Matt's Biology Blog: Osmoregulation in Bull Sharks

All sharks that live in marine environments have adapted their osmoregulatory systems in order to survive. If they didn't, then they would die. Most sharks survive in saltwater due to a high concentration of urea and TMAO in the bloodstream as well as urine that contains excess salt. In marine environments, the external environment has a very high concentration of salt. In order to stay in balance with this outside environment, normal organisms would let water out to try and balance the concentrations. However, if a shark did this then it would soon have too high of a concentration of salt inside its body. To deal with this, sharks bodies contain urea and TMAO. Urea and TMAO make the inside environment saltier, and then balance out the concentrations that way. Water is constantly passively entering the shark through the gills and once inside the body, passes along to the kidneys for desalinization. In sharks, the kidneys make do a very efficient job, and send a very high concentration of salt out via urine and retain a lot of freshwater, preventing the inside of the shark from having a concentration of salt that is too high. Bull sharks are unique because they can survive in marine environments as well as freshwater environments. This is possible because Bull Sharks adapt their osmoregulation systems depending on what environment they are in. When they are in marine environments, they osmoregulate like other sharks, using urea, TMAO, and efficient kidneys to keep an isotonic relationship with the external environment. When Bull Sharks are in freshwater, they basically tone down their kidneys and remove not as much salt but more urea and TMAO. It should be said that this process must be gradual and that a Bull Shark just dropped from saltwater into freshwater it will die. Since Bull Sharks have this unique ability to adapt their osmoregulation system from the normal process of getting rid of salt to the process of retaining salt, they are capable of living in freshwater.