Tripneustes gratilla

Identification: the spines are thin and no longer than 2 cm, brown at the base with a white, sometimes orange, tip. They form ten narrow bands alternating with ten broad bare bands, often black but sometimes green, blue, brown or purple, whose velvety look comes from a dense cover of venomous pedicellariae. The colours fade when the animal dies. The Red Sea subspecies, T. g. elatensis, has far fewer spines, large white pedicellariae and long red tube feet. Its close Atlantic relative is Tripneustes ventricosus.

In the wild it ranges from East Africa to Southeast Asia, north to Japan and Hawaii, south to Australia and New Zealand and east to the Galápagos, from 0 to 75 m, mostly in shallow seagrass beds (1 to 30 m), on rubble and in lagoons. Unlike many urchins it grazes continuously, day and night: algae, periphyton, seagrass (Thalassodendron ciliatum, Syringodium isoetifolium), diatoms and even coral tissue scraped from rock. One study estimated it eats about 24% of a seagrass bed's yearly production, up to half in season. The Red Sea subspecies tolerates salinities above 45 ppt in salt ponds.

Reproduction: the sexes are separate and gametes are released together into the water; the larvae spend a few weeks in the plankton. In Hawaii the Anuenue Fisheries Research Center raised 25,000 juveniles of at least 15 mm in about five months from a million larvae; since 2011 these hatchery urchins have been released in Kāneʻohe Bay to control the invasive seaweed Kappaphycus.

Status: it is edible and fished (exported from the Philippines to Japan, eaten on Réunion), and overharvesting has reduced its populations. It has not been evaluated by the IUCN and is not CITES-listed.