Sneaky Squatters

With such a grand and beautiful title, you’d expect the Pearlfish (Echiodon rendahli) to live up to its name and behave in a suitably dignified manner. Yet it seems that this slender little fish’s habits and preferred living arrangements are perhaps not the sort of thing one discusses in polite society. Out in the areas of the Atlantic, Pacific and Indian Oceans where they live, the key thing is for fish to find somewhere to hide so as to avoid predators. Suitable places are not always easy to find, but the resourceful, eel-like Pearlfish has gotten around the problem by making its home inside the anus of the nearest suitable (and one presumes, unsuspecting) sea cucumber.

Living inside another creature’s bottom might sound like the very worst place to be, but it affords the fish with a measure of protection, letting them hide during the day until they come out to feed at night. Most Pearlfish do no appreciable harm, living commensally within the moist, safe colonic cavity of their unwilling hosts. However, it’s thought that there are at least some species such as the Pinhead Pearlfish (Carapus boraborensis), that are actively parasitic and feed on the sea cucumber’s gonads from within. Pearlfish have been known to live inside other invertebrates like clams, sea squirts and starfish, but sea cucumbers seem to be their preferred home, much to the sea cucumber’s discomfort.

Still, there’s something to be said for small blessings – it’s lucky for the sea cucumber that the Pearlfish is so slender…