Chromatophores are irregularly shaped, pigment-containing cells. If the pigment is melanin, they are called melanophores. Chromatophores are common in crustaceans, cephalopod mollusks, lizards and amphibians, and some fishes.
Figure 3.22.1: Chromatophores in the skin of a squid. Image used with permission (CC-By-Sa-2.0 Minette)
Chromatophores are often used for camouflage. Figure 3.22.1 shows a winter flounder resting on a checkerboard pattern. The chromatophores of cephalopods change size (expand and contract) as a result of activity of muscle fibers and the motor neurons that terminate at them. In crustaceans and amphibians, the chromatophores have a fixed shape. Color change comes about through the dispersal (darkening) or aggregation (lightening) of granules within the cell. This is under hormonal control.
Figure 3.22.2: Chromatopores of a Flounder courtesy of the Field Museum of Natural History