Lives inside a pulsing Xenia colony and eats it.
A small crab, about 2 cm across the carapace, smooth and rounded with none of the spines or knobs that most small reef crabs carry. Colour is deep olive-green to near black, sometimes with a paler blotch on the back, and it matches the shaded base of the host colony well enough that the animal is usually found by its outline rather than its colour. The last pair of legs is flattened into paddles, which places it with the swimming crabs, though it is rarely seen to swim. The other crabs found on soft corals are easy to rule out: the candy crab is spiny, pale and decorated with the host’s own polyps and lives on Dendronephthya, and the soft coral porcelain crabs are flattened, translucent and show only three pairs of walking legs.
It lives in pulsing soft corals of the genus Xenia and its close relatives, down among the stalks at the base of the colony rather than up in the polyp crowns, and the host is the way to find it. Xenia carries diterpene compounds that deter most grazers, and this crab is one of a small number of animals recorded feeding on it — it crops the polyps and stalks, so a colony with bare or cropped patches among healthy ones is worth a second look. Most colonies hold nothing; a proportion hold one crab, occasionally a pair.
It is small, dark and set against a pale moving background, which is the main difficulty in photographing it. The crab retreats deeper into the colony as a light comes on and the polyps of a disturbed colony contract around it.
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.
