainland shelf assemblage (10-1 37 m) is dominated by the sea urchin Lytechinus <br /><br />pictus and the prawn Sicyonia ingentis. This assemblage is heterogeneous in space <br /><br />and time, with sub-assemblages that reflect severe contamination and large storms <br /><br />and/or El Nino. The basin slopes are dominated by the echinoids, Allocentrotus <br /><br />fragilis, Brissopsis pacifica, and Brisaster latifrons. The latter two species may exist <br /><br />in kilometer scale herds on the slopes, but they are absent below the basin sills <br /><br />(737 m). Galatheid crabs are the most abundant megafauna in the San Pedro and <br /><br />Santa Monica Basins. Species composition of all assemblages appeared to be <br /><br />seasonally stable, but 6". ingentis and the tuna crab Pleuroncodes planipes increased <br /><br />by several orders of magnitude during El Nino. Numbers of megafaunal species, <br /><br />individuals, and biomass were highest on the basin slopes reflecting elevated levels <br /><br />of organic material in the sediment. Numbers of individuals and biomass increased <br /><br />on the lower slopes (500-800 m), following El Nino. Although organic material <br /><br />is highest in the basins, the numbers of species and individuals were much lower <br /><br />than on the lower slopes. Near anaerobic conditions in the basins probably exclude <br /><br />many species.
