PHYSIOLOGICAL-RESPONSES OF INFAUNAL (MYA ARENARIA) AND EPIFAUNAL (PLACOPECTEN-MAGELLANICUS) BIVALVES TO VARIATIONS IN THE CONCENTRATION ANDQUALITY OF SUSPENDED PARTICLES-I - FEEDING-ACTIVITY AND SELECTION

Placopecten magellanicus, an epibenthic scallop species, and Mya arena ria, an infaunal clam species were studied simultaneously to compare t heir feeding activity and particle selection in response to variations in the quantity and quality of suspended food. These two bivalve spec ies were exposed to four concentrations of seston (1, 3, 7, and 14 mg l(-1)) at each of three levels of organic quality (25, 50 and 80% POM) . Experimental diets consisted of mixtures of microalgae and silica at concentrations typical of those observed in their natural habitats. C learance rates for both species decreased as the concentration and org anic content of the seston increased. Both species have the ability to reject poorer quality inorganic particles, through the production of pseudofaeces, and significantly improve the quality of the particles i ngested. The ability to select organic particles over inorganic ones w as independent of seston concentration but diminished as the organic c ontent of the seston decreased. When exposed to increasing seston conc entrations, scallops reduce clearance rates and increase pseudofaeces production resulting in maximum ingestion rates at approximately 7 mg l(-1). Clams also reduce clearance rates as concentration increased bu t maintain low pseudofaeces rejection rates thus enabling ingestion to continue to increase to at least 14 mg l(-1). M. arenaria appears to be better suited than P. magellanicus to ingest organic material at hi gher concentrations of low quality seston. (C) 1998 Elsevier Science B .V.