Seasonal, geographic and individual variation of okadaic acid content in cultivated mussels in Sweden

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Abstract
In Western Europe the dinoflagellate toxin, okadaic acid (OA) has been the main cause of diarrheic shellfish poisoning (DSP). Chemical determination of OA in mussels by homogenization of the hepatopancreas, extraction, purification, reaction with 9-anthryldiazomethane (ADAM), HPLC-separation, and fluorometric quantification has been used for weekly monitoring of mussel growing farms and to control harvested mussels. Within a week, sebstantial rises (from 0.41 to 5.4 μg OA/g hepatopancreas) as well as great reductions (from 7.2 to 1.8 μg/g hepatopancreas) were recorded. The rapid rise implies that weekly sampling is not sufficient to ensure that mussels are free from toxic levels of OA. The rapid decrease reveals that efficient toxin clearance mechanisms exist in the mussels. Substantial OA clearance occurs also at low temperatures (1.4-3 °C). Within a mussel growing site the OA concentrations could differ considerably between adjacent mussels (0.63 and 4.2 μg OA/g hepatop.) and even more between mussels grown at different depths along the same rope (0.63 and 10 μg OA/g hepatop.). These data emphasize the importance of sampling in studies on DST in mussels. Great differences between the different mussel growing sites were also observed. These data have been discussed with respect to the spread of the toxin by the sea, and the possibilities of reducing the exposure of the mussels to the toxic algae.