The herbicide paraquat (PQ), still widely used in developing countries, represents a serious risk factor for human and environmental health. To test the sublethal effects of PQ on the freshwater bivalve Dreissena polymorpha, mussels were exposed to 0.125, 0.250, 0.500 mg/L for 7 and 14 days and histologically screened. PQ's genotoxic potential was also determined in haemocytes by the micronucleus, MN, assay. At concentrations > or = 0.250 mg/L, severe lesions, such as cellular vacuolation,

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... and thinness of the germinative epithelia were observed in the digestive gland and testis. A positive trend between the number of granulocytes and all PQ concentrations was observed in both gonads and digestive glands, addressing the inflammatory capacity of this herbicide on these tissues. Mussels exposed to PQ also exhibited a significant MN induction. The spontaneous MN frequencies ranged from 2.75 to 0.425 per cent, while PQ-induced MN rates in treated mussels were between 3.50 and 1.250 per cent. The histopathological effects on the digestive and reproductive systems, as well as the MN induction in the haemocytes, confirmed the cytotoxic and genotoxic effects of PQ also in D. polymorpha.