VON DASSOW, Y.J.; Univ. of California, Riverside: Abundance, diversity, and paleoecology of Pleistocene encrusting organisms

The Pleistocene marine invertebrate fossil record of Southern California includes hundreds of mollusc species, which 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 being a major component of the communities. A number of epibionts are soft-bodied organisms that do not fossilize, but those visible in the fossil record include a host of organisms with calcareous shells (�encrusters�), such as barnacles, bryozoans, tube-building gastropods, and tube-building polychaetes. Given the enormous ecological and taxonomic diversity of the molluscan host shells, what can be said about the diversity of the encrusters? The aim of this study is to compare encruster diversity and abundance with that of the hosts, as well as to examine 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 (<130,000 years old): San Diego, Newport Bay, and Goleta. Analysis of material collected from these three sites shows that encrusters can be an important component of organismal abundance even when they are not a major component of species diversity. Also, there is often a marked difference between encrustation patterns on gastropods and on bivalves. For example, spirorbid worms most often appear on gastropods�probably a result of the propensity for empty shells to be occupied by hermit crabs, which often have a commensal relationship with the spirorbids. Also, bryozoans appear more often on bivalve shells than on gastropods, possibly because the shell surfaces of bivalves are generally flatter and smoother.