Spectral and spatial properties of polarized light reflections from the arms of squid (Loligo pealeii) and cuttlefish (Sepia officinalisL.)

Abstract

SUMMARY On every arm of cuttlefish and squid there is a stripe of high-reflectance iridophores that reflects highly polarized light. Since cephalopods possess polarization vision, it has been hypothesized that these polarized stripes could serve an intraspecific communication function. We determined how polarization changes when these boneless arms move. By measuring the spectral and polarizing properties of the reflected light from samples at various angles of tilt and rotation, we found that the actual posture of the arm has little or no effect on partial polarization or the e-vector angle of the reflected light. However, when the illumination angle changed, the partial polarization of the reflected light also changed. The spectral reflections of the signals were also affected by the angle of illumination but not by the orientation of the sample. Electron microscope samples showed that these stripes are composed of several groups of multilayer platelets within the iridophores. The surface normal to each group is oriented at a different angle, which produces essentially constant reflection of polarized light over a range of viewing angles. These results demonstrate that cuttlefish and squid could send out reliable polarization signals to a receiver regardless of arm orientation.

Do cephalopods communicate using polarized light reflections from their skin?

“…A squid in a normal body position in the water column (NB the light intensity in the sea has an angular distribution) will reflect bright polarized light in a horizontal direction. In L. pealeii and Sepia officinalis, prominent polarization patterns can also be seen along the arms (Chiou et al, 2007;Shashar et al, 2001) (Fig. 2C) position in front of the head (i.e.…”

“…(b) The intensity of polarization signals in the UV range is most likely low. Although not specifically examined, there is strong supporting evidence for this claim (Chiou et al, 2007;Mäthger and Denton, 2001;Mäthger and Hanlon, 2007), showing that the per cent polarization of light reflected from cephalopod skin is very low at short wavelengths around 400 nm.…”

“…Cephalopod skin produces polarized reflective patterns (Chiou et al, 2007;Mäthger and Denton, 2001;Mäthger and Hanlon, 2007;Shashar et al, 2001) (Fig. 2).…”

“…Iridophore reflectance changes generally take longer than chromatophores; however, we have observed fast changes in polarization patterns coming off iridophores (less than a second) . Interestingly, although iridescence in cephalopods spans the entire visible spectrum (blue to near-IR), polarized reflections are strongest in the blue and green parts of the spectrum (Chiou et al, 2007;Mäthger and Denton, 2001;Mäthger and Hanlon, 2007;Shashar et al, 2001). That is, iridescence in other wavebands is generally not, or only weakly, polarized.…”

Optical and Structural Characterization of Natural Nanostructures

“…6.2, exhibits areas that reflect polarized light along the arms and in the forehead. However, this pattern disappears during specific situations such as aggression display or when the individual is camouflaged [45,46]. Many beetles also reflect polarized light.…”