Eyes in the back of its head: The deep-sea spookfish is unusual in that it uses downward-pointing mirrors to focus light into its eyes (black, seen from above here). The orange parts of its eyes collect traces of light coming from above.
Credit: Dr Tammy Frank, Habor Branch Oceanographic Institution
LONDON: A fish that uses mirrors, as well as lenses, to find its way in the dark depths of the sea has been discovered by scientists.
The brownsnout spookfish, which lives between 500 and 2,500 metres below sea-level, may have evolved its reflective eye to more easily spot predators in the light-starved deep ocean.
What's more, the spookfish (Dolichopteryx longipes) is the first vertebrate ever found to use mirrors to see, said Julian Partridge, author of a new paper on the find and an ecologist who studies vision at the University of Bristol in England.
Flashes in the darkness
Although the spookfish was first discovered 120 years ago, this is the first time a live one has been caught, allowing scientists to get a closer look at its unique optics.
The fish was caught in a trawl off the Pacific island of Tonga, by Partridge's co-worker, Hans-Joachim Wagner of the University of Tuebingen in Germany.
At first glance, it appears the spookfish has four eyes (see image, right), but in fact, it has two eyes both separated into two compartments.
One half of the eye points upwards to capture the tiny amount of sunlight that makes its way this deep - this half contains a lens, and is similar to a human eye. It looks for shadows passing above the fish, which could be food or a predator, Partridge told Cosmos Online.
The other half of the eye, which looks like a black bump on the side of the head, points downwards into the ocean depths. The researchers believe that this part of the eye is looking for flashes of bioluminescent light that may give away predators lurking beneath. This 'diverticular' eye uses a complex mirror to detect and focus the light.
High contrast image
"A mirror can give a very high contrast or bright image, which is exactly what you need in the deep sea," explained Partridge. "[It] can also gather a lot of light," he said.
The mirror is made from tiny plates, most likely guanine crystals, stacked up in multiple layers. While this arrangement is not unique in the animal kingdom – it gives fish their silvery sheen – the plates are arranged in such a way that they direct the light to a sharp focus on the retina. "There's a kind of very unique micro-assembly going on," said Partridge.
The research is detailed this week in the journal Current Biology.
Dan-Eric Nilsson, a zoologist who was not involved with the research, said the physiology of the eye was an interesting find. However, he was not surprised by its odd construction.
"Deep sea fish often have unusual eyes because their visual world is so different to ours," said Nilsson, who is based at the University of Lund in Sweden.
