ENCRUSTING ORGANISMS ON COOCCURRING DISARTICULATED VALVES OF 2 MARINEBIVALVES - COMPARISON OF LIVING ASSEMBLAGES AND SKELETAL RESIDUES

Study of the assemblage of encrusting organisms on co-occurring disart iculated valves of the bivalves Crassostrea virginica and Mercenaria m ercenaria in Bogue Sound, North Carolina, indicates that there is litt le 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. Encrust ing bryozoans, a dense low mat composed of many species from diverse p hyla, and a unicellular film cover most of the area of both exterior a nd interior surfaces. The encrusting bryozoans most extensively cover both surfaces of C. virginica but are in second place behind the multi species mat on exterior surfaces of M. mercenaria and behind the unice llular film on its interior surfaces. These differences are inferred t o result from different physical stability of valves of the two bivalv e species, which exhibit different frequencies of circumrotatory growt h. Degradation of the assemblage by sodium hypochlorite, to simulate l oss of organic matter during fossilization, results in the complete lo ss of encrusting sponges, erect hydrozoans, erect bryozoans, and ascid ians. Loss of these taxa results in overexposure and more apparently u niform distribution of skeletal taxa with respect to their surface rep resentation in living assemblages and also in complete loss of the hig her tiers present in the living assemblage. However, indications of th e original structural organization of the living assemblage is indicat ed by preservation of the most abundant taxa in the lower tiers and by the retention in the reduced treated assemblage of the patterns of di stribution that characterized the living assemblage.