Physiology of Finfish and Shellfish (2+1)

1.2.3. Water as a Habitat

Life originally evolved in an aqueous habitat, and although life now exists on land,
it still provides a good habitat for a wide variety of life. Water is still an ideal medium for life, and there are many reasons behind this:

1. A large mass of water can surround the organisms, providing a protective shield, which can prevent the cells drying out, as they could on land.
2. It also makes it easier for life by providing support and buoyancy to organisms, so rigid structures, such as woody tissues (plants) and bones (animals) are not required in such quantities.
3. Water's good solvency and mobility favor the supply of nutrients to organisms, and the removal of waste products, due to their concentration gradients and diffusion. It also allows the oxygen required for respiration to be dissolved in the water, from where many organisms can obtain it. For example, fish take in water via their mouths, and it passes over the gills, allowing oxygen to diffuse into the blood capillaries, and carbon dioxide to diffuse into the water, to be expelled.
4. Fertilization is made much easier by a surrounding mass of water, and when offspring are produced, water tends to disperse then, reducing the risk of competition.
5. A large mass also has a fairly constant temperature, due to the high specific heat capacity of water. This is extremely important, as at the surface of oceans, the annual temperature variation is on average 10°C, but at a depth of 20m, this is reduced to only 1 or 2°C. Thus, aquatic organisms have very little need for the temperature control mechanisms which are required by land-based organisms.
6. Water filters out harmful ultra-violet (UV) rays from the sun.