Holothuria atra

Holothuria atra, known as the black sea cucumber or lollyfish, is a marine invertebrate belonging to the family Holothuriidae. In 1914, Pearson assigned it to the subgenus Halodeima, giving it the full scientific name Holothuria (Halodeima) atra, and it serves as the type species for this subgenus.

Description

This sea cucumber can reach up to 60 centimeters in length, though 20 centimeters is more typical. Its skin is smooth, flexible, and entirely black, often coated with sand—especially in smaller specimens. The mouth, located on the underside at one end, is ringed by 20 black, branched tentacles, while the anus sits at the opposite end.

Distribution and habitat

Holothuria atra inhabits the tropical Indo-Pacific, from the Red Sea and East Africa to Australia. It lives on the seabed in shallow waters, including reefs, sand flats, and seagrass meadows, down to depths of 20 meters.

Its dark color makes it stand out, but a sand coating often camouflages it and may also shield it from the sun. It prefers reef flats with good water aeration but limited wave exposure, as well as shallows near rock slabs where cool water seeps out at low tide. It is frequently found in sun-warmed pools above the low tide mark and tolerates high temperatures well, remaining healthy and feeding even at 39 °C.

Biology

As an omnivore, it sifts through sediment with its tentacles, consuming detritus and organic matter. For defense, it releases a toxic red fluid when its skin is rubbed or damaged. Unlike some sea cucumbers, it does not eject Cuvierian tubules when attacked; instead, it extrudes its internal organs through the anus. Males and females cannot be distinguished externally.

Sexual maturity occurs at about 16 centimeters in length, with spawning mainly in summer and autumn, though it may happen year-round in equatorial waters. It takes 18 to 25 days to reach natural competence. Holothuria atra is also fissiparous, reproducing by transverse fission, mostly in smaller individuals. A constriction forms and deepens until the skin separates, creating two relatively wide, short individuals; no sand sticks to the new surfaces because they lack tube feet.

Ecology

Like many echinoderms, this species contributes to sediment bioturbation, which supports coral reef health. It is often associated with the black polychaete worm Gastrolepidia clavigera, which crawls over its skin, and appears to have few natural predators.