Osmoregulation in Different Environments

 

Marine

Most marine invertebrates are osmoconformers. Among the vertebrates, hagfishes are isotonic with the surrounding sea's water. Most cartilaginous fishes maintain lower internal salt concentration than that of sea's water. Their kidneys for osmoregulation excrete salts through gills and also possess salt excreting organs such as rectal glands. These employ an active transport mechanism to remove salt against osmotic gradients. Some fishes have relatively low salts in body fluids but have rendered these hypertonic to that of sea's water by retaining urea in adequate concentration. Because urea in high concentration is damaging, these fishes retain another chemical trimethylamine oxide for protection against urea. Bony fishes, the descendents of fresh water ancestors but later became marine constantly lose water from their hypotonic body fluids to hypertonic environments. These fishes have adapted themselves to drink large amounts of sea water and excrete concentrated urine resulting in maximum salt excretion and minimum water loss.

Fresh Water

Freshwater animals are constantly facing the osmotic flooding of body fluids and loss of salts. Fresh water protozoa, Amoeba and Paramecium pump out excess water by structures contractile vacuoles. Many fresh water animals including fishes remove excess water by producing large volumes of very dilute urine. The loss of salts is compensated by preference of salt containing food and by active uptake of salts by gills and skin.

Terrestrial

The evaporative loss of water leading to dehydration is the major problem for terrestrial life. Arthropods and vertebrates have successfully adapted to terrestrial mode of life. Terrestrial animals are covered by body surface, which prevents loss of water as the waxy exoskeletons of insects and multi-layered dead, keratinized skin cells of most terrestrial vertebrates. Drinking and eating moist foods compensate for the loss of water. These animals also have metabolic and behavioral adaptations. Some desert mammals e.g. kangaroo rats survive without drinking water by feeding on seeds of desert plants containing more carbohydrates, which produce water of metabolism. Terrestrial animals produce concentrated urine in their kidneys that reabsorb most filtered water in the process of excretion. Terrestrial animals can tolerate dehydration and it differs in various animals. This characteristic is known as anhydrobiosis.