Integrated histopathological, physiological, biochemical, and molecular analysis of gill responses to acute salinity stress in red tilapia ( <i>Oreochromis</i> spp.)

Abstract Growing freshwater scarcity in China is driving aquaculture interests toward saline‐tolerant fish species. Red tilapia ( Oreochromis spp.) is a euryhaline species with strong osmoregulatory capacity, making it an ideal model for investigating stress‐induced physiological changes. In this study, we examined the gill responses of red tilapia subjected to acute salinity stress. Fish were maintained in either (0 h, Ctrl group) or saline water (17.7‰ ± 0.1‰) for 192 h; time points (0, 24, 48, 96, 192 h). Gill samples were collected over a time course for integrated histopathological and physiological analyses. Exposure to saline water induced histopathological alterations, including lamellar shortening and thickening, as well as a significant increase in the apoptotic index. Using qRT‐PCR, we confirmed the significant activation in gills under salinity stress of genes associated with apoptosis ( tnfsf1 , birc5a , casp6 , ddit3 ), ABC transporters ( cftr , tap1 ), and Toll‐like receptor (TLR) signaling ( irak1 , nfkbiaa ). These molecular responses reflect three key processes: osmoregulation (ABC transporters), immune and inflammatory regulation (TLR signaling), and cell fate control (apoptosis). Collectively, they underpin the disruption of homeostasis, immune activation, and oxidative stress observed during acute salinity exposure. Biochemical profiling revealed a dynamic osmoregulatory response: early upregulation of Na + /K + ‐ATPase, NKCC1, and antioxidant enzymes (SOD, CAT, GSH‐Px) was followed by a late‐stage oxidative imbalance, characterized by sustained elevation of malondialdehyde (MDA) and reduced antioxidant capacity. Ion homeostasis became progressively disrupted, with significant alterations in Na + and Cl − levels. Molecular analysis further uncovered a coordinated transcriptional sequence: an initial anti‐apoptotic phase, succeeded by delayed upregulation of pro‐apoptotic and immune‐related pathways (including IgM and LZM), occurring alongside modulation of osmoregulatory genes. Together, these findings provide a mechanistic basis for understanding salinity adaptation in red tilapia.