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.
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.
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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 lightning on a golf course" than being bitten by a shark, Carlson said.
Bull sharks can survive in freshwater by regulating the amount of salt in their bodies. 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.
The boy is expected to recover from the incident.
—Tanya Lewis, LiveScience
This is a condensed version of a report from LiveScience. Read the full report. Follow Tanya Lewis on Twitter and Google+. Follow us @livescience, Facebook & Google+.