Popular Science Monthly/Volume 21/July 1882/Dr Gunther on the Study of Fishes
|DR. GÜNTHER ON THE STUDY OF FISHES.|||
MANY difficulties, says the "Quarterly Review," tend to prevent ichthyology becoming a popular study, as the study of shells, insects, birds, or flowers is popular. However it may be with the particular species that anglers seek out and professional fishermen hunt, fishes as a class are not familiar objects. They keep for the most part out of sight, and when at liberty in their element can be detected only by passing glimpses, after which they are nearly always immediately lost. The aquarium, whatever it may have done to aid the study of the lower forms of aquatic animals, has contributed little or nothing to promote a real knowledge of ichthyology; and a preserved specimen of a fish is a most unsatisfactory object, as far as it can be from having anything of the color or the life or the grace of a real fish, and can not by any possibility be made to present a natural aspect.
Another serious difficulty in the way of the student of fishes may be stated thus: In beginning the study of any department of natural history, whether it relates to plants or to animals, the first effort is to find out characteristics of the smaller groups composing it, and to assort them in accordance with those characteristics; in short, to arrange or classify them. The young conchologist, for example, sees in an instant that out of a miscellaneous collection of shells some are bivalve and others univalve, and that some of them exhibit clear distinctions connected with the form of the animal to which the shell belongs. The young entomologist, with still greater ease, perceives the difference between most of the insects that come in his way, and indeed in some cases needs no instigation to look for them—the difference between a grasshopper and a house-fly, a beetle, a butterfly, and a moth, being self-evident to any one with eyes. So with the vertebrates; it requires no previous zoological instruction to enable any child to point out characters that will separate a snake from a tortoise, a rabbit from a sheep, a whale from a camel, and the rough primary division of all these creatures is at once perceptible. But with fishes this is not so. The learner, judging, as he is at first inclined to do, from outward survey, is surprised to find that the essential differences between a lamprey and an eel are deemed to be far greater than between an eel and a salmon, and that a skate is much further removed from a turbot than the latter is from a gudgeon, while a lancelet, which, when immersed in a bottle of spirit, looks so like a small smelt, differs, in the opinion of certain systematists, more from it than the smelt does from a frog, or indeed from any other existing vertebrate. All this, which the learner finds written in the first book on the subject (if he has one of the least authority) to which he has access, is so entirely in contradiction, as he thinks, to the plain evidence of his eye-sight, that he may well be staggered at the outset of his studies and discouraged from their prosecution. The classification of fishes has in truth been a task of no ordinary difficulty, and it is a subject requiring a far greater knowledge of their internal structure than can possibly be expected of a beginner.
One of the most formidable difficulties in the way of arriving at an intelligent classification has been removed by a discovery which Dr. Günther has made concerning the affinities of certain groups of fishes or fish-like animals, the relations of which to each other and to other fishes had been an inscrutable puzzle to all systematists. Among these were the ganoids, a family represented in an indefinite number of fossils, mostly of very ancient date, but few types of which survive to this day, and these restricted to the fresh waters of Eastern Asia, North America, and tropical Africa; other fossil fishes of equal antiquity, which were closely allied to the abundant sharks, dog-fishes, rays, and skates of our own seas, the "Chondropterygians" or "Elasmobranchs"; sturgeons, "Chondrosteans," possessing much of an archaic character; and besides these, there now exist two animals, commonly called "mud-fish," scientifically "Dipnoi," which have been deemed by some great authorities true fishes, by others amphibians. Furthermore, in the early days of the settlement of the Australian colony of Queensland, accounts were given of the existence in the rivers of the country of a large fish which the colonists called a "salmon," from the fact of its having salmon-colored flesh, and of sometimes, it was said, rising to a fly. Mr. Krefft, Curator of the Australian Museum, at Sydney, having examined a specimen of this fish in 1869 or 1870, pronounced it to be allied to the mud-fish, but discovered also that it possessed teeth so closely resembling those of certain fossil fishes attributed by Agassiz to the sharks, that no doubt could be entertained of the generic identity of the two forms. Accordingly, the newly found animal was described as a species of Ceratodus—that being the name which Agassiz had conferred on the creatures whose fossil teeth he had long before made known. Now, all this was in itself sufficiently remarkable, for it proved that Ceratodus, as a genus, had persisted from the Mesozoic era; but its important bearing was not fully perceived till after some more examples had been obtained and sent to Dr. Günther. He described the recent Ceratodus in great detail in the "Philosophical Transactions" for 1871, and was able, furthermore, by its means, to show how the palæozoic as well as the recent ganoids, the sharks and skates—both ancient and modern—the sturgeons, the mud-fishes, with some other forms that had hitherto been irreconcilable, could all be brought together through some essential characters common to the whole of them, and harmoniously placed in a single class, to which he assigned the name of "Palæichthyes"—fishes of ancient type. Professor Huxley had previously pointed out the affinity of the mud-fishes to certain ganoids, but the credit of discovering this comprehensive classification is due to Dr. Günther.
