Migrating Eyes and the Fascinating World of Flatfish Design

If most people were asked to draw a quick picture of a fish, they would likely draw a torpedo-shaped creature with some fins, an eye on either side of its head, and a mouth at the tip. And for the most part, they would have pretty accurate representation. But there are some fish species in the world that don’t fit into this general shape. Flatfish, like flounders and halibut, take the fish design world and turn it on its ear.

You see, when young flatfish hatch, they look like most other fish. They appear to be tiny bullets, swimming up in the water column. But when it is time for them to mature, they go through a transformation that is both cool and creepy. In fact, flatfish puberty makes the human teenage-years look like a piece of cake!

When a flatfish begins to mature, the bones in its skull twist and move around. One eye starts to MIGRATE from one side of its head to the other side so that it has two eyes on the same side of its face. The fish starts to swim in a tilted manner. Additionally, the newly eye-less side of its body bleaches out, while the double-eyed side takes on a patterned coloration so that it blends with the ocean bottom. At the completion of this metamorphosis, the flatfish no longer swims in the water column but settles out to the floor. This way, the colored, two-eyed side faces up, and the blind, bland-colored side faces down. Its transformed head and body enable it to live this way, feeding as a bottom-dweller for the rest of its life.

So we can say that flatfish are extraordinary creatures.

But they help us understand more. Flatfish create a stumbling block for evolutionary theory. Evolutionary theorists try to explain how and why these assymetrical organisms came to be. They think that today’s flatfish evolved from bilateral (commonly shaped) fish. Yet they have a hard time explaining what drove those fish to evolve from a perfectly good torpedo shape to a re-formed sideways pancake, and they didn’t have any examples of fossilized transition organisms.

In recent years, scientists tried to explain flatfish evolution by comparing them to fish fossils found in a petrified coral reef in Italy.1 The fossils had asymmetrical skulls and one eye on the top of the fishes’ faces. They appeared to be pre-flatfish, somewhere between regular fish and modern flatfish.

But there is one problem.

These newly-discovered fossils are found in the same rock layers as the fish they should have evolved into! These “pre-flatfish”, then, are not really evolutionary transitions. If they were, then according to evolutionary theory they should have been found in rock layers lower than modern flatfish. So how can we explain the fossils in Italy?

Well, there is a great diversity among flatfish species, with some fish families exhibiting a right-migrating eye, others exhibiting a left-migrating eye, and still others with eyes migrating in either direction! These fish have internal genetic mechanisms that make them asymmetrical on the inside from birth, even though they appear symmetrical when they are young. Metamorphosis and "face transformation" is just a regular part of life for these guys. That means we should expect to find specimens that were fossilized during transition or even species with a different form of eye migration. That doesn’t prove they are transitions between symmetrical fish and asymmetrical fish.

What we can say is that flatfish are truly unique creatures that are part of the vastly diverse group of ocean organisms in the world.

1Friedman, M. 2008. The evolutionary origin of flatfish asymmetry. Nature. 454: 209-212