Fig. 4. Shell windows use natural fiber optics to project images with very high resolution (100 lines/mm).

a A cross-sectional SEM of a single shell window shows fibrous prisms of aragonite oriented roughly perpendicularly to the shell surface (the “fibrous prismatic layer”). Together, these prisms act like fiber optic cable bundles. b A light microscope image of a polished shell fragment shows the fibrous prisms rotating in orientation between the regions labeled 1–3. In region 1, we see the side view of fibrous prisms; in region 3 we see cross sections of aligned prisms pointing into the page. c–e Shell windows transmit high-resolution images. c A fragment of unpolished shell shows the contrast between windows (transparent regions that transmit the vertical black lines) and the opaque surrounding shell (white regions that occlude the vertical black lines). d Shell windows from an unpolished fragment transmit 10 lines/mm. e Shell windows from a polished fragment transmit 100 lines/mm. f–h To experimentally test whether the windows act like fiber optic cable bundles (Supplementary Fig. 7), we placed a small 0.3 mm thick polished fragment of shell from a heart cockle on top of a glass calibration slide. We focused the microscope on the ruler on the glass calibration slide and then adjusted the focus to refocus on the surface of the shell (~0.3 mm higher than the background glass slide). By doing so, we could test whether images are transmitted through the windows or are actually projected onto the surface of the shell. Shell windows project images onto their surface (rather than merely transmitting images through). The image came into focus on the surface of the shell as we varied the plane of focus. Here we show images and micrographs from (a, c, d) two representative unpolished shell fragments and (b, c, f, g, h) one representative polished shell fragment. In (c–d), shell fragments were immersed in seawater. Illustration in (f) is credited to Nuria Melisa Morales Garcia, Science Graphic Design LTD.