Why do flounder eyes grow on the same side?

People are familiar with the peculiar appearance of flatfish: instead of having eyes on both sides of the body like ordinary fish, their eyes are on the same side of the body. Ancient people believed that male and female flatfish swam side by side, and when swimming, the eyes of the two fish were close to each other. Therefore, there is a folk saying “as closely associated as mandarin ducks” to describe them. In reality, flatfish live alone on the seabed; their flat bodies lie parallel to the water surface, and both eyes are located on the upper side.

Why do flatfish have both eyes on the same side of the body? This is formed during their development. When young fish hatch from eggs, they are no different from other fish, with their eyes properly positioned on both sides of the head. At this time, they are very active, constantly floating to the water surface to play. However, when they are about twenty days old and their bodies grow to about one centimeter in length, due to the imbalance in the development of various parts, they can no longer swim normally and instead lie sideways on the bottom of the water. It is at this point that their eyes begin to move; the uneven growth of the bones on both sides of the brain, especially the frontal and parietal bones, becomes more prominent. The eye under the body moves upwards due to the continuous growth of the soft band below the eye, reaching the upper side and aligning with the original eye on that side. At this moment, the eye socket bone also forms. Afterward, the position of the eyes no longer changes.

This appearance of flatfish is well-suited for living on the seabed. The side of their body with eyes is brownish-gray, making it inconspicuous, while the underside is white or light yellow. This allows them to avoid the sight of enemies and easily obtain food.