Different concentrations of MeHg induce anxiety-like behaviors in Physalaemus ephippifer tadpoles (Anura, Leptodactylidae)

Abstract

Methylmercury (MeHg) is the most harmful organic form of mercury to organisms, especially in the aquatic environment. Therefore, it is crucial to assess the effects of exposure to this contaminant on aquatic biota using multiple biomarkers. In this context, we aimed to analyze the morphophysiological and behavioral effects of sub-lethal exposure to MeHg in tadpoles of Physalaemus ephippifer. To achieve this, larvae of the model species were subjected to a toxicological assay, conducted across five treatments (control; 0.0004 µg/ml; 0.0007 µg/ml; 0.004 µg/ml; and 0.007 µg/ml of MeHg). Following exposure, the tadpoles were subjected to three behavioral assays: escape response, chemical perception, and visual perception. Subsequently, we performed electrocardiographic analysis, cardiac histology, and teratogenic analysis. In the chemical and visual perception assays, tadpoles exposed to MeHg exhibited anti-predator behavior even in the absence of predatory stimuli. Electrocardiographic analyses revealed cardiac hyperexcitability with an increase in heart rate, including tachycardia at the highest concentration, and histological analysis showed physiological cardiac hypertrophy. For teratogenic alterations, sub-lethal concentrations caused an increase in the occurrence of teratogenic effects, such as alterations in the oral apparatus, and body and intestinal morphology. Thus, it can be concluded that the combined data indicate that sub-lethal exposure to MeHg could generate behavioral and physiological changes similar to anxiety in P. ephippifer tadpoles. Therefore, such observed effects are capable of increasing the vulnerability of tadpoles exposed to MeHg, acting as one of the mechanisms leading to the population decline of anurans.