Open AccessGradients of Orientation, Composition, and Hydration of Proteins for Efficient Light Collection by the Cornea of the Horseshoe Crab
Author(s) -
Spaeker Oliver,
Taylor Gavin J.,
Wilts Bodo D.,
Slabý Tomáš,
AbdelRahman Mohamed Ashraf Khalil,
Scoppola Ernesto,
Schmitt Clemens N. Z.,
Sztucki Michael,
Liu Jiliang,
Bertinetti Luca,
Wagermaier Wolfgang,
Scholtz Gerhard,
Fratzl Peter,
Politi Yael
Publication year - 2022
Publication title -
advanced science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.388
H-Index - 100
ISSN - 2198-3844
DOI - 10.1002/advs.202203371
Subject(s) - cornea , limulus , refractive index , optics , refraction , horseshoe crab , biophysics , materials science , biology , physics , evolutionary biology , paleontology
The lateral eyes of the horseshoe crab, Limulus polyphemus , are the largest compound eyes within recent Arthropoda. The cornea of these eyes contains hundreds of inward projecting elongated cuticular cones and concentrate light onto proximal photoreceptor cells. Although this visual system has been extensively studied before, the precise mechanism allowing vision has remained controversial. Correlating high‐resolution quantitative refractive index (RI) mapping and structural analysis, it is demonstrated how gradients of RI in the cornea stem from structural and compositional gradients in the cornea. In particular, these RI variations result from the chitin‐protein fibers architecture, heterogeneity in protein composition, and bromine doping, as well as spatial variation in water content resulting from matrix cross‐linking on the one hand and cuticle porosity on the other hand. Combining the realistic cornea structure and measured RI gradients with full‐wave optical modeling and ray tracing, it is revealed that the light collection mechanism switches from refraction‐based graded index (GRIN) optics at normal light incidence to combined GRIN and total internal reflection mechanism at high incident angles. The optical properties of the cornea are governed by different mechanisms at different hierarchical levels, demonstrating the remarkable versatility of arthropod cuticle.