Updated: Sep 16, 2020
Imagine you could snap your fingers and produce enough energy to be louder than a grenade and hotter than the surface of the sun. This takes place repeatedly, every single day, by millions of shrimp in shallow coastal waters around the world.
Thank goodness water dampens both sound and heat!
Turns out shrimp is definitely not an insult.
There are 425 species of snapping or “pistol” shrimp, all of which are defined by having a snapping claw (or chela) that produces a very distinctive sound. Dip your head under water in just about any tropical reef or rocky ecosystem, and you’re very likely to hear the non-stop crackling of an active population of snapping shrimp.
These shrimp are small, maxing out at only 3-5 cm (1-2 in.). They always have one small pincer and one that is huge, the snapper, which can be more than half as long as the shrimp’s body. It is proportionately larger in male shrimp than females, but they all have some difference in the size of their left and right claws. Further, some shrimp are lefties and some are righties, but if they lose the larger snapper, it will be regrown as a smaller pincer and the opposite side develops into the new oversized claw (1).
So how does the SNAP work?
It’s a lot like the snap of your fingers, in that extremely rapid ( as in 0.0006 seconds!) closure of the snapper claw will produce a fast, well-focused water jet. Along with the jet of water is what we call a cavitation bubble, which immediately collapses to produce the sound.
And it is exceptionally loud- as in 190 to 210 dB. For perspective, thunder is typically 120 dB and 150 dB can rupture your eardrum, that’s as loud as a jet at takeoff (2).
So much energy is released in the bubble collapse that a flash of light is emitted. Scientists have used the flash to estimate that the temperature of the bubble collapse is over 8,500°F (or 4700°C) (3). To be fair, I may have exaggerated the “hotter than the surface of the sun” statement, it’s actually only hotter than the sun at the center of a sunspot, but I still think it’s pretty impressive.
With this much force and energy, how are the shrimp themselves not hurt?
They have adapted to have orbital hoods, extensions of the shell that cover the eyes and protect the eye from potential damage (4). They also have a special way of building their exoskeleton that is reinforced to give them heat-resistance and strong (but still lightweight) protection (5).
As you might imagine, the water jet that results from the snapping action can stun or kill small prey items such as worms, small fish, or other species of shrimp; but it also has important social use. They use it to communicate with each other but aiming chemical signals at specific shrimp on the reef. They can lace the water with pheromones to attract ladies or signal aggression to a rival before blasting it in their direction (6). If the lady likes what she smells, she can follow the trajectory of the jet and form a socially monogamous, long-term mate pair with the flirty male shrimp. They then often build and share a burrow in which to live and raise their young.
The Buddy System
These shrimp are excellent diggers, and the combination of their burrowing prowess and defensive snap helps them to not just attract a mate, but form mutualistic relationships with a variety of organisms including sponges, cnidarians, other crustaceans, echiurans, molluscs, echinoderms, and goby fish.
In these relationships, the shrimp constructs and maintains a burrow which is used by the goby as a temporary shelter during the day and resting place at night, as well as offering a safe place for the fish to breed. Gobies recognize their shrimp by sight, but the poor eyesight of the shrimp means that the fish must offer chemical stimulus to make an offer of buddying up (7).
Some species of shrimp have such bad eyesight that they are reliant upon pairing with a goby to ensure survival. In exchange for the burrowing ability and defensive posturing of snapping shrimp, the goby offers its partner a warning system using the shrimp’s antennae (8).
The goby indicates alarm by a specific motioning of its tail fin on the antennae of the shrimp. And the shrimp always has at least one antenna resting on the goby’s body. They are practically attached at the antenna even when the pair ventures away from the burrow. The rapidity and intensity of the signal is essential for triggering the shrimp’s response (9).
In the Caribbean, the red snapping shrimp, Alpheus armatus, lives with the ringed anemone, Bartholomea annulate. In this mutually beneficial relationship, the shrimp will fight off a harmful worm species that is a primary predator of the anemone. In return, the anemone protects the shrimp from its own predators by housing the shrimp within its stinging tentacles (10).
In another example of mutualism with a cnidarian, the shrimp Alpheus lottini protects a large stony coral called Pocillopora damicornis against sea star predation. The shrimp lives within the coral polyps and feeds on a mucus that the coral secretes especially for it to feed while attacking the soft tube feet of predatory starfish.
They are undoubtedly amazing, but why are they important?
While the snapping shrimp are not among the shrimp species that people eat, they are what we call “soundscape engineers,” which have profound effects on the local ecology. The soundscape of ocean zones is known to influence ecological processes such as reproduction, larval recruitment, and trophic interactions. In the subtidal and intertidal zones, underwater sound helps far off larvae to orient themselves and find the area where they want to settle. The frequency of the shrimp's snap is detectable at least 6 miles (10 km) offshore (11).
Species of damselfish and crustaceans have been known to use the shrimp’s constant noise to cue themselves to the reef.
We can even use the sounds to monitor the environment! Snap rates vary from 1500 – 2000 snaps/min during summer to fewer than 100 snaps/min during winter (12). Even daily and seasonal variation in the density of snaps can serve as a measurement of habitat composition, density, and quality. The number of shrimp snapping can help to determine overall ecological health (13).