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Bull sharks are the least known shark species responsible for attacks on humans.

Bull Shark Threat: They Swim Where We Swim

The predators have been linked to two highly publicized attacks in Florida that left one teen dead and another seriously injured.

Though over 375 shark species have been identified by science, just three species are responsible for most attacks on humans: the great white (Carcharodon carcharias), tiger (Galeocerdo cuvier), and bull shark (Carcharhinus leucas).

Bull sharks are the least known of the three. But experts note that the species's preference for coastal waters less than a hundred feet (30 meters) deep makes bulls potentially the most dangerous sharks of all.

"Bull sharks inhabit quite shallow waters, which means that they do have a great opportunity to interact with humans, because the two species tend to share the same areas," said George Burgess, curator of the International Shark Attack File at the Florida Museum of Natural History in Gainesville.

Bull sharks are among the most common sharks in Florida waters and are often encountered by divers.

Shark Attacks

The sharks are especially at home in areas with lots of freshwater inflow, such as brackish river mouths. The abundance of such habitat along the coasts of the northern Gulf of Mexico east of the Mississippi River makes this area especially suited to the sharks.

Bull sharks happily tolerate the murky water found in estuaries and bays. Such conditions can sometimes play a role in spurring shark attacks on humans.

"Visibility is a huge factor in shark attacks," Burgess said. "That's one of the reasons we suggest that people avoid murky water situations when they go into the water."

For the most part, bull sharks dine on bony fishes or smaller sharks—but they sometimes aggressively tackle much larger prey.

"They are one of the few warm-water, coastal sharks that will attack big prey," said Mike Heithaus, a shark expert at Florida International University in Biscayne Bay. "In addition to small fish, they might attack a sea turtle, another shark, or the occasional dolphin."

"They are one of the few sharks that will tangle with prey that's the same size or even bigger than them," the marine biologist added. "Most sharks only go after prey that's substantially smaller than they are."

Bull sharks generally grow to about 7.5 feet (2.3 meters) long and weigh up to 285 pounds (130 kilograms).

Despite their healthy appetite and aggressive reputation, the animals can be rather docile in some environments. In the clear waters of the Bahamas, for example, divers regularly interact with crowds of bull sharks.

Though the predators may come in close proximity to humans, statistics suggest that swimmers, surfers, and divers have little to fear from bull sharks.

The United States averages just 16 shark attacks each year and slightly fewer than one shark-attack fatality every two years. Meanwhile, lightning kills more than 41 people each year in the coastal U.S. states alone.

Low-Salt Habitat

While bull sharks are commonly found along coastlines, bays, and harbors, they also frequent a most uncommon shark habitat—freshwater rivers.

The species has been spotted 2,500 miles (4,000 kilometers) up the Amazon River in South America and dwell in Lake Nicaragua, a freshwater lake in Central America. Bull sharks have traveled up the Mississippi River as far north as Illinois and are regularly spotted in India's Ganges.

Their ability to tolerate freshwater is rooted in salt retention.

Sharks must retain salt inside their bodies. Without it, their cells will rupture and cause bloating and death. Given this requirement, most sharks cannot enter fresh water, because their internal salt levels would become diluted.

But bull sharks have special physiological adaptations that enable them to live in fresh water. Their kidneys recycle the salt within their bodies and special glands, located near their tails, also aid in salt retention.

While scientists have learned how the animals survive in fresh water, it is less clear why bull sharks, almost exclusively, developed this amazing ability.

Heithaus, of Florida International University, speculates that "probably the biggest reason is that [freshwater tolerance] allows the juveniles, the little guys, to be in a place that's relatively safe from being eaten by other sharks."

Adult bull sharks likely gain their own competitive advantages from salt retention. However, scientists have yet to uncover precisely what those advantages may be.

Heithaus said a big question for him is what bull sharks gave up to acquire their unique ability to survive in fresh water. "If they were a master of all trades, in both fresh and saltwater, we should see bull sharks dominating coastal waters," the marine biologist said. "There must be some cost to having that amazing ability."

Freshwater tolerance could be rooted in competition for saltwater food resources, where perhaps bull sharks suffered and needed to develop an edge. The ability might also be tied to disease susceptibility or other unknown and unstudied issues.

What's certain is that much more research is needed to understand why these unique sharks turn up in such unlikely locales as the Land of Lincoln. "It's amazing how little we really know," Heithaus said.