CRAYFISH VISION IS ENABLED BY A `NEW` TYPE OF ORGANIC BIO-CRYSTAL

The reflection superposition compound eye, discovered in the 1970’s¹ is a remarkably complex natural optical device found in lobsters, crayfish and shrimp. The eye is unusual in that it operates by reflection rather than refraction. Each compound eye is composed of a series of square-faceted eye units (ommatidia) arranged radially about the eye axis. An image is formed when light is reflected off mirrors lining the square-walls of the ommatidia (the ‘distal reflector’) onto the retina where the image is processed. A second reflecting layer lies below the retina – the tapetum reflector, which reflects photons back through the retina layer which have been missed on the first pass, thus enhancing light sensitivity.

Although a significant amount of work has been performed on the anatomy and physiology of this eye,² almost nothing is known about the nature and properties of its reflective materials. In situ WAXS diffraction patterns utilizing a narrowly focused synchrotron X-ray beam, reveal that the multilayer reflectors in crayfish eyes are composed of a new type of organic bio-crystal – the pteridine, isoxanthopterin. By considering the crystal structure of the material together with ultrastructural information (derived from cryoSEM and microCT) and optical simulations, we rationalize the function of the reflectors in the eye.

Understanding the nature of these reflective materials could yield new applications in materials optics³ and deepen our understanding of how organisms control the crystallization of organic materials in order to optimize a particular function ("organic biomineralization").

[1] Land, M. F. Nature 1976, 263, 764.

[2] Vogt, K. Z. J. Comp. Physiol., 1980, 135, 1.

[3] Lee, L. P., Szema, R. Science 2005, 18, 1148.