Cellular, Physiological, and Biochemical Basis of Adaptive Response to Variable Osmotic Environments by the River Shad, Tenualosa ilisha

The river shad, hilsa (Tenualosa ilisha), is an anadromous fish that migrates from marine to freshwater for spawning. This transition/migration poses severe osmotic stress that hilsa needs to successfully minimize. The present study was conducted to evaluate the cellular (ultrastructure of gill and kidney), physiological (hemocyte counts, blood, and water osmolality), and biochemical (free amino acids, free fatty acids, blood glucose, and cortisol levels) parameters of hilsa collected from four different environmental salinity levels (0‰, 10‰, 20‰, and 30‰ salinity levels; using 10 fish samples from each salinity). Results indicate that increased amount of fatty/mucus cells in the gill and lower number of glomerular capsules in kidney were observed at low (0‰ and 10‰) salinities compared to high (20‰ and 30‰) salinities. Water and blood osmolality (total ionic content) showed declining trends with lowering salinities. Total number of blood cells was also found to vary significantly $\left(P<0.05\right)$ among salinities. Salinity-specific blood cortisol and glucose levels of hilsa were observed as 0‰ > 10‰ > 30‰ > 20‰. Total essential and free amino acids of hilsa blood showed significantly declining trends $\left(P<0.05\right)$ with salinity reductions. No significant differences were observed between 10‰, 20‰, and 30‰ salinities for different types of fatty acids, while hilsa collected from 0‰ showed significantly higher $\left(P<0.05\right)$ levels of fatty acids compared to the remaining three salinities. Findings indicate that hilsa rapidly alters cellular, physiological, and biochemical traits for successful transition between different salinity habitats.