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Pistol crabs use cavitation as a Navy shock: High Noon on the sea bed!

The colt rests easy among pistol crabs (Alpheus Saxidomus, from the group of Alpheidae), which roam coral reefs in tropical waters. Who would have thought it? Instead of giving it a second claw Mother Nature has equipped this creature with a shooting device, which it uses to kill fish with cavitation bubbles and fight underwater battles that even the US-NAVY once learned to fear.

Pistol crabs (Alpheus Saxidomus) ready for a shoot out in a saloon, an illustration by Bruno Haberzettl
Illustration by Bruno Haberzettl

You must have seen this animal with your own eyes to believe that it really exists: Instead of conventional claws, pistol crabs or pistol shrimps, as they are also called, have been equipped with a impressive fire-arm on one side by nature.

With the help of a special muscle alignment and a sort of squirt-gun barrel they can accelerate a jet of water on short range to more than 90 km/h. When they shoot, the tensed "pincer-valve" snaps down in less than one thousandth of a second. Through this brute acceleration the water particles in the immediate vicinity heat up to more than 4000 (!) degrees celsius. The resulting ultra hot cavitation-steam-bubble then implodes with a flash of light and a bang that at 240 decibels even dwarfs the roar of a jet fighter (about 150 decibels).

"Don’t move in too close over the water tank!", Dr. Barbara Schmitz, Professor at the Zoological Institute of the Technical University in Munich and pistol shrimp expert, advises her visitors: "It can get you in the eye."

No, it is not that the graceful female pistol crab scurrying nimbly over the gravel floor in her mini-aquarium for testing purposes, could be dangerous to human beings, but: "If they fire their weapons directly upwards the water can sometimes spray into your face. And with a bit of bad luck it could knock out your contact lenses", smiles Barbara Schmitz.

Harmless little creatures then? By no means! On an unbelievable video clip by a Canadian research team you can see how a pistol crab stalks a nearing coral fish of the same size from behind, seizes it at lightening speed with its left claw and then repeateldy shoots it in the head with the right pincer until the fish no longer moves. – big game hunting in a jungle of corals!

When two of these "Claw-and-Order Crabs" come across one another on free hunting-ground it tends to become quite rough: First they shoot then they negotiate. In a pistol duel among fellows the opponents maintain a strict safety distance. Because the underlying goal of their shooting in this case is not to kill or wound the opponent, but to find out who has the bigger gun: the larger the jet of water fired, the more powerful the fire arm and the stronger its owner.

Whilst other animals (e.g. lions or stags) are often injured when fighting for rank, these cunning little creatures fight it out with menacing looks and displays of muscle. And that is not all to be said, as Jens Herberholz, a colleague of Barbara Schmitz, found out: "With the help of these pseudo-duels a pistol shrimp even detects whether he has a female or a male at gunpoint." – A discovery that can quickly lead to a truce and sometimes even to a family together...

Shooting prey, gun fights with the "nice neighbours", impressive displays in front of lovers: that makes for quite a soundscape on such a densely populated coral reef. A noise that, thanks to the good acoustic properties of water, can be heard more than a mile away.

Near such a colony of pistol-shrimps a diver has the impression of sizzling fat. Or as if Neptune hat just put a bag of popcorn in the microwave. The continuous firing by these miniature hotshots can be so penetrating that it affects the sonar systems in submarines: In World War II the US Navy suspected a Russian sneak weapon off the coast of Florida, because they could not explain the ear-piercing noise in the headsets of the submarine hunters. Looking back it was just two gangs of pistol crabs going at each other in their weekly shoot-out. High Noon in the Florida Keys! – Once upon a crab in the west...

With their pistols firing warning shots the reef-ruffians can keep large fish and other enemies relatively well at large. But what happens if such a coral fish eventually closes in with a snap and catches the weapon arm? Then without further ado fascinating nature once again actuates the growth programme for the other arm: A new shooting device forms from the left pincer, the right one grows back as a normal pincer, and the revolver-hero becomes a left-handed shooter. He will only need a bit of target practice then. p>

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© A report by Tobias Micke (22-10-01) – Contact