Evolutionary biology aims to understand diversity and the different mechanisms shaping this organismal variation. Furthermore, several animals vary greatly in coloration patterns and the adaptive mechanisms they have to optimally perceive visual signals in their light environment. The visual system of fish, due to their extensive variation in spectral sensitivities and their numerous adaptations to the underwater light environment, offers a unique opportunity to disentangle this phenotypic diversity.

Throughout this dissertation, I analyze the visual systems of two major groups of Neotropical teleosts: cichlids and characins. Through transcriptome, genome and physiological experiments, I characterized the extant opsin gene complements of their visual system, which is a product of highly dynamic opsin gene evolution, and their color vision, which is based on the expression of at least three spectrally different visual pigments. The diversity of visual pigments found in these fish is the product of several spectral tuning mechanisms, which they use to fine-tune their spectral sensitivities to specific wavelengths. Our results follow the sensitivity hypothesis because the visual sensitivities of cichlids and characins match the available light in Neotropical ecosystems.

Furthermore, through behavioral assays complemented with visual modeling, I show that African cichlids possess true color vision, the capacity of discriminating color regardless of brightness. This is followed by behavioral experiments analyzing the limits of their chromatic discrimination and discussing the adaptive significance of color vision and its relevance in the visual ecology of Lake Malawi.

This dissertation enhances our understanding of color vision in freshwater fish using molecular and behavioral methods. This work encompasses experiments analyzing the genetic complement of visual pigments, builds knowledge in the evolution of these molecules and their relationship with aquatic environments, and analyzes the color dimensionality of visual systems through behavioral trials. Overall, this dissertation demonstrates the evolution of fish color vision with several methodologies highlighting the importance of an integrative and comparative approach in vision research.