Popular Science Monthly/Volume 29/May 1886/The Science of Flat-Fish, or Soles and Turbot
|THE SCIENCE OF FLAT-FISH, OR SOLES AND TURBOT.|
"ONCE upon a time," says that delicious creation of Lewis Carroll's, the Mock Turtle, "I was a real turtle!" Once upon a time, the modern sole might with greater truth plaintively observe, I was a very respectable sort of a young codfish. In those happy days, my head was not unsymmetrically twisted and distracted all on one side; my mouth did not open laterally instead of vertically; my two eyes were not incongruously congregated on the right half of my distorted visage; and my whole body w r as not arrayed, like a Portland convict's, in a party-colored suit, dark-brown on the right and fleshy-white on the left department of my unfortunate person. When I was young and innocent, I looked externally very much like any other swimming thing, except, to be sure, that I was perfectly transparent, like a speck of jelly-fish. I had one eye on each side of my head; my face and mouth were a model of symmetry; and I swam upright like the rest of my kind, instead of all on one side after the bad habit of my own immediate family. Such, in fact, is the true portrait of the baby sole, for the first few days after it has been duly hatched out of the eggs deposited on the shallow spawning-places by the mother-fishes.
After some weeks, however, a change comes o'er the spirit of the young flat-fish's dream of freedom. In his very early life he is a wanderer and a vagabond on the face of the waters, leading what the scientific men prettily describe as a pelagic existence, and much more frequently met with in the open sea than among the shallows and sand-banks which are to form the refuge of his maturer years. But soon his Wanderjähre are fairly over: the transparency of early youth fades out with him exactly as it fades out in the human subject: he begins to seek the recesses of the sea, settles down quietly in a comfortable hollow, and gives up his youthful Bohemian aspirations in favor of safety and respectability on a sandy bottom. This, of course, is all as it should be; in thus sacrificing freedom to the necessities of existence he only follows the universal rule of animated nature. But, like all the rest of us when we settle down into our final groove, he shortly begins to develop a tendency toward distinct one-sidedness. Lying flat on the sand upon his left cheek and side, he quickly undergoes a strange metamorphosis from the perfect and symmetrical to the lopsided condition. His left eye, having now nothing in particular to look at on the sands below, takes naturally to squinting as hard as it can round the corner, to observe the world above it; and so effectually does it manage to squint that it at last pulls all the socket and surrounding parts clean round the head to the right or upper surface. In short, the young sole lies on his left side till that half of his face (except the mouth) is compelled to twist itself round to the opposite cheek, thus giving him through life the appearance constantly deprecated by nurses who meet all unilateral grimaces on the part of their charges with the awful suggestion, "Suppose you were to be struck so!" The young sole is actually struck so, and remains in that distressing condition ever afterward.
This singular early history of the individual sole evidently recapitulates for us in brief the evolutionary history of the entire group to which he belongs. It is pretty clear (to believers, at least) that the prime ancestor of all the flat-fish was a sort of cod, and that his descendants only acquired their existing flatness by long persistence in the pernicious habit of lying always entirely on one side. Why the primeval flat-fish first took to this queer custom is equally easy to understand. Soles, turbots, plaice, brill, and other members of the flatfish family are all, as we well know, very excellent edible fishes. Their edibility is as highly appreciated by the sharks and dog-fish as by the enlightened public of a Christian land. Moreover, they are ill-provided with any external protection, having neither fierce jaws, like the pike and shark; efficient weapons of attack, like the sword-fish and the electric eel; or stout defensive armor, like the globe-fish, the file-fish, and the bony pike, whose outer covering is as effectually repellent as that of a tortoise, an armadillo, or a hedgehog. The connection between these apparently dissimilar facts is by no means an artificial one. Fish which possess one form of protection seldom require the additional aid of another: for example, all the electric fish have scaleless bodies, for the very simple reason that no unwary larger species is at all likely to make an attempt to bite them across the middle; if it did, it would soon retire with a profound respect through all its future life for the latent resources of electrical science. But the defenseless ancestor of the poor flat-fishes was quite devoid of any such offensive or defensive armor, and, if he was to survive at all, he must look about (metaphorically speaking) for some other means of sharing in the survival of the fittest. He found it in the now-ingrained habit of skulking unperceived on the sandy bottom. By that plan he escaped the notice of his ever-present and watchful enemies. He followed (unconsciously) the good advice of the Roman poet: bene latuit.
