This video pretty much does what it says on the tin. The fish in question belongs to a species of pearlfish — small, tropical fishes known to seek out shelter inside various species of invertebrates... and if you happen to be a sea cucumber, "inside" means you're going to have these little guys all up in your anal pore.

The pearlfish treats the cucumber's anus like you might a garage, reversing into the cucumber slowly but surely until it's disappeared from view almost entirely. I just wish I knew how the cucumber feels about all this.

kineticcreative via Ed Yong