Figure 2.

(a) Spectral reflectance of iridophores (L. pealeii) at different angles of incidence and planes of polarization, showing that with increasing angle of incidence (i.e. 40° and 45°) the reflected light shifts towards the shorter end of the spectrum and becomes polarized. Two reflectance spectra are shown for each angle of incidence: the spectrum reflected in the plane parallel to the plane of incidence and the perpendicular plane of incidence. At oblique angles (40° and 45°), the spectral reflectance in the perpendicular plane is much reduced in comparison with the parallel plane, indicating that the reflected light is linearly polarized. (b) The visibility of the ‘red’ stripe of squid from different orientations taking into account the light distribution in the sea. (i) An observer looking down on a squid will not see any iridescence. (ii) An observer looking down at a squid from a 45° angle will see iridescence from the most anterior and posterior ends of the stripe. (iii) An observer looking directly from the side will see strong iridescence from the entire stripe. See Mäthger & Denton (2001) for more details (modified from Mäthger & Denton 2001). (c) Acetylcholine (1 mM) changes iridescence from non-reflective (black lines, reflectance in IR) through various IR steps (black and grey lines) to red reflective in the squid L. pealeii. Measurements taken at 15 s intervals.