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Thursday, 10 July 2008 21:12

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

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.

Dr. Friedman studied fossils that were found in storage in two European museums.

Each fossile was about 45 to 50 million years old. Friedman examined fossils of a new species of flatfish in the Natural History Museum in Vienna, Italy. The new species is called Heteronectes chaneti (“different swimmer”) by Friedman.

In the Heteronectes species, Friedman explained, “When I first noticed the fossil, it was sitting unidentified in a drawer of indeterminate fossil fish pieces from Monte Bolca [in Italy]. And believe me, it didn’t look like much at the time — it was an incomplete specimen covered with dust and soot." [Nature News]

The other species was previously already named Amphistium. It was found in the Natural History Museum in London, England.

The Amphistium species was previously recorded as having a symmetrical skull and a “flatfish-like” appearance, being distorted about the eyes but with the distortion thought to be caused by the fossilization process.

However, Friedman looked more closely to find that one eye was located just slightly off from where it should--if it had a totally symmetrical head. Thus, Friedman used computed tomography (CT) scans to verify what he saw.

He found that the one eye had begun to move but had not yet moved to the middle of the head. He exclaimed "It's not quite in the Cyclops position,” referring to a member of a primordial race of giants, in Greek mythology, with a single eye in the middle of its forehead.

In this position, Friedman says that the species did not lie flat on the seabed but propped itself up with its fins. Friedman proposes that after the fish found this slightly propped positionto be good for finding food it evolved further.

However, Friedman cautions about the use of assumptions, "Our inability to imagine is what got us into this predicament.” [Reuters: “Fish fossils plug hole in evolutionary theory”]

What are the conclusions of the Friedman study and why is it important to the theory of natural selection (evolution) as first proposed by Charles Darwin? Please read page three.

The conclusions of the Friedman study shows that the evolution of the flatfish was gradual, not quick as earlier studies explained.

Friedman states, "The important thing about this study is it delivers evidence of those intermediates.” [Reuters: “Fish fossils plug hole in evolutionary theory”]

For a long time evolutionary biologists could not explain how these flatfish came to have both eyes on one side of their heads. They already knew from fossil records that very early on in their development these flatfish had one eye on each side of their head.

Some studies in the past even suggested that the change from symmetry to asymmetry was done very quickly. This, “macromutation” type of evolution (fast acting change) was thought to occur all at once with a quick type of mutation.

Friedman stated, "There was no macromutation that all of a sudden gave them both eyes on the same side of the head.” [Reuters]

The point that the theory of natural selection (evolution) was missing was “transitional species.” Scientists had yet to find transitional fossils that showed the intermediate step in the evolution of a particular characteristic.

Until now, that is.

In the case of the flatfish, scientists had fossils of primitive flatfish species that showed one eye on each side of the body. And, they also have modern specimens living today on Earth.

 However, they could not find the “transitional species” for the flatfish that showed the transition in the trait.

Until now, however.

Plus, many scientists contended that such “transitional species” could not exist because it would be a detriment to them, and they would not likely survive.

The research by Friedman disproves this contention. It also helps to provide the evidence for "transitional species" in the process of evolution, and that transitional species did exist and survive to evolve even further.

The article “The evolutionary origin of flatfish asymmetry” summarizes the conclusions of this study. It is found in the July 10, 2008 issue of the journal Nature. (Nature 454, 209-212)

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