Development of High-Order Organization of Guanine-Based Reflectors Underlies the Dual Functionality of the Zebrafish 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 demonstrates 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 are widely used by many animal species. One Sentence Summary The dual function of the zebrafish iris as a light barrier and camouflage reflector is enabled by the high-order organization of intracellular guanine crystals and pigments.