Chiou, T. et al. 2007. Spectral and spatial properties of polarized light reflections from the arms of squid (Loligo pealeii) and cuttlefish (Sepia officinalis L.) J Exp Bio. 210, 3642-3635.

Squid (Loligo pealeii) and cuttlefish (Sepia officinalis L.) are notable for having iridiphores that reflect polarized light. Iridophores are a type of chromatophore that reflect iridescent light through the use of stacked plates to diffract light. Reflection by iridophores is generally very dependent on the angle of the viewer, this is not so for iridophores on the arms of these squids and cuttlefish. Researchers theorize that these organisms use their iridophores for intraspecific communication as they can turn the polarization of light from these iridophores on and off and their vision is polarization sensitive. Many of their predators are not sensitive to polarization, which may allow for a private channel of communication.

To study this structure, researchers placed squid and cuttlefish arms with iridophores under a dissecting microscope with a polarization filter and attached spectrometer. Light was produced at either 90° or 45° to the observational axis and the polarization filter was set to 0°,45°, and 90°. The arms were rotated under these conditions and spectra were recorded. A digital camera photographed the iridophores using the same illumination and polarization settings.

With illumination at 45° the iridophores appeared pink (believed to be caused by the pigments behind the iridophores) and exhibited very little polarization regardless of arm orientation. Illumination at 90° showed polarization with a peak at 500 nm. Electric field vectors of the polarized light remained virtually fixed at 90° regardless of arm rotation and across all wavelengths (400-800 nm). Brightness of polarized light and the iridophore strip changed with rotation of the squid arm. Polarization reached a maximum of brightness 60% (compared to a white standard reflector) in squid and a slightly smaller level in cuttlefish. Photography confirmed that the iridophore strip was the source of the polarized light.

Images from a TEM showed that the iridophore is composed of groups of stacks of plates arranged in a parallel manner. Iridophores appear blue or green in color when light is polarized and red or pinkish when light is non-polarized. These iridophores likely use multilayer reflectors with dielectric surfaces (which selectively reflect certain wavelengths, incident angles, and electric field vector angles). The iridiphores plates are probably composed of reflectin, a protein with a refractive index of 1.59, the greatest of any natural protein known.

This article touched on several subjects discussed in class. Firstly, spectrometers were used to analyze the reflected wavelengths of the surfaces being studied, as was done in many of the examples discussed in class. The spectrometer produces remarkably quantitative results for a greater range of wavelengths than can be seen by the human eye, making it the ideal tool here.

The role of polarization as a means of communication was discussed in class. In this case squids and cuttlefish appear to be using their ability to both produce and detect light polarization to create a private channel of communication, another subject discussed in class. As ability to detect light polarization is relatively uncommon, particularly in the predators of these organisms, the advantages of this form of communication are clear. The fact that this polarization is greatest when the angle of illumination is 90° to the viewer as it would be for a viewer at the same depth (such as another squid or cuttlefish) and is almost non-existent when the angle of illumination is 45° to the viewer (as it would be for a predator looking down for pretty from above) highlights the likely purpose and utility of this form of communication. Attenuation of light intensity due to relatively high rates of scattering by ocean water was an issue discussed in class. While these organisms live 150-400 meters below the surface where light levels are low they are routinely most active at depths of less than 50 meters where light is sufficiently bright.

Finally, this paper discusses production of color through both pigments and structural features. In this case pigment is the predominant source of color from the iridophores when there is no polarization and an interference-based structural protein is the major source of color when polarized light becomes dominant.