Shrimp wields strongest club in the world

The snail never saw it coming. One minute it was creeping slowly along the seabed, minding its own business. Then a lightning hammer blow fell, and in an instant its shell was smashed and it was at the mercy of a ruthless predator.

In this case, the phrase "hammer blow" is used literally. The peacock mantis shrimp comes equipped with two built-in hammers that can deliver swift and powerful blows, smashing straight through other animals' armour.

But that raises a question. To break through a hard thing, you have to be even harder, or you'll just break yourself. So what is it about the hammer of the peacock mantis shrimp that makes it so tough?

Clubbed to death

Mantis shrimps are not shrimps, though they do belong to the same group. They are, however, aggressive predators. What's more, some mantis shrimps can see circular polarised light, which is used to create 3D movies. The colourful patterns on their bodies reflect polarised light, which may be used as threatening signals.

There are two types of mantis shrimp, with different modifications to their front claws. Some have turned the claws into spears to impale their prey, while others – like the peacock mantis shrimp – have turned them into clubs.

Peacock mantis shrimps can deliver one of the fastest blows in the animal kingdom. Their clubs can move at up to 23 metres per second, and accelerate at over 10,000 gs. That means they can deliver a force of about 700 newtons – equivalent to the weight of a 70-kilogram mass – or even as much as 1500 newtons.

With a weapon like that, it's no surprise they can batter through the shells of molluscs and crabs. Larger peacock mantis shrimps have even been known to smash their way out of fish tanks. What's more, the blows are so powerful they cause cavitation: they create tiny bubbles in the water that instantly implode, delivering even more energy to their victims.

Tough nut

So why don't their clubs break? The shrimps do eventually replace each club when they moult, but not before they have delivered thousands of powerful impacts. David Kisailus at the University of California, Riverside, and his colleagues looked at their structure to find out.

They found that the head of the club is divided into three main layers. The outer impact region is mostly made of a tough mineral called hydroxyapatite, which is also found in bone. The hydroxyapatite is arranged in neat crystals, making for a strong coating.

In the second layer, the hydroxyapatite is less organised. Beneath that is a layer of chitin, the polymer that crustaceans use to make their shells. The three layers differ in how bendy they are, so it's hard for a crack that forms in one layer to extend into the next, so cracks don't spread far. What's more, the chitin is arranged in spirals. That means any cracks in the chitin tend to grow in spiral patterns, which barely weaken the overall structure.