L Journal of Experimental Marine Biology and Ecology, 219 (1998) 105–125 Physiological responses of infaunal ( Mya arenaria) and epifaunal ( Placopecten magellanicus) bivalves to variations in the concentration and quality of suspended particles I. Feeding activity and selection a a, b * Gregory S. Bacon , Bruce A. MacDonald , J. Evan Ward a Biology Department and Centre for Coastal Studies and Aquaculture, University of New Brunswick, P .O. Box 5050, Saint John, New Brunswick, E2L 4L5 Canada b Department of Biological Sciences, Salisbury State University, Salisbury, MD 21801, USA Abstract Placopecten magellanicus, an epibenthic scallop species, and Mya arenaria, an infaunal clam species were studied simultaneously to compare their feeding activity and particle selection in response to variations in the quantity and quality of suspended food. These two bivalve species 21 were exposed to four concentrations of seston (1, 3, 7, and 14 mg l ) 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. Clearance rates for both species decreased as the concentration and organic 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 ingested. The ability to select organic particles over inorganic ones was independent of seston concentration but diminished as the organic content of the seston decreased. When exposed to increasing seston concentrations, scallops reduce clearance rates and increase pseudofaeces production resulting in maximum 21 ingestion rates at approximately 7 mg l . Clams also reduce clearance rates as concentration increased but maintain low pseudofaeces rejection rates thus enabling ingestion to continue to 21 increase to at least 14 mg l . M. arenaria appears to be better suited than P . magellanicus to ingest organic material at higher concentrations of low quality seston.  1998 Elsevier Science B.V. Keywords: Bivalves; Clams; Feeding; Pseudofaeces; Scallops; Selection * Corresponding author. Tel.: 11 (506) 648-5620; fax: 11 (506) 648-5650; e-mail: bmacdon@unbsj.ca 0022-0981 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII S0022-0981(97)00177-9