One shark species can swim 2,500 miles up the Amazon and be perfectly fine

Bull sharks have specialized kidneys and a rectal gland that let them retain the salt marine animals need, so they can tolerate freshwater that would kill most other sharks. They've been documented far up the Mississippi and Amazon rivers, and even living in landlocked Lake Nicaragua.

— Marine biology literature, e.g. Thorson (1971) on elasmobranch osmoregulation, Bull shark — —