Researchers have discovered the first vertebrate to use mirrors rather than lenses to focus light in its eyes.

In this month’s Current Biology, Hans-Joachim Wagner, of Tübingen University in Germany, reports that the deep-sea spookfish (Dolichopteryx longipes) focuses light onto its retina using a multilayer stack of reflective plates.

“This is the first report of an ocular image being formed in a vertebrate eye by a mirror,” the research team write.

The paper was published early online in December when New Scientist noted that the fish appears to have four eyes – two looking up and two looking down. However, “It turns out that the spookfish actually has just two eyes, but each eye has two parts, one looking upwards and the other down. … The spookfish is the only deep-sea fish with eyes that have been shown to produce a focused image when looking both up and down.”

In a new press release, research member Julian Partridge, of the University of Bristol, says, “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.”

Of course, as the paper notes, invertebrates have been known to use mirrors for ages.

Image: Tammy Frank, Habor Branch Oceanographic Institution