Short answer
The answer in plain English
The barreleye fish has a clear, fluid-filled shield over its head. Its real eyes are the green tubes inside, not the dark spots at the front of its face. The eyes can look upward for silhouettes and rotate forward when the fish moves in to feed.
Why it matters
What to understand
The barreleye lives in the dim twilight zone of the ocean. The dark marks on its face are organs used for smell, while its green eyes sit safely under a transparent dome. Observations by MBARI showed that those tubular eyes can rotate, explaining how the fish can search above itself and still aim at food in front of its mouth.
The face is a visual trick
Look at a barreleye fish from the front and two dark spots seem to occupy the normal position of eyes. They are not eyes. They are olfactory organs—structures associated with smell.
The real eyes are the bright green tubes visible inside the clear top of the head. This inverted-looking arrangement is why photographs can feel like badly assembled special effects. We expect a vertebrate face to follow a familiar plan, while the barreleye has separated smelling, looking upward, and feeding into different parts of the same compact system.
A window built for the twilight zone
Macropinna microstoma lives roughly 600 to 800 meters deep in the North Pacific, according to the Monterey Bay Aquarium Research Institute. This is the mesopelagic or twilight zone: too deep for normal daylight, but not completely dark.
For an animal below potential prey, the remaining light from the surface creates silhouettes. Upward-pointing tubular eyes gather that narrow opportunity. Their green pigment may help filter the downwelling light and improve contrast.
The transparent dome matters because the eyes sit inside it. It lets light reach them while forming a protective shield around delicate tissues. The dome is not a hollow glass helmet; it is living, fluid-filled anatomy.
The feeding puzzle
For years, preserved specimens encouraged the idea that the tubular eyes were fixed upward. That created an obvious problem: how could the fish aim its mouth at food directly in front of it if it could only look toward the surface?
Live observations and collected specimens studied by MBARI researchers supplied the missing movement. The eyes can rotate. The fish can hold them upward while searching, then turn them forward as it approaches food.
The mechanism changes the whole animal from a collection of odd parts into a coordinated hunter:
- hover nearly motionless and watch the faint ceiling above;
- detect a silhouette with the upward-facing tubes;
- rotate the eyes toward the mouth during the approach;
- take food with a small, precise movement rather than a long chase.
A risky place to eat
Researchers have proposed that barreleyes may take small prey associated with siphonophores—colonial drifting animals with tentacles that can sting and capture food. A clear shield could offer some protection while the fish maneuvers close to those tentacles, although details of its feeding behavior remain difficult to observe in the deep ocean.
That uncertainty is important. The transparent head and rotating eyes are observed anatomy. Specific hunting scenarios are evidence-based interpretations, not a complete recording of every meal.
Why the fish looks impossible
The barreleye violates the face template our brains use for most animals. Its apparent eyes are smell organs. Its real eyes are inside its head. They point upward until they do not. And the structure that seems most fragile—the clear dome—is part of the protection system.
Nothing about it requires fantasy. It only requires a different visual problem: finding faint food above you in a dark ocean where chasing is expensive. Once that problem is clear, the impossible face becomes an elegant piece of engineering.