Which fish has a transparent head that allows it to see through its skull?

The fish with a transparent head that lets it see through its skull is the barreleye fish. Its eyes sit inside a clear, fluid-filled dome of tissue, creating one of the strangest faces in the deep sea.

Barreleyes belong to the family Opisthoproctidae and live in dark waters of the Atlantic, Pacific, and Indian oceans. Their barrel-shaped, tubular eyes normally point upward, helping them detect the silhouettes of prey or predators above. The eyes can also rotate forward when the fish feeds.

The best-known species, Macropinna microstoma, was long misunderstood because the transparent dome collapses when the fish is brought to the surface. Scientists discovered in 2004 that the dome is intact in living animals. The clear shield may protect the eyes from the stinging cells of siphonophores, organisms from which barreleyes may steal food. Despite its common name, the barreleye is not blind-looking: its highly sensitive eyes are adapted to collect scarce deep-sea light.