Lives in a branching coral and drives off the starfish that come to eat it.
A flattened crab reaching about 3 cm across the carapace, cream to pale orange, covered over the shell, legs and claws in large round red to purple-brown spots set well apart. It is the largest of the guard crabs and the size and the spot pattern together are usually enough. The confusion is with the other members of the genus that occupy the same corals — Trapezia cymodoce is plain red-brown and unspotted, and the smaller spotted species carry finer, denser and more irregular markings. Branching Acropora also holds the flatter, plainer Tetralia crabs, which are smaller and pale.
It is an obligate resident of live branching coral, mostly Pocillopora but also Acropora and Stylophora, and it does not survive away from one. It feeds on coral mucus and on fat bodies the polyps produce, and in exchange it defends the colony: guard crabs attack and drive off crown-of-thorns starfish and other coral predators, clip settling algae, and pick sediment off the branches. Removal experiments are unambiguous about the value of this — corals stripped of their crabs suffer far heavier predation and can smother under sediment that the crabs would otherwise have cleared.
A colony normally holds a single male and female, which exclude other adults, so finding two is finding a pair. The crab lives deep in the branches and retreats further as a hand or a camera approaches, though it will also come out to the edge and hold its claws open at an intruder. The same colonies hold the coral-dwelling snapping shrimp Alpheus lottini, which defends the coral in much the same way.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
