Carries a small anemone in each claw. Nocturnal and usually under rubble.
Small — rarely more than 2 cm across the carapace, often much less. The carapace is trapezoidal and carries a network of dark polygonal lines over cream and yellow, a stained-glass pattern that is the reliable field character. What everyone photographs is the claws: the chelipeds are slender and useless as claws, each fixed finger drawn into eight or nine spines forming a hook, and in each one the crab holds a small sea anemone. It cannot feed with its chelae at all, and uses the anemones’ tentacles to gather particles, transferring the food with its mouthparts. Every wild boxer crab examined has been carrying anemones, so the association is obligate at genus level; this species is a generalist, recorded with five different anemones.
The famous behaviour needs a caveat. Removing one anemone leads the crab to split the remaining one in half, producing two genetically identical clones; crabs also fight each other for anemones, and small crabs rob much larger ones. It keeps its anemones starved into a bonsai state, flicking food off their tentacles with the opposite leg, so that held animals shrink while free-living ones of the same species expand several-fold. All of that is properly documented — but it was done on a different boxer crab in the Red Sea with a different anemone, and is transferred to this species throughout the popular literature without saying so. It is very likely true here. It has not been shown. Indo-West Pacific, in the shallows to about 30 m.
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.