Chameleon turquoise, and purple. Chameleon skin has a superficial layer which contains pigments, and under the layer are cells with guanine crystals. Chameleons change colour by changing the space between the guanine crystals, which changes the wavelength of light reflected off the crystals which changes the colour of the skin. Colour change in chameleons has functions in camouflage, but most commonly in social signaling and in reactions to temperature and other conditions. The relative importance of these functions varies with the circumstances, as well as the species. Colour change signals a chameleon's physiological condition and intentions to other chameleons. Chameleons tend
Underwater camouflage predators and prey. Natural selection has produced a wide variety of methods of survival in the oceans. In Ancient Greece, Aristotle commented on the color-changing abilities, both for camouflage and for signalling, of cephalopods including the octopus, in his "Historia animalium": Three main camouflage methods predominate in the oceans: transparency, reflection, and counterillumination. Transparency and reflectivity are most important in the top 100 metres of the ocean; counterillumination is the main method from 100 metres down to 1000 metres; while camouflage becomes less important in the dark waters below 1000 metres. Most animals of the open sea use at least