 Dear students, in this topic we shall discuss the osmoregulation in aquatic environments. The osmolarities of aqueous environment have wide range. In freshwater, the osmolarity is only few milliosmoles per litre. Whereas in ordinary sea water, the osmolarity is 1000 milliosmoles per litre. Whereas in land-logged seas, salt concentration is more than that. Animals living in these different environments have distinct problems related to water and salt exchange. The nature of osmotic problems of freshwater and marine animals are different. Mechanisms for dealing with them are also different. As a rule, animal body fluids tend away from the environmental osmotic extremes. Dear students, those animals which can tolerate a narrow osmotic range, they have very limited abilities to withstand osmotic change. They cannot tolerate substantial change in external environments osmolarity. They include most marine invertebrates. Their body cells cannot tolerate dilution. The animals living in high salts and steinohelanes, if they have less concentration of seawater, they cannot tolerate it. High narrow range animals are steinohelanes. If they go into dilute solution, they die quickly. So, they are restricted to living in such environments in which there is no major fluctuation in salinity. For example, marine spider crabs. Yuri-Helline animals, compared to steinohelane animals, are animals which can survive large fluctuations in external osmolarity. They can tolerate wide range of osmolarities. They can carry osmotic regulation when salinity changes. That is why they do not have any problem because as a result of salinity change, they use osmolar regulation to balance their body. Yuri-Helline animals are found along the coasts, in the estuaries and in the river mouths. These are the environments where salt water and freshwater are mixed and changes in salinity are common. Such animals should be able to tolerate changes in salinity. The Yuri-Helline animals include many osmoconformers as well as osmoregulators. Many barnacles and mussels, which are invertebrates, they are Yuri-Helline osmoconformers. Whereas striped bass and salmon fish, they are examples of Yuri-Helline osmoregulators.