Notorious for their sharp teeth and voracious appetites, piranhas inhabit several of the major river basins in South America. These omnivorous fish are known for their taste for meat, although attacks on human beings are quite rare, despite breathless accounts from early explorers.
In a historic visit to Brazil, Theodore Roosevelt famously saw a group of piranhas shredding pieces of a cow carcass in seconds. His dramatic account would color popular imagination for years, even though it was based on a manipulated spectacle in which fishermen blocked off a group of the fish and starved them beforehand.
Still, piranhas are important scavengers and predators in their native rivers, and they often resort to cannibalism if food gets scarce. It's true that local fishermen occasionally have scars from close encounters with them.
It's unknown how many species of piranhas exist, with estimates ranging from 30 to 60. —Brian Clark Howard
Related: Megafishes of the World