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.