The mystery of one of nature's most incredible evolutionary feats, a fish which has grown mirrors to spot its prey, has been uncovered by Bristol University researchers.

The reclusive spookfish, which prowls the seabed at depths of 1km, produces an optical illusion which makes it appear to have four eyes.

But only the colourful upper pair are real eyes, which use a conventional lens to produce an image.

The black lower pair are actually curved mirrors, which allow the unique fish to spot prey and predators on the seabed below.

Professor Julian Partridge, who heads the Ecology of Vision Research Group at Bristol University, hailed the amazing find.

He said: ''In 500 million years of vertebrates evolving, this is the first time an animal has been found which uses mirrors to focus lights into its eyes.

''It is amazing to think that in all this time there are still animals out there which are breaking the mould and do not rely on the traditional method of using a lens and retina to focus images.

''In this sense it is completely unique and a very exciting find.

''We don't know how they have evolved like this but it may be to do with searching for prey or predators."

The 15cm long fish was captured by scientists trawling the Pacific Ocean above the 10km deep Tonga trench half way between Tonga and New Zealand.

It was taken aboard and put under the microscope, where new computer technology recreated its vision and made the breakthrough discovery about its eyesight. A dozen of the fish have been caught before but the eyesight discovery was only possible because of the new computer simulation programs.

Spookfish, which were discovered 120 years ago, live in virtual darkness and have developed to make the most of what little light is available at 1,000m.

The diverticular eyes can spot flashes, which warn the fish of other animals that are active, and otherwise unseen, below its vulnerable belly.

The mirrored eyes work by using tiny plates of guanine crystals arranged in a multi-layer stack which is specifically arranged to focus light and reflect it to the retina.

Use of a mirror has a distinct advantage over a lens as it produces brighter and more high-contrast images.