What do bull sharks, the Zambezi shark, van Rooyen’s shark, and Lake Nicaragua shark have in common?  A scientific name: Carcharhinus leucas.

Bull sharks often travel into freshwaters like Lake Nicaragua. For many years, scientists thought the Lake Nicaraguan sharks were a separate species, landlocked in the lake. When they tagged the sharks in the late 1960s, they saw the sharks could navigate the rapids of the 175 km-long San Juan River and reach the lake within 2-25 days.

Bull sharks are known for river cruisin’ in other parts of the world, too. They’ve even found them 4,000 km up the Amazon River, as well as in the Ganges, Mississippi, and Brisbane Rivers. Nobody’s sure why they make the trip. Maybe the eating’s better in lakes and rivers from time to time.

All fishes have to maintain a salt and ion balance in their bodies, so how does this saltwater species deal with adjustment to freshwater’s salinity? Well, bull sharks pee more in fresh water.

Sharks like to keep themselves a little salty. In oceans and rivers, sharks’ body fluids are saltier (hyper-osmotic) than the water they are navigating. Freshwater voyaging sharks get their briny bodies by huffing more water through their gills; they end up peeing more. Ion transport in the gills helps them reabsorb Na+ and Cl-. 

Other less-familiar sharks and rays take their own river cruises. The Ganges shark, Glyphis gangeticus, has been confused with its cousin, the bull shark, probably due to its propensity for Huck Finning it.


Check our facts!

Branchial osmoregulation in the euryhaline bull shark, Carcharhinus leucas: a molecular analysis of ion transporters Beau D. Reilly, Rebecca L. Cramp, Jonathan M. Wilson, Hamish A. Campbell, Craig E. Franklin. Journal of Experimental Biology 2011 214: 2883-2895

Plasma osmolyte concentrations and rectal gland mass of bull sharks Carcharhinus leucas, captured along a salinity gradient Richard D. Pillans, Craig E. Franklin. Comparative Biochemistry and Physiology, Part A 138 (2004): 363-371

The status of the bull shark, Carcharhinus leucas, in the Amazon River Thomas B. Thorson. Copeia 1972(3): 601-605.

Movement of bull sharks, Carcharhinus leucas, between Caribbean Sea and Lake Nicaragua demonstrated by tagging Thomas B. Thorson Copeia 1971(2): 336-338




Written by Julia Ellis, illustrated by Hoot.

Julia Ellis was once smacked in the face by a large striped bass she was trying to tag. Now recovered, she maintains her interest in fish and wildlife while working as a freelance writer and fact-checker. A nascent tweeter, she can be reached at @seashell_eyes.