Problems of Osmoregulation
The water, some inorganic salts, nutrient molecules are some of the important components of the body fluids. It is essential for the animals to maintain appropriate concentration of these components for the survival. It is agreed that life originated in the sea, and during the course of evolution animals spread from oceans into the estuaries, rivers and onto the land. These new environments are osmotically different from that of the sea.
Since life originated in the sea, body fluids of the animals are more or less similar to that of seawater in its general composition. Therefore animals inhabiting marine environment do not have the problem of osmoregulation, because their body fluids are isotonic to their external environment, the sea. But animals spread over to brackish, fresh water, and terrestrial environments have the problem of osmoregulation because their body fluids are hypertonic to their external environment. Therefore, these animals evolved various physiological and behavioural adaptations to cope up with the rigors of the osmotic environments. The osmoregulatory organs of these animals play a vital role in this effort.