The bay scallop Argopecten irradians (Mollusca: Bivalvia) has dozens of iridescent blue eyes that focus light using mirror-based optics. Here, we test the hypothesis that these eyes appear blue because of photonic nanostructures that preferentially scatter short-wavelength light. Using transmission electron microscopy, we found that the epithelial cells covering the eyes of A. irradians have three distinct layers: an outer layer of microvilli, a middle layer of random close-packed nanospheres and an inner layer of pigment granules. The nanospheres are approximately 180 nm in diameter and consist of electron-dense cores approximately 140 nm in diameter surrounded by less electron-dense shells 20 nm thick. They are packed at a volume density of approximately 60% and energy-dispersive X-ray spectroscopy indicates that they are not mineralized. Optical modelling revealed that the nanospheres are an ideal size for producing angle-weighted scattering that is bright and blue. A comparative perspective supports our hypothesis: epithelial cells from the black eyes of the sea scallop Placopecten magellanicus have an outer layer of microvilli and an inner layer of pigment granules but lack a layer of nanospheres between them. We speculate that light-scattering nanospheres help to prevent UV wavelengths from damaging the internal structures of the eyes of A. irradians and other blue-eyed scallops.
Digital Object Identifier (DOI)
Published in Journal of the Royal Society Interface, Volume 16, Issue 159, 2019.
© 2019 The Authors.
Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.
Harris, O. K., Kingston, A. C., Wolfe, C. S., Ghoshroy, S., Johnsen, S., & Speiser, D. I. (2019). Core–shell nanospheres behind the Blue Eyes of the bay scallopargopecten irradians. Journal of The Royal Society Interface, 16(159), 20190383. https://doi.org/10.1098/rsif.2019.0383