Rare fish uses "mirrors" to see.

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The first vertebrate with eyes that use mirrors rather than lenses to focus light has been discovered by an international research team that included Tamara Frank, a researcher from Florida Atlantic University, Boca Raton, and member of the Center for Ocean Exploration and Deep-Sea Research at Harbor Branch (Fla.) Oceanographic Institute.

The unusual eyes of the spookfish (Dolichopteryx longpipes) are tubular, which are similar in structure to the eyes of many other fish that swim in the ocean's twilight zone where the background light field is very dim. "What makes this animal so unusual is that each eye is divided into two parts, one pointing upwards and one pointing downward, making it look like it has four eyes, and four-eyed fish don't exist," says Frank.

In the few other species of deep-sea fish that possess split eyes, the upper eye has a lens, like in the spookfish, for focusing light. However, at these depths, there is so little upwelling light that a lens would attenuate light too much in its efforts to focus the light for the lower eye. Therefore, in other fish, the lower eye does not have a lens, resulting in a blurred image. The spookfish however, manages to focus light in the lower eye without using a lens.

Light enters the lower portion of the eye and hits a mirror composed of stacks of crystals, which sit roughly parallel to one another, but their angle...