The structure and function of iridescent corneas in teleost fishes

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Abstract
Many fishes possess an iridescent layer in the cornea which is visible when the eye is illuminated from above. Corneal iridescence is most common among shallow water marine teleosts that live among rocks or on the bottom. Iridescence is produced when light traverses a regular multilayer stack, the thickness of each layer being of the order of a quarter of the wavelength of visible light. However, the components of these stacks have different structures in different groups of fishes and may be formed from endoplasmic reticulum, plates of connective tissue, protoplasm, or regular arrays of collagen fibrils. Reflexion and transmission measurements indicate that too little light is lost by reflexion for the cornea to act as an effective colour or polarizing filter to light near normal incidence. When the angle of light incidence is oblique, the plates become highly reflecting and it is thought that very bright downwelling light that reaches the cornea at large angles of incidence is reflected out of the eye and does not contribute to intra-ocular flare. At angles of incidence near to the normal, it is possible that the plates may be so spaced that there is destructive interference of the reflected light and light losses through reflexion would be very small. The intra-ocular flare resulting from such layers has been measured and it is calculated that the iridescent layer gives a significant increase in visual range underwater without sacrifice of sensitivity. The possibility that corneal iridescence may help to camouflage the pupil is discussed.

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