Copper oxide nanoparticles(CuO-NPs) are among the most widely used metal oxide nanoparticles,which increases the chance of their being released into the marine environment.As the applications of these particles have increased in recent years,their potential impact on the health of marine biota has also increased.However,the toxicological effects of these NPs in the marine environment are poorly known.In the present study,the DNA damaging potential of CuO-NPs in the marine eastern mussel Mytilus trossulus was evaluated and compared to that of dissolved copper exposures.Genotoxicity was assessed by the single cell gel electrophoresis(comet) assay in mussel gill and digestive gland cells.The results showed that copper in both forms(CuO-NPs and dissolved copper) was accumulated to different extents in mussel tissues.The mussel exposed to the dissolved copper attained higher concentrations of copper in the gills than in the digestive gland.In contrast to these results,it was found that CuO-NPs could induce much higher copper accumulation in the digestive gland than in the gills.A clear and statistically significant increase in DNA damage was found in both tissues of the Cu-exposed group compared to the control mussels.Our results indicated that the CuO-NP exposure produced remarkable effects and increased DNA damage significantly in mussel gill cells only.It should be noted that the digestive gland cells were prone to accumulation following CuO-NPs when compared to the gill cells,while the gill cells were more sensitive to the genotoxic effects of CuO-NPs.These results also suggested the need for a complete risk assessment of engineered particles before its arrival in the consumer market. Copper oxide nanoparticles (CuO-NPs) are among the most widely used metal oxide nanoparticles, which increase the chance of their being released into the marine environment. As the applications of these particles have increased in recent years, their potential impact on the health of marine biota has also increased. However, the toxicological effects of these NPs in the marine environment are poorly known. the present study, the DNA damaging potential of CuO-NPs in the marine eastern mussel Mytilus trossulus was evaluated and compared to that of dissolved copper exposures. Genotoxicity was assessed by the single cell gel electrophoresis (comet) assay in mussel gill and digestive gland cells. The results showed that copper in both forms (CuO-NPs and dissolved copper) was accumulated to different extents in mussel tissues. mussel exposed to the dissolved copper attained higher concentrations of copper in the gills than in the digestive gland. contrast to these results, it was found that CuO- NPs could induce much higher copper accumulation in the digestive gland than in the gills. A clear and marked significant increase in DNA damage was found in both tissues of the Cu-exposed group compared to the control mussels.Our results indicated that the CuO-NP exposure caused remarkable effects and increased DNA damage significantly in mussel gill cells only. It should be noted that the digestive gland cells were prone to accumulation of CuO-NPs when compared to the gill cells, while the gill cells were more sensitive to the genotoxic effects of CuO-NPs.These results also suggested the need for a complete risk assessment of engineered particles before its arrival in the consumer market.