The Nature of the Producers.—The marine organisms that can perform the work of photosynthesis are ( ) the planktonic Diatoms, ( 2) the planktonic Peridinians, (3) many unicellular Algae and Flagellates, (4) the great seaweeds (Algae) that live on the bottom in the littoral and shallow-water zones, and (5) "Sym biotic Algae" that are commensal with other organisms. (The or der is roughly that of the importance of the various groups of organisms in marine production.) The "symbiotic Algae" require a few words : They are green, chlorophyllian cells that are included among the tissues of animals belonging to various groups, such as the corals, some worms, some molluscs, and possibly other forms; they are not parasitic in the sense that they tend to injure the tis sues of the animal in which they live they use the nitrogenous residues or excretions of their associate and they contribute carbo hydrate (which they synthesize from carbon dioxide and water) to the tissues of the associate. The relation is that of commensals, or messmates, and it is advantageous to both the organisms. The classificatory place of these green cells is not known but we may regard them as of the nature of unicellular Algae which infect the associate in much the same way that certain bacteria infect the roots of leguminous plants. (See SYMBIOSIS.) The Deep-water and Abyssal Zones.—There is no precise distinction between the shallow-water and the deep-water regions. Outside the 20-fathom contour line the sea bottom deepens very gently towards the continental slope, which is the region of transi tion between the continental shelf and the ocean beds. The con tinental shelf may be regarded as the marginal zone of sea bottom which is bounded by the i,000-fathom contour line, and the ocean bed may be regarded simply as the region outside the continental shelf. Sometimes the gradient from the shelf region is steeper than the gradient on the shelf, but study of charts will show that this is not generally the case. The ocean "abysses" we may take to be the limited regions where the depths are greater than about three miles. It is convenient to make these delimitations but they are very often very far from being precise ones in practice.
Deep Water Faunas.—Now wherever we make investigations we find that the general character of the bottom faunas changes as we pass from the shallow-water into the deep-water regions. Such changes are well-marked ones in the cases of marine species that we know well—for instance, the fishes, so that kinds of fish on the markets can generally be recognized as having come from particu lar limits of depth of sea, and even from quite definite geograph ical regions. The same is the case with those Crustacea, molluscs, and echinoderms, etc. that are fairly well known. Here we refer to the benthic animals—obviously it is all the same to a nektic or planktonic organism, whether it lives in water that is shallow or deep—what affects it is distance from the influence of the land.
This subject could be treated in great detail but the reader must here be referred to works on geographical distribution. He should note that marine benthic faunas vary geographically, every sea area having a more or less different bottom life from every other one. But even in the same geographical region there are differ ences in the fauna that are associated with differences in depth. The Abyssal Fauna.—So we proceed at once to consider the of the sea bottom which is, in general, 2,000 to 3,00o fath oms in depth : there are, of course, places where 3,00o fathoms is greatly exceeded. Now there are some quite remarkable physical conditions associated with sea floors that are deeper than 2,000 fathoms. (I) The pressure of the water is very great, amounting, roughly, to about one ton per square inch for every r,000 fathoms (say, one mile) of depth. (2) There is practically no light, the darkness being that of a very well shielded photographer's dark room. Some ultra violet radiation may penetrate to these depths but it must be negligible from the point of view of the physiology of the animals living in the abysses. (3) The temperature is low, being only two or three degrees above the freezing point of fresh water. (4) There is no production of organic matter, for in the absence of light there can be no plant life. (5) There is nearly absolute uniformity of physical conditions : seasons come and go but there is no change in temperature or in any of the other condi tions that affect life. To us these abyssal conditions would seem
intolerably monotonous.
Abyssal animals are of the same general kinds as the shallow water ones, and it is probable that the deep sea has been populated from the shallow regions. Yet these deep sea animals can always be recognized as having had that origin. The fishes are very char acteristic : big heads, long attenuated bodies, large eyes or else very small ones. All abyssal animals are coloured in monotones, so that the absence of the bright, polychromatic markings of the shal low water species is notable. The fishes eat each other and they and all other abyssal animals must live on the ooze at the ocean bottom—to some extent at least. This ooze is said to be nutritive, since it consists of the deqd bodies of planktonic organisms, the putrefaction of which is retarded by the low temperature at the bottom. There are no plants, of course.
But our knowledge of abyssal faunas is meagre in the extreme. Very few hauls with dredges and trawls have been made in corn parison with the fishing on shallow water regions, fishing instru ments are ineffective to some extent in such great depths. There may be great animals there that the little trawls which we use can not catch and retain. In all speculations as to abyssal life this deep ignorance of the details must always be borne in mind.
Marine Faunas and Floras.—Every large part of the ocean and seas has its characteristic fauna and flora—thus the North sea contains many species that are also found in the English channel and the Irish sea, but there are a few that are plentiful in each of these regions and very scarce in the others. In the case of the fishes (which are well known) this is very noticeable. Still greater differ ences exist between the Norwegian seas and the Mediterranean, while if we consider such a region as the Gulf of Siam we should find few species indeed that were common to this area and any part of the Atlantic ocean. This subject is one of enormous detail and the student must consult works on the geographical distribu tion of marine plants and animals. The rule is that differences both in the kinds, and the abundance of marine organisms go along with wide geographical differences, and this is not due entirely to differ ences in physical conditions, for many parts of the Atlantic are similar in these latter respects to many parts of the Pacific, yet the faunas and floras are not identical, or even like each other. In the process of evolution of species there has been segregation in all the great marine regions so that diverse faunas and floras have devel oped in each. While this is so there are still a few truly cosmopoli tan species—some of the great sharks and tunnies among the fishes, and some of the whales. These animals may roam over most of the world ocean. Even some of the planktonic animals—a few of the copepods, for instance, may be widely distributed.
In general, the abundance of life is greater in the temperate and polar seas than in the tropical ones : that is, far more individual animals and plants are to be seen in a unit region of sea, or sea bot tom in the colder, than in the warmer seas. There is, however, a greater diversity of life in the tropical seas, that is, more species may be seen there than in the higher latitudes. The great sea fish eries of the world (round the British Isles, off the coast of Norway, the Faeroes and Iceland, off Newfoundland, off the coast of British Columbia, and in the Japanese seas) are in temperate or sub-polar regions. The great seal and whale fisheries are—or were—in polar waters, and the wealth of life in the Antarctic, in the shape of the penguin rookeries, is well known. The reason for this distinction between the colder and warmer marine faunas and floras has been ascribed to the greater abundance and more vigorous activity of certain bacteria in the warm seas. These organisms destroy the mineral nitrogenous substances that are essential for plant life. If there is less vegetable plankton there must be less crustacean and molluscan plankton and so an abundant source of food for fishes and invertebrates (and even whales) that exists in cold water becomes reduced in the warmer seas.