Royal Sea Cucumber

Pearl fish are small slender fish that take advantage of the fact that sea cucumbers can’t really run away and so they swim up the sea cucumber’s bum to hide.
Some sea cucumber bums however, are not safe, and the fish who enter them are never seen again. Inside of these sea cucumbers is another species of pearl fish, one who sits quietly in the darkness and waits for its next victim.
Guardian: New Plymouth Girls School