EVEN INVERTEBRATES NEED ELBOW ROOM: SHELL SIZE AND AREA EFFECT ON ENCRUSTATION OF BIVALVES AND BRACHIOPODS FROM THE SOUTHEAST BRAZILIAN BIGHT

Paleontologists have long conducted comparison studies between bivalves and brachiopods, but do their immediate neighbors take note of their physiological and ecological differences? Here we compare the encrustation of brachiopods (Bouchardia rosea) and bivalves (Semele sp.) from the inner shelf of the Southeast Brazilian Bight. Cryptic settlement patterns are common among modern epibionts, and valve interiors of bivalves and brachiopods are preferentially encrusted post-mortem, with little increase in epibiont abundance or diversity with shell age. Encrustation frequencies vary among sites in a similar manner whether the host is bivalve or brachiopod, but brachiopods are more frequently encrusted by a greater number of epibionts per shell. Sediment grain size and composition do not appear to exert environmental controls on encrustation, nor does encrustation vary as a function of water depth alone.

Encrustation patterns appear to differ between bivalves and brachiopods of equal length, but this difference disappears when shells are standardized for surface area. Mean epibiont diversity per shell increases logarithmically with shell area, in a manner analogous to island area effect. Direct comparisons between bivalves and brachiopods would suggest a preference among epibiont larvae for brachiopods over bivalves of equal size if simple length measurements were used, due to substantial differences in shell shape and surface area. However, there are differences in the faunal assemblages colonizing bivalves and brachiopods, indicating some role for larval selectivity on the basis of substrate. These results illustrate the need for cautious and careful consideration of factors influencing colonization of shells in comparative encrustation studies. However, if factors such as shell size and composition, host identity and ecology are taken into account, epibiont communities may preserve a wealth of paleoecological information.