Individual variation in post-metamorphic changes in feeding incidence, digestive organ tissues and enzyme gene expression in Japanese eel Anguilla japonica glass eels

• Published in: Aquaculture 2023-11, Vol.576, p.739890, Article 739890
• Main Authors: Hatakeyama, Rui, Sudo, Ryusuke, Higuchi, Masato, Satomi, Masataka, Yatabe, Takashi, Takasaki, Ryutaro, Imaizumi, Hitoshi, Kazeto, Yukinori
• Format: Article
• Language: English
• Subjects: Anguilliformes; Feeding onset; Histology; RT-PCR; Starvation tolerance
• ISSN: 0044-8486; 1873-5622
• DOI: 10.1016/j.aquaculture.2023.739890

Temporal changes of feeding incidences, digestive organ tissues, and mRNA expression of digestive enzymes were investigated in artificially reared Anguilla japonica glass eels. Incidences of daily feeding and cumulative first feeding exceeded 50% at day 40 and 33, respectively, but tended to increase gradually. Time until first feeding varied from day 11–61 among individuals. Glass eels without food after metamorphosis showed notable developmental changes in their digestive system that included increases in esophagus goblet cells and blood vessel diameters, elongation of the stomach, differentiation and increased number of gastric glands, and increases in gall bladder sizes. A distinct qualitative change, which was gastric gland differentiation, occurred in all eels until week 2 after metamorphosis. All quantitative character values showed trends of gradual changes in their averages and had high coefficients of variation, especially for goblet cell numbers. Few deaths and no histological features related to severe starvation such as notably more hepatocyte necrosis or desquamation of intestinal epithelial cells were observed even after 9 weeks. Hepatocyte-vacuole stored glycogens and pancreatic zymogen granules were also found, suggesting that they were not expressing symptoms of starvation. Relative expressions of five digestive enzymes were lowest at week 0 and increased gradually in their averages with the passage of weeks, along with large variations among fish. This study found that half of the post-metamorphic glass eels began feeding after 4 weeks, but that there were large individual variations in the timing of feeding onset, the degree of developmental of their digestive organs, and in digestive enzyme expressions. •Half of post-metamorphic Anguilla japonica glass eels began feeding after 4 weeks.•Quantitative characters in the digestive organs progressively developed over time.•mRNA expressions of digestive enzymes increased gradually over time.•Few deaths and no severe starvation features occurred even after 9 weeks without food.•Large variations in feeding onset time and digestive system development occurred.