Effect of a continuous supply of the toxic dinoflagellate Alexandrium minutum Halim on the feeding behavior of the Pacific oyster (Crassostrea gigas Thunberg)

The Pacific oyster Crassostrea gigas is sporadically subject to summer bloo ms of the toxic dinoflagellate Alexandrium minutum along French coasts. To account for differences in toxin accumulation between oysters and mussels, trials were performed using a recirculating flume. All results were analyze d using two linear models, respectively describing the effect of food chang e (step function) and comparing the effects of A. minutum diet with a contr ol. Feeding oysters with A. minutum for 8 to 15 days induced significant in hibition (as compared with a nontoxic dinoflagellate, Scrippsiella trochoid ea) of shell valve activity, clearance rate, filtration rate and biodeposit ion rate. Nevertheless, some "tank effect" may have been involved. Regardle ss of the exposure period of oysters to A. minutum, no "compensation" pheno menon was observed; that is, no return to higher shell valve activity compa rable to that noted with nontoxic species. When the A. minutum diet was fol lowed by one based on flagellates (Isochrysis galbana, Tetraselmis suecica) or diatoms (Skeletonema costatum, Thalassiosira weissflogii), most respond ed with an immediate and significant increase in shell valve activity. A de creasing trend was observed with flagellate- but not with diatom-based diet s during a 15-day "detoxification" period. Clearance and filtration rates w ere unchanged or only slightly modified with flagellates or diatoms; wherea s, the biodeposition rate was significantly increased with diatoms but not with flagellates. Skeletonema costatum seems to be the most efficient diet if steady increases in both valval activity and biodeposition rates are con sidered as positive physiological changes in oyster feeding behavior.