Physiological and molecular mechanisms underlying acute hypoxia tolerance in hulong hybrid grouper (Epinephelus fuscoguttatus♀×E. lanceolatus♂)

Hulong hybrid grouper ( Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂) is the highest-yielding grouper in Chinese aquaculture and is widely preferred by the market. However, intensified aquaculture, elevated temperatures, and high-density live transport increase the frequency of hypoxic episodes, making hypoxic stress a major constraint on production. Here, we thoroughly explore the mechanisms of hypoxia tolerance in Hulong hybrid groupers from physiological and molecular adaptation perspectives. We simulated acute hypoxic stress to differentiate between hypoxia-tolerant (HT) and hypoxia-sensitive (HI) individuals. Comparative assays of gill and liver showed a pronounced shift toward anaerobic glycolysis in HT fish, with higher activities of pyruvate kinase (PK), phosphoenolpyruvate carboxylase (PEPC), and lactate dehydrogenase (LDH), together with elevated lactate, indicating maintenance of ATP supply under oxygen limitation. RNA-seq of gill, brain, and liver identified differentially expressed genes (DEGs) and pathways distinguishing HT from HI fish. In gill, ddit4 , ldha , egr1 , hif1an , gp9 , and egln3 were up-regulated in HT fish, with enrichment of HIF-1 and PI3K–Akt signaling. Liver transcriptomes showed enhanced amino-acid metabolism pathways, consistent with mobilization of substrates for energy production and macromolecular repair during hypoxia. In addition, SNP scans revealed variants with high differentiation between groups, including sites within metabolism-related genes, suggesting a genetic contribution to tolerance. Together, these physiological and transcriptomic data indicate that acute hypoxia tolerance in Hulong hybrid grouper involves metabolic reprogramming coordinated by HIF-centered signaling, and they outline immunometabolic pathways relevant to stress and host defense under oxygen limitation. • Acute hypoxia triggers strong responses in gill and brain but modest changes in liver of Hulong hybrid grouper. • Transcriptomics identifies HIF-1 and FoxO signaling as key pathways for hypoxia and reoxygenation responses. • Hypoxia-induced energy metabolism is dominated by glycolysis, with limited contribution from lipid catabolism. • Reoxygenation shifts metabolism to aerobic pathways, with hepatic lipid oxidation exceeding normoxic levels.