Female sex inversion as a reason for an unbalanced sex ratio in the neotropical species Brycon orbignyanus

Abstract

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.

Effect of temperature on gonadal differentiation and growth of Leporinus friderici

“…The process of transitioning from undifferentiated gonads to ovaries (150 DAE) occurred earlier compared to the differentiation to testes (190 DAE). The observed pattern of differentiation observed in this study, where differentiation into ovaries occurs before differentiation into testes, is consistent with what has been commonly described in the majority of Neotropical fish species such as piracanjuba ( Brycon orbignyanus ) ( Zardo et al, 2021 ; Quirino et al, 2022 ), pacu ( Piaractus mesopotamicus ) ( Barbosa et al, 2022 ), lambari ( Astyanax altiparanae ) ( Adolfi et al, 2015 ) and pirarucu ( Arapaima gigas ) ( Amaral et al, 2020 ). The differentiation process was initiated at 150 DAE at 25 °C and at 170 DAE at 29 °C.…”

Long-term monitoring reveals a consistent female-biased sex ratio in Pimelodus maculatus from the Upper Uruguay River Basin

“…Until recently, sex reversal was unknown within the Neotropical fishes, particularly after a complete gonad differentiation (Fernandino and Hattori, 2019). A novel study on a captive population of Brycon orbignyanus, a sympatric species to P. maculatus, revealed the gonadal remodeling and sex inversion from females to males with a complete rise to secondary males at ten months of age (1♀:2♂) (Quirino et al, 2022). Like B. orbignyanus, P. maculatus is a differentiated gonochorist teleost without specific sex chromosomes.…”

Reproductive biology of the Amazonian amphibian fish the splash tetra Copella arnoldi with emphasis to histological characterization

“…In some other species, however, the population sex ratio tends to male predominance. In Brycon orbignyanus , it is related to sex inversion of female specimens, forming secondary males, culminating in final sex ratio of 2♂:1♀ (Quirino et al, 2022). Independently of the scenario, the sex ratio unbalance may threat a species conservation status, as in B. orbgnyanus classified as Endangered (ICMBio, 2018).…”