| Summary: | Fipronil is widely used against insects in agriculture and ectoparasites in domestic areas and veterinary medicine. However, fipronil may influence non-target species as a result of the contamination of aquatic ecosystems. The present study aimed to investigate the acute and sublethal effects of fipronil in freshwater mussels (<i>Unio delicatus</i>), a non-target species, with physiological, antioxidant action mechanisms and histopathological observations. The 96-h LC<sub>50</sub> value of fipronil was found to be 2.64 (1.45–4.56) mg/L. Sublethal concentrations were applied at <sup>1</sup>/<sub>10</sub> and <sup>1</sup>/<sub>5</sub> of 96-h LC<sub>50</sub> as 0.264 mg/L and 0.528 mg/L for 48-h and 7-d. Haemolymph samples, digestive gland and gill tissues of mussels were taken after exposure times. While the Total Haemocyte Counts decreased in 48-h of exposure, it was only high at 0.264 mg/L fipronil-exposed for 7-d (<i>p</i> < 0.05). While glutathione values in digestive glands and gills were higher in the fipronil applied groups (<i>p</i> < 0.05), the AOPP values were only higher in the digestive glands at 7-d of exposure (<i>p</i> < 0.05). Moreover, fipronil caused histopathological alterations on gills and digestive glands. These things considered, the principal component analysis revealed that the most pronounced changes in the antioxidant action mechanisms were caused by the fipronil exposure. These results show that sublethal concentrations of fipronil are toxic to freshwater mussels.
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