Making things crystal clear: unravelling how the iridophore’s ultrastructure causes broadband reflection among amphibians

Among amphibians, structural colour is produced by crystalline platelets that form within specialised colour cells known as iridophores that possess varying architectures. While platelet geometry and arrangement are well described, the mechanisms by which they scatter light remains debated. Linking optical function to iridophore ultrastructure remains difficult because scattering physics is often inaccessible to biologists. In this review, we synthesise how light interacts with the ultrastructure of biological materials via scattering mechanisms, as a means of bridging the disciplinary divide between optical physics and pigment cell morphology that has made it challenging to decipher the function of the iridophore in structural colour production. We then apply this framework to explore how the amphibian iridophore may produce white skin, developing hypotheses that predict how broadband reflection is produced via different structural effects. Our synthesis indicates that there are several potential causes for broadband reflection, related to some degree of variability or disorder between iridophore platelets, including variability in platelet size, orientation, and arrangement. We intend for our review to be a foundational piece that can assist biologists in understanding light scatter within the amphibian iridophore but for this to also be a case study that describes biological systems in general given that these structures are found in a wide diversity of taxa. This will support future research in identifying the structural mechanisms underlying broadband reflection leading to whiteness in amphibians and potentially other animal groups.