The reason for this arrangement is the strategy which such fish have adopted in order to eat and avoid being eaten. This requires them to lie flat on the seabed, retaining full vision upwards but minimizing their own visibility to possible predators above them. The latter requirement is met by camouflage; the coloration of their backs matches the seabed with uncanny exactness, and they are in any case often half-buried in the sand. They can ‘disappear’ in this way with remarkable speed.
A fisherman cried, ‘Look out, they’ll sand!’ a capital expression for I found that the fish sunk into the sand with such rapidity that the operation must be seen to be believed. The plaice lifts up its head and the upper third of its body and then brings it down on the sand three or four times with sharp, quick raps; a small cavity is thus made in the soft, wet sand, which at once fills with water; the fish then works its fins on each side of its body with such a rapid motion that they seem almost to vibrate. These combined efforts enable the fish to conceal itself almost quicker than the eye can follow, and nothing can be seen but its eye, which is of a lovely emerald colour.