The fish is the only vertebrate to have developed mirrors to see Photo: PA
The brownsnout spookfish, which lives at a depth of more than 3,000ft, has been identified as the only vertebrate to have developed mirrors rather than lenses to focus images.
The mirrors give the fish the edge over its predators because they allow it to detect flashes of light made by creatures in the deep in more detail than eyes with lenses can.
While the spookfish appears to have four eyes, it technically has two, each of which is split into two connected parts.
Living at such depths, between Samoa and New Zealand, the spookfish needs one half to point upwards giving a view of the ocean and potential food above.
The other half, which looks like a bump on the side of the fish's head, points downwards.
These "diverticular" eyes are fitted with tiny mirrored plates, which are arranged so that the light entering the eye is reflected to a focused point on the retina. This allows the fish to see what lurks below it.
Professor Hans-Joachim Wagner, from Tuebingen University in Germany, made the discovery after examining the first living specimen ever landed, which he caught off the Pacific island of Tonga.
His research team used flash photography to confirm the fish's upward and downward gazes.
Professor Julian Partridge, of Bristol University, who later conducted tests, confirmed it was the only vertebrate to have developed mirrors to see.
Prof Partridge 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.
"Very little light penetrates beneath about 1000m of water and like many other deep-sea fish the spookfish is adapted to make the most of what little light there is.
"At these depths it is flashes of bioluminescent light from other animals that the spookfish are largely looking for. The diverticular eyes image these flashes, warning the spookfish of other animals that are active, and otherwise unseen, below its vulnerable belly."
Prof Partridge made a computer simulation showing that the precise orientation of the plates within the mirror's curved surface is perfect for focusing reflected light on to the fish's retina.
He added: "The use of a single mirror has a distinct advantage over a lens in its potential to produce bright, high-contrast images.
"That must 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."