Translucent in air and iridescent in water: structural analysis of a salamander egg sac

Article Article scientifique Article Translucent in air and iridescent in water: structural analysis of a salamander egg sac eng 2020 author Zabuga Aleksandra 846176 author Arrigo Marcelle 844729 author Teyssier Jérémie 0000-0002-7590-2987 604738 author Mouchet Sébastien R. author Nishikawa Kanto author Matsui Masafumi author Vences Miguel author Milinkovitch Michel C. 679786 1714-1721 10.1039/C9SM02151E 1744-683X http://xlink.rsc.org/?DOI=C9SM02151E Females of some Asian salamanders of the genus Hynobius deposit in streams their eggs embedded in a translucent envelope called an ‘egg sac'. The edges of the envelope exhibit a spectacular blue-to-yellow iridescent glow, which instantaneously disappears when the sac is removed from water. First, our scanning electron microscopy analyses reveal that the inner surface of the 100 μm-thick envelope displays striations (length scale of about 3 μm), which are themselves covered by much smaller (190 ± 30 nm) and quasi-periodic corrugations. The latter could constitute a surface diffraction grating generating iridescence by light interference. Second, our transmission electron microscopy and focused-ion-beam scanning electron microscopy analyses show that the bulk of the egg sac wall is composed of meandering fibres with a quasi-periodic modulation of 190 ± 60 nm along the thickness of the envelope, generating a photonic crystal. Third, Fourier power analyses of 450 electron microscopy images with varying incident angles indicate that changing the surrounding medium from water to air shifts most of the backscattered power spectrum to the ultraviolet range, hence, explaining that the egg sac loses visible iridescence when removed out of the water. Fourth, the results of our photography and optical spectroscopy experiments of submerged and emerged egg sacs rule out the possibility that the iridescence is due to a thin film or a multilayer, whereas the observed non-specular response is compatible with the backscattering expected from surface diffraction gratings and volumetric photonic crystals with spatial 1D modulation. Finally, although we mention several potential biological functions of the egg sac structural colours and iridescence, we emphasise that these optical properties might be the by-products of the envelope material internal structure selected during evolution for its mechanical properties. Dewey 500.2 Dewey 590 Swiss National Science Foundation CRSII3_154406 CH Swiss National Science Foundation EvoDevo and Physics of Skin Colours 162743 CH Project funding 33 Département de physique de la matière quantique 131 44 Département de génétique et évolution 162 false 819 Groupe Milinkowitch Soft Matter 16 7 5b45133a-67da-4aca-b872-97061c278c9a Article (Published version) Public c9sm02151e.pdf 8843022 application/pdf CC BY-NC openAccess Faculté des sciences / Section de physique / Département de physique de la matière quantique Faculté des sciences / Section de biologie / Département de génétique et évolution

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Translucent in air and iridescent in water: structural analysis of a salamander egg sac	Soft Matter			2020	481	424

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Zabuga, Aleksandra