Sharks are a highly diverse group of cartilaginous fishes, with over 500 species inhabiting the world’s oceans. They occupy an immense range of marine habitats, from coastal waters to deep-sea abyssal plains. While most sharks are strictly tied to saltwater, a small, specialized group defies this expectation. These species possess unique biological machinery that allows them to thrive in dramatically different environments. This physiological capability reveals a surprising truth about shark adaptability.
Marine Environments: The Primary Shark Habitat
Over 99% of all known shark species are obligate marine dwellers, meaning they are biologically restricted to high-salinity environments. Oceanic water typically maintains a salt concentration of around 35 parts per thousand (ppt). Sharks have evolved to match their internal chemistry to this concentration, as their cells and organs depend on external salinity to regulate water and dissolved solutes. This requirement means sharks are found across diverse marine ecosystems, from coral reefs to the open-ocean pelagic zone.
The physical properties of saltwater also aid their survival. The higher density of saltwater provides more buoyancy, which is important because sharks lack the gas-filled swim bladders used by bony fish. Instead, they rely on large, oil-filled livers for lift and depth control.
Species like the great white shark, the tiger shark, and the oceanic whitetip are entirely dedicated to the marine environment. If these obligate saltwater sharks enter a low-salinity river, the osmotic pressure difference causes water to flood their tissues, leading to cellular swelling and death. They lack the internal mechanisms to process and expel the massive influx of fresh water, locking them into the oceans.
The Surprising Exception: Sharks That Enter Freshwater
A few remarkable species possess the physiological flexibility to regularly transit between the ocean and freshwater, making them euryhaline. The most widely known example is the Bull Shark (Carcharhinus leucas), found worldwide in warm, shallow coastal waters. These sharks are frequently found in brackish estuaries and have been documented traveling thousands of miles up major river systems. Bull sharks have been reported 1,100 kilometers (700 miles) up the Mississippi River and nearly 4,000 kilometers (2,500 miles) inland in the Amazon River system. This movement into rivers is often a strategy for juvenile survival, as the lower salinity environments serve as nurseries safe from larger marine predators.
While the bull shark is a frequent visitor, the most extreme exceptions are the incredibly rare river sharks of the genus Glyphis. The Ganges shark (Glyphis gangeticus) is one of the few species restricted to freshwater, inhabiting the turbid rivers of India and Bangladesh. These species are distinct from the bull shark, existing as small, critically endangered populations that have evolved solely within these specialized riverine environments.
The Science of Survival: Osmoregulation in Euryhaline Species
The ability of euryhaline species like the bull shark to move between high- and low-salinity water is governed by osmoregulation, which maintains internal salt and water balance. Marine sharks maintain an internal concentration of solutes, including a high amount of urea, making their body fluids slightly saltier than the surrounding seawater. This adaptation prevents them from constantly losing water to the ocean.
When a euryhaline shark enters freshwater, its body faces the opposite challenge: an overwhelming influx of water and loss of internal salts. To counteract this, the bull shark’s physiology shifts. It drastically reduces the production of urea in its liver, lowering the compound’s concentration in its blood to minimize the osmotic gradient between its body and the surrounding river water.
The kidneys take on a larger role, increasing their output to produce a massive volume of dilute urine to expel the excess water flooding into the shark’s body. The rectal gland, which normally excretes excess salt in marine environments, is suppressed in freshwater to conserve internal salts. This coordinated, multi-organ response allows the bull shark to avoid cellular rupture and maintain life in a habitat lethal to almost every other shark species.