Particular kind of coloration, in which animals (not only fishes) have a darker back and a brighter belly. To a certain degree it camouflages the animal: Seen from above - the air- it does less show up against the usually darker bottom of the water, or in deeper water against its dark blue colour. In relatively shallow water with a sandy, more or less bright bottom, the back in consequence is less dark than over deeper, darker ground, respectively. Seen from below, a fish with a white or silvery belly does less show up against the bright surface of the water. Fishes with a pronounced countershading usually are diurnal; nocturnal species, which during the day hide below the river banks, usually are dark brown to black.