Flip a rock in a rockpool and wait. After the disturbed silts and sediments have settled, and the shore crabs and shanny have sidled off to alternative accommodation, the observer may be lucky enough to notice a subtle movement on the underside of the turned boulder as shards of the very rock surface seem to animate and shuffle about.
A closer look and the outlines of small creatures with obvious crustacean credentials materialise from the murk.
What is a porcelain crab?
These are called broad-clawed porcelain crabs (Porcellana platycheles) and despite a superficial resemblance to the more familiar crustaceans of the rockpool are not true crabs, but more closely allied to the equally strange and little-seen squat lobsters.
You can tell these are not true crabs by the long filamentous antennae and a very different leg count: three pairs instead of four. Counting legs isn’t really necessary though as they are unique in their overall appearance.
Flat and fluffy, with small beady eyes that peer out from a fuzz of hair, their low profile is an adaptation to their preferred hangout – the thin and narrow cavities between intertidal rocks. The fine pelage is all about camouflage: this fuzzy felt traps sediment and allows the crustacean to blend in to its muddy home.
Why are they called porcelain crabs?
Porcelain crabs are so-called due to their fragile nature. Many an overzealous rockpooler has tried to prise one from the surface of a rock only to have it fall to pieces in their hand. They are easily damaged, which is partly by design.
Initially, the crab flattens itself even closer to the rock’s surface, making it harder to grasp, while at the same time gripping on with the help of fine claws on the end of each foot. Any further fiddling and the crab concludes it is under mortal attack by one of its many predators.
At which point it deploys a tactic called autotomy: the deliberate severance and shedding of limbs. Better known from tail throwing in some species of lizards, our porcelain crab does it as well – the idea being to leave an attacking true crab or fish with something to snack on while the rest of the crab sneaks away to live another day.
However, if you’re patient and careful, these tiny crustaceans, no more than 1.5cm across, can be gently separated from the rock’s surface. Turn one over and you’ll see a little more of its structure. The underside isn’t covered in hair like the upper side, allowing it to sit even more snuggly to the rock, and is an off-white colour, which is perhaps another reason for its name.
Where do porcelain crabs live?
Porcelain crabs are quite specific in their habitat, preferring a rocky intertidal zone with plenty of organic matter and debris, often slightly silty or muddy. Unlike other true crabs that make their living scavenging or preying on other larger creatures, the porcelain crab is a specialist filter feeder. So sheltered shores are the best places to look for them.
How they achieve this unusual mode of feeding is difficult to see out of the water, as the feeding limbs, or maxillipeds, used for feeding are folded up underneath its head region. But get one settled into an observation aquarium and you might be lucky enough to see it unfurl these long limbs, each topped with a fringe of hairs. These hairy hands are cast out from the crab’s body in alternating grabs, snatching any organic particles from the water and drawing them down to the mouth.
The humble barnacle deploys a very similar method of catching its food to the porcelain crab, using appendages called cirri
Main image © Peter David Scott/ The Art Agency
