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 (Unio delicatus), a non-target species, with physiological, antioxidant action mechanisms and histopathological observations. The 96-h LC₅₀ value of fipronil was found to be 2.64 (1.45–4.56) mg/L. Sublethal concentrations were applied at ¹/₁₀ and ¹/₅ of 96-h LC₅₀ 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 (p < 0.05). While glutathione values in digestive glands and gills were higher in the fipronil applied groups (p < 0.05), the AOPP values were only higher in the digestive glands at 7-d of exposure (p < 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.

Zinc pyrithione (ZnPT) and copper pyrithione (CuPT) are widely used antifouling booster biocides in the shipping industry as a replacement for the banned tributyltin (TBT). Sub-lethal effects of zinc pyrithione (ZnPT; 10 μg L⁻¹), copper pyrithione (CuPT; 10 μg L⁻¹) and combined copper–zinc pyrithione (CZPT; 5 + 5 and 10 + 10 μg. L⁻¹) to freshwater indicator mussel Unio crassus (5.49 ± 0.85 cm in size) were determined by investigation of the total hemocyte counts, biochemical analysis mussel tissues and histopathological observations after 48-h and 7-d exposures. Mean total hemocyte counts differed significantly among all groups, for both exposure durations. In the 7 d, lipid peroxidation levels (malondialdehyde, MDA nmol g⁻¹ wet tissue) and reduced glutathione levels (GSH μmol g⁻¹ wet tissue) were lower than 48 h. However, there were no significant differences among groups. Total protein levels (mg mL⁻¹) were approximately in the same range and differed only among 7-d exposure groups. The gill tissues showed hemocytic infiltration and lipofuscin like structures, while digestive gland showed loss of digestive tubules in the treatments of pyrithione compounds. Histopathological damages were most severe in the combined exposure groups. In conclusion, ZnPT, CuPT and combined CZPT_5/10 are highly toxic on a freshwater indicator mussel species U. crassus.