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

The retina of MelYllnis roosevelti Eigenmann, 19 15, a velY active treshwater 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 photo receptors are double cones, single long cones, single short cones and rods. Cone inner segments are alTanged as mosaics. The outer nuclear layer contains small nuclei of twin cones, long and wide nuclei oflong single cones, spherical large nuclei of short cones, and small dense nuclei of rods. Horizontal, amacrine, bipolar and ganglion neUl'ones are responsible for connections and integration between photoreceptor cells and afferent neurones. The pigmented epithelium comprises a single layer of cylindrical ce lls each wi th elongated nuclei, mitochondri a at the basal region, and melan in grains that can migrate inside long cell processes, depending on light intensity. Tn 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 ailer 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 li ght, caused by hi gher level of activation of rods to allow the fi sh to see in relative darkness. MetYlIllis roosevelti is a native fish from Brazilian tropica l and sub-tropical regions. More recently trials were made to use it in ti sh cultures in difterent regions of the countlY. Its capacity to adjust to different photic environ ments fac ilitates for rearing in vari ed environments.