Electric eels.
Credit: Franco Banfi / Getty Images

Why they're dangerous: Part of the reason these South American fish are so hazardous is that they look relatively harmless. But Wade says a large one can give you a major jolt. "If you accidentally get too close to an electric eel and it perceives that as a threat, then it will produce a big shock," he says. "You don't get a lot of fatalities, but quite a lot of people get zapped by them. You don't even have to be in physical contact with one." And occasionally electric eels do kill people. "[A big shock] will basically paralyze your muscles, and if you're unlucky and face down, you're going to drown very quickly." Wade once heard a story about a man who fell in the water and was attacked by an especially angry electric eel. "The next thing that happened, which is particularly creepy, is the fish wound itself around the man's chest and continued to shock. It actually killed him by stopping the heart."

How to stay safe: Avoid the sort of places where electric eels hang out. "They tend to have definite hideouts," says Wade. "They're quite territorial. They like living in caves and holes and under rocks. If you see an electric eel, don't swim or wash there. That's their home and they're not going to take kindly to uninvited visitors."