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 achievement when we consider that the ratio in question is ordinarily that between the upper and lower halves of the field of vision, or in other words, between the stimulus received by the lower and upper halves of the retina. Experimental evidence is accumulating showing that these two areas of the retina are functionally differentiated in the required manner.
It was early recognized that those conditions which tend to bring about transitory colour changes are the same ones which, if prolonged, produce quantitative changes. The question has been raised whether the state of chromatophore “expansion” (pigment dispersal) per se, promotes pigment production and cell multiplication, and chromatophore “contraction” promotes the reverse processes, or whether both transitory and quantitative changes are the results of a common (probably hormonal) cause. The weight of present evidence probably favours the latter interpretation.
The relative roles of direct nervous control of the chromatophores and control through hormones is still a subject of controversy, both for fishes and Amphibia. It now seems probable, not only that these two classes of animals differ from one another in important ways, but that the two major groups of fishes, elasmobranchs and teleosts, likewise differ from one another. Even within the group of elasmobranchs, moreover, important differences have been claimed. The whole subject is further complicated by the discovery that “direct” nervous control is itself probably mediated through hormones liberated by the nerve terminals. In general, it is now helieved that nervous control (with the reservation just indicated) is the one chiefly involved in the colour changes of teleosts, while control through blood-borne hormones is chiefly involved in the colour changes of both elasmobranchs and Amphibia. But this statement oversimplifies the present state of the problem.