Effects of gonadotropin-releasing hormone analog on milt production enhancement in starry flounder Platichthys stellatus

Abstract

The treatment of exogenous gonadotropin or gonadotropin-releasing hormone analog (GnRHa) is able to solve some problems that are caused by small milt volume or viscous milt in artificial insemination of flatfish species. However, the mechanism of the action of the exogenous hormones on milt production has not been well understood. Therefore, experiments were carried out to study the effects of GnRHa on the milt production and characteristics of seminal plasma in starry flounder Platichthys stellatus. Males were implanted with GnRHa pellets at three different dosages (50, 100 or 200 μg/kg body weight) during their natural spawning season. Milt volume and sperm production increased in a dose-dependent manner after GnRHa pellet treatments. Spermatocrit and sperm concentration were significantly lower in GnRHa treated groups than in the control group. The sperm motility index was high from day 4 to day 49 in all experimental groups including the control group, and it declined after day 49 in all groups except the 200 μg treated group in which it remained high until day 85. During the experimental period, osmolality, K+ and total protein tended to be higher, in the control group than in the GnRHa treated groups. The results suggested that GnRHa treatment increased the milt volume and sperm production ability in starry flounder. The increase of milt volume was twice as rapid as the increase of sperm production. No significant differences in physicochemical composition of blood plasma and seminal plasma were found according to GnRHa dosages, but there were significant differences between GnRHa treated groups and the control group.