FEEDING RESPONSES OF CRASSOSTREA-GIGAS (THUNBERG) TO INCLUSION OF DIFFERENT PROPORTIONS OF TOXIC DINOFLAGELLATES IN THEIR DIET

An experimental flow-through system allowing determination of the feed ing behaviour of individual molluscs was used to study oysters exposed to mixed diets composed of varying proportions of the diatom Thalassi osira weissflogii and two strains (toxic and nontoxic) of the dinoflag ellate Alexandrium tamarense. Our results show that, when compared to a T. weissflogii unialgal diet, even a diatom/toxic dinoflagellate rat io as low in biomass as 90/10 reduced clearance rates and biodeposit p roduction by oysters. Consumption was slightly but significantly decre ased for a 50/50 diatom/toxic dinoflagellate mixture. For the toxic di noflagellate unialgal diet, ingestion, absorption and consumption were completely inhibited. Thus, the inclusion of low amounts of toxic A. tamarense in a diet composed of T. weissflogii significantly altered p seudofaeces production and probably oyster filtering capacity, whereas no significant effect was observed with the non-toxic dinoflagellate.