The Dual Functional Reflecting Iris of the Zebrafish.

Dvir Gur,Gil Levkowitz,Jan-David Nicolas,Dan Oron,Vlad Brumfeld,Omri Bar-Elli

doi:10.1002/advs.201800338

Abstract

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 physical mechanism that enables this dual functionality and the benefits of using a reflective iris have remained unclear. Using synchrotron microfocused diffraction, cryo‐scanning electron microscopy imaging, and optical analyses on zebrafish at different stages of development, it is shown that the complex optical response of the iris is facilitated by the development of high‐order organization of multilayered guanine‐based crystal reflectors and pigments. It is further demonstrated how the efficient light reflector is established during development to allow the optical functionality of the eye, already at early developmental stages.

Highlights

In the eyes of numerous organisms, including mammals, reptiles, birds and fish, light is absorbed by photoreceptors in the dorsally located retina
A key function of the iris is to prevent the passage of unfocused light
Many irises of the absorptive type, including in the human eye, contain melanin pigments that efficiently absorb a broad spectrum of visible light employed by numerous marine organisms, in which the outer surface of the iris is highly reflective

Summary

Introduction

In the eyes of numerous organisms, including mammals, reptiles, birds and fish, light is absorbed by photoreceptors in the dorsally located retina. In order to obtain a high-contrast image, only light that passes through the pupil and is focused by the lens should reach the retina. A key function of the iris is to prevent the passage of unfocused light. Irises can be classified either as absorptive, or as reflective or scattering, according to their operating principle with regard to incident light [1,2]. Many irises of the absorptive type, including in the human eye, contain melanin pigments that efficiently absorb a broad spectrum of visible light employed by numerous marine organisms, in which the outer surface of the iris is highly reflective.

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Conclusion