Mathger, L.M., & Hanlon, R.T. (2006). Anatomical basis for camouflaged polarized light communication in squid. Biology Letters, 2: 494-496.

In this investigation, Mathger & Hanlon raise the possibility that squid use polarized light signals as a “hidden communication channel.” The authors provide anatomical evidence for camouflaged polarized light communication. Squid have two layers in their skin that allow them to manipulate color patterns: an underlying layer of iridophore cells, and a superficial layer of chromatophores that can expand and contract over the iridophores. Squid can regulate the iridophores, which contribute to the reflectance of polarized light at oblique angles of viewing.

The authors measured the reflectance spectra of skin samples from the squid Loligo pealeii with a fibre optic spectrometer attached to a dissecting microscope. Linear polaroid filters were used to analyze polarization. The authors found linearly polarized blue-green iridescent reflection at an angle of 45°. They also discovered that pigmented chromatophores could alter the color of light reflected from the iridophores underneath. For example, red and brown chromatophores would transmit polarized light, but when the brown chromatophores were not fully expanded, they blocked the polarized light transmission. These results provide anatomical evidence of the ability of squid to physiologically control iridescence and polarization.

Our lecture on the properties of light defines polarized light as occurring when the electric and magnetic vectors of light are aligned. Our lecture on production and transmission of light signals is also relevant to this paper. The authors describe how squid use yellow, red, or brown pigments to absorb certain wavelengths of light and transmit others during their camouflage and signaling. Our textbook defines chromatophores as special cells that produce pigments and house them in many small packets, and describes how very intense colors can be produced with an underlying layer of iridosomes. These iridosomes contain guanine platelets that act like little mirrors to reflect all wavelengths of light.

The authors of this paper also emphasize how quickly cephalopods can change their body patterns for camouflage or signaling. Our textbook describes the mechanism that cephalopods use to change color quickly. The chromatophores have radial muscles connected to them, and when these muscles are contracted, the chromatophore is stretched out and the color becomes visible. These muscles are under voluntary control by the central nervous system.

Although we did not talk about polarized light sensitivity in class, our textbook details how this occurs. Rhabdomeric photoreceptors (which cephalopods have, as described in this paper) are inherently more sensitive to polarized light than ciliary receptors. Our textbook discusses polarization sensitivity in flying insects and fish, but does not mention cephalopods. Possible functions for polarization sensitivity in the textbook include orientation through light in the sky, time of day determination, and schooling behavior in fish. However, this paper provides several examples of polarized light used as communication signals (polarized patterns on the wings of butterflies used for mate recognition, and crustaceans that produce polarized patterns for communication). Therefore, it is not unlikely that these squid could be using polarized light as signals to conspecifics that are invisible to their predators who are not sensitive to polarized light.