Animal Color Change

Chamaeleo pardalis, a chameleon species found in the forests of Madagascar. Chameleons can produce a wide range of colors and patterns on their skin, but they do this primarily to express mood, not to blend in with different environments.

Some animals, such as various cuttlefish species, can manipulate their chromatophores to change their overall skin color. These animals have a collection of chromatophores, each of which contains a single pigment. An individual chromatophore is surrounded by a circular muscle that can constrict and expand. When the cuttlefish constricts the muscle, all the pigment is squeezed to the top of the chromatophore. At the top, the cell is flattened out into a wide disc. When the muscle relaxes, the cell returns to its natural shape of a relatively small blob. This blob is much harder to see than the wide disc of the constricted cell. By constricting all the chromatophores with a certain pigment and relaxing all the ones with other pigments, the animal can change the overall color of its body.
Cuttlefish with this ability can generate a wide range of colors and many interesting patterns. By perceiving the color of a backdrop and constricting the right combination of chromatophores, the animal can blend in with all sorts of surroundings. Cuttlefish may also use this ability to communicate with each other. The most famous color-changer, the chameleon, alters its skin color using a similar mechanism, but not usually for camouflaging purposes. Chameleons tend to change their skin color when their mood changes, not when they move into different surroundings.
Some animal species actually change which pigments are in their skin. Nudibranches (a small sea creature) change their coloration by altering their diet. When a nudibranch feeds from a particular sort of coral, its body deposits the pigments from that coral in the skin and outer extensions of the intestines. The pigments show through, and the animal becomes the same color as the coral. Since the coral is not only the creature's food, but also its habitat, the coloration is perfect camouflage. When the creature moves on to a differently colored piece of coral, its body color changes with the new food source. Similarly, some parasite species, such as the fluke, will take on the color of their host, which is also their home.
Many fish species gradually produce different pigments without changing their diet. This works something like seasonal molting in mammals and birds. When the fish changes environments, it receives visual cues of a new surrounding model. Based on this stimulus, it begins to release hormones that change how its body produces pigments. Over time, the fish's coloring changes to match the new surroundings.