Unlike other shark species that stick to either fresh or saltwater, bull sharks (Carcharhinus leucas) manage to thrive in both. They’ve specialized their ability of osmoregulation, or the ability of an organism to maintain its body. Since all fish, fresh and saltwater alike, have to osmoregulate, bull sharks have figured out how to adjust theirs to all salinity levels.
An external and internal environment is separated by a membrane where substances can move. For fish, this is skin. If the internal has more water than the external, and vice versa, water will move from the highest concentration area to the lowest. So marine animals must prevent dehydration, whereas freshwater animals must reserve their salt levels.Bull sharks have managed to balance these processes out.
Urea and other salty substances normally stabilize based on the environment, which is controlled by the kidneys. By regulating the amount of salt released from the kidneys, the bull shark can adjust and balance by releasing less salt and more urea based on the water. Since people often spend time in estuaries and rivers where bull sharks lurk, encounters with them are often surprised and unexpected; Especially when they were found 2,000 miles upstream in the Amazon River!
Photo credit: wirodive