Popular Science Monthly/Volume 60/November 1901/The Fishes of Japan





THE islands of Japan are remarkable for their richness of animal life. The variety in climatic and other conditions, the nearness to the great continent of Asia and to the chief center of marine life—the East Indian Islands—its relation to the warm Black Current or Kuro Shiwo—the Gulf Stream of the Orient—and to the cold current from Bering Sea, all tend to give variety to the fauna of its seas. Especially numerous and varied are the fishes of Japan.

About nine hundred species of fishes are known, from the four great main islands of Japan, and about two hundred more from the volcanic islands (Kuriles and Liu Kiu) to the north and south. Of the eleven hundred, about fifty are fresh water. All these are derived from the mainland of Asia. Two faunal districts, the north and the south, may be recognized among the fresh-water fishes. The mountain region and the region lying to the north of Fuji abound in trout, with salmon, sturgeon, lamprey and other northern fishes. In the southern district these are absent and the chief fresh-water fishes are ayu, or dwarf salmon, chubs, minnows, cat-fishes and loaches.

The marine fishes are far more varied, their distribution being mainly controlled by temperature and currents. Among these, five districts may be recognized, their range sufficiently indicated by the names, Kurile, Hokkaido, Nippon, Kiusiu, Kuro Shiwo and Liu Kiu. Of these, the Kurile Fauna is subarctic, similar to that of the Aleutian Islands, that of the Liu Kiu islands is tropical, that of the promontories, which strike out into the Kuro Shiwo, is Polynesian. The central region (Nippon) contains the forms essentially Japanese. Kiusiu lias much in common with China, and Hokkaido with Siberia and Manchuria. Each of these districts overlaps, by a broad fringe, on the others.

It has been noted that the fish fauna of Japan bears a striking resemblance to that of the Mediterranean, and Dr. Günther has suggested that this can be accounted for by supposing that in recent times a continuous coast line and sea-passage extended from one region to the other, the Isthmus of Suez not existing.

The resemblance consists in the presence in the two regions of certain striking-looking fishes not found in other parts of the world. An analysis of these resemblances takes away much of their impressiveness. Most of the forms in question are widely distributed, ranging from Japan through India to the Cape of Good Hope. Only three genera are restricted to Japan and the Mediterranean. Resemblances equally strong exist between Japan and the West Indies, or between Japan and Australia. The differences are equally marked. The types regarded as of Japanese origin are all wanting in the Mediterranean. Those of Mediterranean origin are wanting in Japan. There are two main reasons why one fish fauna may resemble another; the one, actual connection, so that fishes migrate from one region to another; the other, similarity of physical conditions, favoring in each region the development of similar kinds of fishes. The evidence points toward the theory that similarity of physical conditions is the chief source of resemblance between Japan and the Mediterranean. The resemblance between Japan and the West Indies is due to this cause, while that of Japan to the East Indies is due largely to direct connection. If Japan and the Mediterranean were ever connected, the Red Sea must have been a region of junction. Yet, while the Red Sea in its fishes closely resembles southern Japan, it has almost nothing in common with the Mediterranean. Except a few shallow water or brackish water types, the shore fishes of the two regions are wholly distinct, none of the characteristic genera of either sea being found in the other.

Yet, geologists affirm that in Pliocene or Post-Pliocene times the Isthmus of Suez was submerged. It is made up of Pliocene, deposits with alluvium from the Nile and drifting sand-hills. Admitting this to be true, the nature of the fishes shows that this channel must have been very shallow and probably in part occupied by fresh water. No bottom-fish or rock-fish has crossed it—only sting-rays, torpedoes, eels and mullets appear to have passed from one side to the other. It must have been impossible for Japan and the Mediterranean ever to have exchanged their deep-water fishes in this way. The only other alternative is the Cape of Good Hope, and this barrier is, to this day, passed by many characteristic fishes of both oceans.

