Several species of shark are known to live in a freshwater environment, but whether they should be considered true freshwater fish is debatable.
Probably the most mysterious are the river sharks of the genus Glyphis. Fewer than ten species have been identified in watercourses around south-east Asia and Australia, with some still waiting to be officially classified, and all are extremely rare.
Much better known is the bull shark (Carcharhinus leucas). This worldwide species enters warm water estuaries from the ocean and then swims into fresher water upstream. If you see an adult bull shark from a kayak, you might wish you had a bigger boat: they are large and aggressive predators and have been known to attack humans.
A notable characteristic of bull sharks living in freshwater habitats is that they excrete a large amount of urine. Sharks that evolved in the ocean have a great deal of salt naturally present in their bodies to prevent them from losing water to the sea through osmosis.
In the non-saline water of rivers and lakes bull sharks have the opposite problem: they would swell up like a water balloon without a way to get rid of the excess freshwater that their bodies absorb. Hence, they pee a lot!