USING DATED SHELLS TO ASSESS THE TEMPORAL RESOLUTION OF EPIBIONT ASSEMBLAGES: ARE THEY ECOLOGICAL SNAPSHOTS, OR OVEREXPOSURES?

Individually dated shells have been used to quantitatively estimate the extent of time-averaging in shell accumulations and the relationship between shell age and taphonomy. In this study, we use individually dated articulate brachiopods from the modern southeast Brazilian Bight to investigate the temporal structure of their epibiont assemblages. Epibionts preserve ecological information with high spatial fidelity and resolution, but estimates of their age structure are based on exposure studies and evidence of host-substrate interaction in the fossil record. However, their age structure should be related to the age of the host. For instance, if colonization occurs at a steady rate, older shells with longer exposure histories should host more abundant and diverse epibiont assemblages than younger shells. Alternatively, if colonization occurs within a short interval (either during the life of the host or immediately after its death), epibiont abundance and diversity should not increase as a function of host age.

This study provides a first glimpse into the temporal structure and resolution of epibiont assemblages, using radiocarbon-calibrated amino acid racemization data. The abundance and diversity of epibionts was evaluated for 86 dated valves of the rhynchonelliform brachiopod Bouchardia rosea, spanning an age range of more than 3000 years. Maximum epibiont abundance occurs on shells less than 400 years old, and maximum diversity was observed on shells less than 100 years old. Because the majority of epibionts colonize the interior surfaces of valves, colonization of live hosts by parasites or symbiotes does not explain this pattern. Instead, colonization appears to take place soon after the death of a host, but beyond this initial period, encrustation does not appear to increase over time. This suggests that epibiont assemblages have very high temporal resolution, even when their hosts experience significant time-averaging, and supports arguments that shells are only briefly exposed at the sediment-water interface.