Most marine organisms have body fluids that are nearly the same concentration as the sea water in which they live. So the rate at which water enters and leaves the body cells is the same. However, the body fluids of freshwater organisms are at a much higher concentration than the surrounding water. So more water enters the body cells than leaves them. Successful freshwater organisms have Osmoregulatory structures that get rid of the excess water that enters the body cells. The contractile vacuole in amoeba is such a structure. The adaptation here is mainly functional.
Bony fishes are the most successful aquatic organisms. The body fluid of the marine bony fish is less concentrated than its surrounding water, while the body fluid of the freshwater bony fish is more concentrated.
Unlike most aquatic organisms, terrestrial organisms are faced with a great problem of water loss from their bodies. Plants overcome this by having waterproof cuticles covering their external surfaces. They also have well developed roots for the continual absorption of water from the soil. Terrestrial animals have developed.
• exoskeletons (invertebrates) which envelop the whole body , and coverings such as scales, feathers and hair (vertebrates) such as scales, feathers and hair (vertebrates) which reduce water loss;
• gaseous exchange surfaces enclosed within the body such as lungs (vertebrates) and tracheal systems (insect) so that water loss is greatly reduced; and
• excretory structures like the malpighian tubules and kidneys which are efficient at reabsorbing water and salt.
Thus, to conserve water, terrestrial organisms had to adapt both structurally and functionally.
Note in aquatic organisms where there is no need to conserve water, gaseous exchange occurs through the whole body surface or through special structures called gills.
