The Bull shark is the only known shark with the ability to tolerate both highly saline and completely fresh water. It has been identified as the Lake Nicaragua shark, which comes from (surprise, surprise) Lake Nicaragua, and the Zambezi shark of Africa. The Bull has been found 1750 miles up the Mississippi River, and 2500 miles up the Amazon! The Bull shark was probably responsible for the Matawan Creek attacks (blamed on Great Whites.) (F.Y.I.: Matawan Creek is a creek in New Jersey that flows into the sea. Back in the beginning of the 20th century, an old man spotted a large shark in the creek. No one believed him, and nothing was done about it. A little while later, a few boys were swimming in the creek after work. One was attacked and killed by a shark. In a vain attempt to save the boy's body, another man was killed. The shark continued upstream and took another man's leg off, but he survived.) The Bull shark is considered highly dangerous, but we have no accurate record of how many attacks by Bull sharks have happened, because of the Bull shark's resemblence to other requiem sharks, such as the Tiger shark and the various reef sharks.
