If Buddhists are right about that whole reincarnation thing, it’d be hard to imagine what you’d have to do wrong to die and come back as a sea cucumber. One minute you’re human and the next you’re crawling around the seafloor as what is essentially a mobile intestine, hoovering up food at one end and expelling it through the other.

And then, inevitably, the pearlfish would find you.

You’re breathing through your anus, by the way, and when you take a breath, the pearlfish strikes – squirming up your butt, making itself comfortable in your respiratory organ, and eating your gonads. Or, they’ll go up in pairs and have sex in your body cavity. And that’s when you realize that you must have been a really awful human being in a past life. Like, the type of person who talks on their phone in a movie theater kind of awful.

Such pearlfishes come in a range of species, and don’t necessarily limit themselves to invading sea cucumbers. They’ll also work their way into sea stars, and are so named because they’ve been found dead inside oysters, completely coated in mother-of-pearl. Beautiful, really, though I reckon the pearlfish would beg to differ.

This behavior is the strange product of a housing crisis. You see, shelter is in short supply on many seafloors, particularly those that lack reefs. And there are few better shelters than sea cucumbers, little mobile homes that pearlfishes will enter pretty much as they please, leaving to hunt and returning for protection. If they can’t return to the same one, no worries at all. There’s plenty of decent housing squirming around the seafloor – if you're willing to live in a sea cucumber's bum.

The letters denote the sea cucumber’s increasing level of discomfort. (Click for larger view.) Images: Daniel Bay

The pearlfish finds its reluctant host likely by smell, according to biologist Eric Parmentier of Belgium’s University of Liège. It then must choose the right end to enter, using its lateral line – sensory organs that detect movements in water – to home in on the outflow from the respiratory tree at the anus.

“Two strategies are observed for entering,” Parmentier said. “One, head first by propelling itself with violent strokes of the tail; two, tail first by placing the head at the cloaca of the sea cucumber and moving the thin tail forward alongside its own body at the level of the lateral line,” then slowing backing into the host, though not yet all the way.

“The reason for this second strategy,” Parmentier said, “is that the host has detected the presence of the fish and, in response, closes its anus. But the host has to breathe, so it has to dilate the anus to realize the water flow. The fish blocks the aperture and the host has to enlarge this opening more and more.”

Depending on what species it is, the pearlfish initiates one of two relationships once inside: a commensal one, in which it simply takes up space without either helping or adversely affecting the sea cucumber, or a rather more parasitic one, in which it chows down on its host’s gonads.

The sea cucumber, though, has a trick up its sleeve. Remarkably, it can regenerate complex body parts like intestines and, yes, gonads. And it’s a damn good thing it can, because sea cucumbers defend themselves in what might be described as a fairly unorthodox manner.

“Probably the best thing that sea cucumbers are known for is evisceration,” said marine biologist Christopher Mah, “which is tossing their guts out at predators when they are harassed by them. So you have a crab or a fish or something and what they'll do is literally eviscerate, just take a good chunk of their intestine that will spool out of their body and get shot out at the predator or whatever as a distraction.”

The Great Pearlfish, Conqueror of Sea Cucumber Bums, Devourer of Reproductive Organs, All-Around Decent Critter to Anything But a Sea Cucumber. Image: John E. Randall

So like a disgraced samurai disemboweling himself, the sea cucumber gifts the world with its intestines, whether the world wants them or not. Interestingly, though, the pearlfish doesn’t itself seem to trigger this reaction for reasons that aren’t yet clear. And it’s important to consider that the fish in fact benefits from the evisceration, because by using the sea cucumber as a home, it necessarily adopts its host’s predators. Its survival depends on the sea cucumber’s ability to defend itself, which is quite intriguing from an evolutionary perspective.

“Is it possible to see here a result of natural selection, in which the choice of a host equipped with a defense system could minimize the risk of predation?” Parmentier asked in a 2005 paper.

Some sea cucumber species even go beyond firing their intestines at predators. They're equipped with hundreds of Cuvierian tubules – sticky, toxic tubes that spray out of the cloaca (an all-purpose opening in creatures like birds and reptiles and some invertebrates that releases both waste and reproductive elements), clinging to attackers and immobilizing them. Yet not only does the pearlfish fail to trip this defense when it enters the sea cucumber, it seems to be immune to its toxins while occupying the host, which Parmentier says may be attributable to the unusual amount of mucous coating the fish's body.

You have teeth in your head, but some sea cucumbers have them in their bum. Imagine, if you will, the life of a sea cucumber dentist. Image: Paddy Ryan

But the sea cucumber may not be entirely defenseless against the invasions of the pearlfish. Some species have what could be functioning as a built-in gate in their anus – a handy accessory considering that in addition to the pearlfish, crabs and clams have also been known to make themselves at home inside the poor critters (the sea cucumber, it seems, like any good host, can never really enjoy itself at its own party).

“The part that hasn't really been substantiated is the fact that you have some sea cucumbers that have these kind of spines around the anus called anal teeth,” said Mah. “And it isn't clear if these anal teeth really are active defensive mechanisms that keep things like fish and crabs out, or if they're just there and we assume they're defensive in some way.”

So on that note, if you’ll excuse me I’m going to look into this Buddhism business.

Hat tip to Oceans IQ for suggesting this week's critter. Browse the full Absurd Creature of the Week archive here. Have an animal you want me to write about? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.

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References

Parmentier, E. and Vandewalle, P. (2003) Morphological Adaptations of Pearlfish (Carapidae) to Their Various Habitats. Science Publisher

Parmentier, E. and Vandewalle, P. (2004) Further Insight on Carapid-Holothuroid Relationships. Marine Biology. 146: 455–465