The Story of Life in the Seas

CHAPTER I.. Oceanography.

OCEANOGRAPHY.

One of the most important facts that has been established by modern investigations of the Sea is that there is no region in its vast extent that is entirely devoid of animal life. The surface waters in the Equatorial calms and the ice-cold waters between the ice-bergs of the Arctic regions are densely populated by animals, large and small; the heavy and heated waters of the Mediterranean and Red Seas, and the cold and comparatively fresh waters of the Norwegian fjords, the shallow waters of the coasts and the greatest depths of the ocean-beds all present us with their characteristic forms of living creatures. There is no Azoic region known to us. Wherever we use the trawl or dredge we may expect to find some representatives of the various classes of marine animals. But the seas exhibit so many varying conditions that, as we might have expected, the animals that characterise one region are absent from another; and while, in some places animal life is abundant, in others it is very scarce; just as on land we find the grass-lands and forests teeming with life, and the great deserts and mountain tops inhabited only by a few solitary Lizards, Birds or Insects.

In order that we may fully understand, then, the nature of the problems concerning the distribution of animals through the seas, it is of importance to consider first the conditions under which they must live in the different parts of the ocean. A knowledge of geography is clearly necessary for those who study the distribution of terrestrial and aërial animals, and equally necessary is it for those who wish to learn something about the distribution of the aquatic animals, to consider first of all the rudimentary principles of hydrography. The principal Sea areas of our globe may be roughly divided into two groups: namely, the great oceans--the Atlantic, Pacific and Indian Oceans--and the Inland seas, which are partly enclosed by land, such as the German Ocean, the Mediterranean Sea, and the Red Sea. Taking the areas of the great oceans and the seas together, we find that no less than 141 millions of square miles, or nearly three-quarters of the surface of the globe, are covered with water. Moreover, these great areas are continuous, so that it would be possible for an animal, other conditions being favourable, to pass from any one sea, such as the Black Sea, to any other, such as the Hudson’s Bay, without leaving the water,--an important fact in the consideration of the distribution of marine forms of life.

The depth of the sea varies very considerably in the different parts of the world. In the inland seas the water is comparatively shallow, but in the great oceans it is very deep. In the middle of the North Sea, for example, we should not expect to find a depth much exceeding 250 fathoms, but in the Atlantic or Pacific Oceans we have to pay out more than 2000 fathoms of the sounding-line before the bottom is reached. In some parts of the ocean-basins a few very deep holes or furrows may be found in which the depth exceeds 4000 fathoms, or 24,000 feet. One of these deep holes occurs in the Atlantic Ocean, a little to the North of the Virgin Islands in the West Indies, and there is another in the Pacific Ocean close to the coast of Japan; but the greatest depth that has yet been found is one recently discovered by H.M.S. “Penguin” off the coast of New Zealand of over 5000 fathoms. Apart, however, from the fact that these very great depths are only of local occurrence, the areas of deep water--that is, of more than 2000 fathoms--are so much greater than the areas of shallow water, that when we make a calculation of the average depth of the sea we find it is no less than 2100 fathoms, or 12, 600 feet.

The temperature of the sea is another feature which undoubtedly influences very greatly the character of its Fauna. The main source of the heat of the sea is the sun--for the heat derived from submarine volcanoes must be comparatively so small that we may omit it from consideration. Consequently we find that in the Equatorial regions the surface waters of the ocean are warmer than they are in the Temperate regions. These, again, are warmer than in the Arctic circles. But water is well-known to be a bad conductor of heat, and therefore the direct influence of the sun affects only the most superficial layers. In the Equatorial region of the Pacific Ocean, for example, the surface temperature is sometimes as high as 80° Fahr., at 100 fathoms from the surface it is only 60°, at 400 fathoms only 45°, and at 1000 fathoms only a few degrees above freezing point. On the land the temperature falls as we pass from the coast to the high plateaux and mountains, and we find snow-capped mountains in Central Africa just as in Switzerland or Norway. In the sea the temperature falls as the thermometer is sent deeper from the surface. Just as on the land the snow line of the mountains is reached at high altitudes in the Tropics, at lower altitudes in the Temperate regions, and in the Arctic circle at the level of the sea, so in the sea the cold water that is found 500 fathoms below the surface in the Tropics, reaches a higher level in the Temperate regions, and is at the surface in the Arctic circle.

There is however one important point of difference between the distribution of these low temperatures on the land and in the oceans, in that they are broken in the former, and continuous in the latter. If we were to imagine an aquatic animal that could only live in temperatures below 35° Fahr., it would be able to travel below the surface from one pole to the other, or from one ocean to another; but it would be impossible for a terrestrial animal, exhibiting the same peculiarity, to leave the Arctic circle or the Alpine region without traversing lands where the temperature is higher than that which is necessary for its existence. It might be supposed from what has just been said that the temperature of the water at the bottom is constant for the same number of fathoms of depth. This is not, however, the case. The temperature of the sea-bottom of the great ocean-beds is approximately the same, varying from 28° F. in the Atlantic to 35° F. in the Pacific; but in places where main basins occur, surrounded on all sides by shallower ridges, the temperature of the bottom of the basin is the same as that of the lowest ridge. For instance, the temperature of the bottom of the Sulu Sea, lying between Borneo and the Philippines, is 40° F. at a depth of over 2000 fathoms. Again, the temperature of the Red Sea is as high as 70° F., although depths of 1200 fathoms occur in its central portions; and this is the same as the temperature at the Straits of Babel Mandeb, which are 200 fathoms deep, and form the only outlet to the open ocean. These facts probably cause considerable modification in the character of the animals inhabiting such enclosed basins, but further investigations are needed before we can arrive at any very definite conclusions in the matter.

Another important element that must be taken into consideration in studying the environment of marine animals, is the quantity and character of the salts held in solution by the sea-water. In the first place we must remember that the sea-water normally contains a far greater percentage of salts in solution than the water of rivers and lakes, and this causes it to be very much heavier. If a tumbler be half filled with sea-water, upon which some fresh water is slowly and carefully poured, there will be for some time very little mixture of the two fluids, the heavier sea-water remaining at the bottom, and the lighter fresh water at the surface. Now the density of the sea-water, or in other words the amount of salts in solution, is not the same over the whole world, and the differences that may be observed in this respect are due, in most cases, to the simple physical principle just enunciated. If we could imagine a river pouring its waters into a perfectly calm, tideless sea, we should be able to trace the fresh river water far away from the coast, for it would simply float on the heavier sea-water without mixing with it to any appreciable extent. In most cases, however, the tidal-waves, rushing up and down the river estuaries, stir up the fresh and salt water together, and cause a very considerable mixture, so that the water becomes either distinctly salt or brackish. Where very large quantities of fresh water are poured into the ocean, as, for example, at the mouth of the Amazon or the Mississippi, the surface water remains so fresh that the salt taste can hardly be appreciated at a distance of some miles from the coast. This fact sufficiently indicates the influence of great rivers upon the density or saltness of the sea-water in their neighbourhood, and the reader will be prepared for the statement that many inland seas, such as the Black Sea, are appreciably less salt than the great oceans.

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