. . .the unusual eyes of the spookfish, Dolichopteryx longipes:
These eyes 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.
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. 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. The stacks sit roughly parallel to one another, but their angle changes over the surface of the mirror, giving it an overall concave shape. more