Systemic toxicity of thiamethoxam in the mud crab (Scylla paramamosain): Oxidative stress, neurotoxicity, and growth retardation.

Thiamethoxam (TMX), a widely used neonicotinoid insecticide, poses a potential threat to non-target aquatic organisms. However, systematic toxicity evaluations on economically important crustaceans remain limited. This study comprehensively investigated the toxic effects and possible mechanisms of TMX on the mud crab Scylla paramamosain. Acute toxicity testing revealed a 96-h median lethal concentration (LC50) of 8.78 mg/L, suggesting moderate toxicity. Following a 21-day exposure to sublethal concentrations (8.8, 88, and 880 μg/L), TMX and its metabolite clothianidin accumulated in mud crabs in a tissue- and concentration-dependent manner, with the highest levels detected in the gut. While a two-phase enzyme system appeared to contribute to TMX detoxification, its efficiency may have been suppressed at higher concentrations and over longer durations. Consequently, despite indications of an activated Nrf2-mediated antioxidant response, TMX induced significant oxidative stress and hepatopancreatic damage in a time- and concentration-dependent manner. Notably, neurobehavioral alterations-including limb curling and impaired righting ability-were observed at 880 μg/L TMX. Mechanistic analyses suggested that these deficits may be associated with perturbations in cholinergic signaling and interference in tryptophan metabolism in the thoracic ganglia, potentially leading to neurotransmitter dysregulation. These multiple physiological impacts observed at 880 μg/L TMX collectively corresponded with significant growth inhibition, reflected in reduced survival, molting, and weight gain rates. In contrast, the two environmentally relevant concentrations (8.8 and 88 μg/L) posed relatively low actual ecological risk to mud crabs under the conditions tested. Overall, TMX presents negligible risk at current environmental levels, but projected higher concentrations may cause substantial toxicity.