Gill Tissue Tolerance Remodeling and Compensatory Regulatory Mechanisms Under Acute Hypoxic Stress in Topmouth Culter (Culter alburnus)

Topmouth culter (Culter alburnus) is highly sensitive to hypoxic conditions, but its regulatory mechanisms remain poorly understood. In this study, C. alburnus were exposed to hypoxia (DO: 0.60 ± 0.05 mg·L−1) for 0, 2, 4, 6, 12, and 24 h to investigate the gill tissue responses and the underlying regulatory mechanisms. Results showed that gill lamellae of C. alburnus exhibited distortion and thickening under hypoxic stress for 2–6 h. Between 12 and 24 h of hypoxia, the gill tissue exhibited changes characterized by sinusoidal dilatation and an increased number of red blood cells. Interestingly, the apoptosis rates significantly increased in all experimental groups in response to the hypoxic environment. The plasma glucose (Glu) level increased rapidly at the early stage and then gradually declined. The total protein (TP) level showed a slight elevation. Meanwhile, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) exhibited sustained increases. Hypoxia stress may induce a metabolic transition from early reliance on glucose for energy to later reliance on lipid metabolism. SOD activity gradually increased under hypoxia, while MDA content exhibited an overall upward trend. Oxidative stress-related genes (foxo1b, mapkapk2, irs2, ppargc1b) were significantly upregulated to enhance antioxidant capacity during the early phase of hypoxic stress. However, during the late phase of hypoxia, the expression of these genes was significantly downregulated. In conclusion, this study offers a new theoretical basis for gill tolerance remodeling and molecular regulation in C. alburnus under acute hypoxic stress.