Toxicity of organophosphorus insecticides in the zebra mussel, Dreissena polymorpha P.

The 96-h toxicity of four organophosphates (thiometon, disulfoton, malathion, and demeton-S-methyl, the oxygen analogue of thiometon) in the freshwater bivalve mollusc was tested using different nominal concentrations ranging between 6 and 50 mg/L. No mortalities were observed in mussels exposed to malathion and demeton--methyl (26 mg/L and 6 mg/L, respectively), and at the lowest concentrations of thiometon and disulfoton (6 and 10 mg/L, respectively). At higher thiometon and disulfoton concentrations, mortalities occurred. At the highest concentrations of 50 mg thiometon/L and 30 mg disulfoton/L, mussel mortalities of 88 and 93%, respectively, were determined. Organophosphate concentrations of up to a factor 10 times higher than in the ambient water were found in exposed mussels, irrespective of whether they were alive or dead. The search for organophosphate metabolites via GC/MS analysis of mussel tissue extracts was negative, suggesting lacking or low oxidative activation of the insecticides used. The mollusc is highly resistant to toxic effects of organophosphate insecticides and their biological active oxygen analogues.