A crab that lives in a pit in living coral, seen as a small face at the bottom of a hole.
Very small crabs, the carapace of most between three and eight millimetres, pale cream, pink or yellow, with a thin shell and a body that is often longer than it is wide. The female lives permanently inside a dwelling in the skeleton of a living coral, a pit, a crescent-shaped slit or a swollen gall that the coral grows around her as she settles and maintains. The male is smaller and moves between dwellings.
What is visible from outside is a hole in an otherwise even coral surface, a few millimetres across, with the front of the crab filling it. The animal withdraws when a light or a shadow arrives and comes back after a short wait, so the sequence is a hole that looks empty and then is not. Feeding is on coral mucus and on material suspended in the water, gathered with fringed mouthparts.
Coral barnacles occupy similar pits in the same corals and are the usual confusion. A barnacle shows a pair of hard opercular plates and, when feeding, a fan of curled cirri sweeping the water; a gall crab shows eyes, antennae and small claws. Records stay at the family unless the coral host and the dwelling shape are both noted. Recorded from about 2 to 25 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.
