Roles of lipids and fatty acids through the spermatogenesis of European eel (Anguilla anguilla)

During the last 25 years, eel populations have declined considerably, this species is considered outside safe biological limits. The life cycle of the eel is quite complex including transoceanic migration whose conditions are still so unknown that even the natural spawning process has never been observed. Eels cease feeding during migration and that is why their energy reserves such as lipids play a crucial role and thus their in deep study is essential to obtain gametes (eggs and sperm) quality as the first step for their reproduction in captivity. In the last years, many efforts have been directed at the study of the influence of fatty acids on the reproductive performance in females, but not in male eels. The present manuscript describes the results obtained in two experiments. The first experiment (which led to the three first articles included in the memory) focused on the effect of fatty acids along maturation, their relationship with steroid hormones and their effect on sperm quality parameters. The second experiment involved the comparison of different diets designed, considering the previously acquired insights into the most influential fatty acids on male eel reproduction, and the assessment of their influence on the reproductive performance of males. The first experiment was used to assess the changes in the fat content, as well as fatty acids, in different tissues of male eels hormonally induced to sexual maturation under different thermal regimes (two variables and one constant). Males finished spermatogenesis earlier with a constant temperature of 20 oC, suggesting that eel spermatogenesis is tightly regulated by temperature. The fat content did not change significantly in the muscle, but was increased in the liver and decreased in testes during testicular development. With regard to fatty acids, during sperm maturation, the liver was highlighted as the main site of synthesis. Finally, EPA, ARA and DHA remained constant in testes, while the level of the rest of fatty acids decreased significantly. Using samples from the same experiment, the correlation between the main steroid hormones and fatty acids at different stages of testicular development were investigated. Similarly as occurs in mammals, EPA and DHA were highlighted as possible modulators of androgen synthesis. The set of the results obtained suggests new perspectives concerning the functions and interactions between fatty acids and steroids in fish spermatogenesis. Also, with samples from the first experiment, a study was carried out seeking to establish the possible correlations between different fatty acids and several sperm quality parameters. Correlations between several highly unsaturated fatty acids with parameters such as volume, the percentage of motile sperm and sperm velocity were found. Finally, with the acquired knowledge so far, a second experiment was carried out designing feeds with different fatty acid percentages in order to evaluate the influence of the diets on sperm quality. The results showed that high levels of DHA and EPA in the diets induce higher volumes and high sperm motility. This study allowed obtaining results applicable to the design of optimum broodstock diets for this species.