Sharks are predominantly marine creatures, adapted to the salty expanse of the ocean. However, a select few species possess the extraordinary ability to venture into and even thrive in freshwater environments. This remarkable adaptation allows certain sharks to navigate a surprising range of aquatic habitats, including rivers and lakes.
The Bull Shark
The bull shark (Carcharhinus leucas) stands out as the most prominent example of a shark capable of inhabiting both saltwater and freshwater. This species is found globally in warm, shallow coastal waters, estuaries, and rivers. Bull sharks exhibit a unique tolerance for varying salinities, enabling them to transition between marine and freshwater systems.
These sharks can grow to considerable sizes, with females typically larger than males, reaching lengths of up to 4 meters (13.1 feet) and weighing around 315 kilograms (694 pounds). Their diet is highly opportunistic, consisting of various fish, rays, and even other sharks. They are known to travel significant distances inland.
Other Freshwater-Tolerant Species
Beyond the bull shark, other elasmobranchs are also capable of entering or residing in freshwater. Sawfish, characterized by their long, saw-like rostrums, are among these species. All five species of sawfish in Australia are critically endangered and can be found in coastal marine waters, estuaries, and freshwater rivers. The largetooth sawfish (Pristis pristis), sometimes called the freshwater sawfish, has a strong affinity for freshwater, with young individuals spending their early years far upriver.
True river sharks, belonging to the genus Glyphis, represent another group with a notable freshwater presence. These include the Ganges shark (Glyphis gangeticus), the speartooth shark (Glyphis glyphis), and the northern river shark (Glyphis garricki). The Ganges shark, a critically endangered species, is primarily found in the rivers of India and Bangladesh. Speartooth sharks are found in tropical river systems of Australia and New Guinea, often moving between saltwater and freshwater environments. The northern river shark, while adults may remain in coastal areas, has juveniles and newborns that regularly venture into freshwater or brackish waters in Australia and Papua New Guinea.
The Science of Freshwater Adaptation
The ability of certain sharks to live in freshwater hinges on osmoregulation, a complex physiological process that allows them to maintain a stable internal salt and water balance despite external salinity changes. Unlike most marine fish, sharks generally retain high concentrations of urea and trimethylamine oxide (TMAO) in their blood and tissues, making their internal fluids slightly saltier than seawater. This strategy helps them absorb water through osmosis in a marine environment.
When these sharks, particularly bull sharks, enter freshwater, they face the challenge of their bodies absorbing too much water from the less saline environment. To counteract this, their kidneys increase urine production dramatically, sometimes excreting over 20 times more urine than in saltwater, to remove excess water. The rectal gland, typically responsible for secreting concentrated salt in marine sharks, reduces its activity in freshwater to minimize salt loss. Additionally, these sharks adjust the amount of urea synthesized and retained, lowering their internal osmolarity to better match the freshwater environment. This coordinated effort across multiple organs, including the gills, kidneys, and rectal gland, enables their survival.
Freshwater Shark Habitats
Sharks capable of freshwater excursions have been documented in various river systems and lakes across the globe. The bull shark, in particular, has an extensive reach into inland waterways. They have been observed far up the Mississippi River, reaching as far as Alton, Illinois, approximately 1,100 kilometers (700 miles) from the ocean. In South America, bull sharks are known to penetrate deep into the Amazon River system, traveling over 4,000 kilometers (2,500 miles) inland.
Other notable freshwater habitats include Lake Nicaragua, where bull sharks are a well-established presence, and various rivers in Florida, such as the Loxahatchee and Caloosahatchee, which serve as nursery grounds for juvenile bull sharks. In Australia, bull sharks are found in the Brisbane River, with populations estimated to exceed 500 individuals. Sawfish species also frequent Australian freshwater rivers, including those in Queensland and the Northern Territory. The true river sharks, such as the Ganges shark, are found in major river systems like the Ganga, Hooghly, Mahanadi, and Brahmaputra in India and Bangladesh. Speartooth and northern river sharks inhabit tidal rivers and estuaries in New Guinea and northern Australia.