THE PLEISTOCENE ENCRUSTER SCENE: COMPARISON OF EPIBIONTS ON GASTROPOD AND BIVALVE SHELLS

The Pleistocene marine fossil record of Southern California includes hundreds of mollusc species that have been analyzed with regard to taxonomy, biogeography, and a number of other factors. However, the epibionts on their shells have been largely overlooked, despite the fact that the shells provide an enormous amount of potential epibiont habitat. Because gastropods and bivalves have very different life habits and undergo different post-mortem processes, we asked the following questions: do encrusting rates differ between bivalves and gastropods, and are the same patterns seen in different fossil assemblages? We recorded encruster diversity and abundance, and examined the differences in encrusting patterns between gastropod-dominated and bivalve-dominated shallow marine assemblages. The fossils in this study are from three Southern California sites of Late Pleistocene age: Point Loma (rocky subtidal), Newport Bay (protected soft-bottom), and Isla Vista (protected fine mud). Despite the fossiliferous nature of all three molluscan assemblages, encrusting organisms are somewhat sparsely represented. The same encruster taxa—oysters, spirorbid worms, and bryozoans—are present in all three assemblages, though in different proportions. Point Loma has the highest percentage of encrusted shells (15.4% of 1432 gastropods, 7.5% of 174 bivalve shells; 14.5% of 1606 total molluscan elements), presumably because it has the highest number of gastropod shells available for use by hermit crabs, which promote settlement of spirorbid worms. Isla Vista has the smallest percentage of encrusted shells (6.9% of 29 gastropods, 0% of 84 bivalve shells; 1.8% of 113 total molluscan elements), probably because this locality also has the highest proportion of infaunal organisms. Newport Bay is the only site in which a higher proportion of bivalve shells are encrusted than gastropods (0.9% of 460 gastropods, 3.9% of 1436 bivalve shells; 3.2% of 1896 total molluscan elements). The data suggest that encrusting rates do differ between bivalves and gastropods, but not in a consistent way. The differences among assemblages are likely to be due to differences in habitat and taphonomy rather than differences in availability of shell substrate.