ANiMAL MUNDi: The Strange Case of the Barreleye Fish

The Strange Case of the Barreleye Fish

written by Corinna of TheFrogBag

logo designed by Eva of CocoNme

First discovered in 1939, this half-foot long deep sea dweller was only known from mangled specimens until just a couple of years ago. Like many animals that live at depth, it’s most fragile features never survive the long trip to the surface and the change in pressure differential between the deck of a boat and their home far below. Perhaps it’s no surprise that we didn’t recognize what a truly bizarre creature the barreleye is until we developed the technology to visit it in its natural habitat.

Just how weird is it? To start with, it has a transparent head. Not just a sort-of-translucent-when-you-shine-light-on-it head, but a truly transparent one. Then there’s the fact that its eyes are inside that head. Take a look at the photo: those big green disks at the top of the fish are the lenses, the dark structures under them the eyes proper (hence the name “barreleye”, of course). And those slightly luminescent structures at the front of the fish, where you might expect the eyes to be? That’s actually a pair of olfactory organs (its “nose”, if you like).

Add it all up and this is one of the most unusual animals ever discovered. The weirdness doesn’t stop there though. The fish spends most of its time with those big green eyes pointed upward, looking for food, but it is capable of swiveling them forward, to watch its meals actually enter its mouth, and backwards to look behind it through its own skull.

Okay, so it’s weird. The question is, why? Why have huge eyes when you live in the pitch black? Why have a transparent head when most of us make do with an opaque head? The answer lies in what it eats, and that fact may be the strangest thing of all. It seems that microstoma lives off of plankton, that ubiquitous microscopic food source famous for powering even animals as big as whales. The tried and true method of making a living from plankton is called filter feeding, literally filtering the tiny organisms out of gallons upon gallons of seawater.

That’s just too normal for the barreleye though. Instead, this fish stalks colonial, jellyfish-like animals called siphonophores and steals their food. Siphonophores themselves catch plankton in huge drifting nets of stinging tentacles, which is presumably why the barreleye keeps its eyes under a transparent dome. When working around dangerous materials it’s always a good idea to use eye protection! In addition, siphonophores glow with bioluminescence, making them easy targets for the huge-eyed micro stoma.

Amazingly, almost nothing was known about the barreleye until the Monterey Bay Aquarium Research Institute visited live specimens deep in their natural habitat in 2004. No one could have guessed how the fish really looked based on the distorted specimens brought to the surface in nets. All of which makes you wonder: if an animal this strange is hiding right off the coast of California, what other secrets might the oceans hold?