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 feeding behaviour of individual molluscs was used to study oysters exposed to mixed diets composed of varying proportions of the diatom Thalassiosira weissflogii and two strains (toxic and nontoxic) of the dinoflagellate Alexandrium larnarense. Our results show that, when compared to a T. weissflogii unialgal diet, even a diatomltoxic dinoflagellate ratio as low in biomass as 90/10 reduced clearance rates and biodeposit production by oysters. Consumption was slightly but significantly decreased for a 50/50 diatomltoxic dinoflagellate mixture. For the toxic dinoflagellate unialgal diet, ingestion, absorption and consumption were completely inhibited. Thus, the inclusion of low amounts of toxic A. lamarense in a diet composed of T. weissflogii significantly altered pseudofaeces production and probably oyster filtering capacity , whereas no significant effect was observed with the non-toxic dinoflagellate.