A small Caribbean fish named Anableps dowei likes to feed on tidbits floating on the water’s surface. He must be able to watch both above the surface for food and below the surface for enemies. This would be impossible for eyes with a single focus. But Anableps has “bifocals.” By means of two pupils, he can see above water through the short dimension of the lens and under water through the long dimension of the lens. By this means he takes care of the fact that light travels at different speeds through air and water. To keep the upper pupils moist, he ducks his head under water every few minutes.

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The “Anableps dowei” is equipped with “bifocal” lenses​—he can see food on the water’s surface while watching for enemies below