The iridophores of the Neon tetra Paracheirodon innesi consist of alternating layers of guanine and cytoplasm. In the dark-adapted state the reflected light from constructive interference is in the ultraviolet or blue. When exposed to light the cytoplasm layers increase in thickness and as a result the reflections shift to longer wavelengths and the iridophores appear green. The iridophores are thought to contain a rhodopsin-like molecule and we suggest that the colour-change mechanism involves the light-induced opening of sodium channels in the plasma membrane, leading osmotically to an increase in thickness of the cytoplasm layers. Experimental support for this suggestion was obtained by the substitution of choline chloride for sodium chloride in the perfusing medium, which can be done without altering the osmotic strength of the perfusing medium. This procedure almost abolished the light response and makes it seem likely that sodium ions are necessary for the light response to take place.

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