The image-forming mirror in the eye of the scallop

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Science  01 Dec 2017:
Vol. 358, Issue 6367, pp. 1172-1175
DOI: 10.1126/science.aam9506
  • Fig. 1 The locations and anatomy of scallop eyes.

    (A) The scallop Pecten maximus with numerous eyes lining the mantle (the white arrow points to an individual eye). (B) A magnified image of five eyes. (C) Fluorescence microscopy image of an eye cross section, showing the cell nuclei stained with DAPI (4′,6-diamidino-2-phenylindole). The (i) cornea, (ii) lens, (iii) distal retina, (iv) proximal retina, and (v) concave mirror are indicated. (D) Low-resolution cryo-SEM micrograph of an eye cross section after high-pressure freezing and freeze-fracturing. The lens (blue), distal retina (yellow), proximal retina (orange), and concave mirror (green) are shown in pseudo-colors. The cilia and microvilli of the photoreceptors were used to identify the locations of the distal and proximal retinas. Red boxes in (C) and (D) identify the region of the central mirror explored in more detail in Fig. 2. Yellow arrows in (C) and (D), direction of on-axis incident light.

  • Fig. 2 The ultrastructure of the multilayer mirror.

    (A to C) Cryo-SEM micrographs of high-pressure–frozen, freeze-fractured cross sections through the eye of P. maximus. (A) The mirror viewed perpendicular to the eye axis. White arrow, direction of on-axis incident light. (B) The tiled mirror viewed from above. (C) Crystals in adjacent layers, stacked directly on top of one another, viewed in a fracture through the mirror. (D) TEM micrograph of a single, regular square crystal extracted from the eye. The crystals are 1.23 × 1.23 ± 0.08 μm (N = 20) with internal corner angles of 90.16 ± 2.78° (N = 28) (means ± SD).

  • Fig. 3 The reflectivity of the mirror.

    (A) Cryo-SEM micrograph of the multilayer mirror, showing the crystal stacking. (B) Simulated (solid black trace) and measured (dotted black trace) reflectivity spectra overlaid with the calculated irradiance spectrum (blue trace) at a depth of 20 m in the scallop’s habitat (20). The inset shows the mirror color (black dot) on a CIE (International Commission on Illumination) chromaticity space diagram. The reduced intensity in the measured reflectivity (which reaches a maximum of 75% at 500 nm) is caused by absorption and scattering processes within the optical path through the eye. The optical path involves passing light through the cornea, lens, and retina before it is reflected off the mirror and back through these elements to the detector.

  • Fig. 4 Whole-eye optics.

    (A) Volume rendering of an x-ray micro-CT scan of a whole scallop eye, showing the eye anatomy. (B) Segmentation of the micro-CT in (A). Black, cornea; navy, “iris;” blue, lens; gray, gross retinal volume; green, mirror. Rays traced through the eye from a point source aligned with the axis of the lens (red) are reflected (yellow) and focused on the retina. The border of the best-focused region encompassing all reflected rays denotes a 3D circle of least confusion (COLC; black line). The inset is a side view of the mirror showing the optical axes of the lens (blue), central mirror (green), and center of the visual field (cyan). The lens and mirror axes are offset by 7.3°. (C) Top and side views of COLCs from sources consecutively offset by 20° from the original point source in (B), labeled 0°. The colors of the COLCs indicate which retina the COLCs pass through: yellow, distal; orange, proximal. (D) Simulated retinal images of planar checkerboards displaying increasingly finer details, corresponding to angular periods of 40°, 20°, and 10° (from left to right) (26). The purple borders in the images denote the extent of different visual angles. Red and yellow spots denote the positions of the best images formed on the distal and proximal retinas, respectively; cyan spots denote the center of each image. (E) The modulation transfer functions of the optics (figs. S9 and S10), calculated on both retinal surfaces at the colored spots in (D), indicate how different angular frequencies (26) are modulated in the resulting image on the retina. Diamonds label the points at which the curves are reduced to 50% of the local zero-frequency intensity (the cut-off frequency).

Supplementary Materials

  • The image-forming mirror in the eye of the scallop

    Benjamin A. Palmer, Gavin J. Taylor, Vlad Brumfeld, Dvir Gur, Michal Shemesh, Nadav Elad, Aya Osherov, Dan Oron, Steve Weiner, Lia

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods 
    • Figs. S1 to S11 
    • Caption for Movie S1 
    • References 

    Images, Video, and Other Media

    Movie S1
    Movie of the X-ray microCT data obtained on a whole scallop eye, showing the off-axis geometry of the mirror and the lens/cornea and the complex 3D morphology of the mirror. The movie shows two complete rotations of 360° with the rotation approximately about the optical axis of the eye. The first rotation shows a volume rendering of the microCT scan and the second rotation the surfaces of the segmented structures in the eye. Three regions of high contrast are observed in the volume rendering which correspond to the cornea, iris and mirror. A weak region of contrast lying below the cornea corresponds to the lens and below this a very diffuse and weak region of contrast corresponds to the position of the retinas. In the segmented part of transparency), mirror (green) and retina (transparent grey) are highlighted

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