Cover illustration: Dynamic vestments
Rapid colour change is only one example of the dramatic rapid physiological changes the giant cuttlefish (Sepia apama) can perform. This remarkable cephalopod uses chromatophores, small vacuoles of pigment, to change colours quickly. These tiny organs are directly under neurological control, and each consists of an elastic sacculus with various coloured granules (black, brown, red, orange, or yellow) surrounded by between 10 and 25 radial muscles. With muscular contraction or relaxation the sacculus can be used to express or hide true pigmentary colours, thus creating differences in both colour and pattern. These unique animals also use iridocytes (also called iridophores) around the eyes and elsewhere to create constructive interference, thereby creating other colours such as those of the shorter wavelengths, like blues or greens. Iridocytes consist of thin electron dense platelets alternating with chitin or cytoplasm of the appropriate thickness …
