Eye migration in flatfish
As a young sole or plaice grows, one eye moves across the head so that both end up on the upper side.
Also: metamorphosis of flatfish
Sole, plaice, halibut, turbot and flounder hatch as ordinary upright larvae with an eye on each side. After a few weeks one eye begins to move over the top of the head while the skull twists, and the young fish settles on the sea floor lying on its blind side. The upper side becomes coloured, often able to change pattern to match sand or gravel, and the lower side stays pale. Whether the eyes end up on the right or the left is usually fixed for each species, which helps in identification.