The aim of the present study was to describe the secondary sexual characteristics of adult Hypancistrus sp. A total of 120 specimens were used to evaluate the morphometric characters, to confirm the morphometric data, histological identification of sex and maturation stages of gonads was performed. The results showed that males had higher median total and standard body lengths, open fin length, right and left fin lengths, head length and width and inter‐eye width than females. However, the females showed a high median abdominal width. Both sexes presented odontodes along the body, with thick odontodes present near the interopercular region and pectoral fins. In mature males, small odontodes were present along the snout. Scanning microscopy analysis confirmed that odontodes in females had a thicker base than those in males, conferring the former with a “cone” morphology. Additionally, it was observed that males had a long and broad head with a triangular shape and "V"‐shaped genital papilla, while females had a round head and "U"‐shaped genital papilla. Thus, we conclude that Hypancistrus sp. had secondary sexual characteristics, being possible to differentiate the sex by the size of the body, abdominal width, shape of the head, size of the odontodes and distinct morphology of the ovipositor. This study will serve as a basis for the development of breeding protocols, as it aims to identify sexually mature males and females of Hypancistrus sp. for breeding in captivity.

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Background Hypostominae is a subfamily of the family Loricariidae that has a great diversity of species. Accordingly, testicular studies in fish can contribute to the phylogeny and taxonomy of species and to the comparison of reproductive aspects between species. Thus, this work aimed to characterize the testicular morphology and spermatogenesis of the Hypostominae species Baryancistrus xanthellus, Peckoltia oligospila and Hypancistrus zebra. Results B. xanthellus, P. oligospila and H. zebra had an anastomosed tubular type of testis. The germinal epithelium was continuous with unrestricted spermatogonia, and the proliferative, meiotic and spermiogenic phases were defined in all species. In the spermiogenic phase, spermatids were classified as initial, intermediate and final. Only in B. xanthellus in the final phase was there nuclear rotation. The sperm for the three species had a head with an oval shape and a single flagellum composed of the short midpiece, principal piece and end piece. B. xanthellus and P. oligospila showed a cylindrical flagellum and H. zebra showed projections that produced a flattened appearance. Conclusions On the basis testicular structure and ultrastructural characteristics of the germ cells, there was a greater relationship between B. xanthelus and P. oligospila, while H. zebra had particular characteristics. These aspects show that despite belonging to the same subfamily, the species have distinct biological characteristics.

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