Why they're dangerous: Um, look at the picture. A relative of the piranha, these frightening behemoths live in the central part of the Congo. "There are stories of them being 200 pounds - bigger than a person," says Wade. "They bite lumps out of crocodiles, they will bite lumps out of each other, and they occasionally bite lumps out of people. I've heard of them taking fingers off - people just trailing their fingers off of the side of a [boat]. I saw a woman who had her Achilles tendon severed by one. She's basically crippled for life because this thing latched onto her ankle." And if the stories are true, then she might have been one of the lucky ones. "The myth is that one of the preferred delicacies of this fish is human male genitalia. They strike from below and make off with it. That's what everybody says. We haven't tracked down anybody who that actually happened to."
How to stay safe: Don't trail your hand over the side of the boat, and don't wear shiny watches or other objects that might trigger their feeding reflex. "If you're out in the river and you suddenly think, 'Oh, this isn't a good place to be,' don't act panicked," says Wade. "That splashing and commotion is what could flip the switch."