But, when the father of all soles (turbot, brill, and dabs included) first took to the family trick of lying motionless on the sea-bottom, two courses lay open before him. (That there were not three was probably due to the enforced absence of Mr. Gladstone.) He might either have lain flat on his under-surface, like the rays and skates, in which case he would, of course, have flattened out symmetrically sidewise, with both his eyes in their normal position, or he might have lain on the right or left side exclusively, in which case one side would soon practically come to be regarded as the top and the other side as the bottom surface. For some now almost incomprehensible reason, the father of all soles chose the latter and more apparently uncomfortable of these two possible alternatives. Imagine yourself to lie (as a baby) on your left cheek till your left eye gradually twists round to a new position close beside its right neighbor, while your mouth still continues to open in the middle of your face as before, and you will have some faint comparative picture of the personal evolution of an infant sole. Only you must, of course, remember that this curious result of hereditary squinting, transmitted in unbroken order through so many generations, is greatly facilitated by the cartilaginous nature of the skull in young flat-fish.
When once the young sole has taken permanently to lying on his left side, he is no longer able to swim vertically; he can only wriggle along sidewise on the bottom, with a peculiarly slow, sinuous, and undulating motion. In fact, it would be a positive disadvantage to him to show himself in the upper waters, and for this very purpose Nature, with her usual foresight, has deprived him altogether of a swim-bladder, by whose aid most other fishes constantly regulate their specific gravity, so as to rise or sink at will in the surrounding medium. Some people may indeed express surprise at learning that fish know anything at all about specific gravity; but as they probably manage the alteration quite unconsciously, just as we ourselves move our limbs without ever for a moment reflecting that we are pulling on the flexor or extensor muscles, this objection may fairly be left unanswered.
The way in which Nature has worked in depriving the sole of a swim-bladder is no doubt the simple and popular one of natural selection; in other words, she has managed it by the soles with swim-bladders being always promptly devoured. Originally, we may well suppose, the ancestral sole, before he began to be a sole at all (if I may be permitted that frank Hibernicism), possessed this useful aërostatic organ just like all other kinds of fishes. But when once he took to larking on the bottom and trying to pass himself off as merely a bit of the surrounding sand-bank, the article in question would obviously be disadvantageous to him under his altered circumstances. A bit of the sand-bank which elevates itself vertically in the water on a couple of side-fins is sure to attract the unfavorable attention of the neighboring dog-fish, who love soles like human epicures. Accordingly, every aspiring sole that ever sought to rise in the world with undue levity was sure to be snapped up by a passing foe, who thus effectually prevented it from passing on its own peculiar aspirations and swim bladder to future generations. On the other hand, the unaspiring soles that hugged the bottom and were content to flounder along contentedly sidewise, instead of assuming the perpendicular, for the sake of appearances, at the peril of their lives, lived and flourished to a good old age, and left many successive relays of spawn to continue their kind in later ages. The swim-bladder would thus gradually atrophy from disuse, just as always happens in the long run with practically functionless and obsolete organs. The modern sole bears about perpetually in his own person the mark of his unenergetic and sluggish ancestry.
At the same time that the young sole, setting up in life on his own account, begins to lie on his left side only, and acquires his adult obliquity of vision, another singular and closely correlated change begins to affect his personal appearance. He started in life, you will remember, as a transparent body; and this transparency is commonly found in a great many of the earliest and lowest vertebrate organisms. Professor Ray Lankester, indeed, who is certainly far enough from being a fanciful or imaginative person, has shown some grounds for believing that our earliest recognizable ancestor, the primitive vertebrate? now best represented by that queer little mud-fish, the lancelet, as well as by the too famous and much-abused ascidian larva, was himself perfectly translucent. One result of this ancient transparency we still carry about with us in our own organization. The eye of man and of other higher animals, instead of being a modification of the skin (as is the case with the organ of vision in invertebrates generally), consists essentially of a sort of bag or projection from the brain, turned inside out like the finger of a glove, and made by a very irregular arrangement to reach at last the outside of the face. In the act of being formed, the human eye in fact buds out from the body of the brain, and gradually elongates itself upon a sort of stalk or handle, afterward known as the optic nerve. Professor Lankester suggests, as a probable explanation of this quaint and apparently rather roundabout arrangement, that our primitive ancestor was as clear as glass, and had his eye inside his brain, as is still the case with the ascidian larva. As soon as his descendants began to grow opaque, the eye was forced to push itself outward, so as to reach the surface of the body; and thus at last, we may imagine, it came to occupy its present prominent position on the full front of all vertebrate animals.
