Just off the west coast of the United States, deep down in the Pacific Ocean, there lives
a fish.
And not just any fish.
With its long, bulging eyes and transparent head, the Pacific barreleye fish is a little…
different.
Biologists don’t actually know much about it, because like many deep-sea animals, the
barreleye is hard to catch, and even harder to observe in its natural habitat, 600 to
800 meters below the surface of the ocean.
But over the years, researchers have managed to learn a little more, both from specimens
that are hauled up to the surface in nets, and -- once -- from catching the thing on
camera.
And it turns out that, unsurprisingly, the fish’s adaptations are exactly what it needs
to survive.
First, there are its eyes.
And no, I’m not talking about those little black circles above its tiny mouth, where
you’d normally expect to find eyes.
Because those are not eyes at all; they’re nares, which are basically just nostrils.
The fish’s eyes are actually those green barrel-shaped things behind the nares, which
give this fish its name… and look straight out of the top of its transparent head
The barreleye fish has what are known as tubular eyes, because they’re shaped like tubes.
And the fish uses them in a very particular way.
Eyes are often compared to camera lenses, in the sense that they collect and focus light.
The larger the lens, the more light it can gather, which is important when you hang out
in the dark depths of the ocean but still need to be able to spot your prey.
But for the barreleye, just gathering light isn’t enough.
Its tube-shaped eyes allow it to see farther away, like binoculars in your head.
They give the fish a fairly narrow range of vision, but that’s okay, because most of
the time, it only needs to look in one direction: up.
That’s because the barreleye fish lives just below its prey.
So in their natural resting state, the barreleye fish’s eyes are looking straight up, scanning
for food.
In 2007, researchers from the Monterey Bay Aquarium Research Institute managed to get
a good look at a live Pacific barreleye fish, and they discovered that its eyes have even
more specific adaptations.
For one thing, they’re green, which the biologists think helps them better see their
prey.
We don’t know all that much about what these guys eat, but scientists think they prey mostly
on jellies and small fish.
And jellyfish are often bioluminescent, meaning that the barreleye is looking for their telltale
glow.
But as the fish stares upward looking for food, even at those great depths, there’s
still a lot of interference from sunlight, which can be bright enough to outshine the
bioluminescence.
The green pigment in their eyes might act as a kind of filter, taking the sunlight out
of the picture and leaving only the glow of tasty food.
But once the fish has spotted its next meal, it can still have trouble chasing it down.
Researchers think that barreleye fish steal their food from siphonophores, which often
have tentacles with a powerful sting.
A sting to a giant, sensitive, light-collecting eye would not be a fun time for the barreleye
fish, which is probably why it has that see-through head.
The head is filled with fluid, forming a shield that protects the eyes from any stinging tentacles.
Something else had been puzzling scientists for a while, though: once it got its food,
how did the fish actually eat it?
Many animals with tubular eyes don’t really move them around.
But with its eyes always facing upward, the barreleye’s mouth would never be in its
field of vision.
For a fish, that would make it tough to actually, like, get the mouth on the food thing.
The Monterey Bay researchers discovered something that we’d never known before: the fish’s
eyes can actually turn to look forward, which they usually do if the barreleye’s body
goes vertical.
Meaning that as it swims upward toward its food, the fish can actually rotate its eyes
inside of its head to see in front of its face.
That makes it much easier for the barreleye to chomp down on its food.
We still have a lot to learn about the Pacific barreleye fish.
But aside from the possibility of getting stung in the face at every meal, life with
a transparent head doesn’t sound half bad.
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