A small spotted crab wedged between the branches of a living coral, where it stays for life. Look into any healthy Pocillopora head and there is usually a pair in there, and their presence changes what happens to that colony.
They defend it. When a crown-of-thorns starfish climbs onto a coral head, the crabs come out and go for the tube feet and the spines, pinching and pulling until the starfish moves off - and a colony with guard crabs in it survives attacks that strip a colony without them. They do the same against feeding snails and against smothering sediment, which they clear branch by branch.
The coral pays them. The crabs live on the fatty mucus it secretes and on the tissue caught in it, and they get shelter no predator can reach into. Neither partner does well alone: guard crabs removed from a colony rarely survive, and colonies without crabs are eaten.
They are also territorial about it. One pair holds a coral head and drives off other crabs, which is why a colony has residents rather than a crowd.
- small flat crab, cream to orange with fine red spots
- deep among the branches of a live coral head, never on open rock
- one pair per colony, and they do not leave it
- width up to 2.5 cm
