Kinetics of Alexandrium minutum Halim toxin accumulation in mussels and clams

Mussels (Mytilus edulis) and clams (Ruditapes philippinarum) were contaminated experimentally using cultures of Alexandrium minutum, a toxic dinoflagellate isolated from French coastal waters. Experiments were carried out in continually flushed and open-circuit systems using Alexandrium densities of 100 to 700 cells/ml delivered to tanks containing the shellfish. All experiments indicated an inversion of the relative proportions of gonyautoxins (GTX2 and GTX3) in shellfish meat during decontamination, whereas saxitoxin (STX) only accumulated during mussel depuration. However, in mussels a density as low as 100 cells/ml led within 10 days to bioaccumulation of paralytic shellfish poisoning (PSP) toxins above the public health threshold. Similar results were observed in clams subjected to fivefold higher cell densities, indicating a less effective assimilation of the dinoflagellate than by mussel. Decontamination experiments on PSP toxin-contaminated mussels (360 micrograms STX eq./100 g or lower uptake) fed two nontoxic diets (1,000 and 10,000 cells/ml of Tetraselmis suesica) showed an appreciable reduction in the time needed to decrease toxin concentration below the accepted threshold for human consumption. We suggest that a simple relation can be established between initial toxicity, the concentration of nontoxic alga available, and the time required for depuration once decontamination kinetics becomes linear and corresponds to the inverse of contamination kinetics.