Butthole Surfers

Pearlfish live inside their host’s anus, just like you and me

Out here in the animal kingdom, we’re all about taking things one day at a time. Putting one foot in front of the other. And like all of us, the parasitic pearlfish lives inside of its host’s anus and feasts upon its regenerating organs.

Like you, or me, or anyone we know, the pearlfish’s preferred host is the sea cucumber—a beige cruller that looks a lot like Heimlich from A Bug’s Life. The cucumbers fart along on the sea floor and suck sand in through their mouths, sifting it for things that might be edible with their internal organs, and then pooping it back out in little coils. They’re like very slow Roombas. And apparently they’re very “Eh” taste-wise, because most predators seem to leave them alone, which makes their bodies the perfect open floorplan studio space for a pearlfish. I’ll let Popular Science explain:

It all begins innocently enough. The skinny, eel-like fish approaches the sea cucumber and gives it a sniff, moving up and down the length of its soon-to-be victim with its body pointed almost vertically. It’s looking for the sea cucumber’s breath, because, well, sea cucumbers breathe through their derrières. If the sea cucumber detects the pearlfish, it’ll hold its breath, sealing its butt like a human holding in a fart. But the sad sack is only delaying the inevitable. At some point it has to breathe, and that’s when the pearlfish strikes.

“Hey!” you might be saying. “Normal!” To which I must reply—yes. And it gets normaler.

Once the pearlfish sees its window of opportunity, it has to make one of the toughest decisions in the animal kingdom: whether to enter the sea cucumber’s bum tail first or head first. This, of course, all depends on the size of the orifice… If the opening is big enough to enter head first, the pearlfish goes at it full tilt, jamming its face in and rapidly flicking its tail to fire itself into the sea cucumber. If the opening is too small, the fish first inserts its thin tail, then backs in slowly.

How do you enter your sea cucumber? Head first? Tail first?

Once the pearlfish is in there, it’s free to stick its head out and interact with the world like a dog with a rolled down car window. If pickings are particularly slim in the snacks department, they can pop inside and eat the cucumber’s internal organs, which regenerate when lost just like starfish legs. Understandably, the traditional delicacy is the gonads.

Sea cucumbers aren’t the only animals pearlfish live in—sea stars, oysters, and other bivalves can all also make good homes—but for some reason these guys seem to be the favorite. Reportedly, a single sea cucumber was once cut open by scientists to reveal an extremely reasonable fifteen adult pearlfish living inside.

I will leave you with this footage from the BBC. May it soothe you like an ASMR video: