08 January 2009

Four-eyed spookfish uses mirrors for eyes

This deep-sea fish is the first vertebrate found to use mirrors, rather than lenses, to focus light in its eyes. Previously, only invertebrates, such as scallops, have been found to use mirrors in eyes.
The mirrors are thought to be more efficient in the dark because they reflect more of the available light into the retina, whereas lenses absorb small quantities as the light passes through them. Although the spookfish was first discovered 120 years ago, no one had discovered its reflective eyes until now because a live animal had never been caught.

Spookfish Uses Mirrors For Eyes
ScienceDaily 8 Jan 09;
A remarkable new discovery shows the four-eyed spookfish to be the first vertebrate ever found to use mirrors, rather than lenses, to focus light in its eyes.

Professor Julian Partridge from the University of Bristol, 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.”

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.

Section through the diverticulum eye showing the way incoming light from a distant light source is imaged. (Credit: Image courtesy of University of Bristol)
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.

Although the spookfish was first discovered 120 years ago, no one had discovered its reflective eyes until now because a live animal had never been caught. When Professor Hans-Joachim Wagner from Tuebingen University caught a live specimen off the Pacific island of Tonga, members of his research team used flash photography to confirm the fish’s upward and downward gazes.

Photographs taken by Dr Tammy Frank looking down on the live fish produced eye-shine in the main tubular eyes that point upwards, but not in the diverticular eyes that point downward. Instead, these reflect light when seen from below.

It was when looking at sections of the eye that had been prepared for microscopy that Professor Partridge realised that the diverticular mirrors where something exciting. The mirror uses tiny plates, probably of guanine crystals, arranged into a multi-layer stack. This is not unique in the animal kingdom (it’s why silvery fish are silvery) but the arrangement and orientation of the guanine crystals is precisely controlled such that they direct the light to a focus. Partridge’s computer simulation showed that the precise orientation of the plates within the mirror's curved surface is perfect for focusing reflected light onto the fish’s retina.

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.

The research will be published this month in Current Biology.
Adapted from materials provided by University of Bristol.


Fish with four eyes can see through the deep sea gloom
A remarkable new discovery shows the four-eyed spookfish to be the first vertebrate ever found to use mirrors, rather than lenses, to focus light in its eyes.

Lewis Smith, The Times Online 7 Jan 09;

A bizarre “four-eyed” fish has been found to use a unique system of mirrors to protect itself from being eaten in the dark depths of the sea.

The brownsnout spookfish has been identified as the only backboned creature known to use mirrors rather than lenses to get images into focus.

The mirrors allow the fish to detect flashes of light made by creatures in the deep in more detail than would be achieved by eyes with lenses, giving it an early warning of predators.

Mirrors are better at providing focused images in the deep sea because they are more efficient in the low light levels and they avoid imperfections in images created by lenses.

The brownsnout spookfish, Dolichopteryx longipes, has ordinary eyes with lenses pointing upwards, but alongside them are downward-looking eyes fitted with tiny mirrored plates. The plates, thought to be made of guanine crystals, are arranged so that the light entering the eye is reflected to a focused point on the retina, allowing the fish to see what lurks below it.

Arrangements of mirrors have been found in a few crustaceans but the spookfish is the first vertebrate to have evolved them to help it to see.

“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 the University of Bristol. “It’s an extraordinary animal. It is absolutely unique for a vertebrate. With mirrors it can make a very bright, high-contrast image.”

The mirrors are thought to be more efficient in the dark because they reflect more of the available light into the retina, whereas lenses absorb small quantities as the light passes through them.

Lenses, especially the spherical type found in fish, also suffer from small aberrations that affect the quality of the image. Professor Partridge said that mirrored eyes might be better for vision in other habitats other than just the deep sea but that lenses evolved first in backboned animals. Eyes with lenses were successful enough that there was little point in animals evolving mirrored eyes.

The downward-looking — “diver-ticular” — eyes are linked to the lensed eyes. Professor Partridge said that the mirrors were an “add-on” to the fish’s one pair of genuine eyes.

The mirror eyes are used to see bioluminescent light created by marine animals signalling to each other or trying to lure prey.

Brownsnout spookfish were discovered 120 years ago but little was known about them until one was pulled up from 2,000-2,600ft (600-800m) during a scientific trawl in the Tonga Trench in the southern Pacific 18 months ago. It was the first live specimen to be studied by researchers.

The fish was caught by Professor Hans-Joachim Wagner, of Tübingen University in Germany, as part of an international research expedition and the discovery of the mirror eye is reported in the journal Current Biology.

The specimen was about 4in (10cm) long and had small teeth. Food in its habitat is so scarce that its diet is thought to comprise virtually anything organic that it can catch.

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