The idea of sharks inhabiting freshwater environments sparks curiosity about these aquatic predators. Many wonder if the powerful, cartilaginous fish typically associated with oceans can survive in rivers and lakes. Separating fact from popular misconception is important for understanding the unique biology of certain species.
Actual Sharks That Enter Freshwater
While most shark species are exclusively marine, a few can venture into and survive in freshwater. The bull shark (Carcharhinus leucas) is the most well-known example, found in tropical and subtropical coastal waters globally. Bull sharks are unique for their tolerance of varying salinities, allowing them to travel far up major rivers and inhabit freshwater lakes. They have been documented in the Mississippi River, reaching as far north as Illinois, and are also found in the Amazon River, over 2,500 miles upstream. They are also present in Lake Nicaragua, living for extended periods.
Beyond the bull shark, a small group of species known as river sharks, belonging to the genus Glyphis, are adapted to freshwater and coastal marine environments. These include the Ganges shark (Glyphis gangeticus) from India and Pakistan, the speartooth shark (Glyphis glyphis) found in New Guinea and Australia, and the Irrawaddy river shark (Glyphis siamensis). River sharks are rare, but they can inhabit freshwater. Another example is the Greenland shark, observed in freshwater, though its primary habitat is deep, cold North Atlantic and Arctic waters.
How Sharks Adapt to Freshwater
The ability of certain sharks, particularly the bull shark, to transition between saltwater and freshwater relies on osmoregulation. This complex physiological process maintains a stable concentration of water and solutes within the body, even when external conditions change.
In a marine environment, sharks maintain high concentrations of urea and other organic compounds in their blood. This helps them absorb water from the surrounding salty water and excrete excess salt through their kidneys and a specialized rectal gland.
When a bull shark moves into freshwater, its body adjusts to the lower salinity. The rectal gland, which excretes excess salt, decreases its activity to conserve sodium and chloride. Their kidneys produce a large volume of very dilute urine, ridding the body of excess water due to the osmotic gradient. Simultaneously, the liver adjusts its urea production, allowing the shark to reduce urea content in its tissues. This coordination allows bull sharks to maintain internal balance and survive in habitats with differing salt concentrations.
Fish Mistaken for Freshwater Sharks
Many bony fish are called “freshwater sharks” due to their appearance or behavior, but they are not true sharks. These fish often have a torpedo-shaped body, pointed snout, and prominent dorsal fins, giving them a superficial resemblance. However, true sharks are cartilaginous fish, with skeletons made of cartilage rather than bone, distinguishing them from these species.
One example is the Bala shark (Balantiocheilos melanopterus), also known as the tricolor or silver shark. This species belongs to the Cyprinidae family, which includes carp and minnows. Bala sharks are native to Southeast Asia and are often kept in aquariums, reaching lengths of up to 14 inches (35 cm).
Another misidentified fish is the Iridescent shark (Pangasianodon hypophthalmus), a species of shark catfish from the Pangasiidae family. Native to large rivers in Southeast Asia, these fish can grow up to 4.3 feet (130 cm). The Colombian shark catfish (Ariopsis seemanni) is another fish called a “shark” in the aquarium trade. This sea catfish from the Ariidae family is native to Pacific-draining rivers and estuaries in Central and South America. While often sold as freshwater fish, they transition to brackish or saltwater as they mature.