The vast majority of sharks, rays, and chimaeras (elasmobranchs) are creatures of saltwater environments. Their biology has evolved to thrive in high salinity, meaning a typical marine shark would not survive long in a river or lake. However, a few specialized species have developed the physiological mechanisms necessary to overcome the challenge posed by fresh water.
The Physiological Barrier to Freshwater Survival
The primary obstacle preventing most marine sharks from entering freshwater is the scientific principle of osmosis. Sharks, like all animals, must maintain a specific concentration of solutes, such as salts, within their bodily fluids to keep their cells functioning correctly. Most marine sharks achieve this by retaining high concentrations of urea and trimethylamine oxide (TMAO) in their bloodstream and tissues.
This internal chemistry makes the shark’s body fluids slightly “saltier,” or hyperosmotic, than the surrounding seawater, which prevents the passive loss of water to the environment. This system works well in the ocean, but it causes a constant, slight influx of salt that the shark must excrete primarily by a specialized organ called the rectal gland.
When a typical marine shark swims into fresh water, the situation reverses completely. Its internal fluids are much saltier than the external environment, causing water to rush into the shark’s body across the gills and skin, and cells to swell from the massive influx. Simultaneously, the shark’s vital internal salts begin to diffuse out of the body into the surrounding water.
The shark’s body cannot cope with this rapid osmotic stress, leading to cellular damage, bloating, and a catastrophic loss of necessary salts. Standard marine shark kidneys are not equipped to process the massive volume of excess water required, nor are they capable of retaining the small amounts of salt needed. Without specialized adaptations, the animal would quickly die from dilution and electrolyte imbalance.
Examples of Freshwater-Tolerant Sharks
A select group of sharks has evolved to circumvent this osmotic barrier. The most widely known is the Bull Shark (Carcharhinus leucas), found in warm, shallow coastal waters worldwide. It is classified as euryhaline, meaning it tolerates a wide range of water salinities and moves freely between the ocean and fresh water.
Bull Sharks are frequently sighted far up major river systems globally. Migrations have been documented in the Mississippi River (reaching Illinois) and the Amazon River (thousands of miles inland). They also inhabit Lake Nicaragua and the Zambezi River in Africa, earning them the nickname Zambezi Sharks.
Using freshwater habitats provides a significant advantage, as river mouths and estuaries serve as protected nursery areas for young pups. The low salinity reduces the risk of predation from purely marine sharks. Other tolerant species belong to the River Shark genus (Glyphis), including the Ganges Shark (Glyphis gangeticus) and the Speartooth Shark (Glyphis garricki).
These River Sharks are much rarer than the Bull Shark and often face a high risk of extinction, such as the critically endangered Ganges Shark. The Bull Shark is the only species known to remain in freshwater for prolonged periods, sometimes surviving for years in landlocked systems. This capability is due to physiological adjustments that allow them to control their internal environment regardless of external salinity.
How Specialized Sharks Manage Osmotic Stress
The survival of euryhaline sharks in fresh water relies on a coordinated physiological response involving several organs. When a Bull Shark enters a river, its body immediately begins retaining salts and shedding the continuous influx of water by altering the function of its osmoregulatory organs.
The rectal gland, which normally excretes excess salt in the ocean, is substantially down-regulated to conserve the body’s limited salt supply. Simultaneously, the kidneys take on a dramatically increased workload, actively filtering and excreting the excess water gained through osmosis. This results in producing a massive volume of highly dilute urine, sometimes up to 20 times the rate observed in saltwater.
The shark also adjusts its internal solute concentration by reducing the concentration of urea in its blood by as much as 50 percent when in fresh water. This reduction helps lower the osmotic gradient between the shark’s body and the environment, lessening the influx of water and reducing the energy cost of osmoregulation.
The kidneys become highly efficient at reabsorbing necessary salts like sodium and chloride from the urine before excretion. The gills also actively take up salts from the surrounding fresh water. This combination of down-regulating salt excretion, increasing water excretion, and actively retaining salts allows the Bull Shark to maintain a stable internal environment.