Environmentally Realistic Metal Mixture Concentration Impacts Early Development in Zebrafish Larvae

The dispersion of metals in the environment leads to the presence of metal mixtures in impacted ecosystems, but few studies have addressed their impact on early developmental processes in fish. Here, we investigated the effect of metal mixtures reported in the Doce River basin in Brazil, including Fe, Mn, and Cd, either singly or as a mixture on the early developmental responses, including developmental rate, heart rate, and behavioral and stress response in zebrafish (Danio rerio). Exposure to individual metals led to disruptions in the early development and behavioral markers in zebrafish larvae, with the order of toxicity being Cd > Mn > Fe. Reduced somite counts, slower heart rate and locomotor activity, and alteration in thigmotaxis were observed, especially in fish exposed to Cd and Mn. Exposure to a mixture of these metals at environmentally realistic concentrations impacted early developmental markers, including somitogenesis, hatching, and heart rate, but did not affect the behavioral responses of zebrafish larvae. Also, exposure to metals either singly or as a mixture for 6 days did not affect the whole-body cortisol, glucose, and protein levels. Some of the metal-specific effects on early development, including behavioral responses, were not seen with the mixture, suggesting that individual metal toxicity may not be a reliable predictor of the mixture toxicity for risk assessments. Overall, metal mixtures in the aquatic environment, such as the Doce River basin in Brazil, may impact the early developmental trajectory of native fish species, including compromising their stress coping behaviors.