Sublethal cadmium exposure disrupts cardiac function and tissue structure in zebrafish larvae.

Cadmium remains a primary toxicological concern because it does not degrade and easily enters the food chain, affecting diverse ecosystems. This study aimed to evaluate the effects of sublethal cadmium exposure on cardiac function and on brain and heart tissues in zebrafish (Danio rerio) larvae, utilizing photoplethysmography as a rapid, non-invasive approach. Larvae (1 day post-hatching) were exposed for five days to cadmium sulfate at concentrations of 0.005, 0.025, and 0.05 mg/L under controlled laboratory conditions. Cardiac activity was assessed twice a day using a non-invasive photoplethysmography-based technique. Histological analyses of brain and heart tissues were conducted using standard fixation, sectioning, and staining protocols. At the highest cadmium concentration, histological examination revealed neuronal degeneration in brain tissue and thinning of the cardiac wall. Cadmium exposure led to heart rate suppression, with the most pronounced effect observed at 0.05 mg/L. Lower concentrations exhibited attenuated but still significant effects compared to controls. These findings indicate that cadmium exerts both functional and structural toxicity in zebrafish larvae, affecting cardiovascular development and neural integrity. The results support the use of heart rate, measured via photoplethysmography, as a sensitive and non-invasive endpoint for detecting sublethal cardiotoxic effects.