Movie S1. Neurally activated iridescence in squid iridophores. “Intact fin” preparation yields a green iridophore phenotype when a nearby nerve branch is stimulated through a proximal suction pipette. Fin iridophores at rest have low peak reflectance. After electrical stimulation of a skin nerve in the fin, a field of yellow-green iridophores is clearly visible (see also figure 1). Playback speed is four times faster than real time. Red square indicates electrical stimuli being applied.

​Movie S3. Structural coloration and peak reflectance have separate mechanisms. Neurally stimulating tissue where the fin muscle and ventral skin (both below the iridophore layer) had been removed, results in a red phenotype. Different response dynamics of peak reflectance and colour change can be observed between red and green iridophore responses (see also figure 3). Playback speed is four times faster than real time. Red square indicates electrical stimuli being applied.

Movie S2. Tissue connectivity is essential for structural coloration. A preparation, with the layers below and above the iridophore layer removed, such that only the nerves, iridophores and connective tissue remain. In this preparation, neural stimulation activates iridophores, but the responses now show a red phenotype (see also figure 2). Playback speed is four times faster than real time. Red square indicates electrical stimuli being applied.​

​Movie S4. Iridocyte flickering alters iridophore appearance. Patches of iridocytes within some iridophores were often observed to “flicker”. In this video, the group of iridocytes in the top part of this iridophore flicker, although the neighboring chromatophores remained unchanged (see also figure S3). Playback speed is ten times slower than real time.