The Boundary-Breaking Shark
When we think of sharks, we picture vast ocean dwellers and kings of endless blue horizons. Most species are strictly found in saltwater and are designed to survive in a stable marine environment. Yet, some species quietly defy that rule – moving between sea and river as if the boundary does not exist.
Bull shark (Carcharhinus leucas) is one of the very few species of sharks known to be able to live in freshwater for extended periods of time, thanks to their euryhaline nature – the ability to adapt to a wide range of salinity (salt level) in the water. This remarkable species can be found in various parts of the world in coastal areas, estuaries, and tropical as well as subtropical rivers, though they usually prefer habitats with a salinity level above 11 ‰.
Scientific Classification
- Kingdom : Animalia
- Phylum : Chordata
- Order : Chondrichthyes
- Class : Carcharhiniformes
- Family : Carcharhinidae
- Genus : Carcharhinus
- Species name : Carcharhinus leucas (Valenciennes, 1839)
Mastering Salinity Shifts
How can bull sharks survive in freshwater while other sharks can’t? The key is in their osmoregulation, which is their ability to maintain water and salt levels in their body. Marine elasmobranchs, such as sharks and rays, do osmoregulation by having high concentrations of urea in their blood and removing extra salt with their rectal gland. Bull sharks, on the other hand, can also adjust their osmoregulation in freshwater by reducing urea in their blood, together with removing extra water by excreting very dilute urine. In simple terms, they have amazing control over what stays inside and what must be released to keep their fluids in balance.
The bull shark’s ability to adapt to freshwater environments not only allows them to survive in many habitats, but also supports their role as top predator that poses a potential danger to humans. That ability, combined with their size and proximity to humans, makes bull sharks more dangerous than even great white and tiger sharks. However, their presence around humans also poses some threats to the bull sharks themselves, such as habitat destruction, being caught as bycatch, or even as the main target due to the high market value of their fins. This has caused a decline in the bull shark population, which led to their current classification as Vulnerable, according to IUCN.
Preserve the Predator
Bull sharks, like most apex predators, play an important role in maintaining the balance of the food chains by controlling the populations of their prey, which is beneficial to coral reefs and seagrass beds. Understanding their ecological dynamics is therefore essential as a first step in supporting their long-term survival.
At Marine Safari Bali, bull sharks are cared for as a part of an ongoing commitment to promote education, awareness and responsible stewardship. Through dedicated programs, guests can learn more about these resilient hunters and even swim in the same water with them in a safe, guided environment. Beyond experience, these programs bring you to explore their world and inspire awareness about them to ultimately preserve the world that we share.
References:
Carrier, J.C., Musick, J.A. and Heithaus, M.R. (eds.) (2012) Biology of Sharks and Their Relatives. 2nd edn. Boca Raton: CRC Press.
Florida Museum of Natural History (2024) Bull shark (Carcharhinus leucas). Available at: https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/bull-shark/ (Accessed: 27 February 2026).
Heupel, M.R. and Simpfendorfer, C.A. (2008) Movement and distribution of young bull sharks Carcharhinus leucas in a variable estuarine environment. Aquatic Biology, 1, pp. 277–289. Available at: https://doi.org/10.3354/ab00030 (Accessed: 27 February 2026).
Hight, B.V. and Lowe, C.G. (2007) Movement patterns and residency of bull sharks, Carcharhinus leucas, in a marine protected area of the Gulf of California. Marine Biology, 152(2), pp. 371–381. Available at: https://doi.org/10.1007/s00227-007-0692-1 (Accessed: 27 February 2026).
International Fund for Animal Welfare (n.d.) Bull sharks: Facts, habitat, diet and conservation. Available at: https://www.ifaw.org/animals/bull-sharks (Accessed: 27 February 2026).
Kronman, D. (2021) Species spotlight: The bull shark. Shark Stewards. Available at: https://sharkstewards.org/species-spotlight-the-bull-shark/ (Accessed: 27 February 2026).The Aquarium of the Pacific (2024) Bull shark. Available at: https://www.aquariumofpacific.org/onlinelearningcenter/species/bull_shark (Accessed: 27 February 2026).