Phenotypic and gene expression characteristics of Japanese eel late-stage leptocephali and glass eels after growth on different diets with a focus on nutrient-accumulation status

Abstract

To evaluate the quality of glass eels that were derived from late-stage Anguilla japonica leptocephali raised on an old-type of shark egg-based diet (SE) or a new-type of fish meal-based diet (FSY), specific gravity (SG), feeding tests, and body compositional and RNA-seq gene-expression analyses were conducted. The largest content difference between the two dietary groups was in lipids, with the body-compositional index (BCI) being 29.2% lower in FSY-leptocephali and 43.1% lower in FSY-glass eels at day 3 post-metamorphosis, but the difference between groups narrowed to 21.6% at post-metamorphosis first feeding. FSY-leptocephali had heavier SG, a larger number of differentially expressed genes (DEGs) related to muscle development and/or movement, and higher creatine kinase expression, suggesting that they settled easily, and were forced to swim more against fast bottom currents in the tanks while consuming more energy. It was found that the days until first feeding in FSY-glass eels was significantly shorter than in SE glass eels, with lower variability among eels. The number of DEGs was larger in FSY-glass eels, with the largest difference of 3.6 times occurring at day 10 post-metamorphosis, especially in the metabolism category, but at first feeding, they decreased drastically in both groups narrowing the difference. In the FSY-glass eels, the number of DEGs related to lipid, amino acid, and protein metabolisms were larger at days 3–20 after metamorphosis. Total enzyme expression levels for acylglycerol degradation, ketone body biosynthesis and citric acid cycle pathways also tended to be higher, suggesting that they obtained energy aerobically using lipids and other materials. Based on the patterns of up- and down-regulation of expression levels, growth hormone, ghrelin/obestatin prepropeptide and pro-melanin-concentrating hormone-like were detected as possible orexigenic hormones, and proopiomelanocortin and nucleobindin-2 as possible anorexigenic hormones. These results indicate that diet quality affects not only SG and other characteristics of leptocephali, but even body composition, metabolism and time to first feeding in the post-metamorphic glass eels for use as seedlings for aquaculture.