INFLUENCE OF LIGHT INTENSITY ON RETINAL ADAPTATION IN ATLANTIC SALMON (SALMO SALAR) YEARLINGS

Retinae of fish exposed to 9 × 102 ft-c are more fully adapted to light than those of fish exposed to 10−1 ft-c. Retinae of the former group take a shorter time to attain the same state of adaptation as that of the latter. However, they take a longer time than the latter to attain their own maximum state of adaptation. Retinae of fish subjected to darkness after exposure to 9 × 102 ft-c are not dark-adapted even after 70 minutes. The retinal epithelial pigment of fishes placed in darkness from light of 10−1 ft-c dark-adapts in 45 minutes while the cones do so in 25 minutes. The retinal index indicates that dark-adaptation occurs 45 minutes after subjection to dark. Hypotheses presented in a previous paper are discussed in the light of these results.

HISTOPHYSIOLOGICAL STUDIES ON THE JUVENILE ATLANTIC SALMON (SALMO SALAR) RETINA: II. RESPONSES TO LIGHT INTENSITIES, WAVELENGTHS, TEMPERATURES, AND CONTINUOUS LIGHT OR DARK

Retinal epithelial pigment dark-adapts in intensities below 10−3 ft-c. Cones are in a light-adapted state in intensities of 10−2 ft-c and above, semiadapted state in 10−3 ft-c, and dark-adapted state in 10−4 ft-c and below. The retinal index corresponds to the response of cones. It is suggested that the visible spectrum of the yearling Atlantic salmon ranges from 3640 Å to 6900 Å because the retina is in a light-adapted state in that range. In light, lower and higher temperatures bring about an expansion of the retinal pigment. Cones and retinal index are not affected. In dark, pigment expands with higher temperatures but temperatures exceeding 18.3 °C bring forth contraction. Cones contract with increasing temperatures but beyond 14.8 °C they expand. The retinal index indicates a greater dark-adaptation of retina with increasing temperatures between 5.0 °C and 18.3 °C but higher temperatures make it less dark-adapted. Cones exhibit no rhythm in their positions in continuous light or dark. Pigment demonstrates no rhythm in light but shows one in dark which persists for a day. The significance of the results is discussed. Comparisons with results with Pacific salmon are made.

HISTOPHYSIOLOGICAL STUDIES ON THE JUVENILE ATLANTIC SALMON (SALMO SALAR) RETINA: I. RATES OF LIGHT- AND DARK-ADAPTATION

The structure of the Atlantic salmon retina corresponds to descriptions by previous workers and is also similar to that of the Pacific salmon retina. Retinal epithelial pigment light-adapts in 60 minutes. Cones light-adapt in 45 minutes. Dark-adaptation of the pigment and cones occurs in 70 minutes. Retinal index shows light-adaptation occurring in 60 minutes and dark-adaptation in 70 minutes. Results are compared with those of the Pacific salmon's.

Correlation of Morphological and Intra-ocular Measurements in the Atlantic Salmon (Salmo salar) Yearling

In the Atlantic salmon yearling, correlation among various morphological measurements is of a higher order than correlation of various intra-ocular measurements. While the latter correlation is positive, it is not of a high enough order to serve as a useful predictive device. Correlation between intra-ocular measurements from the left and the right eyes is also of a low order.

OBSERVATIONS ON THE RETINA OF FLORIDA CHAMELEON (ANOLIS CAROLINENSIS)

The structure of the A. carolinensis retina is similar to that of diurnal lacertilians described in the literature. It possesses 10 layers. The visual cell layer is made up of single and double cones. No rods are present. The internal nuclear layer is thicker than the external nuclear layer. The nuclei and ganglion cells of light- and dark-adapted animals do not show any differences in staining. A prominent conus papillaris and a fovea are present. The epithelial pigment layer and the cones do not undergo any photomechanical changes. It was not possible to condition the chameleons to yield any behavioral responses.

THE EFFECT OF TEMPERATURE ON THE JUVENILE SOCKEYE SALMON RETINA

A histological study of the retina of underyearling sockeye salmon, Oncorhynchus nerka, reveals that temperature has no effect on the cones either in light or in dark. In light, the retinal pigment is not influenced by temperature, while in the dark, temperature above 15 °C brings about a slight expansion.

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

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.

Some Observations on the Structure and Photomechanical Responses of the Pacific Salmon Retina

The general structure of the Pacific salmon retina is described. Rods, single cones and twin cones constitute the visual cell layer which, together with the dense pigment layer, undergoes photomechanical changes common to most teleosts. Stages in the contraction and expansion of rods and cones, and the migration of pigments are depicted.In the laboratory, cultured sockeye salmon became well light-adapted in 20–25 minutes, whereas the time for dark-adaptation was of the order of 55–60 minutes.In the Lakelse River, a sample of natural-run sockeye and coho smolts was found to be fully dark-adapted by about 10:00 P.M. when the light intensity at the water surface was 0.03 foot-candle. Pink and chum fry from Hooknose Creek were similarly dark-adapted by about 9:45 P.M. on an evening when the intensity had fallen below 0.002 ft.-c. At the start of evening migration retinal dark-adaptation in sockeye smolts had just started, whereas in pink and chum fry the retina was approximately half dark-adapted when migration commenced.