Sharks are primarily known as inhabitants of vast, salty oceans. However, some shark species can live in freshwater, an unexpected phenomenon. While rare, a few species possess capabilities allowing them to thrive in rivers, lakes, and other low-salinity habitats. This adaptability highlights their physiological versatility.
Key Freshwater-Tolerant Sharks
The bull shark (Carcharhinus leucas) is the most prominent freshwater-tolerant shark. It moves easily between marine and freshwater systems. While the bull shark is often associated with freshwater, it is important to note that it is not a true freshwater shark, as it typically returns to saltwater for breeding.
Other species also exhibit freshwater tolerance or are true river dwellers. The genus Glyphis, commonly known as river sharks, includes species like the Ganges shark (Glyphis gangeticus), which is restricted to freshwater environments, and the speartooth shark (Glyphis glyphis), found in both coastal marine and freshwater habitats. Additionally, sawfish, which are technically rays but share a shark-like appearance and similar adaptations, can also inhabit freshwater rivers and lakes.
How Sharks Adapt to Freshwater
Sharks adapt to freshwater through osmoregulation, a physiological process controlling water and salt balance. Most sharks maintain high urea and trimethylamine N-oxide (TMAO) in their blood, making internal fluids equal to seawater salinity. In a freshwater environment, this causes water to rush in and vital salts to diffuse out.
Freshwater-tolerant sharks, like the bull shark, use specialized organs to counteract this. Their kidneys efficiently produce dilute urine to excrete excess water while reabsorbing solutes back into the bloodstream. The rectal gland, normally excreting excess salt, decreases activity in freshwater to conserve sodium and chloride. Gills also play a role in sodium and chloride uptake from freshwater. The liver adjusts urea production in response to salinity changes, maintaining internal balance. These adjustments allow sharks to maintain internal balance across varying salinities.
Where These Sharks Are Found
Bull sharks are found in coastal waters worldwide, extending far into inland waterways. They travel significant distances up major river systems globally. In South America, they have been observed up to 4,000 kilometers (2,500 miles) up the Amazon River, reaching as far inland as Iquitos in Peru.
In North America, bull sharks are known to ascend the Mississippi River, with confirmed sightings over 1,700 miles inland, including instances near Alton, Illinois, and Festus, Missouri. They also inhabit freshwater lakes, such as Lake Nicaragua in Central America, where a population was once thought to be landlocked but is now known to migrate to and from the Caribbean Sea via the San Juan River. Other notable locations include the Brisbane River in Australia, the Ganges and Brahmaputra Rivers in India and Bangladesh, the Tigris River, and the Zambezi River in Africa.
Interactions in Freshwater Environments
Sharks in freshwater raise questions about human interactions. Bull sharks prefer shallow coastal and estuarine waters, leading to potential human contact. While they are known for their aggressive nature, shark attacks in freshwater are rare.
Understanding their behavior in these habitats is important for coexistence. Bull sharks may use freshwater systems as nurseries, providing a safer environment for their young away from larger oceanic predators. Human encounters, like those in Lake Nicaragua, highlight the need for awareness where these predators reside. Their ability to navigate diverse environments underscores the dynamic nature of aquatic ecosystems.