Discovered in 1939, the barreleye fish (Macropinna microstoma) remained a deep-sea mystery for decades because its delicate dome-shaped head shattered whenever specimens were hauled to the surface. It was not until 2004 that Monterey Bay Aquarium Research Institute researchers captured live footage of the creature in its natural habitat using remotely operated vehicles. What looked like tiny eyes on the front of its face were actually olfactory organs called nares.
The fish's actual eyes are two large, tubular green lenses submerged entirely inside a transparent, fluid-filled shield surrounding its head. These tubular eyes are designed to collect faint ambient sunlight filtering down from the ocean surface, helping the fish detect the silhouettes of overhead prey such as siphonophores. Crucially, Macropinna can rotate its ocular structures from a vertical orientation to a horizontal position.
This rotation is housed entirely within its protective fluid dome, allowing it to navigate transparently while stealing food directly from the stinging tentacles of siphonophores without sustaining ocular damage.