Shaped and coloured like a single segment of the Halimeda alga it lives on, and cuts pieces of it to wear.
Finding this animal is a matter of finding the alga first. Halimeda grows as a chain of flat, lime-green, calcified segments, roughly triangular and stacked end to end, and the female crab has a carapace that is itself flattened into a triangular blade of almost exactly that shape and colour. Among the segments she is not so much camouflaged as duplicated.
The sexes look so different that they were once taken for separate species. The female carries the broad leaf-shaped carapace; the male is narrower, more angular, with a long pointed rostrum and prominent spines, closer to a conventional decorator crab in outline. Both cut fragments of the alga and fix them to hooked setae on the carapace and legs, so an animal that is already the right shape adds the real thing on top.
The confusion to be aware of is with Huenia brevifrons, which shares the habit and the habitat; separating them reliably needs the rostrum and the carapace outline rather than the general impression, and photographs from muck sites are frequently labelled with whichever name the photographer met first. Where an identification cannot be settled from the picture, this entry treats the animal as Huenia sp. rather than guessing.
The segment that is the wrong shade, or the one that moves against the surge instead of with it, is what gives it away. Torchlight from the side picks out the legs where flat frontal light does not.
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.
