RESUMEN
The objective of this study was to investigate the optimal developmental time to perform sex reversal in Ussuri catfish Tachysurus ussuriensis, to develop monosex breeding in aquaculture. Systematic observations of gonadal sex differentiation of P. ussiriensis were conducted. The genital ridge formed at 9 days post fertilization (dpf) and germ cells begin to proliferate at 17 dpf. The ovarian cavity began forming on 21 dpf and completed by 25 dpf while presumptive testis remained quiescent. The primary oocytes were at the chromatin nucleolus stage by 30 dpf, the peri-nucleolus stage by 44 dpf and the cortical alveoli stage by 64 dpf. The germinal vesicle migrated towards the animal pole (polarization) at 120 dpf. In presumptive testis, germ cells entered into mitosis and blood vessels appeared in the proximal gonad on 30 dpf. The efferent duct anlage appeared on 36 dpf and formation of seminal lobules with spermatogonia and lobules interstitium occurred at 120 dpf. Therefore, gonadal sex differentiation occurred earlier in females than in males, with the histological differentiation preceding cytologic differentiation in T. ussuriensis. This indicates that undifferentiated gonads directly differentiate into ovary or testis between 17 and 21 dpf and artificial induction of sexual reversal by oral steroid administration must be conducted before 17 dpf.