Published online 23 August 2001 | Nature | doi:10.1038/news010823-11


Eyes in their stars

Engineers envy brittlestar bones' built-in lenses.

The brainless brittlestar has crystal clear vision.The brainless brittlestar has crystal clear vision.© J. Aizenberg

A relative of the starfish and sea urchin has turned its skeleton into an all-seeing eye. Near-perfect microscopic lenses in brittlestars' bones are more sophisticated than anything humans can produce, say engineers keen to copy the trick.

Plastic microlenses, inferior to those on the brittlestars, control signals in optical fibres and enhance some displays. They may one day be used in optical computers that process light, rather than electricity.

On the top of the brittlestar's arms, are calcite domes about one-twentieth of a millimetre across. These focus light, avoiding the blurring that perfectly spherical lenses produce. The intricate calcite crystals are aligned so as not to split light into multiple images.

The tiny crystal balls "were too similar to lenses to have been formed by chance", says Joanna Aizenberg, of Bell Laboratories and Lucent Technologies, Murray Hill, New Jersey. She happened upon the "incredible structures" while studying the brittlestar Ophiocoma wendtii1.

"It's astonishing that this organic creature can manipulate inorganic matter with such precision - and yet it's got no brain," says Roy Sambles, who works on optics and photonics at the University of Exeter in Britain.

The crystals' growth must be self-organized - emerging from the right chemical environment rather than being engineered by detailed top-down control. "It's starting with a soup of chemicals and pulling out this wonderful microstructure," says Sambles, who fantasizes about emulating the process "in a bucket in a corner of the lab".

Eye up

Although brainless, the brittlestar has a nervous system. The lenses focus light onto nerve bundles that run behind them, which presumably pick up the signal, allowing the animal to

Crystal balls let brittlestars see what's coming.Crystal balls let brittlestars see what's coming.

Together the lenses form a kind of compound eye that covers the animal's upper surface, allowing it to see all around. Aizenberg compares the structure to a digital camera that builds up a picture pixel by pixel.

"It's bizarre - there's nothing else that I know of that has lenses built into its general body surface," says Michael Land, who studies animal vision at the University of Sussex, Brighton, UK.

It's not clear how good an image the eye can form, but it can almost certainly sense direction and illumination - useful for detecting predatory fish and scuttling into the nearest rock crevice. Behavioural experiments are needed to work out what the system is actually used for, says Land. 

  • References

    1. Aizenberg, J., Tkachenko, A., Weiner, S., Addadi, L. & Hendler, G. Calcitic microlenses as part of the photoreceptor system in brittlestars. Nature 412, 819 - 822 (2001). | Article | PubMed | ISI | ChemPort |