SELECTIVE FEEDING IN SHELLFISH - SIZE-DEPENDENT REJECTION OF LARGE PARTICLES WITHIN PSEUDOFECES FROM MYTILUS-EDULIS, RUDITAPES-PHILIPPINARUM AND TAPES-DECUSSATUS

Mytilus edulis L., Ruditapes philippinarum (Adams & Reeve) and Tapes d ecussatus L. were fed particles of the same shape (spherical), the sam e density (2.1 g cm(-3)) and the same chemical composition (SiO2), but which varied in diameter from 5 to 37 mu m. Findings obtained at diff erent particle concentrations (mean +/- SD) of 51 +/- 2, 105 +/- 18 an d 171 +/- 17 mg 1(-1) invariably indicate that significant proportions of all particles with diameters larger than from between 7.5 and 22.5 mu m were preferentially rejected as pseudo-faeces. We define the pre ferential ingestion index (PII) as the ratio between average particle volume in pseudo-faeces and average particle volume in food. Whatever the particle concentration or the species, this PII was always statist ically higher than 1. Irrespective of particle concentration, PII valu es in M. edulis were lower than in T. decussatus (averages of 1.2 and 2, respectively). PII values in M. edulis were also lower than in R. p hilippinarum maintained at particle concentrations above 171 +/- 17 mg 1(-1). We suggest that preferential size-dependent rejection of large r particles could be of significant adaptive value in the natural envi ronment, either if there are large inorganic particles, or if the aver age organic content of smaller particles is higher.