Crustaceans

CrabsBrachyura

Infraorder28 species

Being a crab is less a family than a solution, and the sea keeps arriving at it. At least five separate crustacean lineages have independently flattened the body, folded the tail underneath and finished up crab-shaped without being crabs at all: king crabs came out of the hermit crabs, porcelain crabs out of the squat lobsters. The animals in this section, the Brachyura, are the ones that got there first and never left.

The shape is better engineering than it looks. Tucking the abdomen flat under the body removes the long soft rear end that a lobster has to spend its life defending, and leaves a compact armored disc with nothing worth grabbing. The legs hinge in a single plane, which is why sideways is the efficient direction of travel: the leading legs pull, the trailing legs push, and the animal slides into a crack narrower than its own body length without having to turn around in it.

The claws are usually not a matched pair. On many reef species one is heavy and blunt-toothed for crushing shell and the other is finer and bladed for cutting and picking, and the animal uses them for different work. Size counts as much as function, because a claw held up is a statement about the animal behind it, and most arguments over a crevice are settled by display rather than by contact.

And nearly all of them are small. The heavy crabs get the attention, but the ones doing the work on a reef are a centimeter or two across and spend entire lives inside a single coral head, a sponge, a sea urchin or one strand of wire coral, never crossing open rock at all. Point a torch into a coral colony after dark and the number of crabs on that reef goes up sharply.

How to recognise crabs

InvertebrateNot a fish - coral, mollusc, crustacean, echinoderm and other invertebrate groups.

broad, flattened body covered by hard carapace
small abdomen tucked under body
ten legs, first pair usually forming pincers
walking sideways or forwards on bottom or reef

28 species in Crabs

Calappa

Box crabs, and the name is literal: the claws fold flat across the front of the body like shutters and close the animal into a smooth sealed box with nothing to grip. They spend the day buried in sand with only the eyes showing, and the right claw carries a tooth used as a can opener on snail shells.

Chlorodiella

Rubble and coral crabs with the claw fingers hollowed out at the tips into small spoons, used to scrape algal film off dead coral. The walking legs lock against the segment before them, so a crab wedged into a branch has to be dug out rather than pulled.

Poisonous

Bearded Coral Crab

Chlorodiella barbata

Fused-tooth Coral Crab

Chlorodiella cytherea

Cymo

Small hairy crabs that share a coral head with the guard crabs and sit lower down in it, near the base of the branches. The shell carries short bristles that collect sediment, which is most of the camouflage, and the claw tips are the field mark - black in one of the two species here and not in the other.

Andreossy's Coral Crab

Cymo andreossyi

Black-fingered Coral Crab

Cymo melanodactylus

Etisus

Round crabs with heavy black-tipped claws of unequal size - one for crushing, one for picking - and a shell either divided into raised lobes or entirely smooth. They spend the day jammed under coral and rock, and the larger of the two here reaches 15 cm, which is big for this page.

Tetralia

The same job as Trapezia in a different coral: these hold Acropora where the guard crabs hold Pocillopora. Flatter, paler, and they move sideways along a branch or onto its underside rather than backing away, so the way to see one is to look at the far side of the branch you are already looking at.

Banded Coral Crab

Tetralia cinctipes

Bandit Coral Crab

Tetralia nigrolineata

Trapezia

Guard crabs, and the arrangement is a contract. The coral grows fatty tissue at the tips of its polyps and the crabs graze it; in exchange they come out and fight crown-of-thorns starfish and coral-eating snails off the colony, and shovel out the sediment that would otherwise smother it. One pair holds a colony and keeps every other adult of their species out of it.