Dietary Soy Isoflavones Promote Feminization and Enhance Growth of Juvenile Japanese Eel (Anguilla japonica)

The Japanese eel (<i>Anguilla japonica</i>) is a valuable aquaculture species in East Asia. However, sex differentiation under culture conditions is often male-biased, complicating the production of female broodstock. This study evaluated the effects of dietary soy isoflavone (SI) supplementation on growth performance, sex differentiation, and sex-specific gene expression in juvenile eels. Juvenile eels (initial mean weight: 1.25 ± 0.02 g) were randomly assigned to 15 tanks, each containing 100 g eels, in a 30-week feeding trial. The control diet (SI0) contained 75% jack mackerel meal. The graded levels (2.5, 5, 7.5, and 10%) of α-Starch were included at the expense of SI, referred to as SI0, SI2.5, SI5, SI7.5, and SI10 diets, respectively. Fish were hand-fed to satiation in triplicate groups of each diet, twice daily for 30 weeks. The SI2.5 and SI5 groups exhibited significantly enhanced final weights and specific growth rates compared to those in the control group (SI0). Histological analysis revealed the presence of differentiating gonads with oogonial proliferation and previtellogenesis in the SI-treated groups, particularly at the 2.5% and 5% inclusion levels. Additionally, the expression of key female-related genes (<i>vasa</i>, <i>cyp19a1a</i>, <i>foxl2a</i>, <i>zp3</i>, and <i>zar1</i>) was significantly upregulated in these groups. In contrast, relatively high SI levels (7.5% and 10%) did not provide any further benefits. These results suggest that low-dose supplementation of dietary SI can enhance feminization and promote growth in juvenile eels, thus offering a potential natural alternative to synthetic hormone treatment for broodstock development.