The snap is not made by the claw halves striking one another. Closing the claw ejects a jet of water fast enough to drop local pressure below the vapour pressure, forming a cavitation bubble; the bubble's collapse produces the noise and a shock wave that can stun small prey. Peak source levels have been measured in the region of 200 decibels relative to one micropascal at one metre, and the collapse also produces a faint light emission.
One animal is loud. A reef of them is a wall of sound: dense colonies generate a broadband crackle that dominates the ambient acoustic spectrum of shallow warm-water habitats and interferes with sonar and underwater communications. Wartime acoustic surveys in the Pacific documented the effect, and shrimp noise remains a recognised nuisance in coastal acoustic work today.
The familiar decibel figure needs care. Underwater decibels use a different reference pressure than airborne decibels, so a snapping shrimp is not comparable to a 200-decibel sound in air, which does not exist as an ordinary acoustic wave. The shrimp is genuinely among the loudest animals in the sea, but the number is not transferable.