By James Morgan
Science reporter, BBC News
The spookfish has a pair of mirrors (black) to focus light from below into its eyes
A Pacific fish uses mirrors as well as lenses to help it see in the murky ocean depths, scientists have revealed.
The brownsnout spookfish has been known for 120 years, but no live specimen had ever been captured.
Last year, one was caught off Tonga, by scientists from Tuebingen University, Germany.
Tests confirmed the fish is the first vertebrate known to have developed mirrors to focus light into its eyes, the team reports in Current Biology.
"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 Bristol University, who conducted the tests.
Spookfish is a name often given to Barreleyes - a group of small, odd-looking deep-sea fish species, found in tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans.
A rare live brownsnout spookfish, Dolichopteryx longipes, was caught last year between New Zealand and Samoa, by Professor Hans-Joachim Wagner, of Tuebingen University.
While the animal appears to have four eyes, it technically has two, each of which is split into two connected parts.
The spookfish needs one half to point upwards, to capture faint glimmers of light from the sea surface 1,000m above.
The other half, which looks like a bump on the side of the fish's head, points downwards.
Seen from below, the mirrored eyes (red) focus light from the depths
These "diverticular" eyes are unique among all vertebrates in that they use a mirror to make the image.
Prof Partridge said: "Very little light penetrates beneath about 1,000m 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."
The mirror uses tiny plates, probably of guanine crystals, arranged into a multi-layer stack.
Prof Partridge made up 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.
It its murky world, the spookfish's visual system would give it a head start on its prey whilst at the same time providing those vital few moments of advance notice to escape the clutches of a predator.