Most people associate sharks with vast, salty oceans, but some species have adapted to live in freshwater environments, including lakes and rivers. This challenges the common perception that these powerful fish are exclusively marine. Though rare, certain sharks can transition between drastically different salinities, a testament to their remarkable evolutionary flexibility.
The Bull Shark’s Unique Habitat
The bull shark (Carcharhinus leucas) is the most well-known example of a shark capable of thriving in both saltwater and freshwater. These robust sharks are found in coastal waters worldwide, but uniquely, they can venture far inland into rivers and lakes. For instance, bull sharks have been documented traveling thousands of kilometers up the Amazon and Mississippi Rivers, reaching as far as Illinois. Lake Nicaragua in Central America is another notable freshwater habitat where these sharks reside.
Adapting to Freshwater Environments
The bull shark’s ability to survive in varying salinities is due to osmoregulation. Most marine sharks maintain internal salt levels similar to seawater, but in freshwater, this balance would flood their cells, leading to swelling and death. Bull sharks counteract this by altering kidney function, producing large amounts of dilute urine to expel excess water. Their rectal gland, typically used to excrete excess salt, reduces its activity in freshwater, helping to conserve sodium and chloride. This physiological flexibility allows them to regulate internal salt and urea levels, enabling movement between freshwater and saltwater habitats.
Other Sharks That Tolerate Freshwater
While the bull shark is the most prominent, other shark species tolerate freshwater or live exclusively in it, though they are much rarer. River sharks, belonging to the genus Glyphis, are adapted to freshwater and brackish environments. Species like the speartooth shark (Glyphis glyphis) are found in tropical river systems in northern Australia and New Guinea, often preferring turbid waters. The critically endangered Ganges shark (Glyphis gangeticus) is another river shark found in India’s river systems, known for adapting to low visibility. Sawfish, technically rays but sharing many shark-like characteristics, also demonstrate freshwater tolerance. The largetooth sawfish (Pristis pristis) regularly enters freshwater rivers globally, with juveniles often spending early years far upstream in nurseries. These species, while not as widely distributed as the bull shark, highlight diverse adaptations within cartilaginous fish, allowing them to inhabit environments typically thought exclusive to bony fish.