The hidden language of light: Polarization signals in cuttlefish courtship

The most conspicuous signals are generally the most attractive; this principle underlies the evolution of sexual signal. While the sexual signal design and its exceptional diversity have primarily explored on the color (wavelength) of light, various animals utilize a different property of light for signaling: polarization.
In short, polarization is a third physical property of light, alongside color and intensity, and refers to the orientation of light waves' vibrations. While most vertebrate species, including humans, cannot perceive polarized light, some invertebrate species, such as crustaceans and cephalopods (e.g., octopus, squid, and cuttlefish), can detect the polarization of light and reflect polarized light from their body surfaces, suggesting that the polarization of light might function as a communication signal. In our study, by focusing on the sexually ornamented trait and the courtship behavior of specific cephalopod species, we found an polarization courtship signal, which is extremely conspicuous from the perspective of cephalopod polarization vision. Additionally, we conducted morphological observations and optical analyses of their polarization-reflective body surfaces, uncovering a novel mechanism for generating complex polarization patterns. In this gethering, I will provide a general introduction to the role of polarization as a visual cue and signal, followed by an overview of our study on the unique courtship behavior involving polarization signaling in the cuttlefish Sepia andreana.