A weaponized water jet
In the coral reefs of Sulawesi, a sound like sizzling bacon fills the water. This is the collective noise of the pistol shrimp, a small crustacean, typically 3-5 cm long, belonging to the family Alpheidae. This family contains over 1,000 species, all known for an oversized claw that can grow to half their body size. This claw is not for pinching; it is a sophisticated biological weapon.
The claw has two parts: a stationary half called the propus and a mobile half called the dactyl. The dactyl has a plunger that fits perfectly into a socket on the propus. When the shrimp contracts a muscle, the dactyl cocks open. When released, it snaps shut in as little as 650 microseconds, forcing the plunger into the socket. This action ejects a narrow jet of water at speeds up to 100 kilometers per hour (about 62 mph).
The speed of this water jet enables the shrimp's ability. According to Bernoulli's principle, the fast-moving fluid creates a region of extremely low pressure directly behind it. The pressure drops so low that it falls below the vapor pressure of the water. This causes the water to boil at an ambient temperature, forming a small vapor-filled void known as a cavitation bubble.
The star in a bubble
The cavitation bubble exists for only a moment. As the water jet slows, the surrounding higher-pressure seawater violently crushes the bubble. The collapse is so rapid and forceful that it produces extreme physical phenomena. The implosion generates a powerful shockwave and a loud "snap" that can reach 218 decibels. This is louder than a gunshot, which typically registers between 165-175 decibels. This shockwave produces a pressure of 80 kPa at a distance of 4 cm, stunning or killing the shrimp's prey of small fish and crabs.
The bubble's collapse also creates incredible heat. For a fraction of a microsecond, the temperature inside the imploding bubble reaches approximately 4,700°C (8,500°F), a temperature near the surface of the sun. This extreme event also produces a faint flash of light, a phenomenon known as sonoluminescence. The light, sometimes called "shrimpoluminescence," is not visible to the naked eye but is detectable with sensitive instruments. The entire process, from the claw snap to the bubble collapse, is over in less than a millisecond. The collective snapping of a large pistol shrimp colony is so loud it can interfere with military sonar equipment.