A pigment-containing cell, or in octopus, squid and cuttlefish a small organ, that an animal uses to change the colour of its skin quickly and reversibly.
In fish, amphibians and reptiles a chromatophore is a single branched cell that keeps its shape while pigment-filled organelles are gathered to its centre or dispersed along its branches. In cephalopods it is an organ: an elastic pigment sac pulled open by radial muscles under direct nervous control. Iridophores are the related cells that reflect light from stacked platelets or crystals instead of absorbing it with pigment.
A chameleon's colour change is mostly not pigment moving. Panther chameleons shift colour by changing the spacing of guanine nanocrystals in their iridophores, which is a structural colour being retuned.
A cuttlefish changes its whole body pattern in well under a second by contracting the muscles around thousands of pigment sacs.
It separates fast, reversible colour change from the slow kind, in which pigment or cells are added or lost over days or weeks, and from seasonal moults, in which the coloured tissue is replaced.
Chromatophores are studied by imaging the skin and by measuring the light it reflects. For camouflaged cuttlefish, hyperspectral images taken on natural substrates were analysed with models of the two- and three-cone vision of fish predators, to ask how good the match is to the eye it is aimed at. For structural colour change the measurement is made on a single reflector: in the neon tetra, the spectrum and the direction of light reflected by one stack of platelets were recorded together. Speed separates the mechanisms too: a muscle-driven cephalopod organ responds in a fraction of a second, while pigment granules carried inside a fish cell take seconds to minutes.
An article describes any colour-changing animal as doing 'what a chameleon does'.
Three different mechanisms are run together: sacs of pigment stretched open by muscle in cephalopods, granules moved inside fixed cells in fish and frogs, and reflecting crystals respaced in chameleons. They differ in speed, in control and in what colours they can reach.
A cuttlefish's colour match to its background is taken as proof that it sees colour.
Cuttlefish are reported to be colour-blind, even though the match is good as modelled for fish vision. How a colour-blind animal achieves it is unresolved, so the match cannot be used as evidence about the animal's own eyes.