Light-reflecting properties of the iridophores of the neon tetra, Paracheirodon innesi

Abstract 1. 1. The change in color of the lateral stripe of the neon tetra, Paracheirodon innesi , is due to the motile activity of the iridophores which are sensitive to light and adrenergic stimuli. 2. 2. The light-reflecting platelets within the iridophore were found to be arranged regularly, making an acute angle of depression with respect to the median plane of the body. 3. 3. When epi-illumination was applied to the skin piece laid horizontally on the stage of a light microscope (with an angle of incidence of about 40°) and the wavelength of the reflected light introduced into the objective lens was monitored, the spectral peak was found to shift to longer wavelengths with the application of K + -rich saline, with a simultaneous decrease in reflectance. 4. 4. Using the identical fiber assembly for light irradiation and measurements of reflected light, we found that the angle of incident light producing the maximum reflectance, which corresponded to the inclination of the platelets, increased with the shift in the spectral peak toward longer wavelengths. 5. 5. It appears from our results that a change in the angle of inclination of the platelets triggered by adrenergic stimuli may give rise to a change in the distance between the platelets which, in turn, leads to the shift in the spectral peak.