Several exogenous probiotics are applicable in fish culture; however, challenges in isolation and verification have hindered the full utilization of numerous host probiotics. Therefore, this study aimed to apply the host probiotic Exiguobacterium acetylicum G1-33 to hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) cultures and explore its mechanism of action. In total, 360 hybrid grouper were divided into four groups, which were fed the following for 60 days: three received commercial feed with varying concentrations of E. acetylicum G1-33 (10⁶, 10⁸, and 10¹⁰ CFU/g), while a control group received commercial feed. The results showed that supplementation with 10⁶ and 10⁸ CFU/g of E. acetylicum G1-33 enhanced gut morphology, upregulated growth-related genes (ghr1, igf-2, s6k1, tor), and promoted growth, with supplementation with 10⁸ CFU/g resulting in the most notable enhancement. However, supplementation with 10¹⁰ CFU/g inhibited growth, possibly because of changes in intestinal morphology. Additionally, supplementation with E. acetylicum G1-33 upregulated the expression of immune-related genes (c3, myd88, Cu/Zn-sod, tlr3, and tnf2) in the liver and head kidney but led to an increase in malondialdehyde content, as well as a decrease in alkaline phosphatase and acid phosphatase activities, in the liver and serum, indicating increased oxidative stress. Moreover, supplementation with 10⁶ and 10⁸ CFU/g E. acetylicum G1-33 enhanced the widespread expression of immune-related genes in the head kidney and liver, respectively, and improved resistance to Vibrio harveyi, whereas supplementation with 10¹⁰ CFU/g weakened this resistance. In conclusion, E. acetylicum G1-33, particularly at 10⁸ CFU/g, emerged as an effective probiotic, optimizing growth performance and immunity in hybrid grouper. This research is pioneering in its application of E. acetylicum in mariculture, potentially broadening the range of probiotic strategies in aquaculture.

Grass carp is an economically important cultured species in China. Triploid embryo production is widely applied in aquaculture to achieve reproductive sterility, improve somatic growth, and reduce ecological risks associated with uncontrolled breeding. In this study, a simple cold shock method for inducing triploid grass carp was developed. The triploid induction rate of 71.73 ± 5.00% was achieved by applying a cold treatment at 4 °C for 12 min, starting 2 min after artificial fertilization. Flow cytometry and karyotype analysis revealed that triploid individuals exhibited a 1.5-fold increase in DNA content compared to diploid counterparts, with a chromosomal composition of 3n = 72 (33m + 36sm + 3st). Additionally, embryonic transcriptome analysis demonstrated that, in the cold shock-induced embryos, genes associated with abnormal mesoderm and dorsal–ventral axis formation, zygotic genome activation (ZGA), and anti-apoptosis were downregulated, whereas pro-apoptotic genes were upregulated, which may contribute to the higher abnormal mortality observed during embryonic development. Overall, this study demonstrates optimized conditions for inducing triploidy in grass carp via cold shock and provides insights into the transcriptomic changes that take place in cold shock-induced embryos, which could inform future grass carp genetic breeding programs.

Oocyte maturation is a pivotal event in teleost reproduction that directly determines egg quality, fertilization success, and the developmental competence of early embryos. However, the transcriptional and post-transcriptional regulatory mechanisms operating within oocytes during maturation in marine teleosts remain poorly understood. In the present study, the orange-spotted grouper (Epinephelus coioides), an economically important marine aquaculture species, was used as a model. Oocytes at four distinct maturation stages were obtained by microscopically removing the surrounding follicular layers, followed by integrated mRNA-seq and miRNA-seq analyses to characterize the molecular regulatory landscape underlying oocyte maturation and hydration. The results showed that, as maturation progressed, oocyte diameter and wet weight increased significantly, accompanied by a marked decrease in Na⁺ content, a significant increase in K⁺ content, and the continuous accumulation of most free amino acids, indicating the gradual establishment of an osmotic basis favorable for oocyte hydration. Transcriptomic analysis further revealed extensive transcriptional remodeling during both the early and late phases of maturation. Differentially expressed genes were significantly enriched in pathways related to oocyte meiosis, cytokine signaling, lipid metabolism, DNA replication, cell cycle regulation, ribosome biogenesis, spliceosome function, oxidative phosphorylation, and mitochondrial activity, suggesting that oocyte maturation is a dynamic process characterized by a shift from basal growth maintenance to metabolic reprogramming, maternal transcript remodeling, and terminal maturation responses. miRNA profiling identified a large number of stage-specific differentially expressed miRNAs, including let-7d-5p, miR-22a-3p, and novel-miR-20/27/118, whose predicted target genes were mainly enriched in ribosome-related pathways, oxidative phosphorylation, DNA replication, transcriptional regulation, and signal transduction. Moreover, the miRNA–TF–mRNA regulatory network demonstrated that miRNAs may not only directly repress target genes, but also mediate hierarchical regulatory cascades through transcription factors, thereby coordinately participating in cell cycle progression, cytoskeletal remodeling, vesicular transport, and immune- and cell communication-related responses. Collectively, this study provides the first systematic temporal atlas of mRNA and miRNA regulation during oocyte maturation and hydration at the oocyte level in a marine teleost, thereby deepening our understanding of the molecular basis of meiotic resumption and egg quality formation, and offering valuable theoretical support for the optimization of artificial breeding and the identification of key molecular targets in grouper reproduction.