Sublethal impact of short term exposure to the organophosphate pesticide azamethiphos in the marine mollusc Mytilus edulis

Concern has been raised that the increased use of pesticides in intensive aquaculture practices may cause adverse sublethal effects to non-target aquatic species. Azamethiphos is an organophosphate (OP) pesticide used to combat sea lice infestations in farmed salmonids. Here, the sublethal impact on the blue mussel, Mytilus edulis, of short term exposure to azamethiphos was determined. The testing regime included biomarkers of exposure (acetylcholinesterase activity), cytotoxicity (neutral red retention), immune function (phagocytic index) and physiological condition (feeding rate). The distribution and sensitivity of M. edulis acetylcholinesterase to inhibition by azamethiphos was first determined, yielding IC50 values of 0.736 and 1.30 mg l−1 for gill and haemolymph, respectively. Exposure of mussels to 0.1 mg l−1 azamethiphos for periods of up to 24 h caused a significant reduction in acetylcholinesterase activity in both the haemolymph (P < 0.0002) and the gill (P < 0.002), alteration in cell viability (P < 0.02) and decrease in phagocytic index (P < 0.03). The feeding rate remained unaffected. The results support the hypothesis that, in addition to its neurotoxic effects, azamethiphos can modulate haemocyte function and immune defence in M. edulis at environmentally relevant concentrations after only a few hours.