Former American president and traveller Theodore Roosevelt came back from South America in 1914 with a (probably apocryphal) tale about a school of piranhas (Pygocentrus nattereri) eating a soldier down to the bone in seconds after he fell off his horse into a river. Word spread and the piranha, admittedly a fast eater and somewhat aggressive, has had a bad press ever since. Piranhas, with razor-sharp teeth, hunt in groups of 5-30, taking chunks out of anything moving near them - usually fish, birds and mammals. Once the first bite releases the scent of blood they all join in the feeding frenzy, lashing the water into a red froth. The piranha is a popular aquarium pet worldwide but has been banned in China, where it is feared that released fish would devastate indigenous species. Fish released into North American rivers are assiduously exterminated by the authorities.