A very flat crab with long spined legs that runs sideways over rock and hides under urchins.
Carapace to about 3 cm across. A flattened, near-circular disc, dark with a pale line down the middle, the legs banded and yellow at the joints, and a row of spines along the leading edge of every walking leg.
The other flat crab on the same rock has a domed, knobbly carapace where this one is a flat disc.
One to twenty-three metres on and under boulders and rubble, often on or beside long-spined urchins. It is strictly a herbivore. It moves fast and sideways over open rock, which is where the name comes from, and it is harmless.
It has been established in the Mediterranean since 1999 and is treated there as an invasive species.
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.
