Physical and Chemical Characteristics of the Marine Environment

Water is essential to the maintenance of all life. It constitutes 80 per cent or more by weight of active protoplasm. It is the most efficient of all solvents and carries in solution the necessary gases, oxygen and carbon dioxide, as well as the mineral substances necessary to the growth of plants and animals, and it is itself one of the essential raw materials in the manufacture of foods by plants.

Organisms living in the terrestrial environment have devised means, such as impervious integuments, to conserve water, and the land plants have roots and special vascular systems for transport of water to all growing parts. In the marine environment there is freedom from dessication, except at high-tide levels, and therefore no highly specialized means are provided for conservation of water or for its transport in plants.

Also of biological importance are the high heat capacity of water and its high latent heat of evaporation, both of which obviate the danger that might result from rapid change of temperature in the environmental medium. Owing to the high degree of transparency of water it is possible for the sea to sustain plant life throughout a relatively deep layer, and in animals the development of organs of vision and of orientation has progressed to a marked degree.

Sea water is a buffered solution; that is, changes from acid to alkaline condition, or vice versa, are resisted (p. 195). This property is of vital importance to the marine organisms, mainly for two reasons: (1) an abundant supply of carbon can be available in the form of carbon dioxide for the use of plants in the synthesis of carbohydrates without disturbance to the animal life that may be sensitive to small changes in pH, and (2) in the slightly alkaline habitat the many organisms that construct shells of calcium carbonate (or other calcium salts) can carry on this function much more efficiently than in a neutral solution.

The support offered to the bodies of marine organisms by the specific gravity of the surrounding medium obviates the need of special supporting skeletal structure in many forms. Striking examples of these are the jelly fishes, unarmored molluscs, unarmored dinoflagellates, and even the large marine mammals with their heavy skeletons, which could not survive in their present bulky state except in an aquatic habitat. The hard shells of crabs, clams, snails, and so on, doubtless serve as support, especially in some burrowing and intertidal forms, but these hard parts may be looked upon also as protective and as a framework for attachment of muscles used in digging, creeping, or swimming.

Sea Water and the Body Fluids. Sea water is a most appropriate environment for living cells, since it contains all of the chemical elements essential to the growth and maintenance of plant and animal protoplasm. It has been shown that sea water is a solution of a large number of salts, and it is important here to consider how it is related as an external fluid medium to the “internal medium”—namely, the body fluids (blood, coelomic fluid, and so on) of the organisms. The ratios of the major salts to each other, and usually their total concentration also, are strikingly similar in sea water and in the body fluids of marine invertebrates. The similarity of composition is not confined to marine animals, however, but is also in evidence in modified form in both terrestrial and fresh-water animals, including the lower and higher vertebrates, as is shown in table 55, which is from data compiled by Pantin (1931) and expanded by Dakin (1935).

Osmotic Relationships.It is well known that when solutions of different osmotic pressure are separated by a semipermeable membrane that allows the passage of water but not of the solutes, there is a movement of the water through the membrane into the more concentrated solution. The cell membranes of organisms are just such semipermeable membranes through which a movement of fluids occurs inward or outward, depending upon whether the osmotic pressure of the external medium is less (hypotonic) or greater (hypertonic) than the internal medium. The internal and external media are isotonic when they are of equal osmotic pressure.

The osmotic pressure of a solution can be computed from the freezing-point depression (p. 67). This computation is possible because the salts that increase the osmotic pressure of a solution also depress its freezing point. The freezing-point depression below 0°C has been designated by Δϑ, (p. 67), but will here be abbreviated to Δ. Sea water having a salinity of 35.00 ‰ freezes at − 1.91°, owing to depression by the substances in solution. In other words, the value for Δ is 1.91°. Similarly, we obtain a Δ of 0.56 for human blood with a freezing point of −0.56°C.

On the basis of Δ values, the osmotic relations of the body fluids of marine and fresh-water animals to their external environmental medium are compared in table 56, from data compiled by Dakin (1935), to whose review the reader is directed for much greater detail and historical treatment.

From the few examples in the table it is evident that the body fluids of marine invertebrates are isotonic or nearly so with their fluid environment, whereas in the fresh-water forms the body fluids are hypertonic to the dilute external medium. For this reason the marine environment in its osmotic relations fails to exact of its inhabitants as great an expenditure of energy in maintaining the proper concentration of body fluids as does the fresh-water environment. The exact mechanism whereby the fresh-water animals are independent of the external medium and are able to maintain a homoiosmotic condition (that is, steady value for Δ) in the presence of the hypotonic water is not known (see Δ for the eel Anguilla anguilla in fresh and salt water, table 56). Their existence under these conditions, however, requires a constant expenditure of energy in eliminating, through the kidneys and other excretory organs, the excess water taken in by osmosis. Marine invertebrates are poikilosmotic (Δ changing with that of the external medium) only within rather narrow limits (Dakin, 1935); hence, they, too, must have some regulating mechanism. Except in estuarine conditions, however, the range of salinity in most parts of the sea is perhaps within the limits of poikilosmoticity of the invertebrates living there. For example, the lugworm, Arenicola marina, in Helgoland waters with a Δ1.72 has an internal medium Δ1.7, but in the Baltic Sea with a water of Δ0.77 the same species has a Δ value of 0.75 for the internal medium.

It should be mentioned here that the teleost (bony) fishes in marine waters are definitely hypotonic and, therefore, in order to keep their body fluids down to the required osmotic pressure for the species, they secrete chloride through the “chloride cells” of the gills (Keys, 1933). This function is a regulation toward a low osmotic pressure of the blood, as opposed to regulation toward a high one as performed by the kidneys of animals in fresh-water environments. That this group of aquatic animals has achieved a marked degree of independence of the osmotic pressure of the external medium is evidenced especially by such forms as the salmon and eel, both of which, though practically homoiosmotic, spend their lives partly in hypotonic and partly in hypertonic environments, The elasmobranchs—namely, the sharks and rays—are isotonic with sea water, but in these the high osmotic pressure of the blood is due not only to the presence of such salts as occur in sea water, but also to high urea content. For further discussion of salinity as an environmental factor, see also p. 839.