bio

Iridophore

A reflective cell layer beneath the pigment tier that works by structure rather than dye: high-refractive -index plates of reflectin protein (n ≈ 1.59) alternating with low-index fluid (n ≈ 1.33), forming a Bragg mirror that reflects one tuned band of wavelengths. The remarkable part is that the band is adjustable — acetylcholine signalling phosphorylates the reflectin, plate spacing shifts, and the reflected colour moves across blue, green and polarised light. This is colour produced by geometry rather than chemistry, and like the pigment cells above it, it is useless once there is no light left to reflect.