Popular Science Monthly/Volume 26/January 1885/Protective Mimicry in Marine Life
|PROTECTIVE MIMICRY IN MARINE LIFE.|
BY mimicry we understand the assumption by animals of a deceptive similarity answering a protective purpose, not only to other animals, but also to lifeless objects, and, in color, to the surroundings. In a biological application, this definition of the term, though different from the common one, is well founded; for similarity of an animal with any object affords it protection, by enabling it to approach its prey unobserved; by facilitating its escape from enemies; or by shielding it, under cover of its resemblance to unpleasant objects, from hostile attacks. A number of observations have been published, by various well-known authors, upon the interesting phenomena of mimicry, but they have related generally to land animals, while the cases of the occurrence of similar phenomena among the inhabitants of the sea have been less extensively noticed. A few have been mentioned by Haeckel and Carus Stern, but I have others, of not less high interest, to describe.
On my voyage from Brazil to England in July, August, and September, 1883, I had many opportunities to secure and examine closely specimens of pelagic life. From the 30th of August to the 5th of September, we crossed the Sargasso Sea, between latitude 25° 12' and 34° 39', and longitude 33° 52' and 35° 52' west. The sea-weeds were not massed in extensive fields, but were distributed in single groups of larger or smaller size, and these were driven by the wind in nearly straight lines, that could be followed with the eye to considerable distances. The linear arrangement was also made distinct to me by its pelagic life, particularly by its great colonies of radiolaria, or polycyttaria, salpæ, and other orders. Thus, I find in my notes such items as, "September 3d, polycyttaria in colossal masses, thick, wide bands of them stretching along for miles; September 14th, immense masses of little salpæ and polycyttaria, causing the water to display milky bands."
I did not neglect to fish up masses of the Sargasso sea-weed every day and examine it through and through for its fauna, and whenever I did this I found many notable cases of mimicry. The color of the younger plants is a yellowish green, while the older stalks are of a more or less dark brown. A luxuriant animal life flourishes on the stems, leaves, and air-vessels of the Sargasso-weed. Little actiniæ, sometimes of a light, sometimes of a dark-brown color, were very numerous on the plants I most closely examined; often so thick as to completely cover the stems. On the same plants, I also found numerous specimens of small, naked snails. These minute gasteropods, a centimetre or a centimetre and a half long, bore on their backs numerous retractile tentacles, arranged in cross-rows at various intervals. In color, they were of various shades of brown, like the actiniæ; when they drew themselves up so that the tentacles stood thickly together, they so much resembled the actiniæ that it would be a matter of difficulty to a person not acquainted with both animals to tell them apart. Another snail, whose tentacles were arranged in rows along each side of the back, was still more difficult to distinguish when any danger threatened it from the actiniæ. Of what use can the resemblance to the actiniæ be to the little mollusks? They are, it is true, great eaters of the actiniæ, for I have seen one of them devour four or five of those animals in an hour; but it does not appear that their access to them is greatly facilitated by the resemblance, for the actiniæ are so confined by the limitation of their movements as to be unable, in any case, to escape their more facile enemies. We are, therefore, reduced to consider the likeness a case of mimicry for the protection of the snails against animals which pursue them, but avoid the actiniæ, whose nettle-cells are by no means pleasant morsels. But as I have not been able to discover what special enemies the snails have, and whether they really dislike the actiniæ, my attempted explanation must remain an attempt, to be confirmed or disproved by some future observer in the Sargasso Sea, who can begin where I have had to leave off. Other cases of mimicry on the part of mollusks have come under observation. According to Dr. H. von Ihering, the Chromodoris gracilis lives associated with a sponge (Suberites), and is colored like it, blue.
On the same sea-weed I found other larger mollusks, which, not resembling other animals, so strikingly resemble the forms of the stems and leaves of the plants that it is extremely difficult to find them in the tangle of brush. They have developed flaps all around their bodies, before and behind, and on either side, the edges of which are irregularly serrated, with the tips of the serratures of a brown tint like the older alga-stems. The surface of the flaps, and of a part of the rest of the body, is beset with numerous small similarly brown tipped teeth, while the color of the animal as a whole is olive-green, like that of the plant in which it lives.
