This is an example of a pearl fish. It is a quite small, quite slender, scaleless marine fish. The pearl fish are actually a family of fish, encompassing 36 different species. Many of the members of this family have a… well… interesting habitat of choice. They live in the sea, yes, but in a very particular part of the sea.
Can you guess where they live? Go on, guess.
Pearl fish live up the butt of sea cucumbers.
That’s right. For some bizarre reason, evolution brought the pearl fish and the sea cucumber together, and the pearl fish spend most of their time happily hanging out in a rectum. They will poke their heads out, eat a bit, and when they want back in they tickle the sea cucumber’s anus with their tail fin. The sea cucumber obligingly opens up, and back up the pooper they go. Different species also tend to prefer specific species of sea cucumber, but don’t ask me why. Also the sea cucumbers don’t seem to mind them, which I suppose is all for the best.
Need more proof? Here are some amusing pictures of pearl fish in their, uh, natural habitat