Meeting Abstract
The field of biologically-inspired design is interested in photonic nanostructures that manipulate light to produce colors by preferentially scattering certain wavelengths. Structural color often results in iridescence because the angle of illumination influences how different wavelengths scatter. The iridescence of the blue eyes of the bay scallop Argopecten irradians invites the hypothesis that photonic nanostructures may be responsible for their appearance. To address our hypothesis, we used transmission electron microscopy to identify structures that may be associated with eye color in scallops. We found that epithelial cells from the blue eyes of the bay scallop A. irradians have three distinct layers: a distal layer of microvilli; a middle layer of small, tightly-packed, electron-dense spheres; and a proximal layer of much larger pigment granules. Computational modeling suggests that the size (120-140 nm in diameter) and packing density (60-80/μm2) of the spheres are consistent with a maximal scattering of shorter wavelengths of light. Using energy-dispersive X-ray spectroscopy, we ruled out the possibility that these spheres are mineralized, suggesting that they are made of dense populations of proteins. In the epithelial cells from the black eyes of the sea scallop Placopecten magellanicus, we found a distal layer of microvilli and a proximal layer of pigment granulates, with no layer of spheres between them. We conclude that the iridescent blue color of eyes from A. irradians comes from the preferential scattering of short wavelengths by the small, tightly-packed spheres and the absorption of longer wavelengths by the pigment granules behind them. In future work, we will address the evolutionary history and ecological relevance of eye color in scallops.
