A heavy crab whose shell is knobbled, pitted and irregular enough to pass for a weathered lump of coral rock, up to about 11 cm (4 in) across. The claws are massive and unequal, folded flat against the front when the animal is at rest, which completes the impression of a stone rather than an animal.
Camouflage here is structural rather than added: unlike the decorator crabs it attaches nothing, relying on the shape and texture of its own shell. It feeds at night on hard-shelled prey - snails, hermit crabs, small urchins - crushed with the larger claw. By day it wedges itself among rubble and does not move, and divers who find one usually do so by accident, having put a hand down beside it. Big, slow and armoured, it has little to fear and behaves accordingly: a disturbed animal raises its claws and stands its ground rather than fleeing.
- shell knobbled and pitted like a weathered stone
- massive unequal claws held folded against the body
- sits motionless among rubble, easily taken for rock
- width up to 11 cm
