Researchers from Tuebingen University in Germany have made a startling discovery by finding the first known vertebrate to have evolved mirrors, which focus light into the eyes. This odd vertebrate is a species of fish (Dolichopteryx longipes), commonly known as the ‘spookfish‘ or ‘barreleye‘. Although the spookfish has been recognised for over 120 years, it was only recently (last year) that a live specimen was captured of the coast of Tonga.

The reason for their ellusive nature is that spookfish are resident in mesopelagic to bathypelagic zones (~400 to 2,500m depth), inhabiting the border between the photic and aphotic zones. In order to enhance visual acuity at such depths where sunlight barely penetrates, the spookfish has evolved an elaborate system of two sets of connected double eyes. One half of each eye points upwards, capturing the faint light from the surface, whilst the other half point downwards.

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The main eyes of spookfish (with the orange-yellow eyeshine from flash photography) are offset with reflective mirrors (indicated here in black) to focus light from above.

These “diverticular” eyes use an elborate mirror constructed of tiny crystals that focus reflected light onto the retina of the eye. Professor Julian Partridge, a co-author on the paper reasons that this system is the reason that the spookfish thrives in such dimly lit regions of the ocean:

“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.

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.”

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When viewed from the top of the head, the mirror inside of the diverticulum (marked with an arrow) is clearly visible through the transparent cornea of the eye.

Nature has evolved some absolutely incredible adaptions to environmental conditions – the really neat thing about this study is that it proves that image formation in vertebrate eyes isn’t limited to refraction, a trait that has evolved in nearly every other vertebrate (including mammals).

“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” (Link)

Click here to read the full paper in Current Biology.

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