International Journal of Molecular Zoology, 2025, Vol.15, No.2, 48-57 http://animalscipublisher.com/index.php/ijmz 49 makes it one of the ideal species for aquaculture in low-oxygen environments. It is worth noting that the tolerance to hypoxia varies among catfish species and populations, which is closely related to the long-term selection pressure of their living environment (Tripathi et al., 2013; Mohindra et al., 2016). This study will summarize the physiological and genomic research progress on catfish hypoxia tolerance, explore its evolutionary adaptation mechanism, analyze the main characteristics of catfish hypoxia physiological adaptation, analyze the key genes involved in hypoxia tolerance and their regulatory changes from a genomic perspective, and introduce the genomic selection signals and epigenetic responses of catfish populations in long-term hypoxia environments. This study will also introduce the progress of functional verification of hypoxia tolerance mechanisms using omics and gene editing technologies, compare the hypoxia tolerance of catfish under different species and ecological conditions through typical cases, and look forward to the prospect of applying these studies to the selection of hypoxia-tolerant varieties and the sustainable development of freshwater fisheries. This study hopes to deepen the comprehensive understanding of catfish hypoxia tolerance and provide a reference for future related research and applications. 2 Physiological Adaptation Characteristics of Catfish toHypoxia 2.1 Behavior and respiratory regulation mechanism Under hypoxia stress, fish usually take a series of behavioral adjustments to increase oxygen intake or reduce oxygen consumption. Catfish are no exception. When the dissolved oxygen in the environment decreases, catfish often first show behaviors such as decreased activity and immobility to reduce energy consumption (Wang et al., 2017; Zhong et al., 2017). At the same time, many catfish will start aquatic surface respiration (ASR): they swim to the surface of the water body, "surface" to obtain water with higher oxygen content or swallow air directly. For example, channel catfish were observed to increase the frequency of buoyancy and perform ASR when the dissolved oxygen dropped below 2 mg/L, which helped to prolong their survival time in hypoxic water. In addition, some catfish species with auxiliary respiratory organs (such as catfish) can also perform pulmonary breathing: they periodically surface to swallow air and store the air in the supragillary organs for gas exchange. When the hypoxic environment persists, catfish will also change their gill morphology and ventilation pattern. 2.2 Metabolic adaptation Under hypoxic conditions, catfish show unique metabolic regulation strategies to reduce dependence on oxygen and maintain energy balance. Studies have shown that catfish will actively reduce their basal metabolic rate when hypoxic, belonging to "oxygen-adaptive" fish, prolonging their survival time by reducing oxygen consumption. At the same time, the glycolysis pathway in their body is significantly activated: the glucose level in the blood increases, lactic acid accumulates rapidly, and the activity of enzymes such as lactate dehydrogenase increases, indicating that catfish compensate for insufficient energy supply by strengthening anaerobic glycolysis. This mechanism of obtaining ATP by enhancing glycolysis helps catfish maintain the necessary energy supply in short-term hypoxia (Xiao et al., 2024; Xing et al., 2025). In addition, a study compared the differences in the utilization preferences of energy substrates in hypoxia among different fish species and found that fish species that prefer to use carbohydrates for energy tend to have a stronger tolerance to hypoxia, while fish species that rely mainly on fat oxidation for energy have poorer tolerance (Figure 1) (Ma et al., 2023). This suggests that hypoxia-tolerant fish species such as catfish may survive hypoxia by reducing fatty acid decomposition and increasing sugar anaerobic metabolism. In fact, catfish will experience "metabolic depression" phenomena such as decreased body activity and loss of appetite when exposed to hypoxia, in order to reduce ATP consumption and oxygen demand, which is one of its important adaptive strategies (Mandic and Regan, 2018). 2.3 Regulation of cardiovascular and hemoglobin systems Catfish maintain tissue oxygen supply to the maximum extent through a series of cardiovascular adjustments such as increasing hemoglobin content, increasing the number of red blood cells, and improving circulation and microcirculation. These adaptive changes are similar to the strategies of plateau birds and mammals to increase
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