Pearlfish: The eel-like fish that lives up a sea cucumber's butt

Name: Pearlfish (Carapidae family)

Where it lives: Inside invertebrate hosts in shallow, tropical waters around the world

What it eats: Plankton, small particles and sea cucumbers' gonads

Why it's awesome: This long, slim fish doesn't look unusual until you see it return home — into a sea cucumber's butthole.

Pearlfish don't have scales or any way of protecting themselves, so instead they have to find a safe hideaway. But rather than sheltering in a seagrass meadow or hiding in the crevices of a rock, they have chosen an unusual refuge: sea cucumbers' anuses.

"The rear end of a sea cucumber may seem like undesirable real estate, but for a pearlfish, it does just the trick," according to the Smithsonian Ocean website.

Sea cucumbers breathe through their butts, giving pearlfish an easy opportunity to sneak into their unwitting host. The pearlfish sniffs out its host, then just has to wait "for the cucumber to open for a breath and swim inside," Smithsonian Ocean said. They do this every time they need to re-enter their home, which they leave to find food.

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Even the sea cucumbers' Cuvierian tubules — special sticky threads that sea cucumbers eject from their bums in self-defense — don't affect pearlfish. And cucumber species with anal teeth still succumb to these pesky butt-dwellers. "I have found a new species that is smaller that was able to go inside, even with the teeth," Eric Parmentier, a pearlfish researcher at the University of Liège, Belgium, told Live Science in an email.

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These eel-like fish often — but not always — make a home for one inside their host. "As far as we know, there is usually one [pearlfish] to a sea cucumber, but some species have been reported to pair up in a single cucumber," Matt Girard, a researcher in the Division of Fishes at the Smithsonian National Museum of Natural History told Live Science in an email.

The number of pearlfish shacked up in a single sea cucumber can hit double figures. In 1975, scientist Victor Benno Meyer-Rochow found a leopard sea cucumber (Bohadschia argus) with 15 pearlfish inside it.

Some species can live inside sea cucumbers without doing them harm, Girard said. They live in symbiosis and "neither host nor invader is harmed."

But there are also parasite species that "eat the sea cucumber's gonads," Parmentier added. "It does not kill the host but it can disturb its reproduction."