Analysis of <i>Ochetobibus elongatus</i> (Kner) Dietary Habits Based on Digestive System Morphology, Histology, and Intestinal Content Sequencing Technology

<i>Ochetobibus elongatus</i> (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 <i>O. elongatus</i> based on high-throughput sequencing of its intestinal contents, and its digestive system morphology, and its histology. Results showed that the digestive system of <i>O. elongatus</i> 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 <i>O. elongatus</i> revealed the following phyla based on 18S V4 meta-barcoding: Chlorophyta, Diatoms, Arthropoda, Basidiomycetes, and Ascomycetes, with the genus <i>Hypophthalmichthys</i> 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 <i>Chlorophyta</i>, <i>Scenedesmus</i>, <i>Pectinodesmus</i>, and zooplankton such as <i>Pseudodiaptomus</i>. 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 <i>Cetobacter</i>, <i>Pseudomonas</i>, <i>Acinetobacter</i>, <i>Aeromonas</i>, and <i>Clostridium</i>. This study indicates that <i>O. elongatus</i> is an omnivore with carnivorous tendencies. Basic biological research on <i>O. elongatus</i> 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.