The idea of a shark attacking someone in the ocean is scary enough, but this week, a 7-year-old boy was bitten by one of these fearsome fish in a lake.
The boy was swimming in Lake Pontchartrain in Louisiana, when something bumped into him in the water and chomped down on his foot, USA Today reported. The bite's appearance suggests it was probably a bull shark measuring about 5 feet (1.5 meters) long, experts say. The boy is expected to recover from the incident.
While it may seem unusual for a shark to turn up in this part of Louisiana, Lake Pontchartrain isn't strictly a lake -- it's an estuary, a coastal body of brackish water connected by rivers or streams to the open ocean. The lake is connected to the Rigolets strait and Chef Menteur Pass to Lake Borgne, which opens into the Gulf of Mexico. [8 Weird Facts About Sharks]
Bull sharks can survive in both saltwater and freshwater, and have been known to frequent the lake. But shark attacks are extremely rare, said John Carlson, a research biologist at the National Oceanic and Atmospheric Administration's Fisheries Service in Panama City, Florida.
"I can't recall the last time there was a shark attack in Lake Pontchartrain," Carlson told Live Science.
Sharks don't usually attack humans. In fact, "you have a better chance of being struck by lighting on a golf course" than being bitten by a shark, Carlson said.
When it does happen though, as in this case, a shark will commonly mistake the flashing movement of a human foot for its natural prey -- a mullet or catfish, for example. The shark will go up and bite it, and then realize it's not food. "We call it 'bite and run,'" Carlson said.
Bull sharks can survive in freshwater by regulating the amount of salt in their bodies. That's why when some sharks are cut open, they smell like urine, Carlson said. Salt in a shark's body normally has a tendency to make it swell up with water, but bull sharks have a special salt gland that keeps the concentration of sodium chloride inside their bodies in balance with the concentration outside -- a phenomenon known as osmoregulation.
In addition to the bull shark in Lake Pontchartrain, there have been accounts of other sharks found in lakes. In July 2013, a man and his grandson spotted what may have been a great white shark in Lake Macquarie, Australia's largest coastal saltwater lake, the Newcastle Herald reported.
Great white sharks generally aren't found in estuaries, however -- they usually stick to more coastal regions, Carlson said.
If you're going to swim in places that sharks are known to frequent, however, it's best to avoid areas or times of day when the sharks are more active and water visibility is low, such as dawn or dusk, Carlson said. Avoid wearing shiny jewelry, he said, because it can resemble a struggling baitfish. And there is safety in numbers, so swim in groups, and don't swim near fishermen, he said.