The Mystery of the Barreleye Fish

The Mystery of the Barreleye Fish

August 13, 2019 100 By Kailee Schamberger


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. Thank you for watching this SciShow Dose,
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