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The Sea as a Biological Environment
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Other Characteristics of the Environment

In addition to the chemical and physical properties of sea water, certain other biologically important characteristics are inherent in the marine environment as a whole. These result from the magnitude of the ocean itself, its great depth, and its expanse.

In considering the ocean in its entirety as an environment, we are at first impressed by the wide ranges of living conditions, the salinities varying from those of dilute estuarian waters to concentrations of 37 ‰ or more in the open sea, temperatures from 30°C to freezing point, light intensities from brilliant sunlight at the surface to absolute and perpetual darkness in the deeper layers, and pressures from a single atmosphere at the surface to about 1000 atmospheres in the greatest oceanic deeps.

Marine Animals
Species Internal medium Δ°C External medium Δ°C
Arenicola marina 1.72 1.7
Arenicola marina 0.77 0.75
Ostrea edulis 2.23 2.11–2.14
Mytilus edulis 2.26 2.11–2.14
Octopus vulgaris 2.16 2.11–2.14
Homarus americanus 1.82 1.80
Cancer pagurus 1.84–1.91 1.91
Hyas aranea 1.83 1.80
Limulus polyphemus 1.90 1.82
Ascidia mentula 2.08 1.98
Teleost fishes
Pleuronectes platessa 0.787 1.9
Conger vulgaris 0.77 2.14
Gadus aeglefinus 0.74 1.92
Fresh-water Animals
Anodonta cygnea 0.09
Unio pectorum 0.15
Limnaea stagnalis 0.22–0.23 0.02–0.03
Telphusa fluviatile 1.17
Daphnia magna 0.20–0.67
Potamobius astacus 0.80
Eriocheir sinenais 1.09
Astacopsis 1.1
Teleost fishes
Salmo fario 0.57
Anguilla anguilla 0.62
(in sea water) 0.73 1.87
Barbus fluviatilis 0.50
Cyprinus carpio 0.50
Anabas tetudineus 0.64
Dipnoi fishes
Epiceratodus fosteri 0.42


Impressive as these ranges may be, nevertheless very uniform conditions do prevail over extensive areas of the environment, and many organisms may, by reason of the monotony of these extensive areas, be very delicately attuned to the prevailing unvarying conditions. Hence, it follows that faunal areas characterized by specific forms can be recognized. On the other hand, a wide range of conditions may be encountered in more restricted areas, especially in coastal regions. These conditions may be due to the physiographic character of the coastline, depth to bottom, topography and nature of the bottom, inflow of land drainage, meteorological conditions, and so forth. Specially adapted and tolerant forms occur here in profusion, for, as will be shown in later chapters, the shallow depths and varying conditions are frequently favorable to abundant production of primary food.

It must not be overlooked that the gradients of salinity, light, and temperature that exist in the sea are favorable to a number of sensitive animals that possess the ability, through swimming or otherwise, to adjust themselves to optimum conditions.

Depth and Light. Inherent in the vertical range or depth of the open-sea habitat are a number of important features of far-reaching biological effect. Of prime importance is the relatively great vertical range of the euphotic zone available for production of floating microscopic plants. But the gradient of light, both as to quantity and quality, resulting from depth of water also allows adjustment of many animals to the optimum condition with respect to this factor and, indeed, is associated with diurnal migrations of many forms to lighter or darker situations.

Pressure. Pressure in itself does not exclude life from the abyssal regions of the sea, for water is but little compressed and equilibrium exists between the inner and outer pressure affecting the body tissues. However, pressure may limit the vertical range of motile forms, although some eurybathic animals apparently are not seriously affected and are known to make daily vertical wanderings of up to 400 m, corresponding to pressure variations up to 40 atmospheres. Harpooned whales are said to “sound” to a depth of 800 m, and the sperm whales must descend normally to great depths, since the large squids upon which they feed inhabit very deep water.

Water Movements. The sea must be viewed as an environment that for the most part is in constant motion with both regular and irregular patterns of flow. The principal biological benefits derived from the circulation are (1) oxygenation of subsurface water, (2) dispersal of wastes resulting from processes of metabolism, (3) dispersal of plant nutrients and other variable elements essential to plant and animal

growth, and (4) dispersal of spores, eggs, larvae, and also many adults. On the whole, the circulation of water is of direct benefit, yet instances may be noted where some adverse conditions result either as incidental or as permanent features. Incidental disturbances may be due to unseasonable shifts in the regular current system, such as give rise to the appearance of “El Niño” off the west coast of South America (p. 704). In this instance, warm water of the Equatorial Countercurrent is carried southward along the coasts of Ecuador and Peru, which are normally bathed by cold currents. The result is a wholesale destruction of animal life along the coast, including many guano birds that depend upon the sea for food. Permanent or semipermanent features of current systems that take a regular toll of life are found where the moving water carries the inhabitants into areas of less favorable living conditions. For example, Gulf Stream inhabitants ultimately perish as they are swept northward into regions where the temperature of the water is lowered by admixture of cold water or by cooling in higher latitudes. Larvae of neritic forms are frequently dispersed to offshore or other locations uninhabitable to the adult animals. Surface currents sometimes strew the shores with defunct bodies of normally oceanic or offshore forms such as the coelenterate Vellela or the pelagic snail Janthina.

Extent of the Marine Environment. That part of the earth which is capable of sustaining life, both plant and animal, is known as the biosphere. The biosphere is subdivided into three principal divisions or habitats known as biocycles. These are the terrestrial, the marine, and the fresh-water biocycles. Each has its characteristic types of ecological features and associations of plants and animals. A few animal species may at times migrate freely from one to another, as is witnessed especially by the salmon or the eel.

The oceans cover some 71 per cent of the earth's surface. Thus, the area of the oceans is about two and one half times the area of the land, but, when considering the space in which life might conceivably exist, account has to be taken of the relative vertical range provided by the two main environments, the terrestrial and the marine. On this basis it is estimated (Hesse, Allee, and Schmidt, 1937) that the marine environment actually provides about three hundred times the inhabitable space provided by the terrestrial and the fresh-water biocycles together; for, whereas the terrestrial environment provides space only in a shallow zone mainly at the immediate surface and to a depth of a few feet at the most, the marine habitat provides livable space for at least some form of life from the surface even to the abyssal depth of several miles. The fresh-water biocycle constitutes only a small fraction of the other two. The aerial portion of the globe is not properly considered a separate biocycle, since entrances into it by birds, insects, and so forth may be considered mainly as temporary journeys.


Owing to the difficulties attendant on the study of the oceans, the marine biocycle is the least known of all.

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The Sea as a Biological Environment
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