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.
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| The pearlfish |
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 hone 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