To return to our sole, however, whom I have left too long waiting in the sand to undergo his next transformation: as soon as he has selected a side on which to lie, he begins to grow dark, and a pigmentary matter forms itself on the upper surface exposed to the light. This is a very common effect of exposure, sufficiently familiar to ladies and others, and therefore hardly calling for deliberate explanation. But the particular form which the coloring takes in the true sole and in various other kinds of flat-fish is very characteristic, and its origin is one of the most interesting illustrations of, natural selection to be found within the whole range of animated nature. In every case it exactly resembles the coloration of the ground on which the particular species habitually reposes. For example, the edible sole lies always on sandy banks, and the spots upon its surface are so precisely similar to the sand around it that in an aquarium, even when you actually know from the label that there is a sole to be found in a particular tank, you can hardly ever manage to spot him as long as he lies perfectly quiet on the uniform bottom. Turbot, on the other hand, which prefers a more irregular pebbly bed, is darker brown in color, and has the body covered on its upper side with little bony tubercles, which closely simulate the uneven surface of the banks on which it basks. The plaice, again, a lover of open, stony spots, where small shingle of various sorts is collected together in variegated masses, has its top side beautifully dappled with orange-red spots, which assimilate it in hue to the party-colored ledges whereon it rests. In this last case the brighter dabs of color undoubtedly represent the bits of carnelian and other brilliant pebbles, whose tints of course are far more distinct when seen in water by refracted light than when looked at dry in the white and common daylight. We all know how much prettier pebbles always seem when picked up wet on the sea-shore than under any other circumstances.
Some few flat-fish even possess the chameleon power of altering their color, in accordance with the nature of the bottom on which they are lying. The change is managed by pressing outward or inward certain layers of pigment-cells, whose combination produces the desired hues.
The origin of this protective coloration must once more be set down to that deus ex machinâ of modern biology, natural selection. In the beginning, those flat-fish which happened to be more or less spotted and speckled would be most likely to escape the notice of their ever-watchful and rapacious foes; while those which were uniformly colored brown or gray, and still more those which were actually black or light pink, would be at once spotted, snapped up, and devoured. Hence in every generation the ever-surviving sole or turbot was the one whose spots happened most closely to harmonize with the general coloration of the surrounding bottom. As these survivors would alone intermarry and bring up future families of like-minded habits, it would naturally result that the coloration would become fixed and settled as a hereditary type in each particular species. Meanwhile, the eyes of the enemies of flat-fish, ever on the lookout for a nice juicy plaice or flounder, would become educated by experience, and would grow sharper and ever sharper in detecting the flimsy pretenses of insufficiently imitative or irregularly colored individuals. Natural selection means in this case selection by the hungry jaws of starving dog-fish. When once the intelligent dog-fish has learned to appreciate the fact that all is not sand that looks sandy, you may be sure he exercises a most vigilant superintendence over every bank he happens to come upon. None but the most absolutely indistinguishable soles are at all likely to escape his interested scrutiny.
The mere nature of the bottom upon which they lie has thus helped to become a differentiating agency for the various species and varieties of flat-fish. Soles, which easily enough avoid detection on the sandy flats, would soon be spotted and exterminated among the pebbly ridges beloved of plaice, or the shingly ledge especially affected by the rough-knobbed turbot. Flounders, whose coloring exactly adapts them to the soft ooze and shallow mud-banks at the mouths of rivers, would prove quite out of place on the deep pools of the channel, covered with pale-yellow sand, where the pretty lemon sole is most at home. In the case of the true sole, too, the long, graceful, sinuous fringe of fins is so arranged that it can fit accurately to the surface on which the fish is lying, and so add in a great measure to the appearance of continuity with the neighboring sands. A sole, settling down on a ribbed patch of sand, can thus accommodate its shape to the underlying undulations, so that it is almost impossible to distinguish its outline, even when you know exactly where to look for it. Soles are very clever at choosing such deceptive hiding-places, and very seldom openly expose themselves on a flat horizontal surface. Moreover, whenever they settle, they take care partially to bury themselves in the sand, with a curious sidelong flapping motion, and so still more effectually screen themselves from intending observers.
