Light is a primary driving force in nature, and most organisms have some type of light-detection system, often one that uses lenses. But it comes as a surprise to learn from Joanna Aizenberg and colleagues, writing elsewhere in this issue (Nature 412, 819– 2001), that a species of brittlestar, Ophiocoma wendtii, possesses a remarkable microlens array. Brittlestars belong to the group of marine invertebrates known as echinoderms, and Aizenberg et al. find that the calcium carbonate (calcite) that makes up the external skeleton of O. wendtii also forms light-sensing arrays.

Credit: J. AIZENBERG

The arrays are found in the skeletal plates that protect the upper-arm joints on each of the brittlestar's five arms. In some light-insensitive species, these plates constitute a relatively open, three-dimensional mesh of single-crystal calcite, with mesh pore sizes of around one-hundredth of a millimetre. But in O. wendtii the outer surface of this mesh has a characteristic array of larger, spherical protuberances, each about one-twentieth of a millimetre in diameter and linked to six neighbours (see the figure). When seen in cross-section, each protuberance has the appearance of a double lens: the radius of curvature of the upper face is about 20 to 30 μm, that of the lower face rather less. This combination gives a focal length of about 10 μm, with a focal-spot size of less than 3 μm. There are bundles of nerve fibres of about that size at each focal spot, and the authors suggest that these bundles are responsible for the documented sensitivity of O. wendtii to light stimuli.

The construction and operation of microlenses have strict requirements. First, there has to be exquisite control of calcite growth to form the lens structures. Second, calcite is optically anisotropic, with different refractive indices for light polarized in different directions. So, to avoid birefringence effects, it has to grow as single crystals with the optical axis parallel to the axis of the double lens. Third, each microlens should ideally have minimal optical aberration, and that seems to be the case. The authors have checked this last point both in experiments using an extracted array of lenses as the focusing elements and by modelling the optical response of such structures.

Human ingenuity came up with microlens arrays only a few years ago, and they are used in directional displays and in micro-optics, for example as signal-routing connectors for signal processing. Once again we find that nature foreshadowed our technical developments. The same applies to photonic solids, structures that can selectively reflect light in all directions. Photonic materials have stimulated much research over the past ten years because of their potential in light manipulation, yet they are to be found in opals and in the wings of butterflies. But then, nature has been in the business of developing functioning optical structures for a very long time.