Whole-body bioconcentration and biochemical and morphological responses of gills of the neotropical fish Prochilodus lineatus exposed to 2,4-dichlorophenoxyacetic acid or fipronil individually or in a mixture

•2,4-dichlorophenoxyacetic acid (2,4-D) and fipronil bioconcentrate in whole body fish.•2,4-D increases catalase and glutathione-S-transferase (GST) activities.•2,4-D generates oxidative stress in the gills.•Fipronil increases GST activity and the mitochondria-rich cell density in the gills.•The mixture of 2,4-D and fipronil induces mostly antagonistic effects in the gills. The herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) and the insecticide fipronil have been used widely in agriculture and detected in aquatic ecosystems, where they threaten wildlife. This study evaluated the whole-body bioconcentration and the biochemical and morphological changes in the gills of the neotropical fish Prochilodus lineatus exposed for 96 h to 2,4-D or fipronil as single compounds or as a mixture (2,4-D + fipronil). Fish exposed to either compound alone bioconcentrated 2,4-D (77 ± 23 ng g − 1 fish dry mass) and fipronil (789 ± 178 ng g − 1 fish dry mass). Fish exposed to 2,4-D + fipronil bioconcentrated fipronil (683 ± 73 ng g − 1 fish dry mass) but not 2,4-D. In the gills, catalase (CAT) and glutathione-S-transferase (GST) activities and the lipid peroxidation (LPO) level increased after exposure to 2,4-D. GST activity increased after exposure to fipronil. Conversely, no changes occurred in CAT and GST activities and LPO upon exposure to 2,4-D + fipronil. Histopathological changes such as hyperplasia, cellular hypertrophy, epithelial lifting, and vascular congestion were frequent in the gills of fish exposed to 2,4-D or fipronil individually or 2,4-D + fipronil. The mitochondria-rich cell (MRC) density increased on gill surface in fish exposed to fipronil or 2,4-D + fipronil. Only exposure to 2,4-D alone induced oxidative stress in the gills. Most morphological changes showed defense responses against the pesticides; however, hypertrophy and the change in MRC indicated compensatory responses to maintain the gill osmoregulatory function. The 2,4-D + fipronil mixture showed antagonistic interaction, except for the MRC fractional area at gill surface, which showed synergistic interaction. This is the first report showing antagonistic interaction of 2,4-D and fipronil in the gills after exposing fish to the mixture of both pesticides. The biochemical and morphological changes in gills endanger the gill functions, a phenomenon that implies an energy cost for fish. Graphical Abstract [Display omitted]