Dispersion of hooks on the anal fins of primary and secondary males in Brycon orbignyanus (Characiformes: Bryconidae): a secondary sexual trait for breeder selection

Brycon orbignyanus exhibits seasonal reproduction where males can be distinguished from females by the roughness present in the anal fin of the specimens that represents a secondary sexual characteristic known as hooks. This study aimed to describe the appearance and morphology of hooks on the anal fin in B. orbignyanus and relate them to the phases of the reproductive cycle of these animals to determine a parameter that can be used for the selection of suitable males for use in induced reproduction. Monthly male specimens of B. orbignyanus of different ages were collected (n = 50 total; n = 10/month) and the specimens were euthanised, biometrically measured, and the testes and anal fins were collected. As sample were processed according to the typical techniques for light microscopy, stereomicroscope, diaphanization and scanning electron microscopy. Except for specimens in the testicular stage of Regenerating, hooks were observed in all other stages of the reproductive cycle in B. orbignyanus males; however, specimens that possess six or more rays with hooks are considered Spawning Capable and can be selected for breeding. No differences were observed in hooks development patterns as related to secondary sexual characteristics among intersex individuals, primary males, or secondary males.