Assessment of the sub-lethal impacts of Karenia brevis on hard clams, Mercenaria campechiensis

Karenia brevis, a toxin-producing dinoflagellate, has been implicated in frequent harmful algal blooms (HABs) in the Gulf of Mexico. Shellfish species, such as Mercenaria campechiensis, have high resilience to K. brevis exposure causing high accumulation and retention of brevetoxins within their tissues. The current study aimed to analyze the accumulation and depuration of brevetoxins in M. campechiensis exposed to 100,000 cells L^-1 for two weeks. The second aim was to identify the potential detoxification enzymes in M. campechiensis as well as potential enzymes representative of stress to see if moderate concentrations lead to stress in exposed M. campechiensis. This was done by exposing M. campechiensis to a continuous flow of K. brevis, followed by a 47-day depuration period. There was an increase in brevetoxin concentration during the exposure with a rapid decrease at the onset of the depuration period. This was followed by a gradual decrease with detectable toxins in the exposed clam tissue for the duration of the experiment. While there was detectable toxin during the depurations, the concentrations were below the NSP level for BTX-2 by day 14 indicating M. campechiensis would potentially be safe for consumption after two weeks following exposure to a moderate K. brevis concentration. Fold change of AChE activity significantly increased in the exposure tank when compared to the controls following the exposure period. Fold change of GST activity from control significantly decreased in the exposure tank compared to the controls on 14 d exposure. Both changes in enzyme activity support the findings that a moderate concentration of K. brevis may induce stress in M. campechiensis. Ultimately, the results of the current study further highlight the importance of the K. brevis regulatory limits as it can prevent possible neurotoxic shellfish poisoning in people who eat contaminated shellfish as it prevents the consumption of shellfish exposed to moderate and high concentrations of K. brevis.