Brittle Stars Are All Eyes

Eye opener. Brittle stars' skeletons turned out to be visual systems, too.

One of Mother Nature's strangest marvels of engineering has been discovered in a most unlikely place. In the 23 August issue of Nature, a team of scientists reports that a brittle star species has turned its skeleton into a vast array of microscopic lenses that cover half its body. The lenses are the core of a remarkable visual system that allows the animals to find dark hiding places on the ocean bottom.

Naturalists had always assumed that the visual system of brittle stars--cousins of the starfish--was rudimentary. But in the 1980s, Gordon Hendler, now at the Natural History Museum of Los Angeles County, discovered that the brittle star Ophiocoma wendtii can spot dark areas from several centimeters away, enabling it to seek refuge from predators. That led Hendler to suspect there might be more to the brittle star than meets the eye.

But if it had "eyes," where were they? Electron micrographs of the brittle calcite that covers the animal's upper body suggested that it might be shaped into tiny lenses. To explore this possibility, Hendler teamed up with materials scientist Joanna Aizenberg and her colleagues at Lucent Technologies' Bell Laboratories in Murray Hill, New Jersey. The team carefully removed a thin slice of skeleton from a brittle star and mounted it on a silicon wafer with a light-sensitive coating. They discovered that the calcite crystals of the skeleton act like remarkably regular lenses, 40 to 50 micrometers across. Each focuses light on a point about 10 micrometers below the skeleton. That's exactly where the brittle star has nerves that fire when stimulated by light, Hendler says. Not yet clear is exactly how much the brittle star can see.

It's a "clever" study that once again reveals that nature is very strange, says optical physicist Roy Sambles from the University of Exeter, United Kingdom. "Why on Earth would anybody suppose that a creature had turned its skeleton into an eye?"

Related sites

Brittle Star Research Page

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