A small squarish crab with a smooth shell scattered with round dark spots, living wedged between the branches of Pocillopora and Stylophora. It reaches about 1.5 cm (0.6 in) across and backs deeper into the colony as a diver comes closer, which is why the usual view is a pair of claws and nothing else.
Guard crabs earn the name. A pair lives in one coral head and defends it, and their main opponent is the crown-of-thorns starfish: the crabs pinch its tube feet and spines until it moves off, and colonies holding crabs survive attacks that strip their neighbours bare. In return they take mucus and fat-rich tissue from the coral. Several species share this habit here, separated by colour and pattern rather than shape, and this one is told by the dark spotting on an otherwise plain shell. Every Trapezia in the catalogue is a photograph taken on these reefs.
- smooth squarish shell scattered with round dark spots
- wedged between coral branches, backing deeper when approached
- stout claws held forward
- width up to 1.5 cm
