Contamination of shellfish by paralytic shellfish poisoning (PSP) toxins poses an economic threat to shellfish farmers. As contaminated shellfish cannot be harvested for long periods of time, it would be very useful to develop processes to optimise and shorten their detoxification. In this study, Pacific oysters Crassostrea gigas were first experimentally contaminated over a period of 13 days with a continuous flow of toxic Alexandrium minutum cultures at concentrations ranging from 150 to 200 cell ml-1 (toxin content after 13 days of contamination 438 $\mu $ g STX equiv. 100 g−1 wet weight). Then, two different detoxification treatments were tested and showed detoxification rates greater than those observed in coastal environments. The first treatment consisted of feeding oysters on Skeletonema costatum, at a concentration of 2000 cell ml−1 to speed up detoxification rates. The second detoxification method used the same Skeletonema costatum diet, supplemented with silt particles at a concentration of 20 mg L−1. A control was also set up by placing contaminated oysters in seawater with no additional algal food. The detoxification experiment lasted 8 days. Toxin contents were analysed by liquid chromatography with fluorescence detection (LC-FD). The S. costatum diet significantly reduced the time needed for oysters to reach the sanitary threshold (80 $\mu $ g STX equiv. 100 g−1 wet weight), but no effect of the silt supplement could be demonstrated conclusively. These different detoxification methods did not influence toxin biotransformations as observed in oyster tissues, i.e. epimerisation and decarbamoylation of gonyautoxins 2 and 3.

Adult oysters Crassostrea gigas were experimentally fed with Alexandrium catenella and Alexandrium minutum which are responsible for recurrent toxic blooms in French coastal waters. C. gigas produced faeces and pseudofaeces containing intact and viable temporary pellicular cysts of these two Paralytic toxin producing species. When incubated in favourable conditions, these pellicular cysts were able to germinate at high rates (between 74 and 94%) and the resulting vegetative cells divided with growth rates close to the non- ingested cells (control). The toxin profile of the vegetative cells originated from the germinated temporary cysts was analyzed by liquid chromatography/fluorescence detection. Total toxin content of newly germinated cells was lower than that of cultured cells. Besides, cell contents of C2, B1, B2 and dcGTX3 toxins featured some changes. Our results suggest that the increased spreading of toxic dinoflagellates through the transfer of shellfish from contaminated towards pristine coastal areas cannot be ruled out. We also suggest that pellicular cysts and newly germinated cells could represent a potential way for the transfer of paralytic toxins toward the higher trophic levels.

Pacific oysters and king scallops placed individually in a recirculating flume were fed for 2 weeks with a constant concentration (120 cell ml–1) of a toxic strain of Alexandrium minutum. Fluorescence at the outlet of each experimental unit was measured continuously, and biodeposits were recovered twice daily to evaluate feeding time activity (FTA) and rates of organic filtration (OFR), ingestion (OIR) and organic absorption (OAR). Ion-pairing high performance liquid chromatography (IP-HPLC) was performed concurrently to quantify paralytic phycotoxins both (i) individually in shellfish at the end of the contamination period and (ii) in A. minutum cultures to estimate cellular toxin concentration. These data allowed comparison of the actual tissue toxin concentration (TOX) with the theoretical toxin accumulation rate (TAR) calculated from the linear relations between OAR, cell number, fluorescence and cell toxicity. The results show high FTA/TAR and FTA/TOX correlations (R2 = 0.78) for both oysters and scallops. The TAR/TOX relation, though not yet clearly defined, suggests the minimum quantity of absorbed toxin at which the accumulation process is induced.