Quantitative Changes in Pigmentation, Resulting From Visual Stimuli in Fishes and Amphibia1

Abstract

Summary Fishes and, in lesser degree, Amphibia respond to backgrounds in such a manner that their shade, and to a certain extent their colour, tend to conform to that of the substratum on which they lie, or over which they swim. The integrity of the eyes and of major portions of the nervous system is essefitial to these phenomena. The immediate, transitory or “physiological” dour changes are due to the rearrangement of pigment particles already present. When the effective stimuli are continued for some days or weeks, changes become evident both in the number of chromatophores and in the pigment contents of each (quantitative or “morphological” colour changes). All three of the types of chromatophores (melanophores, guanophores, lipo‐phores) are affected by these changes. Dark backgrounds favour the production of melanin and inhibit the production of guanin. Pale backgrounds have a reverse effect. In fishes at least, production (or retention) of the yellow pigment xantho‐phyll is favoured by black backgrounds and retarded by white ones, agreeing thus with melanin. To what extent there is any specific effect of coloured backgrounds (sensu stricto) upon the quantity of xanthophyll is not clear at present. Intensity of illumination, above a rather low level, has very little effect upon pigment formation in fishes. There is some evidence, however, of a slight degree of positive correlation between light intensity and melanin formation. Total darkness leads to pigment reduction both in fishes and Amphibia. Blinding of both eyes, in both of these groups, results in a marked increase of melanin, but only in animals which are kept in the light. Experiments involving illumination from below are known to have resulted in considerable increases in pigmentation of the ventral surface, both in fishes and Amphibia. It is not certain in these cases whether optic stimuli have been concerned, or whether the effects have been due to direct illumination of the skin. The response of a fish to its background is primarily a response to albedo, this being defined as the proportion of incident light which is reflected or dispersed from a given surface. On the basis of considerable evidence, a rule has been formulated which has been found to hold approximately, at least for certain fishes. This rule is that, when the animals are subjected to a variety of backgrounds, under uniform illumination, the amount of melanin (or the number of melanophores) produced varies inversely as the logarithm of the albedo of the background. The close analogy between these pigmentary responses of fishes, and the phenomena of sense perception in man for which the “Weber‐Fechner Law” was formulated was pointed out. The question of how a fish recognizes, and responds to, a given albedo, regardless of the absolute degree of illumination present, resolves itself into the question as to how the animal perceives the ratio between the source of light and the light reflected from the bottom and surrounding objects. This last does not seem to be so difficult an achiev...

Pigmentation plasticity enhances crypsis in larval newts: associated metabolic cost and background choice behaviour

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Benthic fish exhibit more plastic crypsis than non-benthic species in a freshwater spring

“…Coarse color change also may be induced in adult organisms with long incubation periods. For example, several species of fish and amphibians darken over dark backgrounds and lighten over light backgrounds if held in captivity for weeks or months (Sumner 1940;Wente and Phillips 2003). Although these examples of slow cryptic color changes are inducible, they are relatively permanent once induced.…”

Morphological color changes in fish: Regulation of pigment cell density and morphology

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