Which deep-sea fish has a transparent head and tubular eyes that point upward?

The barreleye fish has a transparent, fluid-filled head and tubular eyes that normally point upward.

Barreleyes belong to the family Opisthoproctidae and live in the deep ocean, commonly at several hundred metres below the surface. Their unusual eyes are highly sensitive to the faint light produced or reflected by animals above them. The upward-facing arrangement helps the fish detect silhouettes, while the green pigment in the eyes may reduce glare from bioluminescence.

The transparent head covering was not clearly understood until specimens could be observed intact. Earlier collected examples often lost the delicate dome, leading scientists to miss an important part of the animal’s anatomy. Researchers later found that the eyes can rotate forward when the fish positions itself to capture prey.

The best-known species, Macropinna microstoma, is sometimes called the Pacific barreleye. It is not the same animal as an anglerfish, whose famous lure attracts prey in darkness. The barreleye’s see-through head and movable eyes are adaptations to the visual challenges of the deep sea.