HOME ON THE HALF-SHELL: ENCRUSTATION OF BRACHIOPODS AND BIVALVES FROM THE SUBTROPICAL SHELF OF THE SOUTHEAST BRAZILIAN BIGHT

The macroevolutionary history of encrusting organisms (epibionts) cannot be thoroughly evaluated without an assessment of the role played by the substrates available to them through geologic time. Encrustation patterns have been studied for both modern bivalve mollusks and for Paleozoic articulate brachiopods, but are these two systems truly comparable?

In order to investigate this question, we performed a quantitative analysis of encrustation on the shelly benthos of the Southeast Brazilian Bight. The inner shelf of this tropical to subtropical passive margin includes abundant articulate brachiopods as well as bivalve mollusks. This setting therefore provides an unparalleled opportunity to study the naturally occurring encrustation patterns and ecology of epibionts colonizing brachiopods and bivalves.

The encrusting fauna of the inner shelf is similar to those reported elsewhere for modern bivalves, dominated by bryozoans and calcareous worm tubes (serpulids and spirorbids), with minor and variable roles played by bivalves, barnacles, foraminifera, algae and other taxa. Infaunal habits among bivalves could result in lower encrustation frequencies than in epifaunal bivalves, but this may not affect the patterns observed as valve interiors are preferentially encrusted, and therefore occurs post-mortem. The relationship of shell size to epibiont diversity and encrustation intensity can be evaluated independently for bivalve and brachiopods, in order to identify substrate preferences among epibiont taxa.