Hypoxia and reoxygenation induced distinct patterns of response in antioxidant capacity between two cyprinid fish species.

Antioxidant defense represents a critical biological capacity enabling organisms to counteract homeostasis disruption and mitigate oxidative stress induced by environmental stressors, such as oxygen fluctuations. However, comparative studies investigating the differential responses of antioxidant defense systems to oxygen fluctuations across diverse animal species remain limited. In the present study, two common cyprinid species-common carp (Cyprinus carpio) and Chinese hook snout carp (Opsariichthys bidens)-maintained under normoxic conditions (> 7 mg L O) were exposed to hypoxia (1.04 ± 0.2 mg L O) for 3 h, followed by a 3-h reoxygenation period (> 7 mg L O). A comparative analysis of key enzymes within the classical enzymatic antioxidant system was subsequently performed. The results demonstrated that the total protein (TP) concentration in the liver, brain and gill of both fish species was significantly altered under hypoxic and/or reoxygenation conditions. In common carp, hepatic superoxide dismutase (SOD) and total antioxidant capacity (T-AOC) activity, cerebral SOD, catalase (CAT) and glutathione peroxidase (GPx) activity, as well as branchial CAT and GPx activity, were significantly elevated during the hypoxic period. Except for hepatic SOD activtiy, which remained elevated during reoxygenation, these parameters recovered to control levels. These findings support the preparation for oxidative stress (POS). However, in Chinese hook snout carp, hepatic CAT and GPx activity, as well as branchial GPx and T-AOC activity, exhibited a distinct decreasing trend during hypoxia, followed by recovery to control levels during reoxygenation. Furthermore, cerebral SOD, T-AOC activity, along with branchial SOD activity, remained unchanged under hypoxic conditions but increased significantly during reoxygenation. These responses did not align with the POS strategy. The malondialdehyde (MDA) content remained stable or decreased in both tested fish species during hypoxia and reoxygenation, except for an increase in MDA levels under hypoxic conditions. The result indicates that neither species experienced oxidative damage. These results suggest that the common carp tends to employ a proactive strategy to cope with environmental stress. The tendency may be associated with its frequent exposure to adverse environmental conditions, which has likely facilitated the development of enhanced hypoxia tolerance and the capacity to anticipate future stressors. In contrast, the Chinese hook snout carp, which exhibits lower hypoxia-tolerant, inhabits rapid-flowing habitats where dissolved oxygen concentrations remain relatively high and stable. This has suppressed the development of capacities for predict future changes in environmental stressors.