An analysis of the sessile, structure-forming invertebrates living on California oil and gas platforms

Using video transects of oil and gas platform crossbeams off central and southern California, we characterized the structure-forming invertebrates (with a height of at least 20 cm) found around 23 oil and gas platforms at depths between 20 and 363 m. We observed 20,357 individual invertebrates, comprising 19,800 Cnidaria and 557 Porifera of at least 15 species or species groups. Metridium farcimen (Brandt, 1835) was by far the most commonly observed cnidarian, forming 97.6% of all invertebrates catalogued. The alcyonacean, Leptogorgia chilensis (Verrill, 1868), and the scleractinian, Lophelia pertusa (Linnaeus, 1758), were the most commonly observed corals. White vase sponges (most or all in the family Aphrocallistidae) were the most abundant of the sponges (comprising 38.4% observed). We also documented a variety of unidentified foliose, barrel, and other various-shaped sponges. The height of these invertebrates ranged from 20 to 80 cm. Taxa displayed a variety of depth patterns. Some, such as M. farcimen, unidentified white vase sponges, and L. pertusa, were found throughout most or all of the survey depth range, while others (notably the gorgonians L. chilensis, Placogorgia spp., and Acanthogorgia spp.) were found over a relatively narrow range. Invertebrate assemblages tended to be similar among many platforms reflecting species similarities over a broad range of platform depths. Based on these relationships, it is apparent that the assemblages of structure-forming invertebrates varied by depth rather than geography.