A bizarre “four-eyed” fish has been found to use a unique system of mirrors to protect itself from being eaten in the dark depths of the sea.

The brownsnout spookfish has been identified as the only backboned creature known to use mirrors rather than lenses to get images into focus.

The mirrors allow the fish to detect flashes of light made by creatures in the deep in more detail than would be achieved by eyes with lenses, giving it an early warning of predators.

Mirrors are better at providing focused images in the deep sea because they are more efficient in the low light levels and they avoid imperfections in images created by lenses.

The brownsnout spookfish, Dolichopteryx longipes, has ordinary eyes with lenses pointing upwards, but alongside them are downward-looking eyes fitted with tiny mirrored plates. The plates, thought to be made of guanine crystals, are arranged so that the light entering the eye is reflected to a focused point on the retina, allowing the fish to see what lurks below it.

Arrangements of mirrors have been found in a few crustaceans but the spookfish is the first vertebrate to have evolved them to help it to see.

“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,” said Professor Julian Partridge, of the University of Bristol. “It’s an extraordinary animal. It is absolutely unique for a vertebrate. With mirrors it can make a very bright, high-contrast image.”

The mirrors are thought to be more efficient in the dark because they reflect more of the available light into the retina, whereas lenses absorb small quantities as the light passes through them.

Lenses, especially the spherical type found in fish, also suffer from small aberrations that affect the quality of the image. Professor Partridge said that mirrored eyes might be better for vision in other habitats other than just the deep sea but that lenses evolved first in backboned animals. Eyes with lenses were successful enough that there was little point in animals evolving mirrored eyes.

The downward-looking — “diver-ticular” — eyes are linked to the lensed eyes. Professor Partridge said that the mirrors were an “add-on” to the fish’s one pair of genuine eyes.

The mirror eyes are used to see bioluminescent light created by marine animals signalling to each other or trying to lure prey.

Brownsnout spookfish were discovered 120 years ago but little was known about them until one was pulled up from 2,000-2,600ft (600-800m) during a scientific trawl in the Tonga Trench in the southern Pacific 18 months ago. It was the first live specimen to be studied by researchers.

The fish was caught by Professor Hans-Joachim Wagner, of Tübingen University in Germany, as part of an international research expedition and the discovery of the mirror eye is reported in the journal Current Biology.

The specimen was about 4in (10cm) long and had small teeth. Food in its habitat is so scarce that its diet is thought to comprise virtually anything organic that it can catch.

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