A Two-Inch Shrimp Snaps Its Claw and Briefly Makes a Star in the Water

The pistol shrimp is a finger-sized crustacean with one comically oversized claw that it uses, essentially, as a small underwater rifle. The animal cocks the claw open, then slams it shut so fast — the closing edge moves at around 25 meters per second — that the water in front of it can't get out of the way. A low-pressure void opens behind the jet. When that void collapses a thousandth of a second later, the shrimp gets a free weapon.

The collapse is a textbook case of inertial cavitation. As the bubble implodes, the water inside is briefly compressed to enormous pressure and heated to temperatures comparable to the surface of the sun — measurements put the peak above 5,000 K, hot enough to release a faint flash of light. Hydrophones place the snap at about 218 decibels relative to one micropascal, which makes a colony of pistol shrimp one of the loudest sounds in the ocean and a recurring problem for submarine sonar operators in coastal waters.

A shrimp's prey, usually a small fish, doesn't experience any of that nuance. The shockwave alone is enough to stun it. The shrimp then drags it back into the burrow with its smaller claw. Species that snap hardest have evolved a thickened orbital hood over their own eyes to dampen the wave they emit, because the only thing that can hurt a pistol shrimp is, reliably, the next pistol shrimp.