Gonadotropin-inhibitory hormone, GnIH, is named because of its function in birds and mammals; however, in other vertebrates this function is not yet clearly established. More than half of the vertebrate species are teleosts. This group is characterized by the 3R whole genome duplication, a fact that could have been responsible for the great phenotypic complexity and great variability in reproductive strategies and sexual behavior. In this context, we revise GnIH cell bodies and fibers distribution in adult brains of teleosts, discuss its relationship with GnRH variants and summarize the few reports available about the ontogeny of the GnIH system. Considering all the information presented in this review, we propose that in teleosts, GnIH could have other functions beyond reproduction or act as an integrative signal in the reproductive process. However, further studies are required in order to clarify the role of GnIH in this group including its involvement in development, a key stage that strongly impacts on adult life.

RFamide peptides are expressed in the early stages of development in most vertebrates. Gonadotrophin‐inhibitory hormone (GnIH) belongs to the RFamide family, and its role in reproduction has been widely studied in adult vertebrates, ranging from fish to mammals. Because only 3 reports have evaluated GnIH during development, the present study aimed to characterise the ontogeny of GnIH in the fish model Cichlasoma dimerus. We detected the presence of 2 GnIH‐immunoreactive (‐IR) cell clusters with spatial and temporal differences. One cluster was observed by 3 days post‐hatching (dph) in the nucleus olfacto‐retinalis (NOR) and the other in the nucleus posterioris periventricularis by 14 dph. The number of GnIH‐IR neurones increased in both nuclei, whereas their size increased only in the NOR from hatchling to juvenile stages. These changes occurred from the moment that larvae started feeding exogenously and during development and differentiation of gonadal primordia. We showed by double‐label immunofluorescence that only GnIH‐IR neurones in the NOR co‐expressed gonadotrophin‐releasing hormone 3 associated peptide. In addition, GnIH‐IR fibre density increased in all brain regions from 5 dph. GnIH‐IR fibres were also detected in the retina, optic tract and optic tectum, suggesting that GnIH acts as a neuromodulator of photoreception and the integration of different sensory modalities. Furthermore, there were GnIH‐IR fibres in the pituitary from 14 dph, which were in close association with somatotrophs. Moreover, GnIH‐IR fibres were observed in the saccus vasculosus from 30 dph, suggesting a potential role of GnIH in the modulation of its function. Finally, we found that gnih was expressed from 1 dph, and that the pattern of variation of its transcript levels was in accordance with that of cell number. The results of the present study comprise a starting point for the study of new GnIH roles during development.