The four-eyed spookfish may have seemed strange enough. Now researchers say it doesn't really have four eyes. Instead, it is the known first vertebrate to use mirrors, rather than lenses, to focus light in its eyes.

“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 Julian Partridge from the University of Bristol.

While the spookfish 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 down. These diverticular eyes, as they are called, are unique among all vertebrates in that they use a mirror to make the image, Partridge and colleagues found.

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