Relationship between pumping rate and particle capture efficiency in three species of bivalves

For suspension-feeding bivalves, capture efficiency (CE) is the proportion of a given type of particle that is cleared from the water by the gill filaments of suspension-feeding bivalves compared to other particles that are inhaled. The majority of research on CE variability in suspension-feeding bivalves has focused on particle characteristics (e.g. size, surface properties). This study was designed to explore CE as a function of particle size (of natural seston) and pumping rate (PR), as a proxy for fluid velocity. Bivalve species from different families were chosen for their differences in gill structure: Mytilidae (Mytilus edulis), Ostreidae (Crassostrea virginica), and Pectinidae (Placopecten magellanicus). Structural variation in the gills was hypothesized to influence any relationship between PR and CE. Experiments estimating PR and CE were replicated in both laboratory and field conditions. Results demonstrated that PR may influence CE in bivalves and that this relationship is dependent upon particle size and bivalve species (i.e. gill structure). For C. virginica, CE increased with PR (range: 0.4-6.9 l h(-1)) for particles between 2.25 and 7.25 in laboratory experiments and 4.75 and 8.25 mu m in the field. However, for M. edulis and P. magellanicus, no relationship was observed between PR and CE. Among the mechanisms by which particles can be removed from a fluid by a filter, these findings agree qualitatively with the capture mechanism of direct interception applying to all species where CE depends on particle size, and inertial impaction additionally applying to C. virginica where CE depends on fluid velocity and particle size in the 2.25 to 8.25 mu m range.