Behavioral and physiological responses to PSP toxins in Mya arenaria populations in relation to previous exposure to red tides

Paralytic shellfish poisoning (PSP) poses a severe human health risk worldwide and can also adversely affect bivalve populations. This study investigates the intraspecific variation in sensitivity to paralytic shellfish toxins (PSTs) and in toxin accumulation capacity between 2 populations with contrasting histories of PSP in the softshell clam Mya arenaria, a species widely distributed in Atlantic North America. We determine the magnitude and potential ecological consequences of intrinsic variation in toxin susceptibility in M. arenaria, known to have a genetic basis, and the implications for prediction and management of PSTs in regions affected or threatened by PSP expansion. Burrowing, feeding, oxygen consumption (VO2), toxin uptake and survival of 2 test populations were compared during 2 to 3 wk of laboratory exposure to a high-toxicity Alexandrium tamarense strain. Most clams from Lepreau Basin, Bay of Fundy (BF), an area with a long-term history of annual PSP events, exhibited high resistance measured by these parameters, relative to naive clams from the Lawrencetown Estuary (LE). These were highly sensitive to PSTs, as reflected in significantly reduced clearance and VO2 rates; they also failed to acclimate to the presence of toxins. BF clams attained significantly higher (up to 10-fold) tissue toxicities than LE clams. Toxicity of individual clams from the 2 populations varied up to 40-fold under the same experimental conditions. Toxin-induced mortalities were consistently higher among LE clams (up to 30%) compared to BF clams (2 to 8%). Our findings support the hypothesis that red tides result in natural selection for resistance to PSTs in natural populations.