Chromatophore(redirected from Iridophores)
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A pigmented structure found in many animals, generally in the integument. The term is usually restricted to those structures that bring about changes in color or brightness. A majority of chromatophores are single cells that are highly branched and contain pigment granules that can disperse or aggregate within the cell. However, in coleoid cephalopod mollusks (all mollusks except Nautilus), the chromatophores function as miniature organs, and changes in the dispersion of pigment are brought about by muscles. Although the mode of action of the two types of chromatophore is completely different, the effect is the same: pigment either is spread out over a large area of the body or is retracted into a small area.
The movement of pigment takes place in many chromatophores simultaneously, so that the effect is a change in the quality of light reflected from the surface of the animal. The color change functions as a camouflage from predator or prey, but it may also serve for regulating temperature, protecting against harmful radiation, and in signaling. Light stimulates the responses of chromatophores, generally indirectly via the eyes and central nervous system.
Single-cell chromatophores are found in some annelids, insects, and echinoderms. They are much more conspicuous in crustaceans (shrimps and prawns), in fishes (especially in bony fish and teleosts), in anuran amphibians (frogs and toads), and in a few reptiles. The chromatophores may be uniformly distributed in the skin (chameleons), or they may occur in patches (flounders) or lines (around the abdomen in shrimps). Chromatophores of various colors may be distributed unevenly across the body, and occur at different depths in the skin.
Chromatophores produce their colors by reflection after absorption of light. Generally, the light comes from above, but it may come from below after reflection from an underlying structure. The most common type of chromatophore contains melanin (and is, therefore, often called a melanophore), which absorbs all wavelengths so that the chromatophore appears black; other types have red (erythrophores) or yellow (xanthophores) pigments. These pigments generally derive from carotenoids in vertebrates.
Chromatophores contain pigment granules that move within them, giving them an appearance that ranges from spotted to fibrous on the five-stage scale that is widely used to measure the degree of chromatophore expansion. If the pigment within the particular cell is black or brown, the integument takes on a dark appearance when most of the chromatophores are in the last stage of dispersion (stage 5). If the pigment color is yellow or cream, the animal tends to look paler if all the chromatophores are at that stage.
In crustaceans, elasmobranch fishes, anurans, and lizards, control of the chromatophores is thought to be exclusively hormonal. Such hormonal control is true also of some teleosts; in others the control is part hormonal and part neural; while in still others control is purely neural, as in the chameleon. Where nerves are involved, the speed of the response is faster, the chromatophores responding in minutes rather than hours. See Neurosecretion
Each cephalopod chromatophore organ comprises an elastic sac containing pigment granules. Attached to the sac is a set of 15–25 radial muscles that are striated and contract rapidly. Associated with the radial muscles are axons from nerve cell bodies that lie within the brain. Active nerve cells cause the radial muscles to contract and the chromatophore sac expands; when the nerves are inactive, energy stored in the elastic sac causes the chromatophore to retract as the muscles relax. The chromatophores receive only nerve impulses, and there is no evidence that they are influenced by hormones. The chromatophores are ultimately controlled by the optic lobe of the brain under the influence of the eyes.
Two consequences follow from the fact that cephalopod chromatophores are under the direct control of the brain. First, color change is instantaneous. Second, patterns can be generated in the skin in a way impossible in other animals. Thus, cephalopods can use the chromatophores not just to match the background in general color but to break up the body visually (disruptive coloration) so that a predator does not see the whole animal. Because the chromatophores are neurally controlled and patterns can be produced in the skin, they can also be used for signaling. See Pigmentation, Protective coloration
(1) In animals and humans, a pigment cell.
(2) In plants, an organelle of brown and green algae that may be filamentous (as in Spirogira) or stellate in form. Like the chloroplasts of higher plants, chromatophores are separated from the cytoplasm of the cell by a two-layered protein-lipid membrane. They contain chlorophylls, carotenoids, and other substances. Photosynthesis occurs in chromatophores.
(3) In microorganisms, an organelle of photosynthesizing bacteria, usually not separated from the cytoplasm by a membrane. Chromatophores contain bacteriochlorophylls, carotenoids, and a number of electron carriers, as well as enzymes that help synthesize pigments. Photosynthesis occurs in them.