Ammonia is an important water quality factor, which causes several adverse effects in fish. In this study, the 5-month-old hybrid sturgeon (Acipenser baerii ♀ × Acipenser schrencki ♂) with a mean body weight of 50 ± 0.5 g was subjected to ammonia stress (0 mg/L, control group; 10 mg/L, low-concentration group; 50 mg/L, high-concentration group) for 48 h and followed by recovery for 48 h, changes of liver tissue, antioxidant enzyme activity and relative expressions of inflammation-related genes were observed. The results showed that, with an increasing ammonia concentration and exposure duration, hepatocytes showed swelling, congestion and indistinguishable cellular outline. After 48 h of recovery, the degree of swelling decreased, nuclear shifted, and vacuolation reduced; however, the normal levels before stress application were not reached again. Additionally, compared with the control group, the activities of superoxide dismutase, catalase and glutathione peroxidase were significantly upregulated 6 h after stress. Malondialdehyde reached the maximum at 24 h in the low-concentration group and at 12 h in the high-concentration group, indicating the activity of the antioxidant system trying to protect cells from oxidative stress. Furthermore, the relative expression levels of interleukin 1 beta, tumour necrosis factor α genes and heat shock proteins (both HSP70 and HSP90β) in the liver were induced by ammonia stress. These results provide direct evidence for the damage caused by ammonia stress and the self-recovery ability of the hybrid sturgeon.

Ammonia nitrogen is one of the major pollutants in aquaculture. However, the mechanism of ammonia stress on ovarian development and follicular atresia is unknown. In this study, goldfish (Carassius auratus) were exposed to different ammonia concentrations, including 0 (control, C) and 50 mg/L (high, H) for 48 hr at 21 ± 0.5°C. Histology of the ovarian tissue sections revealed that the high ammonia concentrations inhibited the maturation of oocytes, and induced broken and wrinkled radiation bands, follicular membranes, and the accumulation of rod‐shaped yolk particles. From the transcriptome data, a total of 1,405 differentially expressed genes (DEGs) were detected between H and C groups. Most DEGs were highly enriched in low‐density lipoprotein particle‐mediated signaling, oxidative stress‐induced premature senescence, and ovarian steroidogenesis pathway. Furthermore, a total of 19 genes were selected and determined by quantitative real‐time polymerase chain reaction (qRT‐PCR). The changes in oxidative stress and antioxidant activity indicated the excessive accumulation of reactive oxygen species (ROS) in ovarian tissue. The ovarian apoptosis was induced in the H group. The downregulation of genes involved in the ovarian steroidogenesis pathway indicated that ammonia nitrogen stress prevented the synthesis of sexual hormones. These results suggest that high‐concentration ammonia stress induces ovarian damage in goldfish, affecting its antioxidant function, leading to excessive ROS accumulation, ovarian apoptosis, and hindered steroid synthesis, adversely affecting reproductive performance.

Excess ammonia can damage the growth and development of fish. Pseudobagrus ussuriensis is a scaleless fish with important economic value that is more sensitive to ammonia stress. In this study, P. ussuriensis was explored using different ammonia concentrations [control (0 mg/L), CL; low stress (10 mg/L), T1L; and high stress (50 mg/L), T2L] for 48 h. Compared to the control group, the liver cells in the T1L group showed slight damage, while the T2L group was severely damaged, with the cells being loosely arranged, with nuclei lysis and cell vacuolization. The activities of superoxide dismutase, catalase, and glutathione in the T1L and T2L groups were significantly lower than those in the CL group (p < 0.05), and the malondialdehyde reached the maximum at 48 h. Furthermore, 9301 differentially expressed genes (DEGs) (4583 upregulated and 4718 downregulated) were detected by transcriptome sequencing. Most DEGs were highly enriched in cellular processes (GO:0009987) and cell parts (GO:0044464). Especially, the phosphatidylinositol 3-kinase/protein kinase B (PI3K-Akt) signaling pathway had the maximum quantity of DEGs in all the three groups. In-depth analysis revealed the stress caused multiple substitutions of SNP sites in pik3ca and kras, blocking the PI3K/Akt signaling pathway to prevent cancer cell proliferation and spread, accelerating the apoptosis of damaged cells. These results suggest that ammonia stress induces liver damage in P. ussuriensis, causing genetic mutations and cellular carcinogenesis, thereby accelerating cell apoptosis.

