THE OCULAR STRUCTURE, RETINOMOTOR AND PHOTO-BEHAVIORAL RESPONSES OF JUVENILE PACIFIC SALMON

Abstract

A histological study of the eyes of juvenile sockeye, coho, pink, and chum salmon in fresh water shows that the cones and external nuclear and plexiform layers of the retinae of embryos and alevins are poorly differentiated and do not attain normal histological or physiological proportions until the emergence of fry from the gravel. From a histophysiological study it is evident that only the emerged fry and older stages are capable of retinomotor responses and that these responses become more marked with age. Differences in rates of adaptation are found among the species and stages. Generally, the pigment layer shows a latent period before contraction in dark. Sensitivity to light is independent of the complete light adaptation of the retinal pigment or visual cells, while full acuity of vision is dependent upon the complete light adaptation of cones. Threshold values of cones and rods are indicated by the feeding and schooling responses. At light intensities between the cone and rod thresholds the thicknesses of pigment and cone layers obey the Weber-Fechner law. There is no diurnal rhythm in the positions of retinal pigment and cones of juvenile Oncorhynchus either under constant light or dark. Results are discussed in relation to the migratory, schooling, and feeding behavior. The rapid downstream migration of juvenile salmon during a relatively short period in the night may be related to a semi-dark-adapted state of the eye.