How a tiny shrimp fires a savage shock wave using just its claw

27 October 2017

By Aylin Woodward

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 Phoevos Koukouvinis 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.

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