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Fish Uses Mirrors to See Underwater

January 8, 2009

The ocean is deep, dark, and murky but with the help of mirrors a fish in the Pacific gets along just fine.

Scientists from Tuebingen University, Germany, discovered a rare, living brownsnout spookfish, Dolichopteryx longipes, last year off the coast of Tonga.
 
They found that it was 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.”

The findings were published in Current Biology.

Spookfish is a name often given to Barreleyes, a group of small, odd-looking deep-sea fish species, found in tropical-to-temperate waters of the Atlantic, Pacific, and Indian Oceans.

While the spook fish appears to have four eyes, in actually only has two, each one 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 mirrors to make the image.

Partridge said, “Very little light penetrates beneath about 1,000m 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.”

Researchers say the mirror uses tiny plates, probably of guanine crystals, arranged into a multi-layer stack.

Patridge said, “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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Image:blogs.nature.com

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