Brittlestar Optics: Calcitic Microlenses as Part of the Photoreceptor System

Photosensitivity in echinoderms has generally been attributed to "diffuse" dermal receptors. The absence of specialized "eyes" cannot, however, be reconciled with the strong responses to light shown by some species, such as color-change an escape reactions. here we report that certain single calcite crystals used by brittlestars for skeletal construction are also a component of discrete photosensory organs, conceivable with a compound-eye function. The analysis of arm ossicles in Ophiocoma showed that in light-sensitive species, the periphery of the labrynthic calcitic mesh extends into a regular array of spherical microstructures that have a characteristic double-lens design. Photolithographic experiments, in which photoresist was illuminated through the lens array, showed the selective exposure of photoresist under the lens centers. These results provide the first experimental evidence that microlenses are optical elements that guide and focus the light inside the tissue. The estimated focal distance (4-7 microns below the lenses) coincides with the location of nerve bundles - the presumed primary photoreceptors. Each lens is designed to minimize spherical abberation and detect the light from a particular direction. The optical performance is further optimized by a diaphragm action of phototropic chromatophores that regulate the illumination dose reaching the receptors. These structures represent a remarkable example of a multifunctional biomaterial, which fulfills both mechanical and optical functions.