Seawater is evidently an excellent medium for an abundance and variety of life. We know from geological findings that the seas have been well populated since the earliest time for which we have fossil records. It is widely thought that life originated in the sea, most likely in pools on the seashore where many different solutions of varying composition and concentration could accumulate in various conditions of temperature and illumination. Alternatively, some scientists are now postulating that life could have begun around deep-sea hydrothermal vents (see Section 6.4.4). The seas have now been populated for so long that it has been possible for marine life to evolve in great diversity.
Probably all natural elements are present in solution in the sea, and all the constituents needed for the formation of protoplasm are present in forms and concentrations suitable for direct utilization by plants (see Section 4.3.3). The transparency of the water and its high content of bicarbonates and other forms of carbon dioxide (see Section 4.3.2) provide an environment in the upper layers of the sea in which plants can form organic materials by photosynthesis, and in this way great quantities of food become available for the animal population. However, light penetrates only a short distance into the water. Marine plants must therefore be able to float close to the surface or, if attached to the bottom, are limited to shallow depths. Because water is relatively opaque to ultraviolet light, this property gives protection against the harmful effects of this part of the spectrum.
In an aquatic environment very simple and fragile forms of life can exist because the water affords them support, flotation, transport and protection, thereby permitting very simple reproductive processes, and minimizing the need for structural complications, such as locomotor organs, skeletons or protective coverings. In aquatic organisms there are several advantages in small size. For example, a large surface-to-volume ratio retards sinking, facilitates absorption of solutes at great dilution and favours light absorption. Also, small organisms can usually reproduce rapidly to take advantage of favourable conditions.
We shall discuss later (in Chapter 4) how organisms in the sea may be influenced by environmental conditions, notably the temperature, composition, specific gravity, pressure, illumination and movements of the water. However, we have already said enough about the circulation of the oceans to indicate that the water is kept well mixed, and this ensures a generally homogeneous environment. The composition of seawater (see Section 4.3) remains almost uniform throughout its extent despite considerable differences in the rates of evaporation and addition of fresh water in different localities. The composition of present-day seawater may differ in some respects from that of the remote past; but if so, marine organisms have been able to evolve and adjust to changing conditions. The body fluids of all the major groups of marine invertebrates are virtually isosmotic with seawater, and of a generally similar composition (see Section 4.3.1).
The high specific heat of water and the great volume of the oceans provides a huge thermal capacity, and the thorough mixing of the water ensures a fairly even distribution of heat. Consequently the temperature range of the oceans is relatively restricted and temperature changes occur slowly (see Section 4.2.1). The sinking of surface water due to cooling at high latitudes carries well-oxygenated water to the bottom, and thereby makes animal life possible at all depths. Despite biological activity, the buffer properties of the water (see Section 4.3.2, Carbon dioxide) are sufficient to keep the pH stable. It is therefore a notable feature of the marine environment that conditions are remarkably constant over great areas, and many marine plants and animals have correspondingly wide distributions. Such changes as do occur take place slowly, giving time for some organisms to acclimatize. However, stable conditions permit the evolution of a diversity of forms whose environmental requirements are very precise and whose range is limited by quite slight changes in their surroundings. It must therefore be evident that this combination of properties offers propitious conditions for a great variety of marine organisms of many types and sizes.
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