International Journal of Aquaculture, 2024, Vol.14, No.3, 154-164 http://www.aquapublisher.com/index.php/ija 156 specific recombinant gonadotropins to induce spermatogenesis and spermiation in European eels (Anguilla anguilla). In a study, recombinant European eel follicle-stimulating hormone (aarFsh) and luteinizing hormone (aarLh) were used to stimulate androgen synthesis and induce spermiation. The best results were achieved with a combination of constant doses of aarFsh and increasing doses of aarLh, leading to high-quality sperm production with over 50% motile cells (Peñaranda et al., 2018). Additionally, the use of phytoestrogens such as genistein has been shown to induce feminization in Japanese eels (Anguilla japonica) by altering the expression of sex-specific genes, thereby enhancing the female ratio which is commercially desirable (Inaba et al., 2022). 3.2 Embryonic development and growth Embryonic development and growth in eels can be significantly influenced by hormonal treatments. The administration of recombinant gonadotropins not only induces spermatogenesis but also ensures the production of high-quality sperm, which is essential for successful fertilization and subsequent embryonic development (Peñaranda et al., 2018; Kottmann et al., 2021). Furthermore, the feminization of eels through dietary phytoestrogens like genistein can lead to a higher proportion of females, which are known to achieve greater body weight and better growth rates compared to males (Inaba et al., 2022). This approach can be strategically used to enhance the overall growth performance in eel aquaculture. 3.3 Environmental influences on development Environmental factors such as diet and hormonal treatments play a pivotal role in the developmental processes of eels. The study on Japanese eels demonstrated that dietary inclusion of soy isoflavones like genistein can significantly influence sex differentiation and growth by modulating the expression of sex-specific genes (Inaba et al., 2022). Similarly, controlled administration of recombinant gonadotropins in European eels under aquaculture conditions has shown to effectively induce reproductive maturation and improve sperm quality, which are critical for successful breeding and development (Peñaranda et al., 2018; Palstra et al., 2023). These findings highlight the importance of optimizing environmental conditions to enhance reproductive success and growth in eel aquaculture. 4 Enhancing Reproductive Success 4.1 Breeding programs and techniques Breeding programs for eels, particularly the Japanese eel (Anguilla japonica) and European eel (Anguilla anguilla), have focused on overcoming the challenges of inducing maturation and successful reproduction in captivity. Techniques such as hormone injections have been employed to stimulate ovarian growth and spermatogenesis. For instance, the use of salmon pituitary homogenate (SPH) combined with methyltestosterone (MT) has shown to enhance synchronous follicle development in Japanese eels (Lee and Lou, 2019). Similarly, recombinant gonadotropins have been used to induce spermatogenesis and spermiation in European eels, with varying doses of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) yielding significant results (Peñaranda et al., 2018). 4.2 Genetic improvement strategies Genetic improvement strategies have been pivotal in enhancing reproductive success and growth in eels. The high-quality genome assembly of the Japanese eel has provided insights into gene duplication and chromosomal reorganization, which are crucial for understanding the genetic basis of traits related to reproduction and growth (Wang et al., 2022). Additionally, the study of sex differentiation at the molecular level has identified key genes involved in gonadal development, which can be targeted for genetic improvement (Horiuchi et al., 2022). The transfer of maternal steroids and mRNA transcripts to eggs has also been shown to influence embryonic development, suggesting that genetic selection for favorable traits could enhance reproductive outcomes (Kottmann et al., 2021). 4.3 Case studies of successful reproduction Several case studies have demonstrated successful reproduction in eels through various interventions. For example, Lee and Lou (2019) studied the regulatory role of androgens in the primary ovarian growth of the Japanese eel
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