Sharks are predominantly creatures of the ocean, thriving in its salty depths. The vast majority of shark species inhabit marine environments exclusively, making saltwater their natural habitat. Their physiology has evolved over millions of years to adapt specifically to the ocean’s unique chemical composition.
The Salty Truth
Sharks, like all marine life, must manage the salt concentration within their bodies. This process, called osmoregulation, is particularly intricate for sharks. Unlike many bony fish that constantly drink seawater and excrete excess salt, sharks employ a different strategy to maintain their internal balance. They retain high concentrations of urea and trimethylamine-N-oxide (TMAO) in their blood and tissues, which makes their internal fluid concentration nearly equal to or slightly higher than the surrounding seawater. This adaptation prevents water from flowing out due to osmosis, avoiding dehydration.
Sharks still absorb some salt from the ocean water. To manage this excess, they possess specialized organs. Their kidneys work to filter waste, but their primary role in salt excretion is supported by a unique organ called the rectal gland. This gland, located at the end of the intestine, actively removes excess sodium chloride from the shark’s bloodstream and secretes it for expulsion. This integrated system of urea retention, kidney function, and the highly efficient rectal gland allows most sharks to maintain a stable internal environment in saltwater.
Freshwater Exceptions
While most sharks are strictly marine, a few species can tolerate brackish or freshwater environments. The bull shark (Carcharhinus leucas) is the most well-known example, found in tropical rivers and lakes worldwide, sometimes traveling thousands of kilometers inland. This adaptability is due to a sophisticated osmoregulation system that allows them to adjust their internal chemistry when moving between different salinities.
When a bull shark enters freshwater, its body undergoes changes to prevent excessive water intake and salt loss. Hormonal signals reduce blood flow to the rectal gland, decreasing its salt-excretory activity, thus conserving sodium and chloride. Simultaneously, their kidneys increase urine production, making it very dilute to expel the excess water absorbed from the freshwater environment. This ability to modify their internal balance, including lowering urea content in their tissues and increasing urine output, helps them survive in low-salinity conditions.
Beyond the bull shark, other rare species, such as the river sharks (genus Glyphis), including the Ganges shark and speartooth shark, also inhabit freshwater or coastal marine environments. These species are genuine freshwater sharks, with some, like the Ganges shark, being found exclusively in river systems. These instances are rare, however, and represent unique evolutionary adaptations rather than a general characteristic of sharks. The vast majority remain saltwater inhabitants.