Jan. 13, 2009 -- In the barely lit depths of the ocean, the four-eyed spookfish has caught scientists by surprise. This fish -- caught alive for the first time -- is the only vertebrate known to use a mirror to focus an image in its eye instead of a lens.
Moreover, the fish's two eyes (not four, despite its name) are each split into two parts, one looking up and one looking down, and each part is capable of focusing an image.
Only one other fish has been found with such an ability, but it uses two lenses, while the spookfish uses a lens on top and a mirror below, said Julian Partridge of the University of Bristol. Partridge was an author of a study on the fish's vision in the journal Current Biology.
The research team caught the four-inch fish while trawling for specimens between 2000 and 2,600 feet below the surface of the South Pacific, near the north island of New Zealand. The fish had been described as early as 1888, but none of the five researchers, with decades of deep sea diving experience among them, had ever seen one, and no live specimens had previously been recorded.
The team realized they had something special when they photographed the fish from above and below and saw light reflected from eyes in both directions. When they dissected the eye, they realized the lower part contained a mirror, but no lens.
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Little light penetrates to the depths where the spookfish dwells, and many other fish living there have evolved eyes on the top of their heads to capture what little light filters down from the surface. But many organisms in the deep ocean are bioluminescent, so a top-only view misses a lot of potentially useful information about predators or prey that may be flashing below or to the side.
A few other fish have developed downcast extensions of their retinas that allow them to detect the presence or absence of light sideways or beneath, but the spookfish is unique in being able to create an image as the incoming light hits the mirror, which focuses an image on its retinal extension.
Focusing light with a mirror gives an image that's brighter than one using a lens, Douglas said, which may be one reason why the mirror system evolved in the fish, rather than using a second lens.
"Having an eye that looks in two directions is a good idea. Using a mirror is a good idea. I'm not sure why more animals don't do it," said study co-author Ron Douglas of City University London. "I'm surprised that the deep sea isn't stuffed full of them."
"It's saying something about the importance of chance in evolution, I think," said Partridge.
Partridge also speculates that it may take less tissue to make a mirror-based eye than a lens-based eye. In the deep ocean, where food is scarce, that difference may provide a significant advantage.
Some invertebrates, like scallops and shrimp, use mirrored eyes to make images, but the four-eyed spookfish is the first vertebrate found to do so.
"It's a really nice finding. It's always nice to find examples of animals that have sort of mastered physics through evolution," said Justin Marshall of the University of Queensland, Australia, who was not involved in the study.
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