Dr. Günther does not undertake to describe definitely the geographical distribution of fishes in the sense in which the term geographical distribution is used by naturalists of other branches, he holding that "the endeavor to establish, by means of our present fragmentary geological knowledge, the divisions of the fauna of the globe leads us into a maze of conflicting evidence." It is obvious that fishes are not amenable to the laws of geographical distribution which govern land animals. In treating of their relations in this respect it is, moreover, necessary to separate them into categories, of which Dr. Günther makes four: 1. Fresh-water fishes; 2. Brackish-water fishes; 3. Marine fishes, which are furthermore subdivided into shore-fishes and oceanic or pelagic fishes; and, 4. Fishes of the deep sea. Even in the case of fresh-water fishes, which of course live under conditions more similar to those of land-animals than do those of the other categories, he disallows the six great zoögeographical regions which most geologists have accepted, and would arrange them in three zones—Northern, Equatorial, and Southern. These zones, are, however, broken up into regions, which roughly correspond with the six generally received, except that the Australian region of most zoögeographers is split up into two, the "Tropical Pacific" and the "Antarctic" region, the last including the Patagonian seas as well as those of New Zealand and Tasmania. The fishes of the second category (brackish water), owing to the fact of their living in salt-water equally with fresh, and thus being able to spread readily over the globe, can not help in any plan of parceling out the earth's surface into districts. The shore-fishes afford a somewhat better definition, and of them five groups are formed, which inhabit respectively the Arctic Ocean, the Northern Temperate Zone, the Equatorial Zone, the Southern Temperate Zone, and the Antarctic Ocean. The pelagic fishes seem to require separation, but as little can be deduced from them as from the inhabitants of brackish waters, and they insensibly mingle with the fishes of the deep sea.
Thirty years ago no one had the audacity to believe that the abysses of the ocean were tenanted with piscine life. Even animal life of any sort had been supposed to be impossible at a greater depth than that which has now been found to be but the portal of a new world of beings. The discovery of what has since been proved to be deep-sea forms of fishes began indeed long ago, but the abysmal nature of their haunts was hardly suspected, and certainly not recognized till much later, when the fact was established by Dr. Günther, conjointly with the late Mr. James Yate Johnson. On this subject Dr. Günther says: "The knowledge of the existence of deep-sea fishes is one of the recent discoveries of ichthyology. It was only twenty years ago, that, from the evidence afforded by the anatomical structure of a few singular fishes obtained in the North Atlantic, an opinion was expressed that these fishes inhabited great depths of the ocean, and that their organization was specially adapted for living under the physical abyssal conditions. These fishes agreed in the character of their connective tissue, which was so extremely weak as to yield to, and to break under, the slightest pressure, so that the greatest difficulty is experienced in preserving their body in its continuity. Another singular circumstance was, that some of the specimens were picked up floating on the surface of the water, having met their death while engaged in swallowing or digesting another fish not much inferior, or even superior, in size to themselves. The first peculiarity was accounted for by the fact that, if these fishes really inhabited the great depths supposed, their removal from the enormous pressure under which they lived would be accompanied by such an expansion of the gases within their tissues as to rupture them, and to cause a separation of the parts which had been held together by the pressure. The second circumstance was explained thus: a raptatorial fish, organized to live at a depth of between five hundred and eight hundred fathoms, seizes another usually inhabiting a depth of between three hundred and five hundred fathoms. In its struggles to escape, the fish seized, nearly as large or strong as the attacking fish, carries the latter out of its depth into a higher stratum, where the diminished pressure causes such an expansion of gases as to make the destroyer, with its victim, rise with increasing rapidity toward the surface, which they reach dead or in a dying condition."
