A short-spined urchin sitting in a socket it has ground for itself in the rock.
An urchin whose test is oval rather than circular, to about 6 cm on the long axis, carrying stout spines of moderate length with sharp tips. Colour runs pale grey-green, brown, purple-brown and near black, and the spines commonly have a pale ring at the base and a darker tip. It is among the most abundant urchins on Indo-Pacific reefs and one of the first animals on the rocky edge of any muck site.
It lives in a socket. The animal grinds a depression into rock or dead coral with its spines and teeth, wedges into it so that the outline of the burrow matches the shape of the test, and holds position there through the day. After dark it moves out a short distance to graze algae and returns, and the same animal is generally found in the same socket over long periods. The collective effect of a dense population is a real one: this is one of the main agents eroding reef rock in the region.
Records need care. Echinometra mathaei as the name has been used covers a complex of forms that differ in spine colour, test proportions and the colour of the tissue around the spine bases, and several of them have been separated as species on genetic and reproductive evidence. A photograph of an urchin in its socket will not settle which, so a record is sensibly kept at the genus. Recorded from the intertidal to about 20 m, on rock, dead coral, jetty pilings and concrete rather than on open sediment.
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.