Calcitic microlenses as part of the photoreceptor system in brittlestars

Photosensitivity in most echinoderms has been attributed to 'diffuse' derma l receptors(1-3). Here we report that certain single calcite crystals used by brittlestars for skeletal construction(4,5) are also a component of spec ialized photosensory organs, conceivably with the function of a compound ey e. The analysis of arm ossicles in Ophiocoma(6) showed that in light-sensit ive species, the periphery of the labyrinthic calcitic skeleton extends int o a regular array of spherical microstructures that have a characteristic d ouble-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 micr olenses are optical elements that guide and focus the light inside the tiss ue. The estimated focal distance (4-7 mum 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 t o detect light from a particular direction. The optical performance is furt her optimized by phototropic chromatophores that regulate the dose of illum ination reaching the receptors. These structures represent an example of a multifunctional biomaterial that fulfills both mechanical and optical funct ions.