Unraveling the impact of salinity on the osmoregulatory organs of Clarias batrachus: a histochemical and gill ultrastructural study.

Under both natural and cultural environments, the increased water salinity may serve as a significant stressor and have a direct impact on fish metabolism. Therefore, this study assesses the impact of salinity stress on a freshwater fish, Clarias batrachus, when exposed to two different salinity levels (30% and 35% seawater) at three time points (24 h, 3 days and 6 days). Histochemical examination of osmoregulatory organs like skin and gills using Alcian Blue (AB, pH 2.5) and Periodic Acid-Schiff (PAS) staining showed that as salinity and exposure time increased, there was a significant decline in mucus cell numbers, percent area occupancy and the size of both acidic and neutral glycoproteins containing mucus cells. PAS staining also revealed significant histological alterations in kidney tissues, indicating the presence of salinity-induced renal stress. Scanning electron microscopy further revealed structural modifications in chloride cells, underscoring their role in physiological adaptation to varying salinity levels. Even though these adaptations point to a compensatory mechanism, the overall results show that C. batrachus suffers negative effects from prolonged exposure to salinity, including impaired tissue integrity and osmoregulatory efficiency. This study gives significant insights into the physiological challenges encountered by freshwater fish under salinity stress, advancing the knowledge of their adaptive capabilities and limitations.