But the greatest real-life gunslingers have to be the pistol shrimp, aka the snapping shrimp, hundreds of species with an enormous claw they use to fire bullets of bubbles at foes, knocking them out cold or even killing them. The resulting sound is an incredible 210 decibels, far louder than an actual gunshot, which averages around 150.
Pound for pound, pistol shrimps are some of the most powerful, most raucous critters on Earth. Yet at the same time, they are quite vulnerable, allying with all manner of creatures and even forming bizarre societies to protect themselves from the many menaces of the ocean bottom.
The Life Despotic
The pistol shrimp has two claws, a small pincer, and an enormous snapper. The snapper, which can grow to up to half the length of the shrimp’s body, does not have two symmetrical halves like the pincer. Instead, half of it is immobile, called a propus, which has a socket. The other half, called a dactyl, is the mobile part. It has a plunger that fits into this socket. The shrimp opens the dactyl by co-contracting both an opener and closer muscle. This builds tension until another closer muscle contracts, setting the whole thing off with incredible force.
When the plunger slams into this socket it displaces water that jets out at 105 feet a second, a velocity so high that its pressure drops below the vapor pressure of water. Tiny bubbles already present in the water suddenly swell in this low pressure, then collapse when the pressure climbs again.
“You essentially create this cavitation bubble,” said coral reef biologist Nancy Knowlton of the Smithsonian Institute. “And when the bubble collapses, it generates that snap sound,” as opposed to the impact of the claws themselves making the noise.
The creature has weaponized bubbles. (Another ocean critter, the mantis shrimp, produces cavitation bubbles when it rapidly strikes prey with its clubs, though it’s unclear if it evolved to use the bubbles or if they’re just a welcome destructive side effect). And pistol shrimp are far from conservative with their snapping. In fact, vast colonies of them all firing off at each other or their prey have been known to disrupt sonar equipment with a cacophony likened to the crackling of burning twigs. Yet there is an upside to all the rackets.
“Anywhere you go swimming in the tropics, if you listen around you, you can hear these things,” said Knowlton. “And some people have argued that instead of thinking about snapping shrimp sounds like an obstacle to hearing what it is you’re trying to hear, you can also use the sounds of snapping shrimp and other things on coral reefs to assess the health of the reefs.” Lots of noise means lots of life (and lots of pissed off sonar operators).
Like many crustaceans, the pistol shrimp will willingly shed its claws if attacked, because it’s better to lose a pincer than your life, but that won’t stop the noise for long. As complicated a structure as its snapping claw is, the shrimp will grow another—in a surprising way.