The secrets of the iridescent egg sac revealed

Description

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. Combining optical measurements, microscopy and mathematical modelling, we report in the journal Soft Matter that (i) the Hida salamander egg sac wall is a 3D structure with a periodic 1D modulation causing iridescence, and (ii) removal of the egg sac from water causes a shift of the backscattered light to the UV range due to the exchange of surrounding mediums (from water to air) with very different refractive indices, hence, resulting in the loss of iridescence in the visible range. Further analyses are required to identify whether the structural colour and/or iridescence might attract male salamanders for external fertilisation, constitute a visual signal to avoid cannibalism, or are mere by-products of egg sac material internal structure selected during evolution for its mechanical properties.