Piranhas are non-poisonous freshwater fish belonging to the genus Serrasalmus. The body is compressed sideways. They have heavy jaws and long pointed teeth. These predatory fish occur in certain rivers in South America. They feed on fish, chiefly sick and wounded animals, including their own species. They play an important part in the maintenance of a healthy fish population. Of the 18 species three are potentially dangerous to humans: S. piraya (river basin of the Rio San Francisco), S. ternetzi (Rio Paraguay) and S. nattereri (Amazon). These species may be dangerous if they are enclosed in a small part of the river, so that they are short of food. Attracted by the smell of blood in the water, they may then be overtaken by a kind of madness, in which a large prey is attacked and eaten (feeding frenzy).