Sharks are predominantly marine creatures, typically associated with saltwater environments. However, some shark species can inhabit freshwater, including lakes and rivers. This phenomenon is not widespread, but it highlights remarkable adaptations that allow certain sharks to venture far from their usual marine habitats. Their ability to transition between saltwater and freshwater challenges common perceptions about where these powerful fish can survive.
How Sharks Can Survive in Freshwater
The ability of certain sharks to survive in freshwater stems from complex physiological adaptations, primarily involving osmoregulation. Marine sharks maintain an internal salt concentration lower than seawater, mainly by retaining urea in their tissues. This makes their body fluids slightly saltier than the ocean, preventing dehydration. When a marine shark enters freshwater, it faces the opposite challenge: water rushes into its body, and salts can leach out.
Freshwater-tolerant species, like the bull shark, manage this by modifying several organs. Their kidneys produce large amounts of dilute urine, flushing out excess water while reabsorbing vital solutes. The rectal gland, which normally excretes excess salt, significantly reduces its activity in freshwater to conserve sodium and chloride.
The liver also adjusts its urea production, lowering its concentration in the blood when the shark is in freshwater. Gills actively take up sodium and chloride from the dilute freshwater. This coordinated effort allows these sharks to maintain a delicate balance of water and salts, enabling their survival in diverse aquatic environments.
Identifying Freshwater Shark Species
While many shark species are strictly marine, a few can inhabit freshwater environments. The most well-known is the bull shark (Carcharhinus leucas). Bull sharks are found globally in warm, shallow coastal waters and are noted for their tolerance to varying salinities, allowing them to travel far up rivers and into lakes. They are described as euryhaline, meaning they thrive in both saltwater and freshwater.
Beyond the bull shark, other species are associated with freshwater, though often less commonly. River sharks, belonging to the genus Glyphis, are considered true freshwater sharks, residing primarily in freshwater and brackish environments in Asia and Australia. Examples include the speartooth shark (Glyphis glyphis) and the critically endangered Ganges shark (Glyphis gangeticus) found in Indian river systems. Sawfish (Pristis species), while technically rays, are discussed in the context of freshwater sharks due to their similar ability to enter and inhabit rivers and lakes. The largetooth sawfish (Pristis pristis) is one such species found in tropical and subtropical freshwater systems globally.
Documented Shark Sightings in Lakes
Bull sharks have been documented in various freshwater lakes and extensive river systems worldwide, showcasing their adaptability. Lake Nicaragua in Central America is a notable example, where bull sharks travel between the lake and the ocean via connecting rivers. In the United States, bull sharks have been observed far inland in the Mississippi River system, with sightings reported as far north as Alton, Illinois, approximately 1,100 kilometers (700 miles) from the Gulf of Mexico. Historical records indicate at least two confirmed bull shark sightings in the upper Mississippi River basin in 1937 and 1995.
Australia provides numerous instances of bull sharks inhabiting freshwater environments. They are commonly found in river mouths, estuaries, and have been documented in major river systems like the Brisbane River in Queensland. During flood events, bull sharks have been observed in unexpected places, including flooded streets or becoming temporarily landlocked in freshwater lakes. These occurrences typically happen in areas with direct river connections to the ocean, or when extreme weather events create temporary pathways, emphasizing that freshwater presence is not common in isolated lakes without such connections.