Acute toxicity of metals in Rhinella diptycha and Leptodactylus fuscus at different temperatures: a perspective for tropical tadpoles.

Amphibians are highly vulnerable to environmental pollutants, especially during the larval stage, when temperature fluctuations in ephemeral ponds can interact with metals and alter their toxicity, a scenario that may be intensified by ongoing climate change. This study investigated the toxicity of five metals on tadpoles of Rhinella diptycha and Leptodactylus fuscus, across two different temperatures (28 and 34 °C). Lethal concentrations for 50% of the population (LC50) were determined to CdCl2, CuSO4, PbNO4, ZnCl2 and NiSO4, under 28 and 34 °C, for 96 h. Species sensitivity distribution (SSD) curves were then constructed for each metal using LC50 data for both tropical species, together with those from the literature on other tadpole and standard fish species. Toxicity ranking followed Cu > Cd > Zn > Pb > Ni in L. fuscus and Cu > Zn > Cd > Ni in R. diptycha. Pb LC50 was not calculated in R. diptycha due to insufficient number of tadpoles. In almost all cases, increased temperature enhanced metal toxicity in a dose-dependent effect. SSD revealed R. diptycha and L. fuscus as among the most sensitive species to metals. Body edema was observed in tadpoles exposed to Pb after acute exposure. Our findings show that metal toxicity can be amplified at higher temperatures, reinforcing the importance of accounting for temperature effects when establishing safe environmental concentrations of potential contaminants, particularly for species inhabiting warmer areas.