Abstract This work aimed to analyze factors affecting secondary sexual characteristics in the yellowtail tetra Astyanax altiparanae. For this, seventy-five specimens were first separated into three different size classes (small, medium and large groups) between two seasons, summer and winter. In all groups, male fish were consistently bigger in the summer. On the other hand, females from both seasons presented in media the same length into the length classes. Afterwards, we performed histological analyses of the gonads to first confirm the genus and investigate the phase of maturation of each animal. During the winter, most of the small animals were males (22), most of the large animals, females (23), and the medium size animals followed a tendency of 1:1 ratio (9 male: 16 female). In the summer, male were the majority in both small (20) and medium (20) size. Larger-size animals were female (23). Then, in order to analyze the influence of genus, phase of maturation, season of the year, the number, and length of the animals spinelets, we diaphanized, counted, and measured them in each animal. Our results demonstrated that the spinelets are a sexual secondary characteristic of male genus independently of the size, season and phase of maturation. However, some tendencies were observed. Males bigger than 48 mm always presented spinelets; their size are in media the double in summer in comparison to winter; and summer males presents more rays with spinelets in the summer. Curiously, the larger specimen sampled was a female presenting spinelets in five rays. Lastly, we performed the gonadectomization of the animals and hypothesized that gonad hormones will directly influence this characteristic. The gonadectomization only initially influence on the size and number of spinelets in the anal fin rays, since the thirty-day-gonadectomized animals presented few and smaller spinelets against the control ones. However, the spinelets normalized in ninety-day-gonadectomized specimens. Such a work showed spinelets can be considered a secondary sexual characteristic to distinct male from female and can be used in the management in specimens bigger than 48 mm, but cannot indicate fish sterility. Summary statement This study elucidated whether the size, sex, environmental conditions, and gonadal development affect the development of spinelets, a bony structure presented in anal fins in mature fish. Additionally, gonadectomized fish were used to elucidate the effect of gonad on the rise of such structures. Interesting new data showed that such a secondary sex characteristic is influenced by sex, size, gonadal development, and season of the year, but spinelets arose even within gonadectomized fish; this suggests that such a structure is not indicative of sterility in this species.