COMPETITIVE INTERACTIONS AND THEIR TAPHONOMIC CONSEQUENCES AMONG THE ENCRUSTER GUILD

Encrusting organisms provide valuable environmental information for the paleontologist, from paleo-depth inferences to paleoecological reconstructions. Encrusters on hard substrata also provide a database for community analysis, as biota are preserved in situ within their habitat. One such hard substrate, gastropod shells occupied by hermit crabs, provides a novel mobile habitat for modular suspension and filter-feeding encrusting organisms, such as skeletonized bryozoans and hydractinians. Some of these encrusters are restricted to only hermitted shells, while other encrusters are more facultative in their distribution (occurring on rocks, and other hard substrata). An evolutionary question arises as to whether the obligate modular organisms have competitive advantage over facultative encrusting organisms.

To solve this question, overgrowth interactions were used to compare between two encrusting strategies: that of obligate versus facultative encrusters occupying hermitted shells. McKinney's overgrowth equation (McKinney, 1995, Palaios 10: 279-282) was developed to determine relative rates of interactions among modular organisms on hard substrata, taking into account time averaging. As he stated, if hard substrates remain available for settlement for several seasons, then taphonomic processes should provide for a conservative estimate of overgrowth relationships and competitive superiority. Using modern hermit-crab inhabited shells from Sapelo Island, Georgia, McKinney's overgrowth equation was used to test whether two obligate encrusters were statistically more likely to have more overgrowth relationships than facultative encrusters, and were thus competitively superior. Evolutionary predictions suggest that the obligate encrusters would be competitively superior, having evolved for millions of years (see Walker, 1992, Journal of Paleontology 66: 535-558) with their symbiotic host (the hermit crab) while the facultative species would be least competitive. Results will be presented in relation to the taphonomic and biotic factors (i.e. shell species encrusted, hermit crab species) that affect the competitive outcomes for obligate versus facultative modular encrusters.