ALBERT

Description: Study of the assemblage of encrusting organisms on co-occurring disarticulated valves of the bivalves Crassostrea virginica and Mercenaria mercenaria in Bogue Sound, North Carolina, indicates that there is little or no substrate specificity among the encrusting organisms but that the shape of the shells has an important influence on how extensively members of each higher taxon collectively inhabit the shells. Encrusting bryozoans, a dense low mat composed of many species from diverse phyla, and a unicellular film cover most of the area of both exterior and interior surfaces. The encrusting bryozoans most extensively cover both surfaces of C. virginica but are in second place behind the multispecies mat on exterior surfaces of M. mercenaria and behind the unicellular film on its interior surfaces. These differences are inferred to result from different physical stability of valves of the two bivalve species, which exhibit different frequencies of circumrotatory growth.Degradation of the assemblage by sodium hypochlorite, to simulate loss of organic matter during fossilization, results in the complete loss of encrusting sponges, erect hydrozoans, erect bryozoans, and ascidians. Loss of these taxa results in overexposure and more apparently uniform distribution of skeletal taxa with respect to their surface representation in living assemblages and also in complete loss of the higher tiers present in the living assemblage. However, indications of the original structural organization of the living assemblage is indicated by preservation of the most abundant taxa in the lower tiers and by the retention in the reduced treated assemblage of the patterns of distribution that characterized the living assemblage.