This paper presents the results from an experiment, where uptake and elimination of diarrhetic shellfish toxins (DST) of the okadaic acid (OA) and pectenotoxin (PTX) groups were compared between blue mussels (Mytilus edulis) and European oysters (Ostrea edulis). Caged mussels and oysters were suspended in the water column and exposed to a dense bloom of Dinophysis acuta (500–2000 cells L-1) for 4 wk, which was followed by detoxification in the laboratory during 7 wk. Weekly sampling and analysis of OA-group toxins including fatty acid esters (‘DTX-3’) as well as PTX in individual shellfish and plankton samples were performed. The results showed that mussels rapidly accumulated OA-group toxins to levels about 10 times above the regulation limit (160 µg OA kg-1 mussel) whereas concentrations never reached this limit in oysters during the field exposure. Overall, levels were 10–50 times greater in mussels. The OA-group toxins were mainly in the form of esters (>90%) in oysters, whereas in mussels, the esters constituted only a minor proportion of total OA toxin levels. Reduction rates were estimated for each OA toxin to evaluate if faster elimination could explain the lower toxin retention in oysters. However, no consistent species-specific difference in reduction rates were observed, but esters of OA appeared to be reduced at a faster rate in oysters (t1/2 = 23 days) compared with mussels (t1/2 = 35 days). In both species, the free form of OA was eliminated at a faster rate (t1/2=15–17 days) compared with free DTX1 (t1/2 = 23–31 days) and DTX2 (t1/2 = 28–33 days). Slightly slower elimination rates were estimated for the ester forms (t1/2 = 23–42 days). Regarding PTX, PTX2 seco acid (PTX2 SA) was the major PTX detected in both species, but small amounts of PTX2, PTX12 and PTX12 SA were also found. As for the OA-group toxins, oysters generally contained lower total amounts of PTX compared with mussels, but the difference was much less apparent. Estimation of reduction rates of the different PTX compounds showed that these toxins were rapidly eliminated in both oysters and mussels (t1/2 = 6–13 days). We propose that differential rates of gut assimilation and/or biotransformation of the OA-group and PTX explain some of the observed differences in retention and toxin profiles between the bivalves, rather than differences in elimination rates. However, models related to differences in feeding rates, particle selection and behavioural response to toxic algae should be tested in future experiments to evaluate the importance of preingestive mechanisms to the differential toxin retention in these bivalves.From the industrial perspective, our results suggest that O. edulis may be regarded as a low-risk species for DST contamination, which should be taken into consideration by regulatory authorities in charge of sampling frequencies and monitoring programs for shellfish toxins.
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Vol. 28 • No. 2