EPIZOIC ZONATION ON GROWING SCALLOP SHELLS IN MCMURDO SOUND, ANTARCTICA

Shells of living molluscs provide replicate natural substrata for inte rpreting the distributions, abundances and dynamics of fouling assembl ages. Sessile epizoic species on growing Antarctic scallop shells (Ada mussium colbecki Smith) were analyzed in this study across a nearshore depth gradient from 0 to 30 m. Significantly lower epizoic biomasses were found at depths shallower than 20 m. Byssally-attached scallops l ess than 35 mm in shell height were not found above 10 m. The proporti ons of epizoic annelida, chordata, cnidaria, and porifera also were si gnificantly reduced above 10 m. The depth zonation of these epizoic sp ecies suggests that they were limited in shallow water by environmenta l factors such as siltation or geochemical changes associated with sea sonal pulses of glacial meltwater. Between shell habitats, epizoic mac rofaunal biomasses distinctly increased on scallops larger than 65 mm which were recessing into the sediment surface. Within these older she ll habitats, the arborescent epizoic demosponge (Homaxinella balfouren sis Ridley and Dendy) aggregated in a zone near the peripheral margin of the shells. The estimated growth of Homaxinella ''trees'' on the 1- 2-yr-old growth bands of the Adamussium shells averaged 60.8 +/- 33.0 mm . yr-1 with a maximum exceeding 130 mm . yr-1. These Homaxinella gr owth rate estimates were comparable to those determined by independent fouling experiments with inanimate substrates. This study demonstrate s that epizoic assemblage variability can be interpreted between and w ithin habitats on growing mollusc shells.