Molecular adaptations of the estuarine fish Etroplus suratensis in response to salinity fluctuations

Abstract

Climate-driven shifts in salinity regimes pose significant challenges to aquatic organisms, particularly those inhabiting dynamic coastal ecosystems. Etroplus suratensis (Pearl spot), a euryhaline cichlid capable of thriving across fresh, brackish, and marine environments, offers an excellent model for understanding molecular strategies underpinning salinity acclimation. This study investigated the transcriptomic reprogramming in the gills of fish acclimated for 21 days to freshwater (0‰), brackish water (18‰), and marine water (36‰) conditions. Freshwater exposure elicited a coordinated metabolic response characterized by activation of mitochondrial energy pathways, including oxidative phosphorylation and electron transport, to support ATP-intensive ion uptake, alongside enhanced ion transport functions and epithelial remodeling. In contrast, marine water exposure triggered a more pronounced osmoregulatory shift, with activation of calcium-dependent exocytosis and vesicular trafficking pathways central to maintaining ion balance under high salinity. However, prolonged marine water exposure also led to suppression of key antioxidant and immune pathways, revealing a potential trade-off whereby resources are redirected toward ion regulation at the expense of physiological defense mechanisms. Concurrent enrichment of cortisol synthesis, autophagy, apoptosis, and other stress-responsive pathways further reflects cellular remodeling and adaptive stress management under salinity stress. Collectively, these results demonstrate the remarkable metabolic plasticity and molecular resilience of E. suratensis , highlighting its capacity to deploy distinct, context-dependent mechanisms to maintain homeostasis across fluctuating salinity environments.

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