"Chameleon" is the title we usually give to great camouflage artists, but cephalopods (octopuses, cuttlefish and, to a lesser degree, squids) are much better at active camouflage than chameleons. From the moment they are born, octopuses and cuttlefish are capable of almost instantaneously changing the colour and texture of their skin to match their surroundings. They can create patterns in their skin, such as spots and stripes. (No, they can't quite do tartan). These remarkable abilities are the result of thick translucent skin densely covered with layers of chromatophores and iridocytes.

Camouflage Mechanisms:
Chromatophores are elastic pigment cells contained within the skin, underneath a transparent outer layer of skin. They can also be found on some of the cephalopods' internal organs, including the inner walls of the ink sac. The chromatophores contain pigmented fluid and are connected to each other and to the neighboring tissues with a network of muscle fibers, generally between 4 to 24 fibers in the circumference of each chromatophore. When these fibers are extended, the chromatophore shrinks to a pinpoint, so that it is hardly visible. When the fibers are contracted, the chromatophore expands into a flat disc, up to 60 times its original size, tinting the surface of the skin with the pigment it contains. These pigments generally cover a range of variations on brown, black, red, yellow, and orange-red.

Iridocytes are reflector cells found in the skin and inside the body. They are iridescent platelets which cause a light play of colours when light falls on them. Together with the chromatophores, they make a wide range of colours possible for the cephalopods.

The texture of cephalopod skin can also be changed. While at most times, octopus skin is smooth, it can be raised into ridges and bumps. These papillae are often erected when the octopus is especially content, such as when resting after a meal, but it can also be controlled to resemble the surface surrounding the octopus for even more effective camouflage.

Cuttlefish Displays
Cuttlefish are even better than octopuses at camouflage, producing the most varied patterns and colour ranges in the animal kingdom. The cuttlefish possesses three layers of chromatophores, with yellow cells near the skin surface, orange and red ones in the middle, and dark (brown, brown-red, or black) at the base of the skin. When the chromatophores are extended, the colours overlap, working in combination with the iridocytes to produce a range of compound colours like green and gray.

Cuttlefish at rest display a striped pattern much like zebra striping (disruptive camouflage, for the military-minded readers). Generally the natural striping is dark brown alternated with grayish white, but faint waves of colour move over the skin almost continuously. The skin colour can change almost instantly to match any type of shading. Cuttlefish swimming slowly over variegated rocks change their colour in waves, going from brown to red to sandy gray to match the surfaces they are swimming over. They can also imitate patterns - in one experiment, a cuttlefish was placed into a black tank with one white porcelain square on the bottom. The cuttlefish quickly turned black except for a small white spot, a narrow white band, and a rough square of white in its middle.

When extremely irritated or frightened, cuttles will display rapid colour changes and quick evasive swimming before discharging ink to escape. The colour changes start with two dark spots in the middle of its back, followed by the entire body darkening. If further irritated, stripes may appear rapidly, followed by another complete darkening. Generally, at this point the cuttle will discharge a cloud of ink, darken and hide.