I like to tell people that truth is stranger than fiction. Everyone always smiles a little and goes on with their life. No one ever realizes the true extent of that weirdness. If someone were to make up even half the strangeness that goes on in nature theirs would be considered fiction of an unintentional comedy.
Pearlfish are a perfect example of this. They look like eels but what makes them unusual is that most of them live out their adult lives inside other creatures, mostly invertebrates. Most of the time they’re pretty harmless, just hanging around inside occupied clam shells and so on. But about a third of the species make their way in life inhabiting the asshole of a sea cucumber. That’s right; there really exists a fish that lives its life in the anus of another animal, eating its sperm or eggs depending on whether it inhabits a male or females sea cucumber. I rest my case.