Domoic Acid Uptake and Elimination Kinetics in Oysters and Mussels in Relation to Body Size and Anatomical Distribution of Toxin

Toxin accumulation by suspension-feeding qualifier depends on a balance between processes regulating toxin uptake (i.e. ingestion and absorption of toxic cells) and elimination (i.e. egestion, exchange among tissues, excretion, degradation and/or biotransformation) during exposure to toxic blooms. This laboratory study compares the size-specific uptake and elimination kinetics of domoic acid (DA) from Pseudo-nitzschia multiseries in two co-occurring bivalves, the oyster Crassostrea virginica and the mussel Mytilus edulis. Domoic acid concentrations were measured in visceral and non-visceral tissues of different-sized oysters and mussels during simultaneous long-term exposure to toxic P. multiseries cells in the laboratory, followed by depuration on a non-toxic algal diet. Mussels attained 7-17-fold higher DA concentrations than oysters, depending on the body size and exposure time, and also detoxified DA at higher rates (1.4-1.6 d(-1)) than oysters (0.25-0.88 d(-1)) of a comparable size. Small oysters attained markedly higher weight-specific DA concentrations (maximum=78.6 μg g(-1)) than large, market-sized individuals (≤ 13 μg g(-1)), but no clear relationship was found between body size and DA concentration in mussels (maximum=460 μg g(-1)). Therefore, differential DA accumulation by the two species was, on average, approximately 3-fold more pronounced for large bivalves. An inverse relationship between DA elimination rate and body size was established for oysters but not mussels. Elimination of DA was faster in viscera than in other tissues of both bivalves; DA exchange rate from the former to the latter was higher in oysters. The contribution of viscera to the total DA burden of mussels was consistently greater than that of other tissues during both uptake (>80%) and depuration (>65%) phases, whereas it rapidly decreased from 70-80% to 30-40% in oysters, and this occurred faster in smaller individuals. Residual DA concentrations (≤ 0.25 μg g(-1)) were detected at later depuration stages (up to 14 d), mainly in viscera of oysters and non-visceral tissues of mussels, suggesting that a second, slower-detoxifying toxin compartment exists in both species. However, a simple exponential decay model was found to adequately describe DA elimination kinetics in these bivalves. The lower capacity for DA accumulation in oysters compared to mussels can thus only be explained by the former's comparatively low toxin intake rather than faster toxin elimination.