Considerable difficulty exists in determining from what depths fishes are dredged. Although many species display various structural peculiarities, such as a huge head, or an attenuated ribbon-like body, while special phosphorescent organs are very generally present, yet deep-sea fishes as a whole do not represent an ordinal or sectional group by themselves, but are drawn from a number of families, certain members of which have adapted themselves to an abyssal existence. A preponderance of representatives of the families Macruridae, Ophidiidae and Scorpclidae is, however, noticeable.
Whether light or temperature is the prepotent agency in regulating the limitations of the deep-sea fauna, has long been a debated question. It may be noted that reef-building corals, which require an average temperature of from 70° to 75° F. and one which never falls below 68°, are never found below a depth of 20 fathoms (120 ft.). Nevertheless, there are several areas where a temperature of from 70° to 77° obtains to depths of between 80 and 100 fathoms. It is further remarkable that well-characterized deep-sea faunas are locally met with in comparatively shallow waters, one such area occurring in the European Atlantic and a second in the Mediterranean, where they live within the 100-fathom zone. Light, which was formerly supposed not to penetrate to a greater depth than the 40 to 50 fathom-line, has also been regarded as the chief agent controlling vertical distribution. It appears, however, as Prof. Heilprin has remarked, "more than likely that not a single cause, but a combination of causes, is operative in bringing about the general results. That the deep-sea fauna is a fauna of darkness must be admitted; but this is so from the nature of the case rather than a matter of choice resting with the animals composing it."
After referring to the fact of the dissimilarity between the faunas of the two poles, Dr A. E. Ortmann, in a paper on the origin of the deep-sea fauna,[1] observes that we have reason to believe that each of these faunas had a separate origin, "the north-polar fauna being a derivate of the old Mesozoic Mediterranean, the south-polar fauna of the old Pacific fauna. The first developed along the shores of the northern continents, while the second had its original home on the shores of the Antarctic continent. We know that there is a strange element among the littoral fauna of the southern extremities of the continents, differing entirely from the arctic fauna, and we cannot but think that this is a remnant of the old Tertiary antarctic fauna. The above considerations give us a threefold origin of the present deep-sea fauna: (1) An ancient Mesozoic (or pre-Tertiary) constituent, derived from a transformed part of the old warm-water fauna of the deep sea, adapted to the changed climatic conditions. It is clearly autochthonous. (2) A more modern, immigrant, Tertiary constituent, which came from the north-polar littoral waters, and immigrated into the deep sea together with the cool water (or after it had cooled). This element goes back to an old pre-Tertiary stock that lived in the warm littoral waters of the old Tethys (Mediterranean Sea), but as a cool-water fauna, it is not older than Tertiary. (3) Another Tertiary element, corresponding to the second one, but belonging to the south pole, which is finally to be traced back to the warm waters of the old Pacific Ocean of pre-Tertiary times."
The surface, or pelagic, fauna contains some of the smallest and actually the largest of all living animals, for among its members are included a host of so-called animalcules on the one hand and the whales on the other.The surface fauna. The essential characteristic of pelagic animals is that they pass the whole of their existence swimming at or near the surface of the ocean, and only by accident touch the shores or the bottom. Much information with regard to the smaller pelagic creatures will be found in the article Plankton. Among the groups included in the pelagic fauna may be mentioned the radiolarian animalcules, together with certain representatives of the Foraminifera; the siphon-bearing jelly-fishes, such as Physalia (Portuguese man-of-war), Velella, Porpita, &c.; all the pteropod molluscs, such as Clio, Clione and Cavolinia (Hyalaea), together with less aberrant gastropods, like Janthina (violet-snail), Atlanta and Glaucus; a few cephalopodous molluscs, such as the paper-nautilus (Argonauta) and Spirula; and a number of social ascidians, like Salpa and Pyrosoma. Crustaceans belonging to the entomostracous (shelled) and schizopod divisions abound; and there is a group of insects (Halobates), belonging to the order Hemiptera, whose home is on the ocean-surface at, practically, any distance from land. Fishes form no inconsiderable portion of the pelagic fauna, among these being the true flying-fishes, or flying-herrings (Exocoetus), herrings, mackerel, tunny, flying-gurnards (Dactyloptera), sword-fishes (Histtophorus), sea-horses (Hippocampus), pipe-fishes (Fistularia) and many of the sharks. With the exception of the comparatively few fluviatile species, the whole of the cetacean mammals—that is to say, whales, grampuses, porpoises, dolphins, &c.—claim a place among the surface-fauna of the ocean. Whether the sea-cows (Sirenia) should likewise be included is doubtful, as they hold a somewhat intermediate position in regard to habits between cetaceans and seals. While they agree with the former in never (or very rarely) landing and in bringing forth their young at sea, they come inshore to feed. Turtles certainly cannot be considered truly pelagic, since they come ashore to lay their eggs.
