Neonicotinoid insecticide thiamethoxam compromises early larval development of the Amazonian tetra fish Astyanax bimaculatus (Linnaeus, 1758).

Thiamethoxam (TMX) is a widely used neonicotinoid insecticide with high water solubility, which increases the likelihood of contamination in freshwater systems. Here, we assessed lethal and sublethal effects of TMX on the early life stages of the Neotropical Amazonian fish Astyanax bimaculatus. Two assays were performed: (i) embryonic exposure under static conditions (0, 0.05, 0.5, 5, and 50 mg/L; 200 embryos per treatment) and (ii) 96-h acute toxicity test on 1-day post-hatch larvae under semi-static conditions (0, 5, 25, and 50 mg/L; three aquaria of 100 larvae per treatment). During embryonic exposure, a visible coating over the chorion was observed at 5 and 50 mg/L, coinciding with both reduced normal hatching and increased larval deformities. The proportion of normally hatched larvae decreased from 97.5% in the control to 60.0% at 50 mg/L, while deformities increased to 15.5%, and unhatched embryos reached 24.5%. Post-hatch morphometry showed significant reductions in total length and yolk sac dimensions at concentrations ≥ 0.5 mg/L. In the larval assay, mortality increased in a dose-dependent manner, reaching mean values of 22.7%, 32.3%, and 64.0% at 5, 25, and 50 mg/L, respectively; the 96 h LC50 was 35.63 mg/L (95% CI: 27.83-43.44 mg/L). Overall, TMX impaired the survival and early development of A. bimaculatus, supporting the inclusion of early-life stage endpoints in the monitoring and risk assessment of Amazonian freshwater environments.