Physiological responses to nitrite toxicity in freshwater pearl mussel (Lamellidens marginalis)

Nitrite, a known toxic substance formed from ammonia through disruptions in the nitrogen cycle, can impair the physiology of aquatic organisms, if available in excess. A study was conducted to assess the 96-hour median lethal concentration (LC₅₀) of nitrite by following a static renewal method. Subsequently, acute nitrite toxicity study was conducted where adult L. marginalis were exposed to two different level of nitrite as 0 (control) and 183 mg/L of nitrite (Treatment; half concentration of LC₅₀) for 96 h. Significant changes (P < 0.05) in different stress biomarker of haemolymph including glucose, total protein, ALP, ALP, AST, ALT, SOD, CAT, GPX and MDA levels were observed in nitrite exposure treatment group indicating physiological alteration under nitrite exposure. The concurrent changes in MDA levels along with changes in antioxidant enzymes suggests the induction of oxidative stress. Histological examination further revealed substantial damage to the gills and digestive gland after exposure to nitrite such as enlarged inter-lamellar spaces, exfoliation of cilia, and shrinkage of the digestive gland lumen. These findings improve our understanding of the physiological impacts of nitrite exposure in L. marginalis and provide deeper insight into nitrite-induced toxicity in freshwater mussels.