Sharks in freshwater, especially lakes, often spark disbelief. Sharks are predominantly marine creatures, adapted to the salty waters of the world’s oceans. However, some remarkable exceptions exist, challenging the conventional understanding of shark habitats. These exceptions highlight the adaptability some species have developed to survive in diverse aquatic systems.
The Fundamental Challenge: Osmoregulation
Most sharks are cartilaginous fish, tuned for saltwater. A central aspect of this adaptation is osmoregulation, the process by which organisms maintain the balance of water and salts within their bodies. Marine sharks achieve this by keeping their internal salt concentration roughly equal to that of seawater. They accumulate high levels of urea and trimethylamine oxide (TMAO) in their blood and tissues, creating an osmotic pressure similar to the surrounding ocean.
This minimizes osmotic water loss to the saltier external environment. Excess salt diffusing into their bodies (e.g., through gills or food) is actively excreted by specialized organs like the rectal gland, which removes significant salt from the blood.
This system becomes a disadvantage in freshwater. In a hypotonic (less salty) freshwater environment, water rapidly moves into a marine shark’s body due to osmosis, overwhelming its cells. Essential salts are simultaneously lost. This constant influx of water and loss of salts leads to cellular damage, swelling, and ultimately, death for most shark species.
The Unique Case of the Bull Shark
The bull shark (Carcharhinus leucas) is one of few shark species thriving in both saltwater and freshwater. This species possesses unique physiological adaptations that allow it to effectively regulate its internal salt balance across a wide range of salinities. Its ability to adjust its bodily functions makes it euryhaline, meaning it can move between marine and freshwater habitats.
When in freshwater, the bull shark’s kidneys play a crucial role, producing large quantities of dilute urine to excrete excess water absorbed from the environment. This increased urination helps prevent cellular swelling. The liver also adapts by reducing its production of urea, and the rectal gland decreases its salt-excreting activity to conserve sodium and chloride.
The gills of the bull shark also contribute to osmoregulation in freshwater by actively taking up sodium and chloride ions from the dilute water. This coordinated effort among the kidneys, liver, gills, and rectal gland ensures the bull shark can maintain its internal salt and water balance. This physiological flexibility allows bull sharks to survive in freshwater for extended periods.
Pathways to Inland Waters
Bull sharks access inland freshwater bodies by navigating river systems connected to the ocean. Their ability to tolerate varying salinities allows them to travel through brackish estuaries and hundreds, sometimes thousands, of miles upstream into rivers. For instance, bull sharks have been found over 2,400 miles up the Amazon River and as far as Alton, Illinois, in the Mississippi River, approximately 700 miles from the ocean.
While they can inhabit lakes, these are typically not isolated, landlocked bodies of water. Instead, these lakes are connected to major river systems or the ocean, providing a pathway for the sharks to enter and exit. Lake Nicaragua is a well-known example where bull sharks migrate from the Caribbean Sea via the San Juan River. This journey sometimes involves navigating rapids.
Bull sharks’ presence in inland waters is often linked to their life cycle. Pregnant females frequently move into low-salinity rivers and estuaries to give birth, providing a safer nursery environment for their young away from larger marine predators. Juvenile bull sharks may remain in these freshwater nurseries for several years before venturing into saltier waters as they mature. This migratory behavior highlights that their presence is often part of a broader movement pattern between marine and freshwater habitats.