Abstract
Using quadrat sampling of video transects obtained by the submersible Delta, we characterized the larger invertebrates living on the shell mounds surrounding 15 oil and gas platforms (in waters 49–365 m deep) off southern California. The sea stars Patiria miniata (Brandt, 1835), Pisaster spp., and Stylasterias forreri (de Loriol, 1887); sea anemones (Metridium spp.); pleurobranch sea slug (Pleurobranchaea californica MacFarland, 1966); and rock crabs (Cancer spp.) dominated the assemblage. In addition, spot prawns, Pandalus platyceros Brandt, 1851, and the sea urchin Strongylocentrotus fragilis Jackson, 1912, were abundant at a few shell mounds, and large masses of the non-native foliose bryozoan, Watersipora subtorquata (d’Orbigny, 1852), were observed at one platform. Brittle stars were also abundant in patches on some shell mounds. Over all, echinoderms were the most abundant taxa, with eight taxa of sea stars comprising 77% of the total number of organisms counted individually. Excluding brittle stars, the sea star P. miniata attained the highest densities, up to 10 ind per m2. Except for the brittle stars and Metridium spp., which are suspension feeders, the dominant taxa were all carnivorous or omnivorous predators or scavengers, dependent primarily on the food subsidy of mussels and other fouling organisms growing on the upper reaches of each platform. Tall Metridium spp. were the only large, structure-forming invertebrates prevalent on the shell mounds.