It appears that as you go further and further from shore, and into deeper and deeper water, the fewer animals and plants are obtained, and that very few species indeed which live along shore are to be found also at a depth greater than about 100 fathoms.
LANDING THE BEAM-TRAWL ON DECK.
Almost all animals, moreover, have a limited distribution in the sea, as is the case among those on land, though we cannot always, or perhaps often, say why the limits we find should exist; one sort of crab, or mollusk, or polyp, appearing here and another different one exclusively there, when the conditions seem to us very similar, and no barrier is perceptible. It is not easy to explain why a certain sort of cowry, for example, should be found only along a particular strip of coast, when nothing that we can see prevents its extending its range much further. It is believed that the temperature of the water is the chief fact which sets these invisible boundaries to the wanderings of animals living near the surface, only a few of which are very wide-spread in their distribution. The direction and character of the ocean currents have much to do with the geographic distribution of oceanic life, as has been mentioned in Chapter II ([page 25]).
Now in deep-sea life the case is different. Here temperature cannot be of so much account, since only a short distance down, the water becomes almost as cold as ice, and preserves this uniform chill all around the globe. The life found at a great depth, too, is very wide-spread, instead of restricted in its range, often occurring in two or more ocean basins; but here the restriction is an up-and-down one, rather than horizontal, and the secret is found in the word pressure. Few animals are able to live both in the shallows and under the enormous weight of sea water three or four miles deep.
A TYPICAL JELLYFISH.
This species (Pelagia cyanella) is a characteristic
oceanic discophorous medusa, common along the
Atlantic coast of the United States; it is
semi-transparent and lustrous pink.
This has recently (1897) been summed up very clearly by Prof. Arthur P. Crouch, in an article in “The Nineteenth Century,” from which it will be worth while to quote a paragraph or two:
The conditions under which they [that is, deep-sea animals] have to live in the abysmal areas seem very unfavorable to animal existence. The temperature at the bottom of the ocean is nearly down to freezing-point, and sometimes actually below it. There is a total absence of light, as far as sunlight is concerned, and there is an enormous pressure, reckoned at about one ton to the square inch in every 1000 fathoms, which is 160 times greater than that of the atmosphere we live in. At 2500 fathoms the pressure is thirty times more powerful than the steam pressure of a locomotive when drawing a train.[7] As late as 1880 a leading zoölogist explained the existence of deep-sea animals at such depths by assuming that their bodies were composed of solids and liquids of great density, and contained no air. This, however, is not the case with deep-sea fish, which are provided with air-inflated swimming-bladders. If one of these fish, in full chase after its prey, happens to ascend beyond a certain level, its bladder becomes distended with the decreased pressure, and carries it, in spite of all its efforts, still higher in its course. In fact, members of this unfortunate class are liable to become victims to the unusual accident of falling upwards, and no doubt meet with a violent death soon after leaving their accustomed level, and long before their bodies reach the surface....