Long orange hairs, almost always on bubble coral.
A carapace about 2 cm across on disproportionately long thin legs, the whole animal smothered in long fine red to reddish-brown setae. That coat is the common name and the field identification, and it also hides the rostrum and carapace outline that would be needed to identify the animal properly. Which matters, because the name is shaky. Achaeus japonicus is a valid species described from Japan, but the crab divers photograph across the Coral Triangle may not be it: the standard photographic guide lists it as an undescribed Oncinopus and describes its own identification as tentative, and WoRMS holds a single distribution record for A. japonicus, from New Caledonia. The name is best read as the one in use rather than as settled.
Most often found on bubble coral, sitting almost motionless among the inflated vesicles and retreating between them, and also on Euphyllia, soft corals, anemones and hydroids — an association rather than an obligation. It gathers plankton and particles on the setae of its front legs and probably scavenges what the coral catches. Reef slopes and muck between about 5 and 25 m through the central Indo-Pacific. A separate and better-documented association exists between another Achaeus and a hydroid whose polyps the crab uses as fishing rods; that is a different animal and should not be attached to this one.
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.
