The Very Versatile Crab

Forest and Bird, Issue 306, 1 November 2002, Page 40

The Very Versatile

Crab

ANN GRAEME

Ac is like a walking Swiss army knife! It is made up of segments, each fitted with appendages for different uses. The variety of shapes and specialities allows crabs to carry out every function they need — walking, swimming, fighting, communicating, reproducing, eating, and breathing. Take the common mud crab. Its colour matches the mudflats where it lives, and it eats mud, extracting the

nourishing bacteria and diatoms. The largest of the mud crab’s appendages are its pincers, extra large in male crabs for intimidating rivals. Pincers are used by both sexes for scooping up tiny

spoonfuls of surface mud and passing them to the jaws. They work like knives and forks, sorting and sieving from the mud the microscopic food, putting it into the crab’s , mouth and discarding the leftover mud as neat little balls. Using its four pairs of walking legs, the mud crab scurries in and out of its f burrow, ever alert to escape \ the beaks of the swooping N kingfisher or stabbing heron. 4 And underneath the female : crab’s tail, each segment

carries a pair of forked appendages to which she attaches her eggs. She will carry them there until they hatch into tiny larval crabs. The paddle crab, on sandier shores, leads a different life. Its back legs don’t end in claws

but are flattened into large paddles for swimming. When it is not swimming, the crab lies in ambush.on the sea bottom, its paddle legs digging and shuffling the sand over itself until only the stalked eyes poke up like periscopes. When a small fish cruises by, the crab springs out, its sword-like pincers scything through the water to slash or impale the fish. Then, using its pincers like hands, it tears the fish into bite-sized pieces to put into its mouth. The pincers are so strong that they can even prise open pipi and tuatua — and painfully nip an unwary toe! Squatting inside an empty

snail shell, the hermit crab is cautious. Stalked eyes pick up any movement so you need to sit quite still beside a rock pool to see what looks like an empty seashell jerk into life. When, its feelers pick up the water-borne smell of a crushed limpet, the crab casts caution to the wind. It scuttles towards the food as fast as it can tow its shell. With so many hermit crabs in a pool, it’s ‘first in, best fed’ It’s an orgy as each crab jostles to rip off a chunk of flesh with its big pincer. Then the crab cuts up the chunk with its smaller pincer, shreds and minces it with its jaws and stuffs the food into its mouth.

Living in a shell house has imposed some modifications on the hermit crab’s design. Its body is twisted to match the spiral of the shell, and it has special tail appendages to hang on to the inside. Then, when the crab retreats within its shell, its large pincer folds neatly over to barricade the entrance.

Turn over a rock on any harbour shore and the chances are you will find lacquered grey-green crabs swiftly rustling away to hide. They are s\ * Ry

The crustacean appendage is biramous, meaning it has two sections. Both may be similar, as in the twin stalks of the first antenna of a crayfish, but some-times one serves one purpose while the other serves a different one. For example, the crab’s walking leg is the visible branch of one appendage. Its other section has a very different purpose, how-ever, being a feathery gill, almost invisible under the carapace.

six-legged half-crabs, also known as porcelain crabs (The ‘true’ crabs have eight walking or swimming legs.) Half crabs are common and widespread because they are so hardy; the tough, carapace helping them survive the dangers of shifting stones and wave-lashed rocks. Our native half crab Petrolisthes elongatus

has recently appeared across the Tasman, perhaps carried

as larvae in a ship’s ballast water, to establish itself around the coast of Tasmania. Another reason half-crabs are so numerous is because they are filter feeders, and can harvest the ever-reliable plankton soup brought in on every tide. This has required another gadget on the Swiss

army knife of appendages. Their small feeding limbs are modified into very fancy cutlery, some to fan currents toward other limbs with long bristles which comb the plankton from the water.

The next appendages scrape the plankton off the combs and put it in the half crab’s N mouth. It sounds \ : complicated, but it \e works. Most appealing of all are the decorator or camouflage crabs, which plant seaweeds and sponges on their backs and hide by not hiding. When they shed their outer skeletons (as they grow), most plant : a new garden on the new ZF carapace. One thrifty s* — species, however, actually

S camouflage crab transplants the old garden to the new site. So adapted is the crab to matching its environment that, if its surroundings change, it redecorates its carapace to match. The versatile crustaceans, which include the crabs, have been around for 500 million years, and even today 50,000 species exist. But the

crustacean design has its limitations. An exoskeleton protects you but you have to take it off to grow. Then the naked, vulnerable crab must hide and hope that no predator finds it before its new armour has had time to harden. And the opportunistic hermit crab must not only shed its skeleton from time to time, it also has to find a bigger, vacant shell house as it grows bigger.

1s the national coordinator of Forest and Bird’s Kiwi Conservation Club.