It was also shown that, as the same species and genera are found in the most distant parts of the globe, the deep-sea fishes are not limited in their range, and consequently, as is admitted on other grounds, that the physical conditions of the ocean-depths must be much alike all the world over. That the deep-sea fishes are not of a peculiar order, however peculiarly organized, but are for the most part modified forms of surface-types, was another conclusion arrived at from the scattered evidence available before dredging at great depths was systematically practiced, and a conclusion that has since proved to be right. Nevertheless, it still remained to ascertain more precisely the bathymetrical horizons in which the different kinds lived, and this has been to some extent attained by observations made during the voyage of the Challenger; but these can not be received without further critical examination, for, unfortunately, no precaution seems to have been taken to keep the mouth of the dredge closed, and therefore it is probable, if not in some cases certain, that fishes were occasionally entrapped while the machine was passing through the surface-water. On the other hand, the majority of the examples taken in the dredge offer literally internal evidence that they were inhabitants of the abysses, being so organized as to be unable to live near the surface, and consequently that they were captured at the greatest depth to which the dredge reached, or nearly so.
The physical conditions of the deep sea, affecting the organization and distribution of these fishes, are thus formulated by Dr. Günther:
1. "Absence of Sunlight.—Probably the rays of the sun do not penetrate to, and certainly do not extend beyond, a depth of two hundred fathoms; therefore, we may consider this to be the depth where the deep-sea fauna commences. Absence of light is, of necessity, accompanied by modifications of the organs of vision, and by simplification of colors.
2. "Phosphorescence.—The absence of sunlight is in some measure compensated. for by the presence of phosphorescent light, produced by many marine animals, and also by numerous deep-sea fishes.
3. "Depression and Equality of the Temperature.—At a depth of five hundred fathoms the temperature of the water is already as low as 40º Fahr., and perfectly independent of the temperature of the surface-water; and from the greatest depths upward to about one thousand fathoms the temperature is uniformly but a few degrees above the freezing-point. Temperature, therefore, ceases to offer an obstacle to the unlimited dispersal of deep-sea fishes.
4. "The Increased Pressure by the Water.—The pressure of the atmosphere, on the level of the sea, amounts to fifteen pounds per square inch of surface on the body of an animal; but the pressure amounts to a ton weight for every one thousand fathoms of depth.
5. "With the Sunlight, Vegetable Life ceases in the Depths of the Sea.—All deep-sea fishes are therefore carnivorous; the most voracious feeding frequently on their own offspring, and the toothless kinds being nourished by the animalcules which live on the bottom, or which, 'like a constant rain,' settle down from the upper strata toward the bottom of the sea.
6. "The Perfect Quiet of the Water at Great Depths.—The agitation of the water, caused by the disturbances of the air, does not extend beyond the depth of a few fathoms; below this surface stratum there is no other movement except the quiet flow of ocean currents, and near the bottom of the deep sea the water is probably in a state of almost entire quiescence."
