The character of the population of the sea is unique largely because of conditions inherent in the vast extent and depth of the marine environment, such as uniformity over vast areas; but the chief single factor which makes marine life in general what it is results from the salt content of the water.
It has already been mentioned that a close osmotic balance exists between sea water and the body fluids of the marine invertebrate organisms, and that this precludes the need of impervious teguments or special expenditure of energy in maintaining the body fluids at the proper concentration (chapter VIII). However, the marine invertebrates are usually poikilosmotic only within rather narrow limits and the concentration of salts is not the same everywhere in the sea, but in numerous instances the gradients are sharp and great fluctuations may occur in restricted coastal regions. Yet all conceivable habitats are occupied by a greater or smaller number of animals suited to the circumstances, even under very extreme fluctuations; this dispersion gives rise to a classification of animals as well as plants into two large groups, depending upon their tolerance to changes in salinity.
1. Stenohaline animals are those which are sensitive to relatively small changes in salinity. The animals of this group are especially characteristic of deep water and of the open sea, where over vast areas the
2. Euryhaline animals are those having a great degree of tolerance to wide ranges of salinity. They are naturally characteristic inhabitants of the coastal regions and of estuaries. However, owing to their tolerance, they are also frequently found far from shore as scattered specimens in the domain of more stenohaline forms. Apparently other factors related to the open sea prevent their successful propagation to a point of numerical dominance. The degree of euryhalinity varies greatly in different species. Intertidal animals, because of their periodic exposure to direct rains or runoff from land, are subject to considerable fluctuations in salinity, but when low tide is combined with unusually heavy rains the result may be lethal to much of the life on exposed flats and in small pools. Much damage may occur to oyster beds during excessively rainy seasons even though the adults may survive; major fluctuations in salinity in the Fal Estuary are held responsible for the failure of one fifth of the ripe females to spawn properly (Orton, 1937). Corals cannot live where rivers cause a persistent and great fall in salinity, although some reduction can be tolerated; the damaging effect of rivers may result in part from sedimentation. Vaughan (1919) reports that corals living in the tropics are tolerant to a 20 per cent reduction in the salinity of the water of theri normal environment. An extreme case of euryhalinity occurs in the harpacticoid copepod Tigriopus, which inhabits small pools usually reached only by the spray of breaking waves and therefore subject to great evaporation with attendant concentration of salts, or at other times to great dilution from rains. The calanoid Pseudodiaptomus euryhalinus is yet another copepod subjected to salinity ranges varying at least from 1.8 to 68.4 ‰. It normally inhabits small coastal lagoons or ponds along the southern California coast and from them it sometimes finds its way into the sea; these ponds are periodically diluted or concentrated (Johnson, 1939a). In these instances the animals have not sought the salinity in which they live by reaction to a gradient. They simply tolerate the salinity and their optimum concentration is not known. Completely tolerant forms are exemplified in the salmon and the eel, each of which spends a part of its life successively in marine and fresh-water environments.
In a study of the distribution and biology of animals we see that within the sea the salinity influences the character or type of animals that will be present in any region rather than the rate of reproduction or the total amount of animal organic material produced. However, the
Pelagic organisms, such as radiolarians that secrete silica, find their optimum living conditions in waters of somewhat lower salinity than that of the open oceans, and this provides a suggestion as to the condition of the Pre-Cambrian ocean when, judging from Pre-Cambrian rocks (Murray, 1895), radiolarians were apparently more abundant than now.
The relation between the salinity of the water and the prevailing faunas has been an invaluable aid to the study of paleogeography, and has led to an interpretation and understanding not only of the geographic arrangement of ancient seas, but also of the degree of salinity that must have prevailed when the fossilized animals inhabited the area. Conclusions can be drawn because many of these fossils are represented in present-day faunas by identical or closely related species in communities whose natural salinity demands can be ascertained. For example, it has been estimated by Ekman (1935) that during the Littorina stage the central and innermost parts of the Baltic Sea were about 5 ‰ more saline than at the present time. This is proved by a study of the then inner limits of distribution of certain molluscs. Littorina litorea, the common shore snail, formerly lived as far north as Sundsvall (62°20′N),
Size and Salinity. Among euryhaline animals, individuals living in reduced salinities frequently have a smaller maximum size than do those of the same species inhabiting more saline waters. This may indicate that the greater dilution is inimical and that the animals, although able to survive and perhaps even to reproduce, are living near the lower limit of their range of euryhalinity, and that only a slightly greater deviation from the optimum would cause death. On the other hand, the reduced size may result directly from failure of sufficient food supply of a type normally dependent upon higher salinity. This is to say that the organisms serving as the animal's food must be adapted to live in the same biotope or in one sufficiently near to enable it to drift in with water movements. Whatever the cause may be, it should be noted here that it is a strange and unexplained fact that with few exceptions marine animals from groups with fresh-water representatives are larger than the fresh-water relatives and usually the size difference is enormous. In planktonic forms this may be due in part to reduced viscosity of fresh water, but for the population as a whole the matter of food supply or rate of metabolism needs to be considered. Respiration is more difficult in fresh than in sea water. To this burden imposed by the environment is also added the necessity for fresh-water forms to expend special energy in overcoming the effects of the hypotonic surrounding medium and in maintaining a proper osmotic equilibrium. The fresh-water animals doubtless have suffered many vicissitudes not experienced in the more stable marine environment, but the effect of this is unknown.
To return again to the ecological significance of varying salinity in the sea, passive offshore plankton organisms, such as eggs and larvae having a specific gravity very near that of the water in which they float, are likely to lose their buoyancy, sink to the bottom, and be destroyed when they are swept by wind and currents into the less dense dilute waters near shore or at river outlets (Johnstone, 1908). In a study of the distribution of haddock eggs Walford (1938) concluded that the eggs tend to remain afloat in water of the same density as that in which they were fertilized. When forced into water of a different density, however, they are able at least within narrow limits to adopt a new specific gravity to conform with the water in which they drift. In the presence of
Much experimentation has been conducted relative to the specific biological effect or function of the various ions in sea water. It will not be possible here to give an adequate review of this important subject, which can be found discussed at length in special articles and texts in physiology and physiological chemistry. The function of salts in sea water is obviously not restricted to the maintenance of a favorable osmotic pressure with the body fluids, for many forms, notably fishes, are not isotonic with sea water in this respect. Waters of inland salt lakes may have a favorable osmotic pressure (resulting from concentration of various salts) for marine organisms, but other essential salts are wanting and the waters will therefore not support marine life. Even such forms as the shore isopod Ligia, which avoids being submerged in the sea, from whence it appears to be in process of migration, is unable to live unless its gills are moistened with sea water in order that it may acquire the essential salts and water useful in its respiration (Barnes, 1932).