Melatonin is secreted during the dark and has been suggested to be a controlling factor of reproduction in various animals including fish. Eels show diel vertical movements, rising at night and going down during the day as they migrate toward the spawning ground (Seinen Chow et al., 2015). Thus, it is quite probable to think that the sexual maturation of eels is associated with photic conditions and melatonin. No relevant studies, however, have been conducted to explore any relationship between female eel reproduction and melatonin. This study investigated the effects of SPE (salmon pituitary extract) and melatonin treatment on the maturation induction of female eel.
Female eels were divided into four groups, each treated with saline (control), melatonin (M), SPE (S), or SPE+melatonin (SM) by using osmotic pumps. All osmotic pumps were implanted at the beginning of the experiment except the SM group in which osmotic pumps with melatonin were implanted on day 35. Fish were killed, and the blood, gonad, brain + pituitary gland were sampled on day 65. Gonadal development was estimated by means of histology and GSI. The level of plasma melatonin was measured by ELISA. Expression levels of LH, FSH, and melatonin receptor genes were analyzed from tissues of the brain+pituitary gland.
The delivery of SPE and melatonin by the osmotic pump contributed to the induction of sexual maturation and elevation of melatonin levels in female eels, respectively. Eels treated with SPE showed significantly higher GSI and LH gene expression than control eels. The level of melatonin ranged from 228 to 1123 pg/ml in melatonin treated group which was higher than that of the untreated group. However, the administration of exogenous melatonin did not promote or inhibit maturation at all. On the contrary, the levels of plasma melatonin in eels with mature gonads were significantly lower than the levels of eels with immature gonads. At the moment, it cannot be concluded that the sexual maturation of eel is not associated with melatonin. Further studies are required to prove whether the exogenous melatonin works properly or not in the body of eels.