Wave of chromatophore expansion-retraction. A Naef stage XVIII O. insularis embryo seen from the ventral side. Sudden spontaneous waves of chromatophores expanding and retracting are initiated at the posterior end and travel towards the anterior side. The wave is spontaneously repeated three times within approximately 7 seconds. At the beginning of the movie, other chromatophores (even at the opposite side) are also expanding and retracting, such as the large chromatophore located by the right eye. At the end of the movie, we show a single expansion-retraction wave in slow motion; when the wave reaches the middle region of the body (over the internal yolk sac), chromatophores at the posterior end are already retracting.