Do not be deceived by the small size of this creature--that lumpy shape by the pistol shrimp's head is its claw. It's specially modified to blow the shit out of its opponent by snapping shut so quickly it produces a flash of light and a blast of sound reaching 218 decibels, which science says is louder than a damned gunshot.
The sound isn't caused by the claws snapping together, but rather a jet of water which is shot at 60 mph. Due to the, um, "underwateriness" of the action scene, a low pressure bubble is formed and the sound is created when the bubble collapses. And the sound is so powerful it stuns the opponent into a somewhat retarded stupor, leaving Mr. Pistol to reap the rewards and tuck into a lovely comatose crustacean snack.
Wait, it gets weirder. The sheer force of the bubble collapse means it reaches temperatures up to 18,000 degrees Fahrenheit, a.k.a. hotter than the surface of the fucking sun.
In theory this means if you could train a bunch of these shrimp to shoot at each other they could cook and prepare themselves as a meal.