An introduction to the study of fishes

by Albert C. L. G. Günther

Fig. 102.—Pycnodus rhombus, a Ganoid from the Upper Oolite.

CHAPTER XVI.
THE DISTRIBUTION OF EXISTING FISHES OVER THE EARTH’S SURFACE—GENERAL REMARKS.

In an account of the geographical distribution of fishes the Freshwater forms are to be kept separate from the Marine. However, when we attempt to draw a line between these two kinds of fishes, we meet with a great number of species and of facts which would seem to render that distinction very vague. There are not only species which can gradually accommodate themselves to a sojourn in either salt or fresh water, but there are also such as seem to be quite indifferent to a rapid change from one into the other: so that individuals of one and the same species (Gastrosteus, Gobius, Blennius, Osmerus, Retropinna, Clupea, Syngnathus, etc.), may be found at some distance out at sea, whilst others live in rivers far beyond the influence of the tide, or even in inland fresh waters without outlet to the sea. The majority of these fishes belong to forms of the fauna of the brackish water, and as they are not an insignificant portion of the fauna of almost every coast, we shall have to treat of them in a separate chapter.

Almost every large river offers instances of truly marine fishes (such as Serranus, Sciænidæ, Pleuronectes, Clupeidæ, Tetrodon, Carcharias, Trygonidæ), ascending for hundreds of miles of their course; and not periodically, or from any apparent physiological necessity, but sporadically throughout the year, just like the various kinds of marine Porpoises which are found all along the lower course of the Ganges, Yang-tseKiang, the Amazons, the Congo, etc. This is evidently the commencement of a change in a fish’s habits, and, indeed, not a few of such fishes have actually taken up their permanent residence in fresh waters (as species of Ambassis, Apogon Dules, Therapon, Sciæna, Blennius, Gobius, Atherina, Mugil, Myxus, Hemirhamphus, Clupea, Anguilla, Tetrodon, Trygon): all forms originally marine.

On the other hand, we find fishes belonging to freshwater genera descending rivers and sojourning in the sea for a more or less limited period; but these instances are much less in number than those in which the reverse obtains. We may mention species of Salmo (the Common Trout, the Northern Charr), and Siluroids (as Arius, Plotosus). Coregonus, a genus so characteristic of the inland lakes of Europe, Northern Asia, and North America, nevertheless offers some instances of species wandering by the effluents into the sea, and taking up their residence in salt water, apparently by preference, as Coregonus oxyrhynchus. But of all the Freshwater families none exhibit so great a capability of surviving the change from fresh into salt water, as the Gastrosteidæ (Sticklebacks), of the northern Hemisphere, and the equally diminutive Cyprinodontidæ of the tropics; not only do they enter into, and live freely in, the sea, but many species of the latter family inhabit inland waters, which, not having an outlet, have become briny, or impregnated with a larger proportion of salts than pure sea water. During the voyage of the “Challenger” a species of Fundulus, F. nigrofasciatus, which inhabits the fresh and brackish waters of the Atlantic States of North America, was obtained, with Scopelids and other pelagic forms, in the tow-net, midway between St. Thomas and Teneriffe.

Some fishes annually or periodically ascend rivers for the purpose of spawning, passing the rest of the year in the sea, as Sturgeons, many Salmonoids, some Clupeoids, Lampreys, etc. The two former evidently belonged originally to the freshwater series, and it was only in the course of their existence that they acquired the habit of descending to the sea, perhaps because their freshwater home did not offer a sufficient supply of food. These migrations of freshwater fishes have been compared with the migrations of birds; but they are much more limited in extent, and do not impart an additional element to the fauna of the place to which they migrate, as is the case with the distant countries to which birds migrate.

