I see it now: U.S. scientist shows evolution of flatfish

by William Atkins

Friday, 11 July 2008

Page 1 of 3 ! An American evolutionary biologist has finally explained how some strange-looking species of flatfish, such as flounder, came to have both of their eyes on one side of their head. The discovery helps to clarify a missing point in Darwin’s evolutionary theory. Flatfish are an order (Pleuronectiformes) of ray-finned fish. The name means “side-swimmers” in the Greek language. What is very interesting in many species of flatfish is their physical feature of having both eyes on one side of their heads, rather than the common arrangement of having one eye located on each side. Dr. Matt Friedman, from the Department of Geology at The Field Museum and the Committee on Evolutionary Biology at the University of Chicago (both in Illinois, U.S.A.) is the researcher performing this scientifically important study.   He states in the abstract to his paper, “All adult flatfishes (Pleuronectiformes), including the gastronomically familiar plaice, sole, turbot and halibut, have highly asymmetrical skulls, with both eyes placed on one side of the head. This arrangement, one of the most extraordinary anatomical specializations among vertebrates, arises through migration of one eye during late larval development.” Because flatfish spend most of their time on the bottom of water (seabed), they often are seen with one of their sides upwards (as related to the seabed) and the other side downward. Different species have been seen with different sides (left or right) facing the seafloor. This unique situation allows them to use both eyes while skirting the bottom of seas with one of their sides (the one with both eyes) facing the seabed. Some commonly known flatfish are flounders, halibut, and soles. In all, four to five hundred species of flatfish are known to exist on Earth, both in fresh and salt waters. According to this new study, Friedman has discovered from fossils of primitive flatfish that they started out early in their evolution with a symmetrical look, with one eye on each side of their head, and maintained that look throughout their lives. However, over millions of years of evolution, they gradually moved one eye to the other side from birth to adulthood, joining the second eye on its side of the head for an asymmetrical looking head. This process actually occurs during development of each fish. They are born with an eye on each side of their body, but as they grow one eye moves to the other side, so that by the time they are adult, both eyes are on one side of the head. Please check out the video that shows the evolution of this strange looking fish at the Nature News website “The eyes have it: Fossilized flatfish settle evolutionary conundrum.” In addition, if you have a subscription to Science News, its website “A wondering eye” shows some dramatic videos in the evolution of the flatfish. The theory of evolution has been criticized in some circles for not completely explaining the evolution of species. Matt Friedman states, also in his abstract, “Although the transformation of symmetrical larvae into asymmetrical juveniles is well documented, the evolutionary origins of flatfish asymmetry are uncertain because there are no transitional forms linking flatfishes with their symmetrical relatives. The supposed inviability of such intermediates gave pleuronectiforms a prominent role in evolutionary debates, leading to attacks on natural selection and arguments for saltatory change.” Thus, he conducted this study as he explains, “Here I show that Amphistium and the new genus Heteronectes, both extinct spiny-finned fishes from the Eocene epoch of Europe, are the most primitive pleuronectiforms known. The orbital region of the skull in both taxa is strongly asymmetrical, as in living flatfishes, but these genera retain many primitive characters unknown in extant forms. Most remarkably, orbital migration was incomplete in Amphistium and Heteronectes, with eyes remaining on opposite sides of the head in post-metamorphic individuals. This condition is intermediate between that in living pleuronectiforms and the arrangement found in other fishes. Amphistium and Heteronectes indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.” How did Dr. Friedman perform his studies on the fossils of two flatfish species? Please read on. << First page <   1 2 3 Next page > Last page - Post your comment >>

 

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