Deep dive 3 min read
The Fish With a Transparent Head: How the Barreleye Sees
A fish with a see-through dome for a head and glowing green eyes inside it. The barreleye looks like a spaceship, and it took scientists decades to understand it.
Imagine a fish with a clear plastic bubble for a forehead, and two glowing green eyes floating inside it. It sounds like a sci-fi movie prop, but the barreleye fish is real. It’s one of the strangest faces in the deep sea, and one of the hardest to figure out.
Meet the barreleye
The barreleye, Macropinna microstoma, is a small fish, only about 15 cm long. It lives in the North Pacific Ocean, roughly 600 to 800 metres down, in the “twilight zone” where only a faint glow of sunlight reaches. It hangs almost motionless in the water, with its large fins spread wide.
Those aren’t its eyes
Look at its face and you’ll see two dark spots where you’d expect eyes. Surprise: those are its nostrils. The real eyes are inside the clear dome, two glowing green barrels pointing upwards. That’s where the name comes from.
The eyes on its face are fake. The real ones are inside its see-through head.
A head like a cockpit
The dome on its head is a transparent shield filled with fluid. It protects the sensitive eyes like the cockpit canopy on a fighter jet. For decades, nobody knew it was there. Barreleyes were first described in 1939, but whenever they were hauled up in nets, the fragile shield was destroyed. Only when scientists filmed living barreleyes with deep-sea robots in the 2000s did they see the fish as it really is.
How does the barreleye see?
The tubular eyes are excellent at collecting what little light there is. Their green colour comes from a pigment that may filter out the last traces of sunlight, helping the fish spot the glow of other animals instead.
For a long time, scientists thought the eyes were fixed, always looking up. Then, in 2009, researchers watched a living barreleye and discovered that it can rotate its eyes inside the shield. It looks up to spot food silhouetted above, then turns its eyes forward to watch as it swims up to grab it.
Strange but trueScientists think barreleyes may steal food from siphonophores, long stinging colonies of jelly-like animals. The see-through shield could protect their eyes from the stings.
Why a transparent head?
Deep-sea life is all about seeing without being seen. A clear shield lets the barreleye look straight through the top of its head while keeping its eyes safe. It’s a design so unusual that most people refuse to believe photos of it are real.
The verdict
The fish with a transparent head is no Photoshop trick. It’s a perfectly evolved deep-sea spy with a built-in cockpit and swivelling eyes. For more creatures you can see right through, explore our See-Through and Deep Sea collections.
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