Moritz Wagner regards the phenomena of mimicry as the consequence of an innate caution in the animal, that causes it to choose those places for its abode with which it most corresponds in color. Dwelling on this point, he remarks that "the instinct of self-preservation native to all animals, which sharpens their senses against incessantly threatening dangers, prompts marine as well as land animals to seek dwelling-places similar to themselves in form and color." Any person who has had opportunity, as I have, to watch the little crabs and shrimps, that swim around in the alga-groups of the Sargasso Sea, for half a day at a time, will have to admit that there is much in this view, though it will not be necessary to throw away the theory of selection. I can not say whether the crab I have observed is the same that Wagner describes as Nautilo graspus minutus. I have collected several hundreds of the animals, and think, after superficial observation, that I can distinguish more than one species among them, while the variability, especially in color, is wonderful. The adaptation of the innumerable tints to every grade of change in the color of the sea-weed is really marvelous. The younger, lighter green crustaceans are always to be found on the young, verdant fronds of the plant, while the older parts of the weed are inhabited by older, brown animals. The older stems are often incrusted with the white shells of bryozoa, and corresponding with these we are sure to find white spots on the brown armor of the crabs. The legs of the animals are frequently of an olive green ground with brownish spots, deceptively like the slender seaweed-leaves that are just beginning to turn brown. If one will, as I did, pull one of the large plants upon the deck, leave it in a cask of sea-water for an hour or two, and then look through it for crabs without disturbing it, he will find it very hard to discover three or four of the animals, although he may be sure there are a quarter of a hundred of them there; and, if he gives the mass a lively shake, he will find a curious assemblage of the most varied sorts tumbling off the bush, whose behavior will go far to verify Wagner's view; for, if they are allowed the opportunity, they will all swim back to the sea-weed, and each will seek a part of the plant most like it in color. I tried the experiment forty or fifty times, and never saw a little green crab settle on a dark-brown stem. The crustaceans keep to their color, and the brown ones will, with amazing speed, dart through the thick net-work of stems and leaves, to the darkest spot they can find, where they quickly escape observation.
I remarked another striking example of what might be considered intelligent mimicry in a crab, on the 11th of September, after we had got out of the Sargasso Sea. Toward night a piece of dark-brown bark, about as large as one's hand, floated close by the ship. Thinking I might find something upon it, I fished it up with a scoop I had prepared for such purposes and which I had found very useful, and put it in a pail of fresh sea-water for observation. While I stood looking at it, I perceived motions of legs and tentacles, and then discovered a crab, so precisely of the color of the bark, that it might have lain on it a long time, had it not moved, before I would have noticed it. I called the attention of the captain of the ship to the object, and he examined it for some minutes, the crab not moving, before he saw anything but bark. How came this brown crab, on a piece of wood of precisely the same color, in the middle of the ocean? Must we believe that several crabs got on the bark, among them the brown one, and that the struggle for existence resulted in his being the only one left? I am satisfied that we can come to no other conclusion than Wagner's, and that we must believe that a conscious or perhaps an instinctive choice governed the animal in settling upon an object so like it in color. The bark was probably occupied while floating among the sea-weed, then drifted away to the spot where it was found, and where it furnished so singular an isolated example of selective mimicry. I found numerous slender fishes in the algae of the Sargasso Sea, likewise protected by their resemblance in color to the plants among which they lived. One afternoon, after I had examined a plant for an hour for crabs, I took it out of the pail to throw it into the sea, when a fish about the size of a lead-pencil fell out of it. I put the fish into another pail, in which there was also a sea-weed. It instantly vanished from my sight, and I had to look for some time among the thick stems before I could find it again.
I do not know whether any of these observations have been recorded before. During my residence in Brazil, I had but limited opportunities to keep acquainted with recent zoological literature. But I found my sail through the Sargasso Sea full of interest, and I believe that a voyage there would be fraught with pleasure and profit to every naturalist. It is not so very expensive, either; and can easily be made on a schooner or bark sailing from a European port to the West Indies. My passage from Porto Allegro in Southern Brazil, to Falmouth, England, from the 15th of June to the 25th of September, cost me, board included, less than seventy-five dollars. Such a voyage furnishes, moreover, at the same time, an opportunity to make a personal acquaintance with the natural history of tropical regions.—Translated for the Popular Science Monthly from Kosmos.