The guppy (Poecilia reticulata) represents a particularly informative model for understanding the molecular, physiological, and ecological foundations of vertebrate color vision. This mini-review synthesizes current evidence on the expanded visual opsin repertoire of guppies, retinal specialization, phototransduction, developmental and environmentally induced plasticity, and the ecological consequences of variation in visual sensitivity. Guppies possess an exceptionally diverse complement of cone opsins, including ultraviolet-, short-, middle-, and long-wavelength-sensitive classes, with pronounced molecular diversity among LWS paralogs. This diversity provides the molecular substrate for fine spectral discrimination, particularly within the red-orange region that is relevant to sexual signaling. However, the functional consequences of opsin diversity depend on their spatial expression within the retina, photoreceptor physiology, neural integration, chromophore availability, and adaptive regulation of visual sensitivity. Opsin expression changes with age, sex, illumination, and dietary carotenoid availability, demonstrating that the visual phenotype is both developmentally dynamic and environmentally responsive. The review further emphasizes that visual performance is constrained by the high metabolic demands of photoreceptors, oxidative stress, photoprotection, chromophore recycling, and retinal cellular maintenance. At the ecological level, variation in visual sensitivity is associated with mate preference, foraging behavior, predator detection, and population-level differences in sexually selected traits. Overall, guppy color vision emerges as an integrated phenotype in which molecular diversification, retinal organization, physiological homeostasis, environmental plasticity, and behavioral selection interact to shape visual information processing and fitness.
