High-Order Organization of Guanine-Based Reflectors Underlies the Dual Functionality of the Fish Iris

Many marine organisms have evolved a reflective iris to prevent unfocused light from reaching the retina. The fish iris has a dual function, both to camouflage the eye and serving as a light barrier. Yet, the mechanism that enables this dual functionality and the benefits of using a reflective iris have remained unclear. Using synchrotron micro-focused diffraction, cryo-SEM imaging and optical analyses on zebrafish at different stages of development, we show that the complex optical response of the iris is facilitated by the development a high-order organization of multilayered guanine-based crystal reflectors and pigments. We further demonstrate how the efficient light reflector is established during development to allow the optical functionality of the eye, already at early developmental stages. These results shed light on the evolutionary drive for developing a compact reflective iris, which is widely used by many animal species. Significance Statement The fish iris is an exquisite example of nature’s remarkable engineering where specialized cells, dubbed iridophores, produce an efficient light reflector made of guanine-based crystals. This unique structure of the fish iris serves a dual function: In addition to its role as a light barrier, the iris has a second role of camouflaging the eye by creating a silvery reflectance, which merges with the fish skin. The underlying mechanism that enables the aforementioned dual functionality of the fish iris as well as the structural morphogenesis of the guanine reflector during embryonic development, remained unclear. We show that complex optical response of the iris is facilitated by the establishment of a high-order organization of multilayered guanine-based crystal reflectors and pigments.