The effects of a commercial probiotic (Clostridium butyricum) on growth, gut microbiota, digestion‐related enzyme activity, antioxidant capacity and water quality in genetically improved, farmed tilapia (GIFT; Oreochromis niloticus) were assessed in a closed‐circuit, container culture system. A basal diet containing commercially available C. butyricum at 3.0 × 1010, 1.5 × 1011 and 3.0 × 1011 CFU/kg, and a control diet were fed to tilapia fries for 90 days. Growth performances and water quality were improved by probiotic treatment. Activities of digestive enzymes (amylase, lipase and trypsin) were improved, especially with 1.5 × 1011 CFU/kg (p < 0.05). After feeding for 45 days and 90 days, antioxidant capacity in the liver, spleen and head kidney, and the plasma immunity in the treatment groups improved. The analysis of 16S rRNA indicated that C. butyricum did not significantly increase its abundance in the hindgut, but could modulate gut microbial communities and significantly increase functions relating to nitrogen metabolism, phosphorylation and proteinases. These results support the administration of C. butyricum, especially at 1.5 × 1011 CFU/kg. As a probiotic for high‐density tilapia, C. butyricum improves water quality, growth performance, antioxidant capacity and the immune response of GIFT under crowding stress.

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Hypoxia is a critical problem in intensive Epinephelus coioides aquaculture systems. In the present study, the physiological responses of E. coioides muscle to acute hypoxic stress (DO = 0.6 ± 0.1 mg/L) and reoxygenation (DO = 6.0 ± 0.1 mg/L) were analyzed by transcriptome sequencing (RNA-seq) and quantitative real-time PCR (qRT–PCR). RNA-seq was conducted on the muscle tissues of E. coioides in the hypoxia-tolerant (EMS), hypoxia-sensitive (EMW), and normoxic (CM) groups. Among the three groups, a total of 277 differentially expressed genes (DEGs) were identified. KEGG analysis revealed that the pathways significantly enriched after hypoxic stress are involved in the immune response, glycolysis/gluconeogenesis, energy metabolism, vasodilation and proliferation, cell proliferation, and apoptosis. qRT‒PCR verified that the differentially expressed genes FIH-1, PHD-2, PPARα, BCL-XL, LDH-A, and Flt-1 were significantly upregulated after hypoxic stress and returned to normal levels after reoxygenation, suggesting that these DEGs play important roles in responding to hypoxia treatment. In addition, the HIF-1 signaling pathway was also activated under hypoxic stress, and qRT‒PCR confirmed that the expression level of HIF-1α was significantly elevated under acute hypoxic stress, indicating that the HIF-1 signaling pathway is the central pathway in the E. coioides hypoxic response mechanism and activates other related pathways to adapt to hypoxic stress. These pathways jointly regulate energy metabolism, substance synthesis, blood vessel proliferation, cell proliferation, and differentiation and prolong survival time. These results provide ideas for understanding physiological regulation after hypoxic stress and reoxygenation and provide basic insights for the future breeding of hypoxia-tolerant E. coioides.