One of the strangest fish in the world is Anableps anableps, commonly called the ‘four-eyed fish’ because of the unique configuration of its eyes. These are large and bulging, like those of a frog, and are located on the top of its head so that it swims with its eyes half in and half out of the water.

At the waterline, the eyes are divided by a band of epithelium (tissue) into upper and lower parts, with separate corneas and retinas, which function for aerial and underwater vision respectively. The lens is more egg-shaped than convex and can focus two images simultaneously, one from above the water and the other from below (see diagram). This means that the fish’s two eyes actually function as four eyes; hence its name…

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