The objective of this study was to temporally identify sex differentiation to morphologically characterize the formation of female and male germinal epithelium and verify the occurrence of the sex disproportion reported for this species in aquaculture. Specimens of Brycon orbignyanus cultivated in water recirculation systems with a constant temperature of 27 ± 0.5°C were used. Monthly samplings of B. orbignyanus (n = 15) were performed from two months of age until 1 year of age, and posteriorly isolated sampling was performed on specimens of two and three years of age. The gonads of the specimens were removed and processed for light microscopy. In the beginning, the undifferentiated gonads develop into ovaries (females; at three months of age) and functional testicles (primary males; at five months of age); however, some of the females go through a sex inversion process creating intersex individuals and posteriorly secondary males (at five months of age). Due to the processes of sex inversion and gonadal remodelling, the population sex proportion fluctuates until it is finally established at 1♀:2♂. Females above 15 cm are differentiated, while in males (primary and secondary), differentiation will only occur after the animal reaches 20 cm. At the end of the first year of age, primary males and secondary males reach sexual maturity, while female sexual maturity is only observed at three years of age.

Piracanjuba, Brycon orbignyanus, is an important rheophilic species for aquaculture studies on genetic diversity, spermatogenesis, semen cryopreservation and taxonomy.Due to anthropic actions its population has been declining, resulting in the loss of natural stocks, thus pushing the species to near extinction. No stereological analysis of spermatogenesis in piracanjuba has been performed. The present study aimed to evaluate spermatogenesis in B. orbignyanus using stereological analysis to quantify the number of germ cells and Sertoli cells, spermatogonial generations, spermatogenesis yield, meiotic index and the percentage of cell death. Testes of five 1-year-old B. orbignyanus specimens were collected, fixed in 2.5% glutaraldehyde and 4% paraformaldehyde and processed for light microscopy. Three types of spermatogonia were identified. Based on the number of B spermatogonia per cyst, it was estimated that spermatogonia underwent approximately nine mitotic divisions before differentiating into primary spermatocytes. Three types of spermatids were identified. In the final spermatid cyst, there were 1204 germ cells, indicating a spermatogenic yield of 58%. An increase in germ cells resulted in a subsequent increase in the number of Sertoli cells, with growth halted at the spermiogenic phase. Thus, stereological analysis of spermatogenesis is essential for better understanding of the reproductive processes in this species.