Squinting, I can just make out individual skeleton shrimp clinging to a hydroid bush. They are part of a colony of hundreds, with the largest measuring no more than a half-inch long. Knowing that Anna shares my appreciation for such weird creatures, I motion her over, and we settle in for a stay.
There’s little wonder we are so captivated. Skeleton shrimp are oddities with attitudes, appearing both primitive and futuristic, toylike and menacing at the same time. Think of a thousand praying mantises in miniature or, better yet, straw-thin robots on speed armed with claws that would make a lobster envious.
The first to catch our eye are the larger males stampeding through the colony like motorized inchworms looking for someone to wallop. But underlings don’t cower from such bullying; they strike back with a whirlwind of blows. Without question, if skeleton shrimp were quadruple their size they would rank among the most popular attractions in the sea.
Everything about skeleton shrimp is unconventional. For starters, they are not shrimp at all. They belong to an entirely different order of crustaceans, amphipods, but they don’t really resemble others of that lineage either. In fact, their peculiar morphology doesn’t fit comfortably anywhere. You have to burrow deep into their classification, passing through suborder, infraorder and something called the parvorder before you finally arrive at family Caprellidae, into which a head-scratching taxonomist back in the 1800s stuck them along with whale lice, which bear little resemblance to their supposed next of kin.
Although skeleton shrimp are tiny and inconspicuous, divers willing to make the effort can usually track them down. They inhabit most inshore waters around the world. During the summer, they can be prolific in seagrass and on dock pilings and mooring lines along both coasts of the United States and into Canada. In Indonesia we typically find them on hydroids, sponges, gorgonians and algae, but they occasionally gather in unexpected places. In the Gulf of California we found them living on the heads of scorpionfish, and we have heard tales of them taking up residence on frogfishes, sea stars and nudibranchs.
Wherever there are colonies, there are big males controlling the high ground, where the hunting is best. The shrimps’ primary prey is a tiny crustacean they pluck from the currents with their claws, but if given the opportunity they won’t hesitate to tackle bigger game. One night Anna’s video lights attracted a whirlwind of planktonic worms to a colony she was photographing. In the blaze of her beam, a male skeleton shrimp snagged a worm several times its size and wrestled with it for more than a minute before the monster finally broke free.
Small, short-lived animals such as skeleton shrimp are particularly susceptible to boom and bust cycles. From the look of things, the colony we are investigating is approaching its population zenith and heading for an inevitable crash. Such trying times bring out the worst in any animal, and skeleton shrimp have a lot of worst to bring out. Predictably, much of the animals’ bad behavior revolves around sex.
Like other crustaceans, females become reproductively active immediately following a molt. The process produces pheromones that drive the males wild. There is no courtship for these bad boys; the right to pass on genes is settled by a winner-take-all round of fisticuffs. The stakes are high — a well-placed claw can slice an opponent in half — and to add to the peril, each claw is tipped with a poisonous spine capable of inflicting a fatal wound. The last male standing moves fast, helping strip away the female’s old exoskeleton in its haste to spawn.
Contrary to the majority of marine creatures, which release thousands of immature larvae into the open sea during a single spawn, skeleton shrimp — ever iconoclasts — take the less-traveled path, producing a small number of offspring that require extended care. Among the mass of bodies on the hydroid we can make out what appear to be balls of fuzz. As we lean closer, the fuzz becomes the limbs, claws and antennae of several dozen babies clinging to their mother — each an exact replica of its parent.
From the moment hatchlings emerge from the brooding pouch and clamber onto their mothers’ backs they are under siege. And from what we can see, the tykes need protecting. Baby-toting females, far from pushovers, fight like banshees to safeguard their offspring. Surviving juveniles grow rapidly, passing through several molts before they are ready to fend for themselves. It seemed fitting that toward the end of our dive Anna noticed a mother flinging her mature babies off her back with the coldhearted demeanor one would expect of a skeleton shrimp.
© Alert Diver — Q4 Fall 2013