The barreleye fish with a transparent head: the deep-sea creature that sees through its own skull

At depths between 600 and 800 meters in the Pacific Ocean lives one of the most baffling creatures in marine biology: the barreleye fish (Macropinna microstoma). Although the species was first described in 1939 from specimens damaged by trawl nets, it was not until 2004 that scientists from the Monterey Bay Aquarium Research Institute (MBARI) managed to film it alive in its habitat, revealing an unusual anatomical design.

The top of the barreleye's head is not covered by scales or opaque bone, but by a transparent shield filled with fluid resembling the cockpit of a fighter jet. What appear at first glance to be two eyes on the front of its face are actually nares, or olfactory organs. Its true eyes are two large, emerald-green tubular structures located entirely inside its translucent skull.

This arrangement addresses an extreme evolutionary challenge in the deep twilight zone:

• Ultra-specialized vision lenses: Its eyes contain green pigments that filter out faint downward sunlight, allowing it to accurately detect the bioluminescent silhouette of jellyfish and siphonophores floating above it.
• Internal eye rotation: For decades, it was believed that its tubular eyes were fixed pointing upward, but MBARI showed that it can rotate them forward inside its transparent shield when maneuvering to catch prey with its small mouth.

The transparent shield also protects its delicate visual organs from the stinging tentacles of cnidarians from which it often steals food, showing how evolutionary pressure in the abyss can completely reinvent the anatomy of a vertebrate.