A Possible Biomineralized Light Guiding Structure in the Ossicular Skeletons of Starfish Protoreaster Nodosus

Skeletal elements at the distal end of each arm ray known as terminal plates in Starfish, <i>Protoreaster nodosus</i>, possess truncated cone-shaped structures embedded within the rather stochastic porous stereom. Previous works have shown some starfish species possess compound eyes at the arm tip and are capable of image formation. In this study, we hypothesize that the unique biomineral-based structures at the arm tip serve as Light Guiding Structures (LGSs) for photoreception purposes in addition to their mechanical protection of the underlying tissue. Combining quantitative three-dimensional (3D) structural characterization along with X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) measurements, we showed that the entire terminal plate, though with an irregular micro-lattice, diffracted as a single crystal calcite. The channeling of light within individual LGS was achieved by total internal reflection. The radial arrangement of LGSs allowed the guided light to converge at the center of the interior cavity beneath the terminal plate. Ray-trace simulation on a single LGS and optical experiment on the entire terminal plate confirmed the light transmitting and converging capability of the LGS assembly. Our findings may provide another example, in addition to brittle stars, blue-rayed limpet, and certain chiton species, of how organisms modify their biomineralized structure to achieve multifunctionality, specifically, serving dual optical-mechanical roles.