The Bull Shark
Able to thrive in seawater, fresh water, and anywhere in between, this shark has a well-earned reputation for being very aggressive, especially towards humans. In addition to their top-three spot on the attacks-by-species list, they're a prime suspect in the majority of attacks where the shark was unidentified. Offically weighing nearly 700 pounds, they are regularly measured at 11 feet long but are believed to reach up to 13 feet. Bull sharks have been seen as far upriver as Illinois via the Mississippi. If all this wasn't scary enough, they're frequently known to cannibalize their own.
(Corbis/RooM the Agency/jchauser)