Mechanism of osmoregulation is very important for all groups of animals whether inhibting land or water. Unlike plant cells, animal cells when placed in hypotonic solution burst due to the continuous absorption of water. On the contrary they would shrink and die if constantly placed in hypertonic solution. Normally uptake and loss of water are in balance for proper survival of cell.
(1) Osmoregulation in terrestrial animals:
Terrestrial animals are more likely to loose water by evaporation through their permeable surfaces exposed to amsophere. Among various animal groups only Arthropods and vertebrates became the most successful land dwellers. They have developed number of strategies to maintain osmoregulation of their body fluid.
(a) Water proof external coverings: To prevent water loss through external surfaces, vertebrates like reptiles, birds and mammals have water proof keratinized epidermis. Similarly the insects have developed external water proof layer called cuticle.
(b) Storing and excretion of solid wastes: Reptiles, birds and insects excrete uric acid as nitrogenous waste, which is insoluble in water. It is stored temporarily in cloaca where water is reabsorbed from it before its removal from the body in semi fluid form.
(c) Use of metabolic water: Some mammals like camel, kangaroo, rate etc make use of water production during the breakdown of body fats.
(d) Storing the wastes: Mammals do retain some urea in their kidneys where it helps in re-absorption of wastes.
(2) Osmoregulation in freshwater animals:
(a) Osmoregulation by contractile vacuole: Fresh water protests like Amoeba, Paranecium etc bear one or more membrane bound tiny sac called contractile vacuole. Since such freshwater protests have higher osmotic pressure than their surrounding water, so the water constantly comes in by osmosis. If it is not regulated, the organism would burst. Therefore the excess water is stored in contractile vacuole. After it is completely filled, water is discharged out of the cell through a pore into the surrounding water.
(b) Osmoregulation by producing dilute urine: Freshwater animals like fishes have a hypertonic body fluids than the surrounding water. Thus they remove excess water by passing large quantity of very dilute urine.
During the excretion process, they lose some essential ions as well. This is over come by actively absorbing selected ions from outside.
Osmoregulation in marine animals:
Marine bony fishes have hypotonic internal environment; so they are liable to lose water. Thus in order to conserve water, they constantly drink water. The salts taken in along with water are actively excreted by special excretory cells in the gills. Moreover, the filtration rate in their kidneys in very low, so small quantity of concentrated urine in excreted.
Unlike marine bony fishes, sharks and rays maintain relatively slightly hypertonic osmotic pressure of body fluids than their surroundings by storing high concentration of urea in their bodies. Thus they do not have problem of water loss. Excess salts are removed by special glands in their rectum. Marine invertebrates as well as hag fishes have isotonic body fluids, so they do not have osmoregulation Mechanism. Such animals are termed as osmoconformers.