Letters to Nature

Nature 412, 819-822 (23 August 2001) | doi:10.1038/35090573; Received 29 March 2001; Accepted 25 June 2001

Calcitic microlenses as part of the photoreceptor system in brittlestars

Joanna Aizenberg1, Alexei Tkachenko1, Steve Weiner2, Lia Addadi2 & Gordon Hendler3

  1. Bell Laboratories/Lucent Technologies, Murray Hill, New Jersey 07974, USA
  2. Department of Structural Biology, The Weizmann Institute of Science, Rehovot 76100, Israel
  3. Natural History Museum of Los Angeles County, Los Angeles, California 90007, USA

Correspondence to: Joanna Aizenberg1 Correspondence and requests for materials should be addressed to J.A. (e-mail: Email: jaizenberg@lucent.com).

Photosensitivity in most echinoderms has been attributed to 'diffuse' dermal receptors1, 2, 3. Here we report that certain single calcite crystals used by brittlestars for skeletal construction4, 5 are also a component of specialized photosensory organs, conceivably with the function of a compound eye. The analysis of arm ossicles in Ophiocoma 6 showed that in light-sensitive species, the periphery of the labyrinthic calcitic skeleton extends into a regular array of spherical microstructures that have a characteristic double-lens design. These structures are absent in light-indifferent species. Photolithographic experiments in which a photoresist film was illuminated through the lens array showed selective exposure of the photoresist under the lens centres. These results provide experimental evidence that the microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4–7 microm below the lenses) coincides with the location of nerve bundles—the presumed primary photoreceptors. The lens array is designed to minimize spherical aberration and birefringence and to detect light from a particular direction. The optical performance is further optimized by phototropic chromatophores that regulate the dose of illumination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical functions.