Physiology of Finfish and Shellfish

2.4.3. Osmoregulation

Life on earth originated and flourished in 3 major environments viz., seawater, brackishwater and freshwater depending on the variation in salinity. Fishes being confined only to water exhibit several adaptive measures to maintain their internal environment constant. The conditions of life are different in fresh and marine water and so the nature of osmoregulatory mechanism also differs in fishes inhabiting the 2 kinds of environment.

Osmoregulation is a physiological activity for proper maintenance of water and salt balance of the body tissues in fishes, whether they live in fresh, marine or brackish waters or osmoregulation is the active regulation of the osmotic pressure of an organism 's fluids to maintain the homeostasis of the organism's water content; that is it keeps the organism's fluids from becoming too diluted or too concentrated. Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis . It is a physiological activity very closely related to their survival. Kidney and gills perform the function of osmoregulation also in addition to excretion of nitrogenous waste.

Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion). Organisms in both aquatic and terrestrial environments must maintain the right concentration of solutes and amount of water in their body fluids; this involves excretion : getting rid of metabolic wastes and other substances such as hormones that would be toxic if allowed to accumulate in the blood via organs such as the skin and the kidneys ; keeping the amount of water and dissolved solutes in balance is referred to as osmoregulation.