Dynamics of reproductive hormones during downstream migration in females of the Japanese eel, Anguilla japonica.

The profiles of sex steroids (estradiol-17β, testosterone and 11-ketotestosterone) and the mRNA levels of gonadotropins (luteinizing hormone and follicle-stimulating hormone) were investigated before and after downstream migration in females of the Japanese eel species Anguilla japonica, which were collected in the brackish Hamana Lake and its inlet freshwater rivers. Eels were separated into three groups using otolith microchemistry: 'migrants' that grew in the inlet rivers and then made a downstream migration to Hamana Lake mainly in October and November; 'non-migrant' yellow eels caught in rivers during the same season; and 'residents,' which were yellow eels caught in rivers in August. Sex steroid levels, especially those of testosterone and 11-ketotestosterone, were higher in migrants than in non-migrants and residents. Real-time quantitative PCR analysis indicated that mRNA levels of luteinizing hormone (LH) β-subunits were significantly higher in migrants than in other groups, whereas those of follicle-stimulating hormone β-subunits did not show significant changes during downstream migration. The high levels of these hormones during downstream migration raise the question about if they also play a role in motivating the migratory behavior of eels.