The brownsnout spookfish, which lives at a depth of more than 3,000ft, has been identified as the only vertebrate ever found to use mirrors, rather than lenses, to focus light in its eyes. 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.
"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.
The brownsnout spookfish has been known for 120 years, but no live specimen had ever been captured until last year, when one was caught off Tonga, by Professor Hans-Joachim Wagner, of Tuebingen University.
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.
While the spook fish looks like it has four eyes, in fact it only has two, each of which is split into two connected parts. One half points upwards, giving the spookfish 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 into the abyss below. These 'diverticular' eyes are unique among all vertebrates in that they use a mirror to make the image.
Prof Partridge told BBC: "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.