You know it's particularly mysterious when a puzzle stumps Charles Darwin and all the egghead evolutionary biologists that have come since. But, how both eyes of flatfish came to be on the same side of the head was such a bafflingly mysterious puzzle that it needed to be solved twice. By the same person.
|The turbot, Psetta maxima (image Wikipedia)|
But, behind every popular science article beat-up of stumped boffins and puzzling riddles, there's usually some interesting science. And that's the case here.
Flatfish are fascinating creatures. Adults live on the bottom, lying on one side, with both eyes gazing up from the same side of their head. At hatching, though, their larvae look unremarkable in comparison to other fish larvae. Their eyes are on opposite sides of their head and they swim vertically. But, late in their larval development one eye begins to migrate upwards and over the top of the head until it sits near the other eye.
Far from being stumped, several scientists put forward their explanations, including Darwin. Saltationists, such as Goldschmidt, saw it as evidence that some speciation events were the result of large mutations that revolutionised morphology. While others thought that the eye must have gradually migrated, as it does at the end of the larval period.
The evidence seems to have been more strongly in the gradualist camp. And not only because the new synthesis largely killed off the idea of saltation in evolution. It was already known that the more ancestral flatfish groups, such as spiny turbot and flounder, were less asymmetrical and less strongly associated with the bottom than the more derived groups, such as sole. The only thing that was lacking was truly smoking gun evidence.
|Three species of flatfish. From top to bottom, the spiny turbot, Psettodes belcheri, the flounder, Citharus linguatula, and the sole, Achirus klunzingeri. As you move top to bottom, the wandering eye moves further down the head (Pictures from FAO, via FishBase).|
|The two sides of the fossil fish Heteronectes chaneti. Note the eye on the left side (right hand image) is higher than the eye on the right (from Friedman 2012).|
Interestingly, Amphistium and Heteronectes were alive at them same time as flatfish with the modern asymmetrical morphology. Which indicates that they aren't the direct ancestors of the modern flatfish and that the origins of flatfish are much older. This, in turn, suggests that the transitional morphology provided some advantages, since it persisted for so long in the presence of more modern eye arrangements.
|The fossil flatfish Eobothus that was alive at about the same time as Amphistium and Heteronectes, but, like modern flatfish, had both eyes on the same side of its head (image the Fossil Forum).|
|The European plaice, Pleuronectes platessa, using its dorsal and anal fins to lift itself off the bottom (image EOL).|
The analysis suggested that the Latids are the most closely related family of fish. But, Friedman cautions that his analysis was necessarily coarse. Some of the characters identified as uniquely shared by the Latids and Heteronectes may actually be general to a larger group of fish. And, because Friedman didn't examine other flatfish in the study (he must have another paper in the works), the characters identified in Heteronectes may not be shared with other flatfish.
So, two interesting papers. But, although we now know that evolution of the asymmetrical flatfish eye was gradual and, therefore, that transitional flatfish morphologies clearly were not useless, a lot of questions remain. For instance, we can only speculate about the selective pressures that drove eye migration and we don't yet know what the flatfish common ancestor looked like.
Friedman, M. (2008). The evolutionary origin of flatfish asymmetry Nature, 454 (7201), 209-212 DOI: 10.1038/nature07108
Friedman, M. (2012). Osteology of †Heteronectes chaneti (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships The Journal of Vetebrate Paleontology, 32 (4), 735-756 DOI: 10.1080/02724634.2012.661352
Martinez, G. M. and Bolker, J. A. (2003). Embryonic and Larval Staging of Summer Flounder (Paralichthys dentatus) Journal of Morphology, 255, 162-176