Meeting Abstract
Many animals produce structural colors, but taxa vary in how and why they do so. Scallops, for example, have dozens of mirror-based eyes and, in some species, these eyes are a bright iridescent blue. In other species, the eyes are a non-iridescent brown or black. We hypothesized that the blue eyes of scallops obtain their color, at least in part, from the scattering of short-wavelength light by photonic nanostructures. Using transmission electron microscopy, we found the epithelial cells surrounding the blue eyes of the bay scallop Argopecten irradians contain close-packed nanospheres that are absent from the black eyes of the sea scallop Placopecten magellanicus. The nanospheres in the eyes of A. irradians are ~ 180 nm in diameter and consist of electron-dense cores ~ 140 nm in diameter surrounded by less electron-dense shells 20 nm thick. These core-shell nanospheres are packed at a volume density of ~ 60% and energy-dispersive X-ray spectroscopy indicates they are not mineralized. Using optical modeling, we calculated that the nanospheres in the eyes of A. irradians are an ideal size for producing angle-weighted scattering that is bright and blue. From these results, we have learned how scallops make their eyes blue, but we have yet to learn why they do so. We hypothesize that light-scattering nanospheres help prevent UV wavelengths from damaging the internal structures of the eyes of blue-eyed scallops. If we are correct, we would expect to see a correlation between eye color and habitat depth across scallops, with blue-eyed species tending to live in shallower, more UV-rich environments than black-eyed species. To test our hypothesis, we used comparative phylogenetic methods to ask if eye color in scallops correlates with environmental features related to light conditions, such as depth.
