Fish
Barreleye
Macropinna microstoma · also called barreleye, barrel-eye, spook fish
The barreleye is a fish that looks like a science-fiction prop: a small silver body topped by a transparent, dome-shaped forehead, with two glowing green eyes suspended inside it. Those eyes normally stare straight up — the fish literally looks out through the top of its own see-through head.
Overview
The barreleye lives in the twilight zone of the temperate North Pacific, around 600–800 metres deep, hanging almost motionless in the water on large flat fins. It feeds on small jellyfish, siphonophores, and the plankton caught on their tentacles, and may steal prey from siphonophores as it drifts among them.
The transparent head
The top of the barreleye’s head is a clear, fluid-filled dome, like a cockpit canopy. The fish’s actual eyes are the two green domes inside it — the dark spots on its face that look like eyes are just nostrils. The dome protects the delicate eyes (possibly from jellyfish stings) and lets light reach them from many angles. Its structure was only understood in 2009, when live specimens were first filmed; every earlier netted fish had the fragile dome torn off.
Rotating tubular eyes
The eyes are tube-shaped — highly light-sensitive but narrow in view. Their default aim is straight up, scanning for prey silhouetted against the faint surface light. When the barreleye moves in to feed, the eyes rotate forward inside the dome so it can watch the prey and its own upturned mouth. A yellow lens pigment helps it filter sunlight and pick out the glow of bioluminescent prey.
Status
The barreleye has not been assessed for conservation. It is hard to study — small, delicate, and 700 metres down — and most of what is known comes from MBARI’s deep-sea vehicles filming living fish in the 2000s.
Barreleye: common questions
Why does the barreleye have a see-through head?
The transparent, fluid-filled dome protects and houses the fish's unusual tubular eyes while letting light pass through to them from almost any angle. The eyes themselves are extremely light-sensitive and are shaped like short tubes rather than balls, which gives excellent sensitivity and a narrow field of view — ideal for a very dark environment but limited in where it can look. By sitting inside a clear dome, the eyes can rotate freely to aim in different directions without the fish having to move, and the dome shields them, possibly including from the stinging cells of the jellyfish and siphonophores the barreleye feeds among. The structure was only understood in 2009, when researchers first filmed intact live specimens; earlier netted fish always arrived with the fragile dome destroyed.
Can it really look through its own forehead?
Yes. The default position of the barreleye's tubular eyes is pointing straight up, so the fish looks up through the transparent top of its own head, scanning for the shadows of small prey animals silhouetted against the dim light filtering down from the surface. When it detects something and moves in to feed, the eyes rotate forward within the dome so the fish can keep watching the prey — and its own upturned mouth — as it manoeuvres and strikes. This solved a long-standing puzzle — with fixed upward-pointing eyes, how could the fish see what it was eating? The answer is that the eyes are not fixed.
What are the two dark spots on the front of its face, if not eyes?
They are the fish's nostrils (olfactory organs). In photographs and drawings of the barreleye they sit roughly where you would expect eyes to be, above the mouth, which has fooled many people — including early describers — into thinking the fish had ordinary forward-facing eyes and the green domes were something else. In fact the green domes on top of the head are the eyes, and the "eyes" on the face are just the sense of smell. The mix-up is understandable given how strange the layout is.
How do we know so little about it?
The barreleye lives 600 to 800 metres down in the open ocean, a habitat that is expensive and difficult to study, and it is a small, delicate fish. For decades the only specimens came up dead and mangled in trawl nets, with the soft transparent dome torn away, so its most important feature was literally missing from every museum specimen. It was only when the Monterey Bay Aquarium Research Institute's remotely operated vehicles filmed and carefully collected living barreleyes in the 2000s that the intact head, the rotating eyes, and the feeding behaviour could be observed. Sightings remain rare enough that each new one is notable.