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The retina of Metynnis roosevelti Eigenmann, 1915, a very active freshwater fish, was investigated by light and electron microscopy and was found to have a complex neuronal structure that allows rapid responses of visual stimuli. Retina photoreceptors are double cones, single long cones, single short cones and rods. Cone inner segments are arranged as mosaics. The outer nuclear layer contains small nuclei of twin cones, long and wide nuclei of long single cones, spherical large nuclei of short cones, and small dense nuclei of rods. Horizontal, amacrine, bipolar and ganglion neurones are responsible for connections and integration between photoreceptor cells and afferent neurones. The pigmented epithelium comprises a single layer of cylindrical cells each with elongated nuclei, mitochondria at the basal region, and melanin grains that can migrate inside long cell processes, depending on light intensity. In darkness, pigment is concentrated in the basal region of the cells and in daylight it is concentrated in the processes, surrounding and protecting the outer segments of photoreceptors. When exposed experimentally to monochromatic red light, expansion of melanin pigments was provoked at the beginning of light period, followed by their withdrawal after exposure to long wave lengths. No active movements of cones or rods were observed. Considerable renewal of photoreceptor membrane discs occurred after one week in red light, caused by higher level of activation of rods to allow the fish to see in relative darkness. Metynnis roosevelti is a native fish from Brazilian tropical and sub-tropical regions. More recently trials were made to use it in fish cultures in different regions of the countiy. Its capacity to adjust to different photic environments facilitates for rearing in varied environments

Morphology of the retina in the freshwater fish Metynnis roosevelti Eigenmann (Characidae, Serrasalminae) and the effects of monochromatic red light