51����

This reef ain’t big enough for the both of us. Two pistol shrimp face each other, each spreading open its giant snapping claw – nearly half the size of its body. One or both of them then snaps the claw shut in its opponent’s direction, firing off a powerful water jet at speeds up to 30 metres per second.

These shrimp shootouts are rarely fatal, but can leave the loser retreating with missing claws or puncture wounds. But the high-speed squirt isn’t what harms their target – it’s the resulting shock wave. Now we have glimpsed how this unfolds in fine detail.

If you stick your head under coastal tropical waters, you may hear a sound like chestnuts crackling as they roast. At a volume of about 200 decibels – louder than a .22 calibre rifle shot – these pops are some of the loudest in the ocean, second only to sperm whale clicks.

Originally, marine scientists thought the sounds were produced by the impact of the shrimps’ claws closing. Now, we know that they ring out when an air bubble collapses around the watery salvo, much as when bubbles form in our joints and rapidly collapse as we crack our knuckles.

Prior work didn’t explore precisely how this bubble forms, says at the City University of London. “We knew the bubbles were there, but we didn’t know what they looked like.” So his team sought to unravel the mystery by simulating what happens after the shrimp shuts its claw at different speeds.

They found that when the claw snaps shut, friction between the fast-moving jet and the still water surrounding it creates a swirling vortex. Once the vortex starts spinning rapidly enough, it leaves a void in the center, like the dimple in your tea as you stir it.

Eventually that void collapses, and as it does, it releases a powerful pressure wave. All of this happens in less than half a millisecond, Koukouvinis says.

Because these shrimp only grow to about 5 centimetres, their underwater weapons are limited to targeting animals of a similarly minuscule size. But at the Smithsonian Institution in Washington DC doesn’t recommend sticking your hand in any tank holding the shrimp.

“You wouldn’t feel anything if you’re just snorkelling near a reef, but if you have your hand in front of a shrimp’s claw you’re definitely feeling it when it goes off,” she says. “It hurts.”

And that’s the point. Pistol shrimp are scavengers, eating detritus on the sandy ocean floor, so they don’t often use their bubble-gun claws to stun prey, Knowlton says. Instead, they use their shock waves to protect their mates and homes in some of the tiniest gunfights on earth.