These fish can see through their foreheads.

In the ocean’s twilight zone, sunlight fades to perpetual midnight. Animals have evolved an array of adaptations for surviving in an environment of dim light and shadows, but one fish stands out as one of the most unique.

The barreleye fish (Macropinna microstoma) has enormous eyes that rotate inside a transparent head. This unique eye structure allows the barreleye to peer up at potential prey or focus forward on a meal.

Like many fishes in the ocean’s twilight zone, Macropinna scans the waters above for food. Most of the time, the barreleye lies in wait in a horizontal position, eyes oriented upwards. Their large, flat fins allow them to remain nearly motionless in the water and maneuver very precisely.

The tubular shape of their eyes concentrates light to enhance their vision, revealing the silhouettes of their prey in the faint downwelling sunlight. Green lenses help Macropinna distinguish natural sunlight from the bioluminescent light their prey produces for camouflage.

Since Macropinna was first described in 1939, scientists had been perplexed by how this fish actually consumed prey when the eyes and mouth point in different directions. MBARI scientists combined video footage recorded of barreleye fish in their natural habitat with lab observations to solve this paradox. 

After spotting a tasty morsel, Macropinna rotates those big eyes from upward-looking to forward-looking to keep prey in sight and grab a bite. 

Macropinna eats a variety of zooplankton, including small crustaceans and jelly-like siphonophores. MBARI scientists speculate that the transparent shield offers protection as the barreleye maneuvers among the stinging tentacles of siphonophores to snag a meal.

Deep-sea animals capture the imagination. These are our neighbors who share our big blue planet. MBARI research and technology are revealing our close connection to the ocean and how human actions threaten the future of deep-sea ecosystems. By sharing our encounters with the amazing animals of the deep, we hope to raise awareness about the deep sea and inspire stewardship of the largest living space on Earth.