Pseudobagras ussuriensis is a valuable freshwater fish species with enormous breeding potential. To clarify the relationship between the main visceral indices and body weight in this species, 139 cultured individuals were randomly selected to measure body weight and six major organs (namely the intestine, liver, swim bladder, kidney, spleen, and gonadal), and then the causal network of internal organs and body weight of one-year-old P. ussuriensis was analyzed by path analysis, and sex-specific regression models were developed. The results showed that the correlations between body weight and the masses of the intestine, liver, swim bladder, kidney, and gonad were significant, while the spleen mass showed a significant positive correlation with body weight. Path analysis revealed that the direct path coefficients of the intestine, liver, swim bladder, kidney, and gonad on body weight were significant, and that of the spleen was significant. Through regression analysis, multiple linear regression equations were established. Importantly, the swim bladder had the greatest direct effect on body weight in males, whereas the intestine exhibited the strongest direct effect in females. These findings provide valuable insights for the selection and breeding of P. ussuriensis based on visceral indices.

Ammonia is a key factor in the water, impacting the physiological functions of aquatic organisms. To explore the effect of ammonia stress on mature eggs, female A. baerii at the end of the fourth stage of ovarian development were subjected to varying ammonia concentrations (0 mg/L (control, C), 10 mg/L (low concentration, T1), and 50 mg/L (high concentration, T2)) for 96 h. After 96 h of stress, histological analysis revealed that the follicular membranes of group T1 remained intact and clear compared to group C, although the vacuole fusion had begun. In contrast, the T2 group exhibited ruptured follicular membranes and adhered yolk granules compared to the C group, indicating structural damage. Transcriptome analysis generated 97.89 Gb of clean data, with each sample yielding over 6.09 Gb. A total of 5576, 3719, and 9446 differentially expressed genes (DEG_S) were screened from T1 vs. C, T2 vs. C, and T2 vs. T1 comparisons, respectively. Gene Ontology analysis (GO) functional enrichment analysis showed that DEG_S were significantly enriched in multicellular organism processes (T1 vs. C), cell surface receptor signaling pathways (T2 vs. C), and immune system processes (T2 vs. T1) during biological processes. It indicates that ammonia exposure may enrich cellular components in the extracellular space, potentially disrupting the function of the extracellular matrix. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment indicated significant impacts on amino acid metabolism, particularly glutamate and arginine pathways, as well as key pathways involved in steroid biosynthesis and antioxidation. Weighted gene co-expression network analysis (WGCNA) revealed that a total of 26,369 DEGs were divided into 29 distinct modules, displaying obvious associations with their traits. In the T2 vs. C group, antioxidation-related genes such as GST and GCLM were significantly downregulated, and the expressions of key enzymes for steroid synthesis, such as CYP11A1, CYP17, and CYP19A1 were suppressed, indicating that high ammonia nitrogen concentrations impair oocyte function by inducing oxidative stress and disrupting hormone synthesis. This study provides a comprehensive repertoire of candidate genes associated with ammonia stress in the mature egg of A. baerii, which will be useful for development of sturgeon breeding and reproduction.

The genus Pseudobagrus represents a group of economically and ecologically significant freshwater bagrid catfishes in East Asia, yet its taxonomy remains contentious. This study employed the complete mitochondrial genomes of 15 Pseudobagrus species to clarify their phylogenetic relationships. The mitogenomes ranged from 16,526 to 16,647 bp, exhibiting a conserved gene order and significant AT bias (average A + T = 57.85%). All genomes contained 13 protein-coding genes (PCGs), 22 tRNAs, two rRNAs, and a control region. Start codons were predominantly ATG, except for COI (GTG), while stop codons varied among TAA, TAG, and incomplete T--. Codon usage bias favored NNU and NNA codons, and 12 optimal codons were identified in P. albomarginatus. The phylogenetic trees based on concatenated PCGs revealed two major clades. Clade I contained 14 species. Within this clade, P. albomarginatus, P. tenuis, and P. brevicorpus clustered together first, and then this trio grouped with P. ussuriensis. Pseudobagrus trilineatus formed the separate Clade II. These results provide a molecular foundation for species delimitation and systematic revision within Pseudobagrus, supporting the monophyly of the genus while highlighting cryptic diversity and taxonomic complexity.