January 8, 2009

Study: Spookfish uses mirrors for eyes

Barreleyes, also known as spookfish, are the first vertebrates found to use mirrors, rather than lenses, to focus light in its eyes, British researchers said.

In nearly 500 million years of vertebrate evolution, and many thousands of vertebrate species living and dead, this is the only one known to have solved the fundamental optical problem faced by all eyes -- how to make an image -- using a mirror, biologist Julian Partridge of the University of Bristol said.

While the spookfish looks like it has four eyes, in fact it only has two that are split into two connected parts, Partridge wrote in the journal Current Biology.

One half points upward, giving the spookfish a view of the ocean -- and potential food. The other half, which looks like a bump on the side of the fish's head, points downward into the abyss below.

These diverticular eyes are unique among all vertebrates because they use a mirror to make images, he said.

The spookfish -- found in tropical to temperate waters in the Atlantic, Pacific and Indian Oceans -- was first discovered 120 years ago, but no one had detected its reflective eyes until now because no one had caught a live one until recently, Partridge said.

Researchers used flash photography to confirm the fish's upward and downward gazes.

The use of a single mirror has a distinct advantage over a lens in its potential to produce bright, high-contrast images, Partridge said.

This appears to give the fish a great advantage in the deep sea, where the ability to spot even the dimmest and briefest of lights can mean the difference between eating and being eaten, he said.