EXPERIMENTAL-STUDY OF THE EFFECTS OF A TOXIC MICROALGAL DIET ON FEEDING OF THE OYSTER CRASSOSTREA-GIGAS THUNBERG

The aquaculture industry is often faced with the problem of toxic microalgal blooms that can cause human poisoning after contamination of cultivated shellfish. The oyster Crassostrea gigas, because of its particular sensitivity to dinoflagellates that produce paralytic toxins, was chosen as a model for study of the absorption of different monospecific algal diets of varying toxicity. A continuous, flow-through system was used to expose batches of shellfish successively to nontoxic and toxic diets of Alexandrium tamarense (7 200 ng STX eq. per 10(6) cells). The same experiment was repeated with other batches for cultures of Scrippsiella trochoidea (nontoxic) and Alexandrium minutum (500 ng eq. STX per 10(6) cells). The results indicate that the clearance rate decreases in the order Scrippsiella trochoidea > A. minutum > nontoxic A. tamarense > toxic A. tamarense. It was difficult to determine the exact nature of the physiological process enabling oysters to perform selective feeding since particle size probably interfered with toxin effects. However, C. gigas would seem to prefer very toxic Alexandrium cells during absorption in the digestive tube, perhaps because of higher nutritive value or easier digestibility.