Hypophthalmichthys molitrix and Hypophthalmichthys nobilis are economically important freshwater fish species in China. During resource surveys in the Xiangluzhou section of Xiangjiang River, Changsha, in 2020, fish suspected to be natural hybrids of H. molitrix and H. nobilis were identified. To confirm this, we employed morphological and molecular analyses of H. molitrix , H. nobilis , and their suspected hybrids. Morphometric results revealed that most countable and measurable traits of suspected hybrids ( n = 29) were intermediate between those of H. molitrix ( n = 30) and H. nobilis ( n = 26). However, the traits were slightly biased toward those of H. molitrix , representing an intermediate, transitional form indicative of hybridization. Genome resequencing followed by principal component analysis and population structure analysis indicated that the genetic composition of the suspected hybrids ( n = 9) was intermediate between H. molitrix ( n = 10) and H. nobilis ( n = 9), containing nuclear genetic material from both species. These results confirmed the suspected hybrids as true hybrids of H. molitrix and H. nobilis . COI gene barcoding results showed that the suspected hybrids ( n = 28) formed a single clade in the haplotype network and shared haplotypes exclusively with H. molitrix ( n = 29), indicating that H. molitrix was the maternal parent. Furthermore, the asymmetric hybridization pattern suggested potential influence of physicochemical factors, such as water temperature, water quality, and flow velocity, which affect the affinity between sperm and egg cells. We propose that the natural hybridization of H. molitrix and H. nobilis originates from genetic exchange. The influence of Xiangjiang River’s unique aquatic ecological environment and hydraulic engineering projects, which have led to changes in flow regimes and the degradation of spawning grounds, have further altered the species’ ecological reproductive habits and disrupted reproductive isolation and natural hybridization. This study provides direct empirical evidence of natural hybridization between H. molitrix and H. nobilis within their native ranges in China through morphological and molecular research methods, thereby complementing previous hybridization inferences based on genomic data. These findings offer a novel view on fish hybridization theory.

To explore the current state of genetic diversity of in the Chinese soft-shelled turtle (Pelodiscus sinensis) in the Dongting Lake basin, the genetic diversity of five sub-populations consisting of 71 turtles were analyzed through mitochondrial Cytb and 12S rRNA. Our results revealed 13 haplotypes for Cytb and 6 for 12S rRNA. The overall haplotype diversity and nucleotide diversity indices were 0.750 and 0.014, and 0.712 and 0.009, respectively. The Shaoyang sub-population showed the lowest genetic diversity according to both markers. The genetic distances between sub-populations ranged from 0.010 to 0.018 (Cytb), and from 0.002 to 0.014 (12S rRNA), with the largest distance observed between the Shaoyang and Junshan sub-populations. The Junshan sub-population was significantly different from the other sub-populations (p < 0.05), and gene exchange was weak, despite belonging to the same population. Genetic variation within the P. sinensis sub-population was much higher than that between sub-populations. There was no recent expansion event in the history of P. sinensis. Overall, the genetic diversity of P. sinensis was high, whereas it appeared to be homogenous, suggesting a potential decline in genetic diversity. This study provides valuable insights for the conservation and sustainable use of P. sinensis.

Ochetobibus elongatus (Kner) is a migratory fish found in the Yangtze River basin and areas south of it, and listed as a critically endangered (CR) fish on the China Red List of Vertebrates. To achieve group recovery and artificial breeding, this study investigated the dietary characteristics of O. elongatus based on high-throughput sequencing of its intestinal contents, and its digestive system morphology, and its histology. Results showed that the digestive system of O. elongatus lacked a stomach and mainly consisted of the oropharynx, pharyngeal teeth, esophagus, intestine, and anus. The gut index was 0.88, with clear segmentation of the foregut, midgut, and hindgut, and the visceral mass index was 7.35%. Histological analysis of the digestive system revealed the presence of keratinized dental plates or pharyngeal teeth in the pharynx, as well as a high density of taste bud cells in the soft palate of the oral cavity. The surface layer of the intestinal villi contained numerous mucous cells, with the average number of mucous cells per villus gradually increasing from the esophagus to the hindgut, and the foregut having the longest and most abundant mucosal folds. The esophagus exhibited well-developed circular and longitudinal muscle layers, while in the hindgut, both the circular and longitudinal muscle layers were slightly thicker than those in the midgut. High-throughput sequencing of the intestinal contents of O. elongatus revealed the following phyla based on 18S V4 meta-barcoding: Chlorophyta, Diatoms, Arthropoda, Basidiomycetes, and Ascomycetes, with the genus Hypophthalmichthys and algae being the main classifications. In contrast, based on COI meta-barcoding, the study newly identified the phyla Cnidaria and Mollusca, with the genera Chlorophyta, Scenedesmus, Pectinodesmus, and zooplankton such as Pseudodiaptomus. Metagenomic sequencing revealed that the gut microbiota at the phylum level was predominantly composed of Pseudomonadota, Ascomycota, Basidiomycota, Chytridiomycota, and Bacillota, with key genera including Cetobacter, Pseudomonas, Acinetobacter, Aeromonas, and Clostridium. This study indicates that O. elongatus is an omnivore with carnivorous tendencies. Basic biological research on O. elongatus is of great significance for the restoration of the population, artificial breeding, and the development of its artificially formulated feed. It also provides important data for the formulation of biodiversity conservation measures.