Utilization of microbial organic aggregates by bay scallops, Argopecten irradians (Lamarck)

The use of organic aggregates as a food source by bay scallops, Argopecten irradians (Lamarck), was quantified and compared with the use of phytoplankton, a known good food. In two experiments, the first designed to characterize nitrogen incorporation and the second nitrogen release, N-15 was used to trace N into and out of scallops under defined, laboratory conditions. Scallops were fed experimental diets of labelled 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 aggregates (70 mu g . gDW(-1) . h(-1) for aggregates derived from F. vesiculosis and 306 mu g . gDW(-1) . h(-1) for those derived from G. tikvahiae). On both diets, fecal material and N-15 was released rapidly during the first 6 h of depuration. It was estimated that scallops incorporated aggregate N with an absorption efficiency of 77 to 79% as compared with an efficiency of 90% for phytoplankton N. Estimated assimilation efficiency ranged from 14 to 43% for scallops fed aggregates, and 75% for those fed phytoplankton. Despite these differences, aggregates represent a potentially important source of nutrition for suspension-feeding bivalves living in near-shore regions rich in natural seston.