Despite being colour-blind, squid and cuttlefish live in a visually complex world, using polarised light reflected from their mirror-like iridophores to communicate with each other. A stripe of high-reflectance iridophores on their arms reflects the most highly polarised light. But how do the reflections change as the animals move their super-bendy arms? To find out, Tsyr-Huei Chiou and colleagues took samples of squid and cuttlefish arms and examined reflections from the stripe under a microscope as they rotated and tilted the samples through different angles while shining light on them(p. 3624).
They found that the posture of the arm had little or no effect on the E-vector of the reflected light, which describes the plane of orientation of a polarised light wave perpendicular to the direction the light wave is travelling. Arm orientation also didn't affect the partial polarisation, which is how much the light was polarised, and the spectral reflectance, which is the ratio of the light reflected back from the surface compared to the amount of light hitting the surface.
When they changed the angle of the light hitting the arm, however, the partial polarisation and the spectral reflectance changed, making the reflected colour appear less saturated. Interested to know how the reflections were staying relatively constant, the team examined the arms under an electron microscope, finding that the arm stripes are made up of several groups of multi-layer platelets within the iridophores, which are all oriented at different angles. This produces a constant reflection of polarised light over a range of viewing angles, and suggests that cuttlefish and squid can send out reliable polarisation signals to each other regardless of the orientation of their arms.