Bull Shark

This is one of 43 shark species that can live in both seawater and freshwater. That fact alone makes this one of the best known shark species.

But this shark is also known as one of the top 3 sharks most likely to attack humans (the other 2 are the Great White Shark and the Tiger Shark). Of course, we know that shark attacks are extremely rare (you're more likely to be struck by lightning), but nonetheless, this shark has been nicknamed "The Pit Bull of the Sea" because if its aggressive behavior.
Most sharks have the same salt concentration in the blood as the sea water they are swimming in. This isn't the case with Bull Sharks. Instead, they only have 50% of the salt concentration in their blood.

These special hunters are migrants. They are found in many various areas including Oceans, rivers, and even some fresh water lakes! They tend to stay in warm and moderately deep waters around 30 to 150 meters (150 to 500ft) deep.
It seems the Bull Shark favors the Atlantic, Indian, and Pacific Oceans. Some of their favorite rivers include the Brisbane River, Amazon River, Ganges River, Brahmaputra River, Potomac River and the Mississippi River. They have been spotted several hundred miles upstream in these rivers, but typically stay within 100 miles of the Ocean. No attacks on humans have ever been recorded in these areas.