UTILIZATION OF MICROBIAL ORGANIC AGGREGATES BY BAY SCALLOPS, ARGOPECTEN IRRADIANS (LAMARCK)

The use of organic aggregates as a food source by bay scallops, Argope cten irradians (Lamarck), was quantified and compared with the use of phytoplankton, a known good food. In two experiments, the first design ed to characterize nitrogen incorporation and the second nitrogen rele ase, N-15 was used to trace N into and out of scallops under defined, laboratory conditions. Scallops were fed experimental diets of labelle d organic aggregates, produced from the dissolved material released by either a brown (Fucus vesiculosis) or red (Gracilaria tikvahiae) alga , or phytoplankton, Thalassiosira weissflogii (Grunow), for periods of 5-15 h in a flow-through system. More N was incorporated by scallops fed phytoplankton (523 mu g . gDW(-1) . h(-1)) than by those fed aggre gates (70 mu g . gDW(-1) . h(-1) for aggregates derived from F. vesicu losis and 306 mu g . gDW(-1) . h(-1) for those derived from G. tikvahi ae). On both diets, fecal material and N-15 was released rapidly durin g the first 6 h of depuration. It was estimated that scallops incorpor ated aggregate N with an absorption efficiency of 77 to 79% as compare d with an efficiency of 90% for phytoplankton N. Estimated assimilatio n efficiency ranged from 14 to 43% for scallops fed aggregates, and 75 % for those fed phytoplankton. Despite these differences, aggregates r epresent a potentially important source of nutrition for suspension-fe eding bivalves living in near-shore regions rich in natural seston.