Which fish has a transparent, fluid-filled head that reveals its tubular eyes?

The barreleye fish has a transparent, fluid-filled head that reveals its tubular eyes.

The best-known species is Macropinna microstoma, a deep-sea fish found in the North Pacific. Its eyes point upward through the transparent shield, allowing it to detect silhouettes of prey above. Green pigments in the eyes may help reduce the glare of sunlight filtering down from the surface.

Barreleye eyes can also rotate. They generally point upward while the fish searches for prey, then turn forward when it feeds. This unusual arrangement helps the fish track small organisms and gelatinous animals in dim water.

A common mix-up is that the fish’s visible structures are not its nostrils or brain; they are its tubular eyes. Scientists described the species in 1939, but its delicate head covering was not clearly understood until live specimens were observed in the early 2000s.