Selective feeding in shellfish: size-dependent rejection of large particles within pseudofaeces from Mytilus edulis, Ruditapes philippinarum and Tapes decussatus

Mytilus edulis L., Ruditapes philippinarum (Adams & Reeve) and Tapes decussatus L. were fed particles of the same shape (spherical), the same 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 different particle concentrations (mean +/- SD) of 51 +/- 2, 105 +/- 18 and 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 preferential 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 statistically higher than 1. Irrespective of particle concentration, PII values 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. philippinarum maintained at particle concentrations above 171 +/- 17 mg 1(-1). We suggest that preferential size-dependent rejection of larger particles could be of significant adaptive value in the natural environment, either if there are large inorganic particles, or if the average organic content of smaller particles is higher.