Inhabiting tropical and subtropical waters, the pistol shrimp is a small crustacean, only 3-5 centimeters long. These shrimp are found in diverse environments like coral reefs and seagrass beds. They are distinguished by their asymmetrical claws, with one being disproportionately larger than the other. This larger claw, which can be bigger than half the shrimp’s body, produces one of the loudest sounds in the ocean.
The Science of the Snap
The powerful sound originates from a specialized claw featuring a two-part mechanism of a plunger and a socket. When the shrimp contracts a muscle, the “hammer” part of the claw snaps shut with incredible speed. This closure shoots a jet of water forward at speeds reaching 60 miles per hour.
This high-velocity water jet creates a low-pressure pocket, forming a tiny vacuum bubble known as a cavitation bubble. The surrounding higher-pressure water immediately collapses this bubble. It is the bubble’s implosion, not the two parts of the claw hitting each other, that generates an acoustic pressure wave, resulting in a “snap” that can reach over 200 decibels.
The collapse of the cavitation bubble has other effects. For a fraction of a second, the temperature inside the collapsing bubble can reach over 4,700 degrees Celsius, a temperature comparable to the surface of the sun. This process also produces a faint flash of light, a phenomenon known as sonoluminescence, although it is too dim to be seen by the naked eye.
A Hunter’s Tool
The shockwave generated by the collapsing bubble is the pistol shrimp’s primary tool for hunting. This blast is capable of stunning or even killing small prey, such as other fish and crustaceans, from a short distance. Once its target is incapacitated by the sonic blast, the shrimp emerges from its hiding place to retrieve the meal.
This ability is not limited to hunting. The snap also serves as a defense mechanism against predators. The loud sound and concussive force can deter would-be attackers, giving the small shrimp an advantage. Within pistol shrimp colonies, these snaps are also used for communication and to settle territorial disputes.
An Unlikely Partnership
The pistol shrimp often engages in a cooperative partnership with certain species of goby fish. This relationship is a form of mutualism, where both animals receive a benefit. The shrimp, which has very poor eyesight, is an excellent burrower, digging and maintaining a home that provides shelter for both itself and the goby.
In return for a safe place to live, the goby, possessing excellent vision, acts as a sentinel. It positions itself at the entrance of the burrow, keeping watch for approaching danger. The two communicate through touch; the shrimp places one of its long antennae on the goby’s tail. If the goby spots a threat, it signals the shrimp with specific tail flicks, and both partners retreat into the burrow.
This teamwork enhances the survival chances for both species. The goby receives protection from predators and a secure location for mating. The nearly blind shrimp gains an early warning system, allowing it to forage and work outside its burrow with reduced risk.