Four hundred and eighty-three genera of fishes are known from Japan. For the purpose of our present study we must take from this list all the fresh-water types, derived from China; all the northern types, derived from Bering Sea and the general Arctic stock; all the pelagic fishes, at home in the open sea, and all the bassalian fishes, or those inhabiting great depths below the range of climatic changes. After these are withdrawn, we have left the shore fishes of tropical, or semi-tropical, origin. Of these, Japan has 334 genera; the Mediterranean, 144; the Red Sea, 191; India, 280; Australia, 344; New Zealand, 108; Hawaii, 144; West Indies, 299, and the Panama region, 256.

Common to Japan and the Mediterranean are 79, all but two being of wide distribution; to Japan and the Red Sea, 111; to Japan and Hawaii, 82; to Japan and Australia, 135; to Japan and the West Indies, 113; to Japan and Panama, 91. To the Mediterranean and the Red Sea, 40 genera are common, all of wide distribution; to the West Indies and the Mediterranean, 70, 59 being of wide distribution; to the West Indies and Panama, 179, only 101 being of wide distribution.

It is evident from an analytical table that the warm-water fauna of Japan, like that of Hawaii, is derived from that of the East Indies and Hindostan; that the fauna of the Eed Sea is derived from the same source; that the Mediterranean fauna bears no special resemblance to that of Japan rather than to that of the other parts of Eastern Asia with like conditions of temperature and no greater than is borne by the West Indies; that the fauna of the two sides of the Isthmus of Suez have relatively little in common, while those of the two sides of the Isthmus of Panama show a remarkable degree of identity.

When the fishes of Panama were first described, it was claimed that their species were almost entirely identical with those of the West Indies; this statement was followed by speculations on the relation of the depression of this Isthmus to the Gulf Stream, and to the glacial epoch. Further investigations by Jordan, and by Evermann and Jenkins showed the fallacy of this claim of identity. Of about 1,400 species now known from the two sides of the Isthmus, only 70 are identical, or five per cent, of the whole, and about 10 of these are almost cosmopolitan in the tropics. Dr. Paul Fischer finds about three per cent, of the mollusks identical on the two coasts.

Dr. E. T. Hill goes on to show that there is neither geological nor biological evidence of the submergence of the Isthmus of Panama since Tertiary times, and that such a barrier existed as far back as Jurassic times. There is, however, evidence of a brief connection in Tertiary time at the end of the Eocene period.

Assuming this to be true, the actual facts of distribution seem to be in accord with it. The period of depression was before the life-time of most of the present species. It was, however, not earlier than the period of most of the present genera. It was relatively shallow, but wide enough to permit the infiltration from the Caribbean Sea to the Pacific of species representing most of the genera of sandy bays, rocky tide pools and brackish estuaries. Since the channel was closed, the species left on either side have undergone modification in varying degrees, mostly retaining generic identity, while losing some of their specific characters.

Doubtless, local oscillations in coast lines have taken place and are even in operation at present, but the time has passed when a dance of continents can be invoked to explain anomalies in animal distribution. Most of these will be found to have simple causes, when we know enough of the facts in the case to justify a hypothesis.

The laws governing animal distribution are reducible to three very simple propositions:

Every species of animal is found in every part of the earth having conditions fit for its existence, unless

(a) Its individuals have been unable to reach the region in question through barriers of some sort, or,

(b) Having, reached the region, the species is unable to maintain itself through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment, or else,

(c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the parent type.

In general, the different types of fishes are most specialized along equatorial shores. The processes of change through natural selection take place most rapidly there and produce more far-reaching modifications. The coral reefs of the tropics are the centers of fish-life, corresponding in fish economy to the cities in human affairs. The fresh water, the Arctic waters, the deep sea and the open sea represent ichthyic backwoods—regions where change goes on more slowly and in which archaic types survive.

The study in detail of the distribution of the fishes of the tropics, is most instructive. The study of the origin of the fish groups of Japan affords a fascinating introduction to its multifarious problems.