Development of the genital duct system in the protandrous black porgy, Acanthopagrus schlegeli

Protandrous black porgies, Acanthopagrus schlegeli, have a striking life cycle, which includes early sex differentiation, bisexual gonads, and a male-to-female sex change at three years of age. We found novel features of and insights into the development of the genital duct system in relation to the gonadal stage during early gonadal development and natural sex change. We found that the genital ducts developed at 16-20 weeks of age during sex differentiation. The gonad developed "ovarian cavity cracks" and became "four-stranded" during the first prespawning period and then proceeded to the development of genital ducts before 1 year of age. Two ovarian cavities of the paired gonads combined, developed, and extended caudally to form the oviduct, making up the inner duct of the genital ducts. The testicular main cavities also extended and fused together to form the outer duct of the genital duct system, that is, the sperm duct. The coexistence of an outer sperm duct and an inner oviduct constituted a unique "double cannula genital duct" structure. Gradually the inferior walls of the oviduct intermingled with those of the sperm duct, and the circular lumen of the sperm duct changed into an "M-shaped canal." Finally, the sperm duct and oviduct separated completely at the distal part of genital duct system. During natural sex change, the male reproductive passage regressed and degenerated and was replaced by connective tissue. The oviduct arrested as a blunt end during male phase and, finally, extended and connected to the genital pore during the female phase.