Sharks are overwhelmingly creatures of the ocean, with the vast majority of the more than 500 known species requiring high salinity to survive. Their internal biology is finely tuned to seawater, which is why most sharks are known as stenohaline, meaning they can only tolerate a narrow range of salt concentrations. However, a small, highly specialized group of sharks possesses a rare physiological capability called euryhalinity. This is the ability to thrive across a wide spectrum of salinities, from full-strength ocean water to pure fresh water. This adaptation allows certain species to venture far inland, utilizing river systems and lakes otherwise inaccessible to their marine relatives.
The Apex Euryhaline Predator: The Bull Shark
The bull shark, Carcharhinus leucas, is the most renowned and widespread shark species capable of prolonged existence in fresh water. This robust predator uses its euryhaline ability to migrate hundreds of miles up major river systems across the globe. Their presence has been documented deep within the Mississippi River, with historical sightings reaching as far north as Illinois, over 700 miles from the Gulf of Mexico.
Bull sharks maintain permanent populations in large freshwater bodies, such as Lake Nicaragua in Central America, where they navigate the San Juan River system. They are also commonly found in the Amazon River basin and the Zambezi River in Africa, leading to the nickname Zambezi sharks. Adults typically reach lengths of 7 to 11 feet and weigh between 200 and 500 pounds, making them formidable predators in both marine and freshwater ecosystems.
Female bull sharks often use the lower-salinity environment of river mouths and estuaries as nursery grounds. They give birth in these safer waters, and the juvenile sharks benefit from the reduced number of larger marine predators found inland. The young remain there for several years before migrating out to sea as they mature.
The Biological Mechanism of Salt Tolerance
The ability for certain sharks to transition between salt and fresh water relies on a complex physiological process called osmoregulation. Most sharks maintain internal fluid concentrations slightly saltier than the ocean by retaining high levels of urea and Trimethylamine N-oxide (TMAO) in their bloodstream. This high internal solute concentration prevents water loss in the ocean, but in fresh water, it creates a challenge where a massive influx of water would normally dilute their cells.
When a euryhaline shark enters fresh water, its body must rapidly adjust to prevent this osmotic crisis by modulating its primary osmoregulatory organs. The rectal gland, a specialized organ which excretes excess salt in the ocean, dramatically reduces its activity to conserve salt ions within the body. Simultaneously, the liver reduces its production of urea, slowly lowering the overall internal solute concentration of the shark’s blood.
The kidneys play the most active role in fresh water, increasing their output to produce a large volume of highly dilute urine. This watery urine constantly flushes out the excess water passively entering the body across the gills and other membranes. Furthermore, the gills utilize specialized cells to actively absorb salt ions from the surrounding water, helping to replenish salts lost through diffusion. This coordinated change in organ function allows the shark to maintain a stable internal environment despite the extreme change in its external habitat.
The Rare Specialists: True River Sharks
Beyond the Bull Shark, there is a distinct, much rarer group known as the true river sharks belonging to the genus Glyphis. These species are highly specialized and primarily adapted to live within turbid, low-salinity river systems, contrasting with the Bull Shark’s ability to move between habitats. The genus includes critically endangered species such as the Ganges River Shark (Glyphis gangeticus) in India, and the Northern River Shark (Glyphis garricki) and Speartooth Shark (Glyphis glyphis) of the Indo-Pacific.
These Glyphis species are generally less studied due to their elusive nature and limited distribution in remote areas. Their adaptation means they are often restricted to particular river sections for their entire lives. This makes them vulnerable to habitat degradation and overfishing, as their life cycle is highly dependent on the freshwater habitat.
Other cartilaginous fish, or elasmobranchs, also possess the ability to live in fresh water, though they are not true sharks. Certain stingrays and sawfish species can inhabit freshwater systems for extended periods. Their existence highlights that the ability to conquer the osmotic challenge of fresh water has evolved in a few unique lineages within this ancient group of fish.