Photoperiod modulates male sexual maturation in the Emperor tetra ( <i>Nematobrycon palmeri</i> ), a strictly tropical ornamental fish

Abstract Photoperiod manipulation is widely used to regulate reproduction in temperate fish species, but little is known about its effects in strictly tropical species. This study evaluated whether the Emperor tetra ( Nematobrycon palmeri ), a tropical ornamental fish, can modulate sexual maturation in response to artificial light regimes. Juveniles of both sexes (~1‐month old) were exposed for 74 days to 4 light–dark cycles: 6L:18D, 12L:12D (control, representing natural equatorial conditions), 18L:6D, and 24L:0D. Illumination was provided by LED bulbs at 65 lux. Males under the 6L:18D treatment exhibited a significantly higher incidence of sexual maturation compared with the other groups ( p &lt; 0.05). The 18L:6D regime also promoted earlier maturation relative to the control ( p &lt; 0.10), whereas continuous light (24L:0D) delayed the onset of sexual dimorphism ( p &lt; 0.05). Growth did not differ among treatments, confirming that rearing conditions were adequate. On the top of the fish's head, between the eyes, a translucent patch of skin with few melanophores may indicate the presence of a “pineal window,” supporting this species' capacity to perceive photoperiodic cues. These findings provide the first evidence that a strictly tropical ornamental fish can detect and respond to variations in photoperiod. Photoperiod manipulation, therefore, represents a simple and effective strategy to enhance reproductive performance in N. palmeri , with potential applications for sustainable ornamental fish culture.