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.
Why live observation changed the answer
Deep-sea animals often reach laboratories in poor condition. Pressure changes, nets, handling, and preservation can distort soft tissue or leave it missing entirely. A transparent, fluid-filled shield is exactly the kind of structure that may not look normal in a jar. A specimen can still establish anatomy, but it may conceal how that anatomy behaves in life.
Remotely operated vehicles added context that a preserved fish could not: posture in the water, the direction of the eyes, and the animal’s response while swimming and feeding. Bringing live individuals into a controlled observation tank then allowed researchers to see the eye rotation more clearly. Neither method replaces the other. The specimen supplies close anatomical evidence; observation supplies movement and habitat.
The preserved anatomy explains where the eyes are; live observation explains how they move. Together, those findings support a search-and-approach sequence, rather than a complete record of every meal:
- 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.
The fish’s small mouth also matters to the interpretation. It points away from a high-speed attack on large prey and toward careful feeding on relatively small items. Its broad, flat fins can help it hold position and maneuver precisely. Together with the rotating eyes, those features fit a patient search-and-approach strategy better than a long pursuit through dark water.
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.
The dome, rotating tubes, small mouth and positioning fins make sense together when the fish searches for faint prey above it. The proposed link to siphonophores adds a possible feeding setting; it remains less directly established than the eyes’ movement and the shield itself.
Visual guide
How the pieces fit together