I may note in passing that such correspondence in color with the general hue of the surrounding medium is especially common wherever a single tone predominates largely in the wider aspect of nature. Arctic animals, as everybody knows, are always white. Ptarmigan and northern hares put on a snowy coat among the snows of winter. The uncommercial stoat needlessly transforms himself on the approach of cold weather into the expensive and much-persecuted ermine. Imagine for a moment the chances of life possessed by a bright scarlet animal among the snow-fields of Greenland, and one can see at once the absolute necessity for this unvarying protective coloration. Even a royal duke would scarcely venture to approve of flaring red uniforms under such conditions. All the conspicuous creatures get immediately weeded out by their carnivorous enemies, owing to their too great obtrusiveness and loudness of dress; while those alone survive which exactly conform to the fashionable whiteness of external nature. So, too, in the desert every bird, lizard, grasshopper, butterfly, and cricket is uniformly dressed in light sand-color. The intrusive red or blue butterfly from neighboring flowery fields gets promptly eaten up by the local bird, whose plumage he can not distinguish from the sand around it. The intrusive scarlet or green bird from neighboring forests finds the bread taken out of his mouth by the too severe competition of his desert brethren, who can steal upon the native grasshoppers unperceived, while he himself acts upon them like a red danger-signal, and is as sedulously avoided by the invisible insects as if he meant intentionally to advertise in flaming posters his own hostile and destructive purpose.
In short, sand-haunting creatures are and always must be necessarily sand-colored.
A few tropical flat-fish, however, living as they do among the brilliant corals, pink sea-anemones, gorgeous holothurians, and banded shells of the Southern seas, are beautifully and vividly spotted and colored with the liveliest patterns. In this case the necessity for protection compels the fish to adopt the exactly opposite tactics. All those young beginners which happen to show any tendency to plain brown coloring are sure to be recognized as fish, and get promptly eaten up among their bright surroundings; only those which look most like the neighboring inedible and stinging nondescripts stand any chance of escaping with their precious lives. A Quaker garb which would easily pass unobserved in the murky English Channel would become at once conspicuous by contrast among the brilliant organisms of Amboyna or Tahiti. This beautifully proves the relativity of all things, as philosophers put it. Ordinary people express the same idea in simpler language by saying that circumstances alter cases.
Most of our English flat-fish lie consistently on one side, and that the left; they keep their right eye always uppermost. But the turbot and the brill reverse this arrangement, having the left side on top and colored, while the right side is below and white. Two other fish, known as the fluke and the megrim, but not received in polite society, follow the example of their fashionable friends in this respect. But in no case are these habits perfectly ingrained; now and then one meets with a left-sided sole or a right-sided turbot, which looks as though a great deal were left to the mere taste and fancy of the individual flatfish. Some have taken to lying most frequently on one side and some on the other; but it is interesting to note that when a normally right sided individual has happened to lie with his left side uppermost that side becomes colored and distorted exactly the same as in his more correct brethren. This shows how purely acquired the whole habit must be. It points back clearly to the days when flat-fish were still merely a sort of cod, and suggests that their transformation into the un symmetrical condition is merely a matter of deliberate choice on their own part. Indeed, there seems good reason to believe that many young flat-fish never undergo this change at all, but swimming about freely in the open sea assume that peculiarly elongated and strange form known as the leptocephalic.
I don't mean to say that all leptocephali are originally the offspring of flat-fishes, but some probably are; and so a word or two about these monstrous oceanic idiots and imbeciles may not be here out of place.
