Optical constraints shape a polarized sexual ornament in cuttlefish

Ornamented traits involved in sexual signaling are shaped by trade-offs between reproductive benefits and survival costs, yet their evolution may be constrained by physical mechanisms that generate signals. Here, we show that optical constraints shape the morphology and development of sexually ornamented arms in the cuttlefish Doratosepion andreanum, which produces conspicuous polarization signals. Their polarization patterns arise from light transmission through circularly birefringent muscle tissue, which rotates the light's angle of linear polarization (AoLP). Optical experiments showed that AoLP rotation increases linearly with arm width, reaching 90 deg shift at a width of 3.91 mm, which maximizes polarization contrast against underwater background for cephalopod polarization vision. Morphological analyses revealed that growth of the ornamented arm converges on this width, with development deviating from the typical conical shape into a uniform cylindrical shape. These results suggest that optical mechanisms of signaling define a narrow morphological design space for polarized ornaments.