On the Depth at Which Fish Can See

Abstract

At what maximum depth fish can see in the ocean and in lakes has remained a matter for speculation up to the present time, first because little was known regarding the strength of the illumination at various depths and second because no adequate tests of the sensitivity of the fish eye had been made. During the past 10 years, however, the penetration of daylight into natural waters has been carefully studied by a number of investigators in various parts of the world. Furthermore, the recent excellent work by Grundfest ('32) on the sensibility of the sun-fish, Lepomnis, gives us a reliable yardstick for one species of fish. In view of the fact that these two sets of data are now available it occurred to me that it would be of interest to calculate the greatest depth in several different types of water at which the sun-fish could just discern an object of a certain size at a given distance. Although the validity of certain assumptions must be granted in order to make such a calculation, it seems probable that we can arrive at the right order of magnitude for this fish. Whether the photosensitivity of other fish is similar to that of Lepomtiis can be decided, of course, only by the performance of suitable experiments. The vision of fish with telescopic eyes and of other highly specialized species is undoubtedly on an entirely different scale.2 In the experiments on Lepomiis, which were carried out for quite another purpose, Grundfest made use of the rheotropic response. The fish to be tested was confined in a cylindrical glass jar around which was placed a cylindrical screen composed of equal and alternate vertical bars and spaces and illuminated to any desired intensity by a band of light, 2 myp wide, from any part of the spectrum. When this screen was rotated in either direction, the fish responded by a sudden movement in the direction of the rotation, provided that the illumination was above a certain threshold the value of which depended upon the part of the spectrum being used. By repeated trials of ' Contribution No. 100. 2 A correlation undoubtedly exists between the degree of enlargement or of degeneration of the eyes of deep-sea fish and the depth (and consequently the illumination) at which they live. The issue is confused, however, by important differences which apparently depend upon whether the fish is bottom-living or pelagic and whether or not it possesses light organs. This problem has not yet been satisfactorily elucidated (Murray and Hjort, '12, p. 680). 452