LABORATORY STUDY ON THE EFFECT OF DICHLORVOS ON 2 COMMERCIAL BIVALVES

Organophosphorous compounds (metrifonate, dichlorvos) are commonly use d in marine fish farms to control ectoparasitic copepods or monogenean s populations, After fish treatment, these products may be discharged directly in shellfish growing areas. As they are known for their envir onmental toxicity, it becomes necessary to study the effect of these c hemicals on the bivalves. Two commercial bivalves: Manila clam, (Rudit apes philippinarum) and Japanese oyster (Crassostrea gigas) have been exposed for 6 h to 1 and 0.1 mg l(-1) dichlorvos concentrations and th en replaced in drug-free seawater until the end of the experiment (48 h). The behaviour and mortality of the shellfish have been observed, T he specific acetylcholinesterase activity (AChE) was measured in the t otal clam tissue and the oyster gills following the automated method o f Galgani and Bocquene (1988). No abnormal mortality was observed, but all bivalves were opened because of a relaxation of the adductor musc les. AChE inhibitions were 40 and 70% (for 0.1 and 1 mg l(-1) dichlorv os concentrations respectively) for clams and 83 and 87% (for 0.1 and 1 mg l(-1) dichlorvos concentrations respectively) for oysters. Forty- two hours after the end of the exposure, the AChE activity of oyster r emained low and a partial recovery of the initial activity was observe d for clams. These findings illustrated that the concentrations tested are not lethal but have a clear effect on the bivalves physiology. Th erefore it may be considered that because of the dilution and degradat ion processes dichlorvos effect is only limited to bivalves reared wit hin or in close contact with fish farms.