Most sharks would die quickly if you dropped them in fresh water - their bodies are tuned for salty seawater, and the wrong balance would kill them. Bull sharks are different. They’re built to handle both salt water and fresh water, and they regularly swim up rivers far from the sea.
The trick is their kidneys and other specialized organs, which constantly adjust their internal salt balance depending on where they are. When they swim into a river, the kidneys ramp up to keep more salt inside the body and dump excess water. When they go back to sea, they reverse.
Bull sharks have been recorded 700 miles up the Amazon and almost 2,000 miles up the Mississippi. There’s a permanent population of bull sharks in Lake Nicaragua, hundreds of miles from the ocean. A few have even shown up in Australian golf course ponds during floods. Almost nowhere is truly “out of reach” for a bull shark.
