Studies on the Development of Potential Biomarkers for Rapid Assessment of Copper Toxicity to Freshwater Fish using Esomus danricus as Model

Living in an environment that has been altered considerably by anthropogenic activities, fish are often exposed to a multitude of stressors including heavy metals. Copper ions are quite toxic to fish when concentrations are increased in environmental exposures often resulting in physiological, histological, biochemical and enzymatic alterations in fish, which have a great potential to serve as biomarkers. Esomus danricus was chosen as model in the present study and the metabolic rate, gill morphology, total glycogen, total protein, superoxide dismutase and catalase were critically evaluated. The 96h LC50 value was found to be 5.5mg/L (Cu as 1.402 mg/L). Fish groups were separately exposed to lethal (5.5 mg/L) and sub lethal concentrations (0.55 mg/L) of copper sulphate over a period of 96h to examine the subtle effects caused at various functional levels. Controls were also maintained simultaneously. Significant decrease in the metabolic rate (p<0.001) of the fish was observed in both the concentrations studied. Studies employing Automated Video Tracking System revealed gross changes in the architecture of gill morphology like loss, fusion, clubbing of secondary gill lamellae, and detachment of gill rakers following softening of gill shaft in fish under lethal exposures indicating reduced respiratory surface area. Biochemical profiles like total glycogen and total protein in gills and muscle of fish exposed to 5.5 mg/L showed appreciable decrease (p<0.05 to 0.001) from control. Significant inhibition of superoxide dismutase (60.83%), catalase (71.57%) from control was observed in fish exposed to 5.5 mg/L at the end of 96h exposure only. Interestingly, in fish exposed to 0.55 mg/L enzyme activity is not affected except for catalase. Toxic responses evaluated at various functional levels are more pronounced in fish exposed to 5.5 mg/L and these can serve as potential biomarkers for rapid assessment of acute copper toxicity in environmental biomonitoring.