Optically functional isoxanthopterin crystals in the mirrored eyes of decapod crustaceans

Benjamin A. Palmer, Anna Hirsch, Vlad Brumfeld, Eliahu D. Aflalo, Iddo Pinkas, Amir Sagi, Shaked Rosenne, Dan Oron, Leslie Leiserowitz, Leeor Kronik, Steve Weiner, Lia Addadi

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9 Citations (Scopus)

Abstract

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro–X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.

Original languageEnglish
Pages (from-to)2299-2304
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Volume115
Issue number10
DOIs
Publication statusPublished - Mar 6 2018

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Keywords

  • Crystal
  • Eyes
  • Isoxanthopterin
  • Mirror
  • Reflection

ASJC Scopus subject areas

  • General

Cite this

Palmer, B. A., Hirsch, A., Brumfeld, V., Aflalo, E. D., Pinkas, I., Sagi, A., Rosenne, S., Oron, D., Leiserowitz, L., Kronik, L., Weiner, S., & Addadi, L. (2018). Optically functional isoxanthopterin crystals in the mirrored eyes of decapod crustaceans. Proceedings of the National Academy of Sciences of the United States of America, 115(10), 2299-2304. https://doi.org/10.1073/pnas.1722531115