Piranhas are fish that live in the Amazon River and lakes ... isn't formed because the piranhas realise they'll be more dangerous when hunting together - it's actually a defensive tactic.

These Jurassic marauders used their razor teeth to tear chunks of flesh and fins off other fish ... but most scientists believe that the group approach is to protect piranhas from predators.

They also have long venomous spines that carry deadly poison, making them one of the most dangerous fish in the ocean ... attacking boats in recent years. Piranhas also have a bad reputation ...