Meet the Pearlfish, a slim eel-like fish that has found an unusual home - inside a sea cucumber's anus! Despite having no scales for protection, these fish have adapted to use the sea cucumber's sphincter as a safe haven. They even wait for their host to breathe and then swim inside to return home.

Source: https://www.livescience.com/animals/fish/pearlfish-the-eel-like-fish-that-lives-up-a-sea-cucumber-s-butt