Abstract
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.
Highlights
Be they bright or dull, conspicuous or cryptic, the colours of animals are often of adaptive significance [1]
We found that the mirror in the eye of A. irradians has a peak reflectance at approximately 540 nm, this value is almost certainly angle-dependent and influenced by the absorption of light by the two retinas within the eye
Epithelial cells from the black-eyed P. magellanicus have an outer layer of microvilli and an inner layer of pigment granulates but lack a layer of nanospheres between them
Summary
Be they bright or dull, conspicuous or cryptic, the colours of animals are often of adaptive significance [1]. Animals produce colours primarily through the absorption of light by pigments, the scattering of light by nanoscale structures or interactions between the two [2,3]. Colours produced by ordered photonic nanostructures are often iridescent because the wavelengths they scatter towards a viewer depend on both the position of the viewer and the angle of incident light [4]. [7]), molluscs produce colours, in part, through the coherent scattering of light by photonic nanostructures. In many cases, these nanostructures have linear dimensions of the order of a quarter wavelength of light Cephalopods, for example, have light-reflecting organs termed iridophores that scatter light using multilayer reflectors in which high-RI layers of protein approximately (a)
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