H.P. Lovecraft, creator of the infamous Cthulhu mythos, said his dread tentaculate creature slumbered in a sunken city in the South Pacific Ocean. In that very spot (and in other spots around the world) may live a creature with a striking resemblance: the world’s only full-time swimming sea cucumber.

Although first described from a specimen hauled from the deep in 1894, it was only captured on film in recently. Wrote Echinoblogger Chris Mah of this video, “Its not a commonly encountered animal..and we live in a wonderous time that we can see several minutes of HD video of this seldom seen animal swimming by...”

Just a few months ago this footage was acquired by the United States’s own ROV, Deep Discoverer(and you can make the video even larger by watching it here).

Its name is Pelagothuria natatrix, and as far as we know, it is the only sea cucumber -- also called a holothurian -- that does this. Mah says one pair of scientists have described this animal as "...perhaps the most bizarre holothurian species in existence.” (Chris was also my source for the Cthulhu comparison: “[I]t does have that Lovecraftian/Elder Thing-Mountains of Madness vibe going on doesn't it??” Yep.)

P. natatrix is the product of a remarkable series of evolutionary flip-flops. Waaaay back before the Cambrian Explosion -- the burst of diversification that produced most major animal groups we see today -- its ancestors were left-and right-handed(bilaterally symmetrical) animals of some sort, the last common ancestor of the echinoderms (sea stars and friends) and the vertebrates, our own group. Then they became a radially symmetrical sea lily or starfish-like animals. Still later they became bilaterally symmetrical sea cucumbers. And now once more they have transformed into the radially symmetrical jellyfish-like animal you see here.

To understand how unusual this life form is it helps to know something about the sea cucumbers. Holothurians, as mentioned, were once radially-symmetrical animals like sea stars or sea urchins. At some point, they tipped over onto their sides and began a life of deposit feeding – scouring the seafloor (or burrowing into it) with sticky modified tube feet/feeding tentacles for the tastiest bits of goo and fluff that rain down every day from above. Think of them as the Roombas of the sea.

In the process, they abandoned their radial symmetry for a sausage shape with a forward-facing mouth and an anus at the back (on starfish the anus is on top of the animal, and the mouth on the underside). And oh what an anus! Not only do they breathe through their butt (their respiratory equipment branches off of it), but the space inside, the cloaca, is apparently so attractive and capacious that entire fish like to live inside there and some sea cucumbers have evolved “anal teeth” just to keep them out.

If that wasn’t amazing enough, they also possess two more extraordinary gut-related abilities: under stress, they can eject their guts – not gut contents, the guts themselves, often including gonads and other organs – from their mouth or anus, perhaps as a feint to distract predators. Then, they simply regrow their internal organs.

Or they can choose to merely rupture their hindgut and eject the base of their respiratory tree, the hollow sticky Cuvierian tubules. Upon release, these filaments rapidly inflate to up to 20 times their usual length and acquire the ability to lock on to anything they touch, entangling potential predators.

Holothuria forskali Rab2011 h 3798.JPG
Credit: Rpillon Wikimedia (CC BY-SA 3.0)

Writes Colin Tudge of these filaments in “The Variety of Life”, with characteristic British understatement, “As I can attest, having once been caught molesting a wild sea cucumber, these are most off-putting.” Then, the sea cucumber merely detaches the cables and wanders off whereupon, you guessed it, it simply regrows them in a few days.

As sea cucumbers are essentially sitting targets on the seafloor, they have evolved yet a third defense: nasty toxins that make them poisonous to all but a few predators, the most prominent of which is called holothurin.

Yet for all this, they get by on surprisingly little brain power as they essentially have no brain. What they do have is a neural ring, but even surgically removing this ring is apparently no impediment to the sea cucumber as it can still go happily on about its business, ring or no.

Whatever sea cucumbers happen to be doing on the bottom, they are doing it well, because they are the largest source of biomass on the deep sea floor. Sea cucumbers were observed to be a large component of what was found alive on the bottom of the Mariana Trench, as I wrote about a few years ago, although these were yet another type of sea cucumber that specializes in filter feeding the water with feeding tentacles it extends into the current, rather than roving the bottom sucking sand like the spice crawlers in Dune.

There is another class of sea cucumber, however, that specializes in sometimes leaving the bottom, by using its own body and sometimes a webbed collar as well to propel itself into the water column, drift for a time on the current, then land some distance away. These part-time swimming cucumbers are often translucent and fascinating to watch (although, according to Mah, they have never been observed to do an emergency gut dump like their more shallow-water kin).

This montage from the Nautilus Live Expedition gives a good sampler of these:

However, Pelagothuria -- our full-time swimmer -- seems to have taken the next logical step and forsaken the seafloor entirely, transcending sea cucumber-ness and becoming something new. Like jellyfish, it specializes in feeding on the suspended particles floating around it, before they reach the swarms of awaiting sea cucumbers on the bottom.

Perhaps in another 10 million years, should this species survive and diversify, we will have a whole new group of echinoderms called the “cucumber jellies”. Which probably sounds delicious to those who like cucumber. Which I do not (why do you enjoy eating something that tastes like watermelon rind??).