An Image Forming Mirror in the Eye of the Pecten Scallop

The vast majority of both animal eyes and synthetic image-forming devices use lenses to focus light by refraction. However, in exceptional cases in nature and in some man-made applications, images are produced by light reflection.

A case in point is found in the Pecten scallop which has one of the most remarkable visual systems in nature possessing up to two hundred iridescent eyes along the mantle tissue edges (Fig.1 left). An image is produced by reflection of light from a concave mirror lining the back of the eye onto the retina residing in the centre of the eye (Fig.1 middle). CryoSEM microscopy shows that the hemi-spherical mirror is composed of 20 – 30 layers of perfectly tiled square crystals (Fig.1 right). In-situ microspot X-ray diffraction together with electron diffraction demonstrate that these crystals are crystalline guanine and that the organism uses control over crystal twinning to produce the unusual square morphology in which the high-refractive index (100) face is preferentially expressed. This crystal morphology facilitates the tiling of the large area mirror in addition to ensuring that incident light always impinges on a maximally reflective surface. Optical simulations of the mirror’s reflectance spectrum demonstrate that it reflects most efficiently blue light and is thus well optimized for use in the scallops habitat (100 m down) where only blue light penetrates.

This work reveals novel strategies which organisms use to control crystal morphology and organization to optimize a particular optical function. Such knowledge could be applied in several aspects of material science in particular in the development of novel synthetic optical devices.