Mäthger, LM, Hanlon RT. (2007) Malleable skin coloration in cephalopods; selective reflectance, transmission and absorbance of light by chromatophores and iridiophores. Cell Tissue Research 329:179-86.

Mäther and Hanlon (2007) researched the spectrometric characteristics of the coloration of the squid, Loligo plealeii. According to the authors this paper is the first to make spectral measurements of the colors that are produced by the combination of chromatophores and iridiophores in cephalopods. Cephalopods are unique in their ability to rapidly change colors, this rapid change is accomplished by two means. First, the skin of squid, and other cephalopods, contains chromatophores- pigment containing organs that exist in as many as three colors on one individual (red, yellow or brown)- which are directly innervated and controlled by the animal's brain. Chromatophores are contracted and expanded by muscles that allow them to change in size quickly. Second, cephalopods posses iridiophores- light reflecting cells which lie under the chromatophores and structurally reflect light that has passed through the outer layer of the skin. In this paper the authors measured the reflected and transmitted spectra of expanded chromatophores of squid and found a relatively large amount of light (70-90%) passed through the red and yellow chromatophores, but not the brown (20-30%). The authors also found the the iridiophores reflect a wide range of wavelengths (above 700nm, close to infrared, to well below 600nm) and the the wavelength and intensity of reflected light depended on physiological control of the iridiophores by muscarinic cholinergic receptors and can be turned completely off or on at will. Finally the authors investigated the interaction of the two color producing structures and found that chromatophores act not only to adsorb incident light but also to filter light after it has been reflected by iridiophores and leaves the skin to produce an even wider range of outgoing wavelengths than either system could produce alone ranging into the green and even blue wavelengths.

This article is rife with references to material recently discussed in lecture. Foremost is the use of different sources of color. Squid and other cephalopods as noted in the paper utilize both pigment and structural colors. Pigments are molecules that produce color by selectively absorbing and reflecting different wavelengths when excited. In cephalopods these pigments reflect red, yellow and brown color. In addition cephalopods have iridiophores which produce iridescent structural colors through interference. Alone these two systems produce colors in the red to yellow range, but put together light leaving iridiophores can pass through and be filtered by the different chromatophores to produce colors throughout the visible spectrum. Interestingly at the time of the paper all evidence pointed to cephalopods being unable to see color, so the purpose of such a dynamic and wide array of coloration has yet to be fully determined.