ADAPTIVE CHANGES OF COLOR AMONG FISHES

By F. B. SUMNER

Perhaps I owe an apology for coming before this meeting with a discourse which bears so little relation to practical fisheries problems as this one on the color-changes of fishes.. It is to be presumed, however, that the fish culturist and the angler have observed some of the phenomena which I am about to describe and have perhaps even realized in a general way their meaning. And I think you will agree with me that the man who takes an intelligent interest in his stock-be it fishes or cattle-is a far more practical man, in the broad sense of the word, than the man who confines himself to the ordinary routine duties of his profession.

That many fishes are strikingly adapted to their surrounding in respect to their general coloration is a fact familiar to all. A casual inspection of any good aquarium will reveal abundant instances. It has likewise long been known to naturalists that certain species possess the power of changing their colors with more or less rapidity in conformity to changes in the color or the shade of the background. Those, for example, who have had much to do with the common minnow, Fundulus, in our laboratories at Woods Hole, realize that this fish is far paler when kept in a white vessel than when kept in a dark one. The work of Pouchet and of a number of subsequent investigators has shown that the stimuli which call forth these changes are received through the eyes, since blinded fishes no longer respond adaptively. Such fishes may, it is true, undergo changes of color, but these changes have no relation to the optical properties of the environment. And, indeed, normal fishes may undergo rapid changes of color, as a result of what have been called "psychic" stimuli, e.g., fright, sexual excitement, etc. Thus Dr. Townsend has recently published some interesting articles in which he has described and

illustrated the color changes that he has observed among the fishes in the tanks of the New York Aquarium. And it is likely that most of the fish culturists present today have noted cases where a sudden and conspicuous change of color has resulted from a fish having been handled or otherwise disturbed. Such changes do not, however, have any adaptive significance so far as we know. They are probably of no more utility to the animal than are blushing and various other indications of emotional disturbance in ourselves.

It now seems to be fairly certain that the color changes of fishes are brought about by a movement of the pigment granules within the chromatophores or color cells of the skin, and not by an actual contraction and expansion of the chromatophores themselves. The work of Pouchet and others has shown that the efferent nerve tracts which control this action of the color cells pass through the sympathetic trunks. Section of the spinal cord alone will not result in a paralysis of the chromatophore function below this level; section of the sympathetic chain will do so. But just as muscle or gland cells may be called into activity by stimuli applied directly, without the intervention of nerve fibers, so the chromatophores may be caused to contract by tactile stimuli or by the electric current.

Such, in brief, are the main facts which have been recorded regarding the physiology of color changes among fishes. It is my purpose today to discuss certain experiments which I conducted at Naples during the past winter and which I have continued during the present season at Woods Hole.

*

As is frequently the case with those who visit the Naples station for the purpose of biological investigation, I failed to find material suited to the problem upon which I had originally expected to work. I therefore said to myself: "So much the worse for your problem," and set about to

* I take pleasure in acknowledging the facilities and courtesies extended to me by the director and staff of the Naples station.

find another one. Now all of you who have visited the aquarium at Naples doubtless recall the tank devoted to flounders and other bottom-dwelling fishes. No one can fail to be impressed by the harmony of appearance between these fishes and their background, a harmony which relates not only to general color-tone, but to texture and pattern as well. (I will add that one need not go as far as Naples to see beautiful instances of this phenomenon. Our own aquarium in New York is provided with some good examples.) In viewing specimens of one of the common European turbots (Rhombus maximus) I was particularly struck by the detailed resemblance which obtained between the markings of the skin and the appearance of the surrounding gravel. Now, while the pattern of the fish was such as to harmonize wonderfully with this gravel, it would not, on the other hand, have harmonized particularly well with the fine sand, even if similarly colored, and would have been no better adapted to a bottom of large stones. The query at once suggested itself: is it a mere coincidence, this detailed agreement of the fish with its present background, or does the fish have the power of controlling the color-pattern as well as the general color-tone of the body as a whole?

Here, then, was a problem worth looking into. But, strange as it may seem, I found practically no references in the literature to adaptive changes in the color pattern of fishes. And, stranger yet, the possession of such a faculty -which soon become evident when I began my observations was a source of surprise to others at the station, including one investigator who had carried on some important studies upon the color changes of crustacea and fishes. That this power exists I think will be plain to you from the lantern slides which I shall soon show you.

The fish from which I had the most favorable results was a small species of flounder (Rhomboidichthys podas) belonging to the Psettinæ or turbot group. Unfortunately this species is not at all as common as one would desire for

experimental purposes, and during the latter part of my stay the supply gave out completely, so that certain important tests were left untouched.

When just brought into the laboratory by the collectors the fishes were usually of a rather dark brown color, with inconspicuous darker and lighter markings. This appearance makes it seem probable that they were commonly taken upon the mixed sand, composed of finely divided lava and tufa which is so common in the bay of Naples.

In my experiments, I began with natural backgrounds. I used the dark mixed sand just referred to; a fine gravel, composed of gray and brown pebbles of various shades; and again a coarse gravel, made up of stones an inch or more in diameter. I also used several other sorts of material, including a jet-black sand of almost pure magnetite crystals, and a white artificial product, obtained by grinding up marble. Bottoms of slate and of white opaque glass were employed in some of my experiments; and finally plates of glass which had been painted with various colors and with black and white, distributed in various patterns. It is with these last that I obtained some of the most picturesque results.

What these results were can best be told by reference to the lantern slide pictures, which I am going to show you, but it may be well at this point to sum up the leading facts which were revealed during my experiments.

(1) Fishes became nearly white upon a white background; dark brown and in some cases nearly black upon a black one.

(2) The animals were limited, however, to black, white, brown and gray tones.* Bright red or yellow back

*More radical changes of color have, however, been described for certain other fishes, as, for example, one cyprinoid species, which has been found to vary from light green to dark brown, depending upon the condition of the bottom on which it lives; and our own cunner, when young, may be bright green, pink or of less showy colors. I do not think, however, that a transition from one color to the other has ever been observed in the latter case as a direct result of a change in the nature of the background, and the supposition that the same fish can undergo this change is only an inference.

FIG. 1.-A Mediterranean flounder, Rhomboidichthys podas, showing color pattern displayed by the fish upon a bottom of coarse, dark sand, containing white particles.

FIG. 2.-The same specimen as that shown in Fig. 1, after transfer

to a bottom of fine gravel. (Compare carefully the markings in the two cases.)