In its larval stage, after locating a suitable sea cucumber, the [Fierasfer acus] pokes its head inside to see whether it has found the right host, and then turns around and backs in tail first through the rear opening. Being very thin and almost needle-shaped, it fits neatly into the long, narrow, tubelike intestinal tract. With its head usually sticking out and the rest of its body hidden inside the sea cucumber, Fierasfer acus spends the remainder of its larval stage well protected… Apparently the sea cucumber does not always feel comfortable with its live-in partner: it seems that at least occasionally it tries to evict the fish when the non-paying tenant does not behave like a polite guest. There is evidence that the fish sometimes pushes through the walls of the intestines and invades the body cavity of its host. Once there, it proceeds to eat some of the latter’s organs, which is decidedly poor etiquette.
– from Partners, Guests, and Parasites: Coexistence in Nature, by Hilda Simon