Australasian Science: Australia's authority on science since 1938

Mirror-Eyes Reflect Evolution of Vision

A small tropical fish uses mirror-eyes on the side of its head to help see its prey in the darkness of the deep sea habitat, according to research published in Proceedings of the Royal Society B.

It is only the second vertebrate known to use a mirror system to see a focused image by reflection rather than using a lens. The fish use their mirror-eyes to see underneath and to either side, and have conventional eyes on the top of their head to look up towards what little sunlight filters down.

Adjunct Professor Julian Partridge of The University of Western Australia first described the brownsnout spookfish (Dolichopteryx longpipes) 4 years ago, while the glasshead barreleye fish (Rhynchohyalus natalensis) was collected last year. Their habitats can be half a kilometre under the sea surface, where many animals are bioluminescent and much of vision is to do with seeing dim flashes of blue light in an otherwise dark world.

The mirror eyes of the two fish are extremely different even though the fish belong to the same family.

The barreleye fish’s mirrors are made of two transparent layers of guanine and cytoplasm stacked in such a way that they become reflective. The layers create a smooth silvery surface from which light bounces onto the retina. Focus depends on the shape of this surface.

The brownsnout spookfish’s mirrors are also made of guanine and cytoplasm but, instead of being smooth, the surface is composed of angled reflective crystals that shine light onto the retina. The arrangement of the plates allows the light to be focused.

Remarkably, the mirrors have evolved from different layers of retinal tissue in the two species, providing a similar solution by different mechanisms.