Now, the effect of these conditions on some part or parts of their structure is such that all deep-sea fishes are easily recognizable, without positive evidence of their having been caught at a great depth and in many of them the most striking characteristics relate to the pressure of the water they inhabit. Their bones and muscles are comparatively feebly developed; the former "have a fibrous, fissured, and cavernous texture, are light, with scarcely any calcareous matter, so that the point of a needle will readily penetrate them without breaking." They are loosely attached to each other—the vertebræ especially; and, unless carefully handled, the body will almost certainly fall to pieces. But that this is not the animal's normal condition we may be well assured. It is due simply to the absence of the pressure which keeps the whole organization compact; for, as has just been stated, most of these fishes are rapacious, and to indulge their enormous voracity they must execute rapid and powerful movements, to effect which their muscles must be as firm and their vertebræ as tautly braced as in their surface-swimming relatives. Marvelous as this is, it is far from being all that is marvelous in the structure of these dwellers in the profundities. Besides modifications of their eyes, such as are found in several other groups of animals, many of them are furnished with "more or less numerous, round, shining, mother-of-pearl colored bodies imbedded in the skin," of which Dr. Günther says: "These so-called phosphorescent or luminous organs are either large bodies of an oval or irregularly elliptical shape placed on the head, in the vicinity of the eye, or smaller round globular bodies arranged symmetrically in series along the sides of the body and tail, especially near the abdominal profile, less frequently along the back. . . . The organs of one kind consist of an anterior, biconvex, lens-like body, which is transparent during life, simple or composed of rods; and of a posterior chamber, which is filled with a transparent fluid, and coated with a dark membrane composed of hexagonal cells or of rods arranged as in a retina. . . . In the other kind the organ shows throughout a simple glandular structure, but apparently without an efferent duct. Branches of the spinal nerves run to each organ, and are distributed over the retina-like membrane or the glandular follicles. The former kind of organs are considered by some naturalists true organs of vision (accessory eyes), the function of the latter being left unexplained by them."
There can hardly be a reasonable doubt that the functions of these organs, of both kinds, have reference to the conditions of light under which the animals live, but further than that judgment concerning them must be suspended. Dr. Günther briefly states three hypotheses that have been broached as possible: 1. That both kinds are "accessory eyes," to which the objection is offered that several fish having well-developed and even large eyes, perfectly adapted for seeing in the dark, are endowed with them, while in other deep-sea fishes, without external eyes, they are absent. 2. That only the organs with a lenticular body and a retina-like membrane behind it are visual, but that the glandular organs are phosphorescent; and more may be said for this view than for any other, since the glandular organs are certainly luminous. 3. That all the organs are producers of light, in which case it must proceed from the inner cavity and be emitted through the lens-like body as through a bull's-eye lantern. It will be hard to decide which of these suppositions is the right one, for it seems impossible to reproduce in the animals and their environment on the surface the conditions of an uninterrupted deep-sea life. [This subject was fully discussed by Dr. Ernest Krause in the last number of "The Popular Science Monthly."]
The deep-sea fishes display few colors. Their bodies are generally black or silvery, with a most brilliant sheen, which is preserved even after years of immersion in spirits. A few are "picked out" with bright scarlet, either on the fin-rays or the filaments attached to them. These filaments, it may be said, are eminently characteristic of fishes that inhabit still water, and the fact that many of the deep-sea forms are adorned with them perfectly accords with the belief that the abysmal regions are quiet.
Another remarkable property of some of these creatures is the possession of a stomach so capable of distention that it can hold a prey of twice or thrice the bulk of its destroyer. Dr. Günther gives figures of two or three fish with distended stomachs; and Mr. Johnson, his associate in this investigation, writes of a specimen which he procured at Madeira:
"The man from whom I obtained it stated that he had a fish with two heads, two mouths, four eyes, and a tail growing out of the middle of the back, which had astonished the whole market; and the fishermen, one and all, declared they had never met with anything like it before. At first sight it really did appear to be the monster described; but a short examination brought to light the fact that one fish had been swallowed by another, and that the features of the former were seen through the extensible skin of the latter. On extracting the fish that had been swallowed, it proved. . . to have a diameter several times exceeding that of its enemy, whose stomach it had distended to an unnatural and painful degree." The action performed by the fish in these cases is not, however, a real swallowing, but more like the similar process executed by serpents.
