Serum estradiol-17beta and testosterone levels during silvering in wild Japanese eel Anguilla japonica.

To understand the changes of serum levels of sex steroids in the wild Japanese eel Anguilla japonica during silvering process, eels collected from the Kaoping River of Taiwan from August 2000 through June 2001 were examined. The maturational stages of female eels before and during silvering were divided into four stages: juvenile, sub-adult, pre-silver and silver stages based on skin coloration and oocyte diameter. Male eels were investigated only in the silver stage. Radioimmunoassays were employed to measure serum levels of estradiol-17beta (E(2)) and testosterone (T). The mean liver mass of the female eels increased significantly during silvering, but the mean hepatosomatic index remained constant. In contrast, mean ovarian mass and gonadosomatic index increased significantly during silvering. Serum concentrations of E(2) in females increased significantly during silvering (P<0.05), while E(2) was undetectable in silver males. The mean serum T concentrations increased significantly in females (P<0.05) during silvering, with lowest mean values in the juvenile stage and highest mean value in the silver stage. The mean serum T level in the silver males was significantly lower than in silver females (P<0.05). In conclusion, both serum E(2) and T concentrations increased with ovarian development of wild Japanese eels during silvering, while serum E(2) was undetectable in the silver male eels. The findings support the idea that androgen, but not estrogen, plays a major role in silvering process of the eels in both sexes.