Effects of long–term spectral deprivation on the morphological organization of the outer retina of the blue acara ( Aequidens pulcher )

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Abstract
To investigate the developmental plasticity of colour vision, we reared fish with a trichromatic cone system (Aequidens pulcher) under three near–monochromatic lights, differentially stimulating each spectral cone type from the larval stage to the age of at least one year. Control conditions comprised white lights of two intensities. The treatments did not affect the visual pigments, but led to significant changes in cone outer segment lengths. Furthermore, in the blue–reared group the density of single cones within the retina was reduced by two–thirds after 18 months of exposure, while no changes were observed in the other groups. The connectivity of cone horizontal cells with the single cones was influenced by the intensity and spectral composition of the rearing lights: H1 cells were more sensitive to the spectral component, whereas H2 cells responded to intensity cues. In the blue–light group the dynamics of horizontal cell synaptic spinule formation and degradation were severely compromised. These observations show that long–term spectral deprivation leads to significant morphological changes at the level of photoreceptors and horizontal cells. While the reactions of photoreceptors may be interpreted mostly in terms of compensation, the functional consequences of the changes observed on the horizontal cell level remain to be determined electrophysiologically.