Ammonia stress-induced physiological and histological changes in Heteropneustes fossilis (Bloch, 1794)

Ammonia is a major environmental pollutant in freshwater aquatic systems, significantly impacting the survival and growth of aquatic organisms. In the present study, we investigated the changes in blood physiological parameters and gill structure in Heteropneustes fossilis under hyper-ammonia stress (25 mM NH₄Cl) for experimental exposure durations of 1, 3, 6 and 9 h, as well as after a 24 h recovery period in normal water following 9 h of exposure (9h+N). Results indicated a progressive increase in blood ammonia levels up to 6 h, with fluctuations thereafter, however, even after recovery period, it was significantly more than that of control (p<0.05). This was accompanied by a corresponding rise in urea concentration, reaching up to a four-fold increase at 6 h exposure, followed by a decrease at 9 h and to that of control level at 9h+N. Blood glucose exhibited a continuous increase with prolonged exposure till 9 h exposure and then significantly decreased during recovery period. Serum glutamic-oxalacetic transaminase (SGOT) showed significant increase at 6 h, while serum glutamic-pyruvic transaminase (SGPT) at both 3 and 6 h. There was a fluctuating pattern of serum lactate dehydrogenase (LDH). No significant changes were observed for haemoglobin, haematocrit, serum protein and serum creatinine. Histological examination of gill tissues revealed extensive damage, particularly after 9 h of ammonia exposure. At this time point, 78% of the gills exhibited severe structural alterations, including the loss of secondary lamellae tips and 51% showed oedematous changes.