Flatfish skin contains chromatophores whose pigment granules can be dispersed to darken the skin or aggregated toward the cell centre to lighten it. By varying dispersion across a mosaic of these cells, a plaice can shift its overall tone and, more slowly, its coarse pattern to approximate the gravel or sand it is resting on.
The control loop runs through the eyes. Experiments on flatfish going back over a century show that the response depends on what the fish sees of the surrounding substrate rather than on direct light falling on the skin, and that individuals prevented from seeing the ground fail to match it. The fish also buries itself, flicking sediment over its margins with its fins, which does much of the concealment work that colour alone cannot.
The matching is approximate, not photographic. Popular accounts sometimes describe flatfish reproducing chequerboards exactly; what is reliably demonstrated is a good approximation of tone and grain, achieved within minutes rather than instantly, and the neural and hormonal control of the finer patterning is still incompletely described.