Most asymmetrical vertebrates

The most asymmetrical vertebrates are the flatfishes, of global oceanic distribution and belonging to the taxonomic order Pleuronectiformes. As larval forms, flatfishes are bilaterally symmetrical like other fishes, and their bodies are laterally flattened. During this phase, they live above the seabed (pelagic), but as their development towards the juvenile state advances they lose their swimbladder, sink to the seabed, and become uniquely asymmetrical. One eye migrates across the head until it lines up alongside the other eye, becoming inclined towards the twisted mouth, but the flatfish's fins remain laterally flattened (rather than becoming dorsoventrally flattened like those are of genuine dorsoventrally flattened fishes, such as rays and skates). Moreover, the body side bearing both eyes now becomes the fish's dorsal (upper) side and remains pigmented, whereas the body side lacking eyes now becomes the fish's ventral (under) side and loses its pigmentation.

In some species, the two eyes usually or always lie together on the right side of the fish, whereas in certain others they usually or always lie together on the left side. Moreover, there are some species, including the most primitive flatfishes alive today, in which both versions commonly occur – some individuals have eyes that are right-sided, the others left-sided.

There are several fossil species that collectively show the gradual evolution and development of this extraordinary asymmetry, and indicate that it originated as random occurrences but subsequently became fixed, suggesting that it conferred some survival advantage.