Jan 13

Bizarre Animal of the Week: Barreleye

The most unexplored habitat on Earth is the ocean deep. We know more about the surface of the moon than we do about the very bottom of our seas. But from what little we have explored, we have discovered a very strange and mysterious world, filled with creatures that can boggle the mind.

Take this fish, for example. It’s called the barreleye, or spookfish.


Photo: Fishbase

Straight away, you will notice that something isn’t quite right with this animal. For a start, its head is a transparent dome. That alone would make it a worthy addition to ‘Bizarre Animal of the Week’, but the barreleye isn’t finished yet. Can you see those two green, rounded organs within its head? Those are the fish’s eyes. Yes, rather than having eyes that poke through its head like virtually all other animals that possess them, the barreleye has them beneath its skin, looking up through its own head. The transparency, at least, now becomes clear.

The barreleye fish usually hangs motionless in the dark waters of the ocean deep. Its eyes point upwards through its head, absorbing the last vestiges of light that filter down to these depths, and tracking its prey by their faint silhouette above. When it spots food, its eyes can rotate within its head so that they face forwards as the fish turns its body from a horizontal position to a vertical one to grab its prey from below. One type of barreleye, the brown spookfish, even employs a mirror to focus an image in its eye, the only vertebrate known to use such apparatus.


Photo: Monterey Bay Aquarium Research Institute

All of this is great and fascinating and all that, but why exactly have the barreleyes gone down this unorthodox evolutionary route? Well, there are no concrete answers, but there are at least some theories. By having their eyes inside their heads, barreleyes avoid the usual problem that most fish have, which is that they don’t have forward-facing binocular vision because their eyes are located on the sides of their heads. Eyes on the side of your head are great for an animal that needs to monitor different directions for incoming predators, but not so good for active predators like the barreleye. By having eyes inside their head, they can point in the same direction (usually upwards), therefore vastly improving the barreleyes’ depth perception and their ability to spot faint objects.

Of course, surely it would make more sense – and require less bodily modifications – to simply migrate the eyes slightly, rather than making the whole head see-through? But maybe there are other advantages to this quirk. Barreleyes often prey on small jellyfish or the tiny creatures caught in their tentacles. Eyes would normally be very vulnerable to the stinging cells of these animals, but because they are encased within a transparent and protective shield, the barreleyes are hardly affected.

The deep sea is such a mysterious and intriguing place that I’ve decided to dedicate an entire week to exploring it. Next time, we’ll be looking at more strange and unusual fish that make their home in the dark ocean abyss.

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