A FURTHER REPORT UPON THE EFFECTS OF THE VISUAL ENVIRONMENT ON THE MELANIN CONTENT OF FISHES

¹ Scripps Institution of Oceanography of the University of California, La Jolla

The results are presented of recent experiments upon Girella nigricans and Fundulus parvipinnis, together with a comparative discussion of previous similar experiments by the author and collaborators.

In this latest series, Girella was exposed for two and for four months to backgrounds of five albedos (black, three grays and white), lighted by 100-watt electric lamps; while another set was exposed to black backgrounds only, lighted by a 10-watt lamp. In the Fundulus series, only the brighter lights were employed.

The effects of this treatment upon melanin production (or loss) were much more pronounced in Girella than in Fundulus. In Girella, the amount of melanin in the skin, after four months, was 2 times as great in fishes from the black containers as in those from the pale gray containers. This difference in melanin content was trivial, however, in comparison with the difference in appearance of the living fishes. The latter was due, for the most part, to the transitory disposition of the pigment within the chromatophores.

The minimum melanin content in Girella was not obtained from the occupants of the white containers, but from those of the pale gray containers. The four values from "black" to "pale gray" formed, however, a descending series having a distinctly logarithmic arrangement.

In Fundulus no such arrangement was found, there being only one significant difference, that between fishes of "white" history and all of the others. The mean value of the latter is less than 1 times that of the former.

The data from this and similar previous experiments show that in four of the five species studied, the melanin values, when plotted against albedo, form "hollow" curves, and that this arrangement, in some of the cases, is definitely logarithmic. The possible analogy between this tendency and the "Weber-Fechner Law" is discussed.

Fishes kept in black and dark gray bowls showed little change after two months. Those from the other bowls, most of all from the white, showed considerable further decreases between the 2-month and the 4-month periods. It is probable that the differences between the dark-adapted and pale-adapted fishes resulted less from increase of pigment in the former than from decrease in the latter.

No probable difference existed between black-adapted fishes kept under 100-watt lights and the same when kept under 10-watt lights, the lighting here being in a ratio of about 16 : 1.

Reasons are given for believing that the chromatic response of a fish to its background is not much interfered with by the presence of other fishes in the same container.