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THE spindly arms of brittlestars may double as a primitive eye thanks to
microscopic crystals in its skeleton that focus light. These tiny lenses rival
the best that human technology can offer, and their discovery solves a
long-standing mystery about the behaviour of these marine animals.

鈥淭his is a big breakthrough for brittlestars because no one knew how they
detect light,鈥� observes Stephen Stancyk, a marine biologist at the University of
South Carolina in Columbia.

Unlike their starfish cousins, brittlestars can move quickly by walking on
their arms, and they tend to avoid brightly lit places. 鈥淭hey鈥檙e very delicate
animals and they don鈥檛 come out of their hiding places except at night because
they get chewed on by lots of different animals,鈥� Stancyk says.

Gordon Hendler, a brittlestar expert at the Natural History Museum of Los
Angeles County, teamed up with physicists at Bell Laboratories in New Jersey to
investigate the brittlestar鈥檚 visual system. With the help of a scanning
electron microscope, the team found arrays of calcite crystals, each about 10 to
15 micrometres in diameter, incorporated into the animal鈥檚 skeleton.

The researchers embedded a layer of the crystals in a silicone film and
tested their optical properties. They found that each crystal focuses light on a
tiny spot about 5 micrometres below the bottom of the crystal. This corresponds
exactly to the position of nerve bundles in the brittlestar鈥檚 body. Hendler says
the crystal lenses focus light so well that the light intensity at the nerve
bundles is about 50 times greater than that at the surface of the lens. Hendler
says he鈥檚 certain the nerve bundles have photoreceptors because a previous
experiment of his recorded electrical activity in the nerve fibres in response
to light.

The team also found that each lens only collects light oriented within about
10 degrees of its long axis. Hendler says this selectivity should enable a
brittlestar to determine which way a light or shadow is moving across its
body鈥攁 useful trick to avoid predators. Still, the visual experience of
brittlestars is probably far poorer than our own. 鈥淐hances are it doesn鈥檛 allow
them to form an image,鈥� Hendler says.

But for their size, the brittlestar lenses are at least as good as anything
humans can produce, says Roy Sambles, a physics professor at Exeter University.
Sambles says it鈥檚 very difficult to manufacture microlenses that focus light so
effectively. High quality microlenses could find a role as miniature optical
sensors, or focus light tightly enough to etch computer chips and other
intricately patterned materials.

鈥淚t鈥檚 astonishing that it鈥檚 been able to build this essentially out of
coral,鈥� Sambles says. 鈥淗ere you鈥檝e got a primitive organism in the ocean and
it鈥檚 been able to get these materials and fashion them into these fabulous
microlens arrays.鈥