Freshwater Lamellidens marginalis as sentinels of TBTCl toxicity: Changes in the shell composition and biomarker...

Freshwater bivalves are one of the most exposed faunistic groups and show remarkable changes in exposure to different toxicants. Tri-n-butyltin chloride (TBTCl) is an organotin compound, having wide applications ranging from the antifouling properties to its role as a catalyst in the trans-esterification process during chemical synthesis. To address its potential impact, three sublethal concentrations of TBTCl (0.1--EXP I, 0.05--EXP II, and 0.033 mg L^-1--EXP III) were used in a static non-renewal bioassay test on freshwater bivalve Lamellidens marginalis for a period of 96 h. The nitric acid-digested shell powder was used for the analysis of calcium (Ca²⁺), sodium (Na⁺), and potassium (K⁺). In comparison to the control, Ca²⁺ concentration was found to decrease by 38.6%, 54.3%, and 60.0% in EXP III, II, and I, respectively. Calcium is required for maintaining the shell in bivalves, and its ionized form is taken from the surrounding water and gradually transferred through the outer mantle layer toward the region of shell growth. A decrease in the calcium concentration is indicative of the inhibitory effect of TBTCl on the L. marginalis shell. K⁺ concentration increased with increasing toxicant concentration; whereas, concentration of Na⁺ decreased in EXP III and II but increased in EXP I. The integrated biomarker response (17.9, 23.3, 19.8, and 14.7) and biomarker response index values (4, 2, 3, 3.67) obtained for the control, EXP I, II, and III groups, respectively, suggest severe alterations in shell ionic composition of the bivalves with increasing dose concentration.