 Dear students, in this topic we shall discuss osmoregulation in marine animals. The marine invertebrates and primitive chordates i.e. acidians have body fluids which are isoosmotic to seawater. Their osmolarity i.e. overall concentration of the body fluids as well as the plasma concentration of individual ions are similar to the seawater. So they need not expend any energy in regulating the osmolarity of their body fluids. In marine vertebrates, hagfishes have an isoosmotic plasma concentration. However, they regulate concentration of individual ions. They regulate various ions and maintain blood, calcium, magnesium and sulphate ion concentration significantly lower as compared to the concentration of these ions in seawater. Similarly, they maintain the concentration of sodium and chloride ions on a higher level than seawater in the body. Cartilaginous fishes which remain in the marine environment are also isotonic to seawater. However, the way they produce isoosmotic environment is different from hagfishes. They maintain lower concentration of inorganic ions in their body. These hagfishes keep higher level of inorganic ions but sharks, rays and skates keep lower level of inorganic ions. They also keep higher level of sodium and chloride ions in their body as they excrete sodium and chloride from the body through their kidneys and through their rectal glands. Now, how do they maintain body isotonic level when they excrete salts? Because of presence of organic osmolites. i.e. urea and tri-methyl amine oxide, which are present in the fluid of the body and keep the asmolarity of the body at C level. However, keeping the urea level high in the body, that is a toxic substance. How is it possible? Because it denatures proteins and for this purpose, the other compound in the body, tri-methyl amine oxide, actually balances the toxic effects of urea, because it stabilizes the protein structure and prevents them to become denatured. Here students, now we shall discuss the asmoregulation in marine modern bony fishes i.e. tiliosts. The fluid in the body of tiliosts keeps their asmolarity at least at the hypotonic level. That is why tiliost fishes have a tendency to lose water, especially across their gill epithelium. To replenish this lost water, they drink seawater. But seawater has more salts in it, so their body is flooded with excess salts. So, they have to eliminate these excess salts. Sodium, chloride and potassium ions are eliminated by active transport through the gills, while calcium, magnesium and sulfate ions are excreted through their kidneys through urine. Their urine is isotonic according to blood, but it is rich in salts. As a result of combined action of gills and kidneys, the osmotic balance of water and ions in marine tiliosts is achieved.