Epizoic zonation on growing scallop shells in McMurdo Sound, Antarctica

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Shells of living molluscs provide replicate natural substrata for interpreting the distributions, abundances and dynamics of fouling assemblages. Sessile epizoic species on growing Antarctic scallop shells ( Adamussium 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 less than 35 mm in shell height were not found above 10 m. The proportions of epizoic annelida, chordata, cnidaria, and porifera also were significantly reduced above 10 m. The depth zonation of these epizoic species suggests that they were limited in shallow water by environmental factors such as siltation or geochemical changes associated with seasonal pulses of glacial meltwater. Between shell habitats, epizoic macrofaunal biomasses distinctly increased on scallops larger than 65 mm which were recessing into the sediment surface. Within these older shell habitats, the arborescent epizoic demosponge ( Homaxinella balfourensis 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 growth rate estimates were comparable to those determined by independent fouling experiments with inanimate substrates. This study demonstrates that epizoic assemblage variability can be interpreted between and within habitats on growing mollusc shells.