Comparative efficiencies of different non-toxic microalgal diets in detoxification of PSP-contaminated oysters (Crassostrea gigas Thunberg).

Experimental PSP contamination of adult Pacific oysters (Crassostrea gigas) by the toxic dinoflagellate Alexandrium minutum Halim (120 cells.mL-1 continuously maintained in each flume) was carried out in a recirculated seawater system to obtain toxin levels above the safety threshold. In these conditions, 150 to 300 micrograms STX.eq.100 g-1 of shellfish tissues were produced at 16 degrees C within 8 to 15 days, corresponding to field values observed along French coasts. Diets based on non-toxic flagellates or diatoms were then used to detoxify the contaminated oysters. Despite large individual variations in toxin levels at the end of the contamination period, detoxification times were of the same order of magnitude (3 to 4 days), reaching a toxin level equal to or less than the safety threshold. These variations were most likely related to marked individual variability in valve and/or clearance activities. No significant differences in detoxification rates were found when oysters were fed Isochrysis galbana, Tetraselmis suecica, Thalassiosira weissflogii, or Skeletonema costatum. The different biochemical compositions of each algal species appeared to have no significant effect on detoxification rates. GTX2/GTX3 were the dominant compounds found in shellfish tissues during depuration, whereas C toxins were quite low (< 2 micrograms STX.eq.100 g-1) and STX or NeoSTX undetectable. These results do not suggest any bioconversion of paralytic toxins but indicate good correlation between the toxin composition of Alexandrium and oyster tissues.