Megabenthic invertebrates on shell mounds  associated with oil and gas platforms off California

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.