The interest of Dr. Günther's book does not end with the account of deep-sea fishes, but the chapters devoted to that subject and to classification illustrate the most striking discoveries that have been recently made in ichthyology. Among the curiosities of fish-life that please and amuse as well as instruct, is the story of the fighting-fish of Siam, which, on seeing another of its species, or even its own image, in a mirror, becomes suddenly excited, and of which, though it is dull in hue at other times, "the raised fins and the whole body shine with metallic colors of dazzling beauty, while the projected gill-membrane, waving like a black frill round the throat, adds something of grotesqueness to the general appearance." The Siamese are infatuated with the combats of these fishes, staking on the results considerable sums, and sometimes their persons and families, while the license to exhibit fish-fights is farmed, and brings in no small revenue to the royal treasury.
The peculiarity of the flounders, and other flat fishes, by which the eyes, normally situated in the young, move around as the animal grows, until they are both on the same side of the body, is well known, but the manner in which the transposition is effected is still in question. There is, moreover, no end to the wonders to be found in fishes' eyes. Those of the genus Anableps, known in Demerara as "four eyes," have the iris horizontally divided by a black band, which almost justifies their name; and as these fishes frequently swim with the head half out of the water, it is presumed that the upper and lower portions of the cornea are adapted for the different density of the media in which they are respectively used. The "star-gazers" (Uranoscopus), and others, have eyes that can be raised or lowered at will; but the most remarkable instance of mobility in these organs seems to exist in certain gobies of the genus Periophthalmus and its ally Boleophthalmus, which might be called "oglers," as they have the power of thrusting their eyeballs far out of the socket, and turning them as freely as a chameleon rolls his. These fishes are also remarkable for another faculty, toward which their versatile eyes must contribute not a little. At low water they remain on the muddy flats, and hunt for their prey, which consists of small crustaceans and other marine animals, making rapid leaps by the aid of their fins and tails, which are strong; and when their eyes are retracted they are protected by a membranous lid.
Then the fishes that travel over land, the flying-fishes, with the controversy as to whether they really fly or only seem to (with Dr. Günther denying the reality of the flight, and others affirming it from their personal observations), and the fish that build nests, like the stickleback, and the fishing-frog, or angler, and the salmons and the cod, and the herring, afford inviting objects of curious observation, or scientific and economical study or speculation, concerning the later results of which we have kept the readers of "The Popular Science Monthly" informed. The last fish we notice is the eel, the manner of the reproduction of which is yet a puzzle to naturalists. Dr. Günther says of it:
"Their mode of propagation is still unknown. So much only is certain, that they do not spawn in fresh water, that many full-grown individuals, but not all, descend rivers during the winter months, and that some of them, at least, must spawn in brackish water or in deep water in the sea; for in the course of the summer young individuals, from three to five inches long, ascend rivers in incredible numbers, overcoming all obstacles, ascending vertical walls or flood-gates, entering every larger or smaller tributary, and making their way even over terra firma to waters shut off from all communication with rivers. Such immigrations have long been known by the name of 'eel-fairs.' The majority of the eels which migrate to the sea appear to return to fresh water, but not in a body, but irregularly, and throughout the warmer part of the year. No naturalist has ever observed these fishes in the act of spawning, or found mature ova; and the organs of reproduction of individuals caught in fresh water are so little developed, and so much alike, that the female organ can be distinguished from the male only with the aid of a microscope."
- An Introduction to the Study of Fishes, by Albert C. L. G. Günther, Ph.D., F. R. S., Keeper of the Zoölogical Department in the British Museum.