Artificial induction of gonadal maturation and ovulation in the Japanese eel (Anguilla japonica T. et S.)

Abstract

8 Injections of carp pituitary extract (CPE, 5 pit. / fish / 10 days) induced ovarian maturation in female eel, gonadosomatic index (GSI) increasing significantly to 43-55 %. 4 injections of CPE (1-2 pit. / fish / 10 days) induced spermatogenesis and spermiation in male eel, GSI increasing to 2.1-2.8 %. After serial injections of CPE, serum gonadotropin (GtH) levels increased significantly, indicating that the exogeneous GtH stimulated gonadal development and maturation in the Japanese eel. 8 implantations of 17 α-methyltestosterone (MT) or androstenedione (ADSD) (50 μg / g body weight / 15 days) stimulated ovarian maturation in female eel, GSI increasing significantly to 38-49 %. 4 implantations of MT or ADSD (50 μg / g body weight / 15 days) induced spermatogenesis and spermiation in the male eel, GSI increasing to 2.1-3.5 %. MT or ADSD implantations significantly elevated pituitary and serum GtH contents. ADSD or MT implantations resulted in the significant elevation of serum estradiol levels in female eel and serum testosterone levels in male eels. In female eel, ADSD implantations significantly increased mammalean-libe-gonadotropin-releasing hormone (mGnRH) content in brain and pituitary. These results demonstrated that the positive feedback effects of sex steroids on the brain (hypothalamus)-pituitary-gonad axis play an important role on the induction of gonadal development and maturation in the Japanese eel, and the endogeneous GtH is the key hormone during these physiological processes. In the matured female eel, ovulation and spawning can be induced 18-21 hours following an injection of CPE alone or in combination with [D-Ala6, Pro9-NEt]-luteinizing hormone-releasing hormone (LHRH-A) and dopamine antagonist domperidone (DOM).