letters to natureNATURE|VOL 412|23 AUGUST 2001|www.nature.com 819M. Maher, L. Milne, D. de Palacio, K. Redman and C. Roger for recording changeoverrates. We also thank D. Bates, P. Harvey, O. Merne and A. Walsh, the Neale family, D. Shaw,M. and L. Smyth, B. Zonfrillo, the Royal Society for the Protection of Birds and theScottish Seabird Centre for logistical support. S. Bearhop, D. Elston and A. Stien gavestatistical advice, while S. Albon, F. Daunt, M. Harris, R. Moss, J. Sherratt andK. Thompson commented on an earlier draft. This project was funded by a NaturalEnvironment Research Council CASE Research Studentship to S.L.Correspondence and requests for materials should be addressed to T.N.S.(e-mail: [email protected]) or S.W. (e-mail: [email protected])..................................................................Calcitic microlenses as part of thephotoreceptor system in brittlestarsJoanna Aizenberg*, Alexei Tkachenko*, Steve Weiner², Lia Addadi²& Gordon Hendler³* Bell Laboratories/Lucent Technologies, Murray Hill, New Jersey 07974, USA²Department of Structural Biology, The Weizmann Institute of Science,Rehovot 76100, Israel³Natural History Museum of Los Angeles County, Los Angeles, California 90007,USA..............................................................................................................................................Photosensitivity in most echinoderms has been attributed to`diffuse' dermal receptors1±3. Here we report that certain singlecalcite crystals used by brittlestars for skeletal construction4,5arealso a component of specialized photosensory organs, conceivablywith the function of a compound eye. The analysis of arm ossiclesin Ophiocoma6showed that in light-sensitive species, the periph-ery of the labyrinthic calcitic skeleton extends into a regular arrayof spherical microstructures that have a characteristic double-lensdesign. These structures are absent in light-indifferent species.Photolithographic experiments in which a photoresist ®lm wasilluminated through the lens array showed selective exposure ofthe photoresist under the lens centres. These results provideexperimental evidence that the microlenses are optical elementsthat guide and focus the light inside the tissue. The estimated focaldistance (4±7 mm below the lenses) coincides with the locationof nerve bundlesÐthe presumed primary photoreceptors. Thelens array is designed to minimize spherical aberration andbirefringence and to detect light from a particular direction.The optical performance is further optimized by phototropicchromatophores that regulate the dose of illumination reachingthe receptors. These structures represent an example of a multi-functional biomaterial that ful®lls both mechanical and opticalfunctions.Echinoderms in general, and especially the brittlestars (Ophiur-oidea), exhibit a wide range of responses to light intensity, from alargely light-indifferent behaviour to pronounced colour changeand rapid escape behaviour7. Figure 1 compares the appearance andthe skeletal structure of two species of Ophiocoma, which representthe two extreme photosensitivity types. Ophiocoma pumila (Fig. 1a)shows no colour change and little reaction to illumination.Ophiocoma wendtii is a highly photosensitive species, and it changescolour markedly7, from homogeneous dark brown during the day(Fig. 1b, left) to banded grey and black at night (Fig. 1b, right).Another conspicuous behavioural response to light is negativephototaxis: O. wendtii is able to detect shadows and quickly escapefrom predators into dark crevices7, which they are able to identifyfrom several centimetres away8. The latter reaction is particularlyunexpected in these animals as the behaviour is usually associatedwith the presence of discrete photosensory organs. No specializedeyes have, however, been documented in brittlestars and theirreactions to light have been linked to diffuse dermal receptors1±3.The sensitivity to light seems to correlate with the specializedskeletal structure of the dorsal arm plates (DAPs). These ossiclesprotect the upper part of each joint in brittlestar arms (Fig. 1c).Skeletal elements of echinoderms are each composed of a singlecrystal of oriented calcite shaped into a unique, three-dimensionalmesh (stereom)4,5,9,10. The diameter of the typical stereom in theDAPs of Ophiocoma is about 10±15 mm (Fig. 1d). In O. wendtii aswell as in other photosensitive species6, the outer surface of the DAPstereom bears a characteristic array of enlarged spherical structures40±50 mm in diameter (Fig. 1d, f). In cross-section they have aremarkably regular double-lens shape (Fig. 1g). The optical axis ofthe constituent calcite is oriented parallel to the lens axis andperpendicular to the plate surface9. The mean geometry of the lenseswas inferred from the measurements of lens diameter (L) andthickness (t) in 20 random lenses sectioned through the centre (Fig. 1g):t  0:89L  2:2 1with a correlation coef®cient (r2) of 0.91. Similar lenses were also100 µm1 cm1 cmbLSdca eftdLL0a0g10 µm10 µm10 µm10 µmFigure 1 Appearance and skeletal structure of ophiocomid brittlestars. a, Light-indifferentspecies Ophiocoma pumila shows no colour change from day (left) to night (right). b, Light-sensitive species O. wendtii changes colour markedly from day (left) to night (right).c, Scanning electron micrograph (SEM) of a dorsal arm plate (DAP) of O. wendtii cleansedof organic tissue. d, SEM of the cross-section of a fractured DAP from O. wendtii showingthe typical calcitic stereom (S) and the enlarged lens structures (L) that constitute theperipheral layer. e, SEM of the peripheral layer of a DAP of O. pumila showing that it lacksthe enlarged lens structures. f, SEM of the peripheral layer of a DAP from O. wendtii withthe enlarged lens structures. g, High-magni®cation SEM of the cross-section of anindividual lens in O. wendtii. Red lines represent the calculated pro®le of a lenscompensated for spherical aberration. The operational part of the calcitic lens (L0) closelymatches the pro®le of the compensated lens (bold red lines). The light paths are shown inblue.© 2001 Macmillan Magazines Ltdletters to nature820 NATURE|VOL 412|23 AUGUST 2001|www.nature.comfound on the dorsal region of lateral arm plates. Ventral arm platesand most of the surface of lateral arm plates do not develop enlargedspherical structures, and the orientation of the optical axis of theconstituent calcite is not