Lolling about lazily in the open ocean a number of small, long, ribbon-like fish are frequently found, quite transparent and glassy in appearance, with no head at all to speak of, but furnished with a pair of big eyes close beside the tiny snout. They are languid, boneless, wormlike creatures, very gelatinous in substance, and looking much like pellucid eels without the skin on. For a long time these leptocephali (as they are called) were supposed to be a peculiar class of fishes, but they are now known to be young fry of various shore-haunting kinds, which have drifted out into the open ocean, and had their development permanently arrested for want of the natural environment. They are in fact fish idiots, and though they grow in size they never attain real maturity. If, as some authorities believe, many of these queer idiotic forms really represent stray flat-fish, then their symmetrical development once more points back to the happy days when the ancestral sole still swam upright, with one eye on each side of his head, instead of being distorted into a sort of aggravated squinter.
Besides the "reversed" specimens of soles and turbots—right-sided when they ought to be left-sided, and vice versa—occasional double or ambidextrous individuals occur, in which the dark color is equally developed on both sides of the body. Whether these impartial flat-fish are in the habit of turning over in their beds—whether they represent the uneasy sleepers of pleuronectid circles or otherwise—I am not in a position to state; but probably they are produced under circumstances where both sides have been frequently exposed to the action of light, which seems to have a sort of photographic effect upon the pigments of the fish's body. Everybody knows in fact that the upper side or back of most ordinary fish, exposed as it is to the sunlight, is darker than the lower side or belly; and this natural result of the solar rays has indirectly a protective effect, because when you look down into the water from above it appears dark, whereas when you look up from below the surface appears bright and shining; so that a fish is less likely to be observed (and eaten) if his back is dark and his under-surface white and silvery.
Albino soles are far rarer than doubles, and seldom occur except in very young and foolish specimens. Naturally an albino forms an exceptionally sure mark for his enemies to hawk at, and he is therefore usually devoured at an early stage of his unhappy existence, before he has time to develop properly into a good specimen. For the same reason adult white rabbits are very rare in the wild state, because they form such excellent targets for owls in their early infancy. Rabbits, when tamed, as we all know, tend to "sport" in color to a surprising extent; but this tendency is repressed in the wild condition by the selective action of the common owl, which promptly picks off every rabbit that does not harmonize well in the dusk of evening with the bracken and furze among whose stalks it feeds.
All the flat-fish are carnivorous. They live chiefly off cockles and other mollusks, off lugs, and lob-worms, or off small shrimp-like creatures and other crustaceans. In summer-time soles resort to banks and shallow spots near the mouths of rivers to deposit their spawn. They are obliged to do this in shallow waters, because, like most other fish, they are very unnatural mothers, and leave the sun to do the whole work of hatching for them. To be sure, there are some few right minded fish which take a proper view of their parental responsibilities, such as the pipe-fishes, which carry about their unhatched eggs in a bag, sometimes borne by the affectionate mother, but oftener still by the good father, a perfect model to his human confrères. Or again, the familiar little stickleback, who builds a regular nest for the reception of the spawn, and positively sits upon it like a hen, at the same time waving his fins vigorously backward and forward so as to keep up a good supply of oxygen. But soles and most other fish consider that their parental duties are quite at an end as soon as they have deposited their spawn in safety on a convenient sunny shallow.
This fact produces a sort of annual migration among the soles and other flat-fish. In spring, when all nature is beginning to wake up from its winter sleep, the soles seek the shoal water, which forms their spawning-ground; and, therefore, in April, May, June, and July, the British sole is chiefly trolled for off the Dogger Bank and the other great submerged flats of the North Sea. But when November comes on again the soles once more retire for the season into winter quarters in the deep water for the purpose of hibernating during the foodless period. The North Sea soles (in whose habits and manners the London public is most profoundly interested) generally resort for their long snooze to a deep depression known as the Silver Pits, lying close beside the Dogger Bank. These Silver Pits are so called because when they were first discovered (about the year 1843) they formed a sort of Big Bonanza for the lucky fishermen who originally resorted to them. There the soles lay, huddled together for the sake of warmth, like herrings in a barrel, thousands arid thousands of them, one on top of the other, a solid mass of living and sleeping solehood, only waiting for the adventurous fisherman to pull them up and take them to market. Man, treacherous man, crept upon their peaceful slumber unawares, and proceeded, like Macbeth, to murder sleep wholesale in the most unjustifiable and relentless manner. He dropped his lines into the Silver Pits the water there is too deep for dredging and hauled up the hapless drowsy creatures literally by the thousand till he had half exhausted the accumulated progeny of ages. The Silver Pits are still excellent winter fishing-grounds, but never again will they yield such immense fortunes as they did at the moment of their first exploration.
