GONADAL HISTOLOGY AND PLASMA SEX STEROIDS DURING SEX-DIFFERENTIATION IN GREY MULLET, MUGIL CEPHALUS

Gonadal histology and plasma levels of sex steroids were investigated during the period of sex differentiation and the annual cycle of grey mullet, Mugil cephalus. Primordial germ cells were observed in the gon ads of 3-month-old grey mullet and appeared to be undifferentiated in fish at 3 to 6 months of age. At 7 to 14 months of age 23.3% of the fi sh examined had differentiated into males and females whereas 70-90% o f 15- to 17-month-old fish and more than 95% of 18- to 24-month-old fi sh had completed gonadal differentiation. Differentiation into females occurred earlier than males. In 15- to 17-month-old fish, spermatogon ia and primary oocytes in the perinucleolus stage were observed in the testes and ovaries, respectively. Testes underwent intensive spermato genesis at 18 to 24 months whereas ovaries during this period containe d both primary and well developed vitellogenic oocytes. There was no s exual dimorphism in the annual cycle of plasma estradiol-17 beta (E(2) ), testosterone (T) and 17 alpha-hydroxyprogesterone (17 alpha-OH P) i n 7- to 24-month-old grey mullet. Plasma E(2) levels significantly dec reased in 9- to 12-month-old presumptively female and male grey mullet . On the contrary, peak levels of plasma T were observed in both femal e and male grey mullet at 10 to 11 months. Plasma 17 alpha-OH P levels did not show significant profiles in the annual cycle of grey mullet. The data conclude that few gonads at 7 to 14 months begin to differen tiate, but sex differentiation mainly occurs at the age of 15 months. It is 9 to 12 months of age when the decreases of plasma E(2) and incr eases of T levels significantly occur in both female and male grey mul let, M. cephalus. (C) 1995 Wiley-Liss, Inc.