Specialized skeletal structures enable brittlestars to 'see'

Brittlestars are marine echinoderms that characteristically have five long, thin arms radiating from a disc-shaped body. In August 23 Nature Joanna Aizenberg and colleagues at Bell Laboratories/Lucent Technologies, New Jersey, The Weizman Institute of Science, Israel and the Natural History Museum, Los Angeles describe the structure of unusual skeletal structures that could act as a compound eye.

Despite having no eyes it has long been observed that brittlestars of the genus Ophiocoma wendtii can escape from predators by sensing and responding to shadows. Calcite forms the skeleton of the animal and also forms light sensing microlens arrays — hundreds of lenses derived from a single calcite crystal. Aizenberg et al. found that the optic axis of the crystal was perpendicular to the plane of the microlens, enabling the transmission of light without splitting it into different directions (Nature 2001, 412:819-822).

In addition they removed the organic matter from a crystal ...