The puzzle of how the asymmetrical eyes of flatfishes have evolved has been resolved.
All flatfishes have asymmetrical skulls, where both eyes are located on one side of the head.
This allows the fish to maintain the use of both eyes in vision as they lie on their sides on the sea floor.
Larval flatfishes start out with symmetrical skulls (i.e. with both eyes on either sides of the head) like any other fish, but as the fish grows, one eye ~migrates up and over the top of the head to end up directly next to the other eye on the opposite side of the head in the adult fish.
Opponents of evolution have long argued that the asymmetrical eyes on the adult flatfish could not have evolved gradually, citing the lack of apparent evolutionary advantage to a fish with an asymmetrical skull but with eye on both sides of the head; recently published research has finally uncovered such evidence.
Publishing his results in a recent issue of the journal Nature, Matt Friedman of the University of Chicago describes a new genus and species of fossil flatfish (Heteronectes chaneti) that is thought to be the transitional form between flatfishes and other (more symmetrical) fishes.
The new genus is named after a disused collective name for flatfishes (Heterosomata), as well as the incomplete orbital migration characterizing this taxon (Greek heteros, different; nectri, swimmer).
The specific name honours Bruno Chanet for his pioneering studies on fossil flatfishes.
Friedman also showed that another fossil fish (Amphistium) whose evolutionary relationships were unclear is actually a fossil flatfish.
Both Amphistium and Heteronectes have an aymmetrical skull in which one eye is displaced upwards, with both eyes remaining on opposite sides of the head. Although Amphistium has been described for more than 200 years, the nature of its asymmetrical skull was not known until the author confirmed it using CT scans.
According to Friedman, What we found was an intermediate stage between living flatfishes and the arrangement found in other fishes, with the two fossil fishes indicating ...that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.
The evolutionary advantage of a displaced eye is unclear, but according to the author, modern-day flatfishes often prop their bodies above the substrate by depressing their dorsal- and anal-fin raysSimilar behaviour might have permitted Amphistium and Heteronectes"both of which have long median-fin rays"the use of both eyes while on the sea floor.
For more information, see the paper: Friedman, M (2008) The evolutionary origin of flatfish asymmetry. Nature 454, pp. 209"212.