Sea cucumbers are relatives of sea urchins and sea stars (starfish). Generally speaking, they lie about on the seabed and are as inactive as their plant namesakes. In the deep, however, sea cucumbers do things rather differently.
One species, the ‘Spanish dancer’ – a nod to the twirling skirts of flamenco dancers – is a translucent, ruby-red creature that swims about and drifts on gentle currents in the deep sea, propelled by elegant wafts of its webbed cloak. Another name for it is the ‘remarkable dreamer’ – a translation of its scientific name, Enypniastes eximia.
It’s also been given a more grotesque nickname: the headless chicken monster. Admittedly, it does bear a passing resemblance to a plucked fowl carcass that’s been flung into the ocean. It’s roughly the right size, too, growing to about 25cm (9in) long.
The body part that looks like a neck post-decapitation is in fact a mouth, surrounded by a ring of feeding tentacles. When Enypniastes lands on the seabed, it uses those tentacles to shovel sediments into its mouth.
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In common with other sea cucumbers, it feeds on accumulated piles of marine snow – the shower of organic detritus that sinks down from the sea surface, comprising dead plankton and their faeces bound into fluffy particles by a sticky microbial glue.
This swimming sea cucumber was discovered in the 1870s by scientists aboard the HMS Challenger during its famous oceanographic expedition. Enypniastes lives across all oceans, including near Antarctica, at depths below 500m (1,600ft) and down to at least 6,000m (approx 19,600ft).
It’s incredibly fragile, being made up mostly of water and specimens tend to get damaged in collecting nets. As such, scientists only got a good idea of what it actually looks like when it was seen alive for the first time this century, with the help of remotely operated deep-diving robots equipped with video cameras.
Thanks to its watery body, Enypniastes is neutrally buoyant and can lift off the seabed then swim without expending much energy — a key survival strategy in the deep sea, where food is hard to find.
Its coiled digestive tract, filled with pale sediment, is visible through its transparent body. Commonly, just before it launches into the water column, this sea cucumber voids its guts and squeezes out a poop of cleaned sediment – a bit like dumping ballast sandbags from a hot air balloon.
In this way, Enypniastes plays an important ecological role, mixing up and aerating the seabed just as earthworms do for soils on land.
In common with many other animals living in the deep sea, Enypniastes can illuminate its body – another key survival strategy as it floats about in the dark.
If something bumps into it, this sea cucumber’s gelatinous skin lights up and sloughs off like a glowing cloud. It’s thought to act like a burglar alarm, revealing its attacker’s presence to predators of that species, while the sea cucumber itself swims off to safety.
Laboratory studies carried out on living Enypniastes specimens have found that they can quickly regrow their skin and retain their ability to glow in the dark.
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