Belonging to a different category from any of those mentioned are certain Crustacea closely allied to, or identical with, species living in the sea, which inhabit inland lakes where no direct passage from the sea is now possible. Attention was first called to these in the case of some of the large lakes of Sweden, in which Professor Lovén found some Crustacea—Mysis relicta (see [Fig. 16], p. 47), Mesidotea entomon, Pontoporeia affinis—almost or quite identical with species inhabiting the Baltic, the Arctic Ocean, and the North Atlantic. There is geological evidence to show that these lakes were once fjords, or arms of the sea, and have become cut off from communication with the Baltic by gradual elevation of the land. The marine animals which they contained would thus be imprisoned, and as the water became less and less salt, by the inflow of rivers, certain species which were able to accommodate themselves to the altered conditions would survive. Some of the species living in the Swedish lakes have since been found to have a wider distribution. Thus, Mysis relicta, which should perhaps be reckoned as only a variety of the Mysis oculata of Arctic seas, has been found in lakes in Russia, North Germany, and North America (Lake Superior and others), and has lately been discovered in Lough Neagh and some other lakes in Ireland.
The brackish waters of the Caspian Sea contain a very remarkable assemblage of animals, including many Crustacea, which, although now quite isolated from the oceans, are certainly of marine, and in part of Arctic, origin. Among these are some species closely allied to or identical with those of the Swedish lakes already mentioned, together with a great variety of species of Mysidacea, Cumacea, and Amphipoda, which appear to have been evolved from marine forms since the Caspian was cut off from communication with the Arctic Ocean.
To such assemblages of animals derived from marine species and isolated in inland lakes the name of "relict" faunas has been given. It is necessary to use caution, however, in extending this explanation of their origin to every case of peculiar lake faunas. For example, there are difficulties in the way of supposing that Lake Baikal was ever in open and direct communication with the sea, although it contains many animals, such as seals, which are certainly of marine origin. The chief Crustacea of the lake are numerous species of Amphipods belonging to the genus Gammarus, and other genera closely related thereto, and for these, at all events, there is no need to assume a "relict" origin.
One of the most remarkable lakes in the world from a zoological point of view is Lake Tanganyika in Africa. When it was found that this lake contained a fauna very different from that of the other great lakes of Africa, it was rashly assumed that it must be of relict origin, and some remarkable speculations were indulged in as to the former connection between the lake and the sea. Further research, while it has greatly emphasized the peculiar nature of the fauna, has entirely disposed of the view that it originated in this way. The Crabs and Prawns, for example, are not nearly related to marine forms, but belong to groups that are characteristic of fresh waters in the tropics. While Nyassa and the Victoria Nyanza have as yet only yielded a single species of Prawn, and that one of enormously wide distribution (from the Nile to Queensland), Tanganyika contains no fewer than twelve species, all of which are peculiar to the lake, while all except one belong to genera unrepresented elsewhere. Similarly, the Crabs found in the other great lakes of Africa belong to commonplace types of River Crabs of the genus Potamon; in Tanganyika, in addition to some of these, there are three species of a remarkable genus, Platytelphusa, not known from any other locality. The Copepoda and Ostracoda of Tanganyika comprise a remarkably large number of species, many of them peculiar to the lake. A most unusual feature is the entire absence of Cladocera. It is not easy to explain the occurrence of this remarkable fauna in Tanganyika, but the evidence from other groups of animals, such as Mollusca and fishes, tends to suggest that the lake must have been, until recently, completely isolated from the other lakes and river-systems of Africa, that it had no outlet, and that the water was consequently more or less brackish. Under these conditions the fauna of the lake, originally similar to that of the other African lakes, has evolved along lines of its own.
Fig. 62—A Well Shrimp (Niphargus aquilex). × 7. (After Wrzesniowski.)
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A very interesting division of the fresh-water fauna is constituted by those animals which inhabit underground waters. In the South of England there is found not unfrequently in the water of wells a small colourless transparent Amphipod known as the "Well Shrimp" (Niphargus aquilex—[Fig. 62]), distinguished from the common fresh-water Gammarus by the slenderness of its body, by the elongation of the last pair of tail appendages (uropods), and by the absence of eyes. The proper habitat of Niphargus is not actually in the wells, but in the subterranean reservoirs and streams by which the wells are fed. These subterranean channels intercommunicate over wide areas, and are now known in many parts of the world to contain a peculiar assemblage of animals which become accessible to the naturalist in wells and in the streams and lakes of large caves. Further, the scanty "abyssal" fauna of deep lakes is partly made up of species which enter the lakes by subterranean channels, and find a suitable habitat in the deep water. Species of Niphargus, for example, have been dredged in Lough Mask in Ireland and in some of the Swiss lakes.
Several species of blind Crayfishes have been found in caves in North America, the best known being one (Cambarus pellucidus—[Plate XXV].) found in the Mammoth Cave in Kentucky; and blind Prawns belonging to various genera have been discovered in caves in America and Europe.