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 gonads 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 fish examined had differentiated into males and females whereas 70–90% of 15‐ to 17‐month‐old fish and more than 95% of 18‐ to 24‐month‐old fish had completed gonadal differentiation. Differentiation into females occurred earlier than males. In 15‐ to 17‐month‐old fish, spermatogonia and primary oocytes in the perinucleolus stage were observed in the testes and ovaries, respectively. Testes underwent intensive spermatogenesis at 18 to 24 months whereas ovaries during this period contained both primary and well developed vitellogenic oocytes. There was no sexual dimorphism in the annual cycle of plasma estradiol‐17β (E2), testosterone (T) and 17α‐hydroxyprogesterone (17α‐OH P) in 7‐to 24‐month‐old grey mullet. Plasma E2 levels significantly decreased in 9‐ to 12‐month‐old presumptively female and male grey mullet. On the contrary, peak levels of plasma T were observed in both female and male grey mullet at 10 to 11 months. Plasma 17α‐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 differentiate, but sex differentiation mainly occurs at the age of 15 months. It is 9 to 12 months of age when the decreases of plasma E2 and increases of T levels significantly occur in both female and male grey mullet, M. cephalus.