FEED ADMINISTRATION OF ESTRADIOL-17-BETA STIMULATES FEMALE DIFFERENTIATION IN JUVENILE GREY MULLET MUGIL CEPHALUS

The objective of the study was to investigate the control of female di fferentiation by oral administration of varying doses of estradiol-17 beta (E2) in juvenile grey mullet Mugil cephalus. The concentrations o f E2 in plasma, stomach and muscle were also measured after feeding ex periment. Six-month-old juvenile grey mullet were divided into three g roups, control (n = 55), low (n = 35) and high (n = 35) doses of E2, r espectively. The low-dose diet was 1 mg E2/kg feed (period I for 4 mon ths) and then 15 mg/kg feed (period II for another 4 months). The high -dose diet was 8 mg E2/kg feed (period I for 4 months) and then 120 mg E2/kg (period II for another 4 months). Low- and high- doses of E2 st imulated development of a unisexual group of juvenile grey mullet, all in the female direction. The effects of E2 on feminization was dose-d ependent. E2 was able to induce the beginning of sexual differentiatio n at a smaller body size and length in this species. E2 in muscle and blood was not detectable in the control or low-dose groups. Higher lev els of E2 were detected in plasma, stomach and muscle immediately afte r the ending of treatment with high doses of E2. E2 in stomach and mus cle in the high-dose group diminished to control levels after 45 days of feeding with the control diet.