View Images Piranha jaws. Credit: Silk666

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 heaviest specimen weighed 200 kilograms, making it one of the largest freshwater fish in the world. And it is surely one of the toughest.

A few years back, materials scientist Marc Meyers arranged a bloodless bout between arapaima and piranha, in his lab at from the University of California, San Diego. He mounted arapaima scales on a rubbery surface, and pressed into them with piranha teeth attached to an industrial hole-puncher. The teeth managed to penetrate the scale, but only slightly. Before they reached the underlying “flesh”, they cracked.

The scales’ toughness comes from their microscopic structure. A hard outer layer resists the penetration of a piranha’s tooth, but is also relatively brittle. Thankfully, a soft but tough inner layer absorbs the incoming force so that the whole scale doesn’t snap. “It’s how body armour should be made,” says Robert Ritchie from the Lawrence Berkeley National Laboratory, who is working with Meyers to study the scales. “If it was just the hard shell, the thing would shatter.”

View Images Structure of arapaima scales. From Chen et al, J. Mater. Res., 2011

The properties of the outer layer are easily explained. It’s heavily mineralised, which makes it hard. It also has a corrugated shape to give it flexibility—cut through it, and you’d see what looks like a row of dark mountains. That allows the layer to bend without breaking, so the arapaima can flex and move its body while still repelling piranha jaws.

The inner layer is more interesting. It’s made largely of collagen, the same protein found throughout your flesh. Long strands of collagen are bundled to form fibres, each a micrometre (a millionth of a metre) thick. The fibres are arranged in parallel sheets, and each is rotated slightly against the one above it. This is technically known as a “Bouligand-type” structure. “I call it a Liberace-type spiral staircase,” says Ritchie.

That’s the default shape, but it can change. When the team placed the scales in an X-ray beam and applied force to them, they saw that the collagen fibres lose their neat spiral arrangement. Suddenly, they realign to face the direction of the force. This microscopic shape-shifting toughens the scale—imagine trying to break a pencil by pushing or pulling against its tips, rather than on its midpoint.

View Images Arapaima head, by Jeff Kubina.

There are plenty of other examples in nature where microscopic cylinders are aligned to resist incoming forces. Ritchie also studies the abalone, an edible sea snail with a phenomenally hard shell. It has a series of mineral plates, which are all aligned perpendicular to the surface. The arapaima scale behaves in the same way, but the amazing thing is that it does so in real-time. It has adaptable body armour!

Ritchie suspects that the structure of the arapaima’s scales may inspire human engineers who are designing new types of body armour. “Lightweight body armour is something everyone wants,” he says. “The Kevlar armour our troops get is extremely heavy and many people don’t bother wearing them. But nature does it very well.”