In 1848, when the California gold-fever was at its very height, some other lucky smack-owners hit upon a second deposit of solid soles, lying in layers on a small tract of coarse bottom near Flamborough Head, where they retired to hibernate, perhaps, in consequence of the hard treatment they had received in the Silver Pits. This new El Dorado of the fishing industry was appropriately nicknamed California, because it proved for the time being a very mine of gold to its fortunate discoverers. But, like the prototypal California on the Pacific coast, its natural wealth was soon exhausted; and, though it still yields a fair proportion of fish, its golden days are now fairly over.
Driven from the banks and pits by their incessant enemy, the trawler, the poor soles have now taken to depositing their spawn on the rough, rocky ground where the fishermen dare not follow them for fear of breaking their nets against the jagged ledges. These rocky spots are known as sanctuaries, and if it were not for them it is highly probable that sole au gratin would soon become an extinct animal on our London dinner-tables. Even to the sanctuaries, however, they are rudely followed, as Professor Huxley has shown, by their hereditary fishy foes, who eat the spawn, and so deprive the world of myriads upon myriads of unborn soles, consigned before their time to dull oblivion. Formerly, fishermen used to throw away these useless fish when caught; in future, they have strict orders from the inspectors of fisheries to kill them all wherever found.
However, even the remnant left by all enemies put together is quite sufficient to each case the number of turbot in the sea next year would be double what it is this; the year after that there would be four times as many; the next year eight times again; and so on in a regular arithmetical progression. In a very few decades the whole sea would become one living mass of solid turbot. As a matter of fact, since the number of individuals in any given species remains on the average exactly constant, we may lay it down as a general rule that only two young usually survive to maturity out of all those born or laid by a single pair of parents. All the rest are simply produced in order to provide for the necessary loss in infant mortality. The turbot lays fourteen million eggs, well knowing that thirteen million nine hundred and ninety-nine thousand nine hundred and ninety-nine will be eaten up in the state of spawn or devoured by enemies in helpless infancy, or drifted out to sea and hopelessly lost, or otherwise somehow unaccounted for. The fewer the casualties to which a race is exposed the smaller the number of eggs or young which it needs to produce in order to cover the necessary losses.the waters with a pleuronectid population with extraordinary rapidity. The fecundity of fish is indeed something almost incredible. The eggs of soles are extremely small—not so big as a grain of mustard-seed—and the roe of a one-pound fish usually contains as many as one hundred and thirty-four thousand of them. Turbot are even more surprisingly prolific: Frank Buckland was acquainted with one whose roe weighed five pounds nine ounces, and contained no less than fourteen million and odd eggs. It is a sad reflection that not more than one of these, on an average, ever lives to reach maturity. For if only two survived in
In fish generally it takes at least a hundred thousand eggs each year to keep up the average of the species. In frogs and other amphibians, a few hundred are amply sufficient. Reptiles often lay only a much smaller number. In birds, which hatch their own eggs and feed their young, from ten to two eggs per annum are quite sufficient to replenish the earth. Among mammals, three or four at a birth is a rare number, and many of the larger sorts produce one calf or foal at a time only. In the human race at large, a total of five or six children for each married couple during a whole lifetime makes up sufficiently for infant mortality and all other sources of loss, though among utter savages a far higher rate is usually necessary. In England, an average of four and a half children to each family suffices to keep the population stationary; above that number it begins to increase, and has to find an outlet in emigration. If every family had four children, and every child grew up to maturity and married, the population would exactly double in every generation. Even making allowances for necessary deaths and celibacy, however, I believe that as sanitation improves and needless infant mortality is done away with, the human race will finally come to a state of equilibrium with an average of three children to each household. But this is getting very far away indeed from the habits of flat-fishes.—Cornhill Magazine.