Echinometra mathaei

Identification: the slightly oval test is 5 to 7 cm across (8 cm at most according to SeaLifeBase), up to 10 cm with the spines. It is usually black; the spines, all of similar length, range from beige to black through green, pink, bluish grey or brown, sometimes with paler tips, and all carry a pale ring at the base. Genetic studies suggest E. mathaei is a complex of several species. It can be confused with Parasalenia gratiosa (smaller, flatter, with more obvious white) or with young Stomopneustes variolaris or Echinostrephus molaris.

In the wild it ranges from Madagascar, East Africa and the Red Sea to Hawaii, mostly at 1 to 8 m on reef flats and upper reefs but down to 139 m. Using its spines and jaw (Aristotle's lantern) it bores a hole into basalt or limestone, which it leaves at night to graze turf algae, as well as small invertebrates, carrion and debris. Where fishing removes its predators, as in Kenya, its densities can rise more than a hundredfold, leading to overgrazing and reef erosion. Its predators are mainly fish (such as Coris aygula), moray eels, brittle stars and gastropods. The small shrimp Athanas areteformi lives among its spines.

Reproduction: gametes are released into the water, fertilisation is external and the larvae (echinoplutei) live in the plankton before settling. In the Gulf of Suez, at the northern edge of its range, it spawns in summer and autumn; in warmer waters, all year round.

Status: it has not been evaluated by the IUCN and is not CITES-listed. It is seldom seen in the aquarium trade.