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 p roportions of algae and silt, as well as in the natural environment at high seston concentrations up to 64.37 mg/l. Crassostrea gigas did no t adjust retention efficiency according to the food quality, gill poro sity being controlled by the concentration of particulate matter. At l ow seston load (1.34 mg/l), the oyster showed no change in retention f or particles larger than 3-4 mum (Equivalent Spherical Diameter, ESD). But at the highest concentration (64.37 mg/1), it could only retain p articles larger than 12 mum (ESD) with maximum efficiencies. The Japan ese oyster therefore has an active response to high particle concentra tion, and which was described by a model where retention efficiency (E r) varied with suspended particulate matter (Vol mm3/l) as follows: Er = - 12.537 Vol e(-0.304 ESD) + 100