Unveiling cadmium toxicity in Etroplus suratensis through endpoint analysis and biomarker profiling: A step towards coastal water quality guideline development

This study contributes to ecotoxicological data essential for establishing water quality standards for cadmium (Cd). The 96-hour median lethal concentration (LC₅₀) of Cd on Etroplus suratensis was 1.56 ± 0.02 mg L⁻¹ with upper and lower confidence intervals of 1.24 to 1.89 mg L⁻¹ respectively^. The No Observable Effect Concentration (NOEC), Lowest Observable Effect Concentration (LOEC), and chronic values were 0.11, 0.18, and 0.15 mg L⁻¹, respectively. Antioxidants such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione S-transferase (GST), and total glutathione (TG) increased dose-dependently. Lipid peroxidation (LPO) significantly rose (p < 0.05). Acetylcholine esterase (AChE) activity in the brain and muscle decreased significantly at 0.32 (p < 0.01) and 0.47 (p < 0.001) mg L⁻¹. Energy reserves and electron transport system (ETS) activity dropped in Cd-exposed groups. Cellular energy allocation decreased from controls to the highest exposure group. Behavioral changes such as rubbing, surfacing, jerking, burst swimming, and fin flickering intensified with exposure. Hematological analysis showed a dose-dependent red blood cell (RBC) count decrease and a white blood cell (WBC) count increase. Major gill lesions included lamellar aneurism, lamellar fusion, necrosis, hyperplasia, and hypertrophy. Liver abnormalities featured blood vessel dilation, thrombosis, hemosiderin formation, nuclear pyknosis, and necrosis. Retinular layer damage affected pigment and photoreceptor cells. The integrated biomarker index indicated significant effects on GST, GPx, ETS activity, and LPO content. Also, the high sensitivity of Etroplus suratensis to cadmium confirms its suitability as an indicator species for pollution monitoring.