The distinction between freshwater and marine fishes is further obscured by geological changes, in consequence of which the salt water is gradually being changed into fresh, or vice versa. These changes are so gradual and spread over so long a time, that many of the fishes inhabiting such localities accommodate themselves to the new conditions. One of the most remarkable and best studied instances of such an alteration is the Baltic, which, during the second half of the Glacial period, was in open and wide communication with the Arctic Ocean, and evidently had the same marine fauna as the White Sea. Since then, by the rising of the land of Northern Scandinavia and Finland, this great gulf of the Arctic Ocean has become an inland sea, with a narrow outlet into the North Sea, and its water, in consequence of the excess of the fresh water pouring into it over the loss by evaporation, has been so much diluted as to be nearly fresh at its northern extremities: and yet nine species, the origin of which from the Arctic Ocean can be proved, have survived the changes, propagating their species, agreeing with their brethren in the Arctic Ocean in every point, but remaining comparatively smaller. On the other hand, fishes which we must regard as true freshwater fishes, like the Rudd, Roach, Pike, Perch, enter freely the brackish water of the Baltic.[17] Instances of marine fishes being permanently retained in fresh water in consequence of geological changes are well known: thus Cottus quadricornis in the large lakes of Scandinavia; species of Gobius, Blennius, and Atherina in the lakes of Northern Italy; Comephorus, of the depths of the Lake of Baikal, which seems to be a dwarfed Gadoid. Carcharias gangeticus in inland lakes of the Fiji Islands, is another instance of a marine fish which has permanently established itself in fresh water.

In the miocene formation of Licata in Sicily, in which fish remains abound, numerous Cyprinoids are mixed with littoral and pelagic forms. Sauvage found in 450 specimens from that locality, not less than 266, which were Leucisci, Alburni, or Rhodei. Now, although it is quite possible that in consequence of a sudden catastrophe the bodies of those Cyprinoids were carried by a freshwater current into, and deposited on the bottom of, the sea, the surmise that they lived together with the littoral fishes in the brackish water of a large estuary, which was not rarely entered by pelagic forms, is equally admissible. And, if confirmed by other similar observations, this instance of a mixture of forms which are now strictly freshwater or marine, may have an important bearing on the question to what extent fishes have in time changed their original habitat.

Thus there is a constant exchange of species in progress between the freshwater and marine faunæ, and in not a few cases it would seem almost arbitrary to refer a genus or even larger group of fishes to one or the other; yet there are certain groups of fishes which entirely, or with but few exceptions are, and, apparently, during the whole period of their existence have been, inhabitants either of the sea or of fresh water; and as the agencies operating upon the distribution of marine fishes differ greatly from those influencing the dispersal of freshwater fishes, the two series must be treated separately. The most obvious fact that dry land, which intervenes between river systems, offers to the rapid spreading of a freshwater fish an obstacle which can be surmounted only exceptionally or by a most circuitous route, whilst marine fishes may readily and voluntarily extend their original limits, could be illustrated by a great number of instances. Without entering into details, it may suffice to state as the general result, that no species or genus of freshwater fishes has anything like the immense range of the corresponding categories of marine fishes; and that, with the exception of the Siluroids, no other freshwater family is so widely spread as the families of marine fishes. Surface temperature or climate which is, if not the most, one of the most important physical factors in the limitation of freshwater fishes, similarly affects the distribution of marine fishes, but in a less degree, and only those which live near to the shore or the surface of the ocean; whilst it ceases to exercise its influence in proportion to the depth, the true deep-sea forms being entirely exempt from its operation. Light, which is pretty equally distributed over the localities inhabited by freshwater fishes, cannot be considered as an important factor in their distribution, but it contributes towards constituting the impassable barrier between the surface and abyssal forms of marine fishes. Altitude has stamped the fishes of the various Alpine provinces of the globe with a certain character, and limited their distribution; but the number of these Alpine forms is comparatively small, ichthyic life being extinguished at great elevations even before the mean temperature equals that of the high latitudes of the Arctic region, in which some freshwater fishes flourish. On the other hand, the depths of the ocean, far exceeding the altitude of the highest mountains, still swarm with forms specially adapted for abyssal life. That other physical conditions of minor and local importance, under which fresh water fishes live, and by which their dispersal is regulated, are more complicated than similar ones of the ocean, is probable, though perhaps less so than is generally supposed: for the fact is that the former are more accessible to observation than the latter, and are, therefore, more generally and more readily comprehended and acknowledged. Thus, not only because many of the most characteristic forms of the marine and freshwater series are found, on taking a broader view of the subject, to be sufficiently distinct, but also because their distribution depends on causes different in their nature as well as the degree of their action, it will be necessary to treat of the two series separately. Whether the oceanic areas correspond in any way to the terrestrial will be seen in the sequel.