Ecological adaptation in animals to aquatic environment
In the aquatic environment animals are confronted with the problem of excess water. Freshwater and marine animals try to maintain water balance in quite different ways. On account of the difference in the osmotic pressure between body fluids of aquatic organisms and their surrounding fluids aquatic organisms have developed osmoregulatory mechanism to deal with this problem. The salt content to the body fluids in freshwater animals is relatively high as compared to the surrounding water medium. Hence, the water tends to diffuse continuously into the body. Extra water from the body needs to be removed frequently. Most aquatic animals (e.g. protozoans and fishes) excrete the extra amount of water from the body by osmoregulation. Protozoa employ contractile vacuoles and other multicellular invertebrates and chordates use excretory organs such as nephridia and kidney (Figure shown below).
In marine animals the situation is just the opposite. The concentration of salts in the body fluids is low as compared to the surrounding medium (i.e. hypotonic). Under these conditions the body tends to loose water. But through metabolic osmoregulation water is retained in the body and salt is excreted. Animals vary widely in their salinity tolerance. Organisms which have a narrow tolerance and cannot withstand salinity fluctuation are called stemhaline. Organisms which can survive under a wide range of salinity conditions are termed euryhaliie. Usually animals inhabiting cooler fresh water and marine environments tend to have large body size except in a few cases such as diatoms and sea urchins which have relatively larger sizes in warmer waters.