Life on the reef is no picnic. These underwater jungles give life to countless fish and invertebrates, but danger lurks in every nook and cranny. Reef inhabitants defend themselves in any way possible: oversized teeth, neurotoxin, barbed spikes, full-body armor. Two unexpected best friends, however, have devised some particularly unusual security measures.
The rowdy pistol shrimp (family Alphidae) is unique to the underwater world: even newly hatched baby shrimp pack heat. The species’ bizarrely oversized claw isn’t used for crushing prey or impressive mates, but for firing off protective vacuum bubbles. Any critter foolish enough to venture close to the shrimp’s burrow in search of an easy snack will be in for a shock when one of these bubbles is launched in its direction at over 100 km/hr! If that wasn’t enough, the “bullets” subsequently implode, causing a racket that can reach up to 200 dB and produces a visible flash of light. Although most intruders manage to escape with no more than a headache, not all fish survive being shot by a pistol shrimp.
If you’re a scuba diver, you may have come across one of the thousand-plus species of pistol shrimp that grace our oceans. They boast a worldwide distribution, after all, with many species inhabiting the tropical reefs we humans like to frequent. Although the shrimp tend to stay safely tucked into their burrows for most of the day, they’re easy to recognize if they do show themselves: aside from that wacky XL claw, their exuberant colors catch the eye. Alpheus randalli is particularly popular in the aquarium hobby thanks to its white and red candy cane pattern, while Alpheus soror can be told apart from its cousins by its bright orange body, purple claws, and a large eye spot on its side. They’re simply impossible to overlook. When they do catch your eye, you’ll likely notice the pistol shrimp pair—they’re monogamous—comes with a bonus critter.
Many species of pistol shrimp don’t just share their burrow with their life partner, but also keep an unusual housemate. This may sound illogical given that they normally mow down any creature that comes too close to their burrow, but there’s a reason they make an exception for gobies of genera such as Cryptocentrus, Stonogobiops, and Amblyeleotris. Commonly known as the “watchmen”, these gobies make for excellent look-outs, explaining why the shrimp are more likely to adopt than shoot them. Despite their excellent means of self-defense, after all, most of the Alphidae are blind as bats. Pretty tough to take aim when you can’t see what you’re shooting at!
The resulting symbiosis—collaborations between two different species, in which both players fare better together than alone—is one of the most fascinating in the marine world. The shrimp serve as both architects and bouncers, meticulously maintaining the unusual ensemble’s shared burrow while also protecting it against intruders and hungry predators. The second task requires the help of the watchmen, which spend much of their days keeping a close eye on the sandy seafloor surrounding their home. If they spot anything suspicious, a quick flick of the tail fin or body alerts the blind bouncer. The gunslinger shrimp pulls its weapon, there’s a flash and a bang, and the invader slinks away or is eliminated—allowing the BFFs to return to more mundane tasks such as burrow maintenance and foraging.
The remarkable friendship between pistol shrimp and watchman gobies is just one example of the wondrous collaborations hidden below the surface. Life in the ocean isn’t easy, and danger is just another part of daily life—but some reef dwellers learn to trust each other blindly. Want to spot this dynamic duo in the wild? Make some time during your next dive to scour the “boring” sandy patches you’d normally avoid. If you spot a small burrow, perch next to it for a minute. The goby will generally poke its head out first, followed by the shrimp, which rarely ventures far. Given the exploding bubbles’ range is barely half a centimeter, you can observe them at your leisure without worrying about being peppered with gunfire yourself.
Sources & further reading
Anker, A., Ahyong, S. T., Noel, P. Y., & Palmer, A. R. (2006). Morphological phylogeny of alpheid shrimps: parallel preadaptation and the origin of a key morphological innovation, the snapping claw. Evolution, 60(12), 2507-2528.
Werding, B., Christensen, B., & Hiller, A. (2016). Three way symbiosis between a goby, a shrimp, and a crab. Marine Biodiversity, 46(4), 897-900.