A piranha’s jaws contain rows of triangular teeth, which interlock like rows of flesh-shearing scissors. They’re powered by huge muscles that take up most of the space in the fish’s head, giving it the strongest bite (for its body weight) of any back-boned animal.

But the arapaima doesn’t care. This giant South American fish has evolved piranha-proof armour, allowing it to happily share water with schools of these fearsome predators. For the piranhas, it must be like finding that the juiciest steak in the supermarket is coated with an impregnable shield.

The arapaima, with its streamlined body and flattened head, looks like a cross between a torpedo and a doorstop. It can grow to over 2 metres long and the