For aquatic organism salt concentration (measured as salinity in parts per thousand) is a major factor for their survival. Salinity of different aquatic habitats varies greatly. It is less than 5 per thousand parts, in inland waters, 30-35 per thousand parts in the sea and more than 100 per thousand parts in some hypersaline lagoons. Many fresh water animals cannot live for long in sea water and vice versa because of the osmotic problems they would face due to the change in relative tonicity of the surrounding water with cytoplasm. It may result into endosmosis or exosmosis according to the conditions. The tolerance of organisms to the salinity range varies. Some organisms are tolerant of a wide range of salinities and are called euryhaline e.g. salmon, while some can tolerate only a narrow range of salinity i.e., stenohaline e.g., shark.