A large proportion of the smaller pelagic animals are more or less completely transparent, while others, such as the violet-snail, have developed an azure tint which renders them as inconspicuous as possible in the waste of waters. In the case of the larger animals, like mackerel and the finner-whales, the same result is attained by the under surface of the body being silvery white (thus rendering them invisible when looked at from below against the sky), and the upper surface olive or blackish green, sometimes, as in the mackerel, mottled to harmonize with the ripple of the waves.
The distribution of whales and dolphins has been taken by P. L. and W. L. Sclater to some extent as a basis in dividing the ocean into zoological regions. Since, however, such regions were mainly defined on the distributional evidence afforded by seals and sea-cows, they are best considered in connexion with the shallow-water fauna.
The shallow-water, or littoral, fauna includes all marine animals which belong neither to the deep-sea nor to the surface fauna, and is the most important of all three.The shallow-water fauna. In addition to the great bulk of marine invertebrates, the littoral fauna may be taken to include the reef-building corals (whose distributional limitations under the influence of temperature-control have been already mentioned) and likewise seals and sea-cows among mammals, and turtles among reptiles.
"The fauna of the coast," observes Prof. H. N. Moseley, "has not only given origin to the terrestrial and fresh-water faunas, it has through all time, since life originated, given additions to the pelagic fauna in return for having received from it its starting-point. It has also received some of these pelagic forms back again to assume a fresh littoral existence. The terrestrial fauna has returned some forms to the shores, such as certain shore-birds, seals and the polar bear; and some of these, such as the whales and a small oceanic insect (Halobates), have returned thence to pelagic life.
"The deep-sea fauna has probably been formed almost entirely from the littoral, not in most remote antiquity, but only after food, derived from the debris of the littoral and terrestrial faunas and floras, became abundant in deep water. It was in the littoral region that all the primary branches of the zoological family-tree were formed; all terrestrial and deep-sea forms have passed through a littoral phase, and amongst the representatives of the littoral fauna the recapitulative history, in the form of series of larval conditions, is most completely retained."
From the distribution of certain groups of animals, it has been Marine zoological regions.attempted (as stated) to divide the ocean into a number of zoological provinces, or regions. Among the more important of such schemes, the following may be mentioned.
The reef-building corals, whose limitations are defined by conditions of temperature and depth, are necessarily restricted to certain seas and coasts within or near the tropics. "They abound," wrote Dr A. R. Wallace in the ninth edition of the present work, "in and near the West Indies, on the east coast of Africa, in the Coral regions.Indian Ocean, in the Malay and Pacific archipelagoes, and on the coast of Australia; while they are absent from the whole of the west coasts of South America and of Africa, from the Indian peninsula, and from much of the east coast of South America. The coral-reefs of the Bermudas, in 33° N. lat., are the farthest from the equator; in the Red Sea they reach 30° N., in the Pacific 27° N., while they nowhere extend to more than 29° S. of the equator. . . . The coral regions are therefore somewhat peculiar, and differ considerably from those which best exhibit the distribution of other marine animals. The regions adopted by Prof. J. D. Dana are three—the first comprising the Red Sea and Indian Ocean; the second, the whole of the Pacific islands and the adjacent coasts of Australia; and the third the West Indies. This last region is the most isolated in position; and it is not surprising that it should contain the largest proportion of peculiar forms. The corals of the Central Pacific are also very peculiar, as are those of the Red Sea and Indian Ocean."
Prof. J. D. Dana[2] proposed to divide the oceans into three main areas according to the distribution of Crustacea. These areas are respectively termed the Occidental, the Africo-European and the Oriental.Crustacean regions. The first comprises both coasts of America; the second, the western shores of the Atlantic, both African and European; while the third comprehends the vast area from the east coast of Africa to the Central Pacific. Each of these regions is subdivided into climatal and local provinces but the primary divisions can alone be mentioned here. The facts adduced in support of this scheme of distribution are interesting. At the date of Prof. Dana's memoir 47 genera were known to be exclusively American, 15 being common to both the east and west coasts; but as 26 genera were said to be confined to the west and 6 to the east coast, these two provinces are really distinct, even if they do not form primary regions. The Africo-European region had 19 peculiar genera, and only 8 in common with the
