PARTICLE CLEARANCE AND ABSORPTION OF PHYTOPLANKTON AND DETRITUS BY THE SEA SCALLOP PLACOPECTEN-MAGELLANICUS (GMELIN)

Diets of cultured phytoplankton (Tahitian Isochrysis aff.galbana and Chaetoceros gracilis Schütt), fresh and "aged" kelp powder (Laminaria longicruris de la Pyl.) and resuspended sediment were fed to sea scallops (Placopecten magellanicus Gmelin) from Georges Bank. The majority of particles from each diet were within the size range effectively retained by sea scallops (>5/gmm). Based solely on net and gross absorption efficiency (AE) data as an index of nutritional quality, the following ranking was observed: "aged" kelp debris > C. gracilis = I. galbana > fresh kelp > resuspended sediment. While only a small fraction of the total sediment organic matter was utilized (mean net AE = 9 %), an average of 50 % of available nitrogen was absorbed. Resuspended paniculate matter may play an important role in the energy gain and nitrogen demands of this species. Despite the ability of P. magellanicus to efficiently digest "aged" kelp debris, it was consumed at a much lower rate than phytoplankton and did not contribute significantly to the dietary requirements of the sea scallop. These results demonstrate the importance of phytoplankton as a diet for the sea scallop, but indicate that detrital particles can contribute to energy gain during periods when phytoplankton are less available to meet energy demands. © 1990.