The plain brown and black sea cucumbers that turn over the sand at most muck sites.
Holothuria is the largest genus of sea cucumbers, with around 160 species, and it accounts for most of the plain, unremarkable holothurians lying on open sand across the Indo-Pacific. Bodies are usually 20 to 40 cm, cylindrical and soft, coloured black, charcoal, grey-brown or dull red, with short unbranched feeding tentacles around the mouth rather than the heavily branched ones of Pearsonothuria. Three species account for most sightings. Holothuria atra is entirely black and normally coated in adhering sand, which both camouflages it and shades it. Holothuria edulis is black above and pink below and carries no sand coat. Holothuria leucospilota is charcoal or reddish-black, commonly lies with its front half under a rock and the rest extended across the sand, and stretches to about a metre when relaxed. Identification below genus from a photograph is unreliable, and the record is usually best left at Holothuria.
They are deposit feeders. Sand goes in at the mouth, organic material is stripped from it in the gut, and clean sand is deposited behind as a coiled cast. The casts are a standard feature of a muck site’s surface and are often more conspicuous than the animals that made them. Defences differ between species: H. leucospilota ejects a mass of sticky white Cuvierian tubules from the anus and regenerates them over a fortnight or so, while H. atra has no functional tubules and instead releases a red fluid containing holothurin when its skin is rubbed.
On what rides them: the emperor shrimp and the sea cucumber crab are the associates divers look for on holothurians, but the published host records for both are dominated by Pearsonothuria, Bohadschia, Stichopus and Thelenota rather than by Holothuria. A plain black cucumber on open sand is usually just that, and the branched-tentacled species are the more productive ones to check.
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.