Fish
Four-Eyed Fish
Anableps · also called four-eyed fishes, cuatro ojos
A four-eyed fish cruises along the surface of a coastal lagoon with half of each eye poking above the waterline and half still submerged, watching a bird overhead and a meal below at the exact same moment. It only has two eyes, technically, but each one is split into two working halves, an elegant solution to a problem most fish never have to solve at all.
Overview
The four-eyed fish, genus Anableps, is a surface-dwelling livebearer native to coastal and brackish waters of Central and South America. Each of its two eyes is divided into an upper, air-adapted half and a lower, water-adapted half, giving it simultaneous vision above and below the surface.
Two lenses per eye
Because light bends differently in air than in water, each half of a four-eyed fish’s divided eye carries its own differently shaped lens, letting both halves stay in sharp focus at once despite operating in two completely different optical environments.
Status
Least Concern — common and widespread across suitable coastal and brackish habitat throughout its Central and South American range.
Four-Eyed Fish: common questions
Does a four-eyed fish actually have four eyes?
No, a four-eyed fish has only two actual eyes, but each eye is divided by a horizontal band of tissue into an upper half adapted for seeing in air and a lower half adapted for seeing underwater, effectively giving the fish four separate functional fields of vision even though it only has two physical eyes.
How does a four-eyed fish see clearly both above and below water at once?
Because light behaves differently when passing through air compared with water, the upper and lower halves of each of a four-eyed fish's divided eyes have differently curved lenses specifically suited to each medium, allowing the fish to maintain clear focus simultaneously in both the aerial and underwater portions of its field of view.
Why does a four-eyed fish swim right at the surface?
A four-eyed fish swims with the divided upper half of each eye positioned above the waterline and the lower half submerged specifically so it can watch simultaneously for aerial threats such as birds and for food or danger below the surface, a surface-skimming position that makes full use of its unusual split vision.
Does the four-eyed fish lay eggs like most fish?
No, the four-eyed fish is a livebearer, meaning it gives birth to fully developed live young rather than laying eggs, a reproductive strategy shared with several other surface-dwelling fish species and one that allows its offspring to begin surface-skimming life immediately after being born.