NO INFLUENCE OF FOOD QUALITY, BUT RATION-DEPENDENT RETENTION EFFICIENCIES IN THE JAPANESE OYSTER CRASSOSTREA-GIGAS

Relative retention efficiency (Er) in Crassostrea gigas was estimated in the laboratory with seven artificial diets consisting of variable proportions of algae and silt, as well as in the natural environment at high seston concentrations up to 64.37 mg/l. Crassostrea gigas did not adjust retention efficiency according to the food quality, gill porosity being controlled by the concentration of particulate matter. At low seston load (1.34 mg/l), the oyster showed no change in retention for particles larger than 3-4 mum (Equivalent Spherical Diameter, ESD). But at the highest concentration (64.37 mg/1), it could only retain particles larger than 12 mum (ESD) with maximum efficiencies. The Japanese oyster therefore has an active response to high particle concentration, and which was described by a model where retention efficiency (Er) varied with suspended particulate matter (Vol mm3/l) as follows: Er = - 12.537 Vol e(-0.304 ESD) + 100