In 1975, Victor Benno Meyer-Rochow was diving off the Banda Islands in Indonesia, when he collected a leopard sea cucumber—a cylindrical relative of starfish and sea urchins. It was a large and stubby specimen, 40 centimetres long (16 inches) and 14 centimetres wide. He dropped it in a bucket of water, which he placed in a refrigerated room.

Sometime later, a slender, eel-like fish swam out of the sea cucumber’s anus.

It was a star pearlfish, and it wasn’t alone. Another wriggled out. And another. After ten hours, 14 pearlfish had evacuated the animal’s bum, each between 10 and 16 centimetres long. Another one stayed inside.

There are many species of pearlfish. Some live independently, but several make their homes in the bodies of shellfish, starfish, and other marine animals. Indeed, they got their name after one individual was found inside an oyster, dead and embedded within mother-of-pearl.

But sea cucumbers are their most infamous hosts. Having found one by following its smell, a pearlfish will dive into the anus headfirst, “propelling itself by violent strokes of the tail,” according to Eric Parmentier. If the sea cucumber objects and closes down its anus… well, it still has to breathe.

Oh yeah, sea cucumbers breathe through their anuses. By rhythmically expanding and contracting their bodies, they drive water through the anal opening and into a branching, lung-like structure called the respiratory tree. This process creates gentle currents that a pearlfish can use to find its hosts. It also creates a vulnerability, because a sea cucumber that’s clenching its butt is also holding its breath. When it exhales, as it eventually must, it dilates its anus, allowing the pearlfish to thread itself in. This time, it goes tail-first, bit by bit, breath by breath.

Some species just use the sea cucumbers as shelters. But the Encheliophis pearlfishes are full-blown parasites that devour their host’s gonads from within.

Pearlfish are typically found alone, and adults have been known to kill rivals that try to infiltrate the same host. Still, as Meyer-Rochow found, the fish can sometimes be more sociable—or at the very least, tolerant. No one knows why. It’s possible that when sea cucumbers are rare, the fish are forced to share a host. Alternatively, they could have gathered to breed. “If indeed the 15 fish entered for sexual reasons, one cannot help but think of the orgy that must have taken place inside the sea cucumber,” Meyer-Rochow says.

These anal abodes aren’t easy places to live.

Pearlfish should be especially vulnerable, since they are literally swimming inside the saponin-producing structures. And yet, when Igor Eeckhaut exposed various fishes to sea cucumber saponins, the pearlfish survived 45 times longer than other species. How do they cope?

Sea cucumbers resist their own poisons because their cell membranes comprise special chemicals that interact less strongly with saponins. But Lola Brasseur from Eeckhaut’s team found that pearlfishes don’t use such fancy chemistry. Nor do they have any tricks for detoxifying the saponins.

Instead, they rely on mucus, which they secrete onto their skins. The mucus helps to lubricate them on their way into their hosts, but it also acts as a physical barrier against the toxic saponins. It’s especially effective because the pearlfishes make so much of it—six to ten times more than other fishes that have no interest in sea cucumber bums.

Of course, none of this explains why the sea cucumbers don’t use their most effective defence. When threatened, they can expel their respiratory trees at their attackers, relying on their regenerative powers to re-grow the lost organ.