Some years ago I was idly watching some fish swimming about in a clear park stream, the bottom of which was peculiarly marked with large areas of contrasting light and dark spots.
As the fish swam about searching for food, they would occasionally stop for a short time over one of these areas and within a few minutes, much to my amazement, they would become almost invisible. I watched for an hour or more, and they always changed color to harmonize with the background.
Many species of fish have, this power of color change but some are much more adept than others. Some of the flatfish, including the flounders and the turbots, are probably the champion blenders of the fish world.
Experiments have shown that if they are placed even on a checkboard background, within a short time it is almost impossible to tell where background ends and fish begins.