13 September 2008

'Cock-eyed' fish: how did it get that way?

The flatfish is well and truly cock-eyed. Like an animated leaf, this fish slithers over the sea bottom, with both googly eyes on top and a crooked mouth to slurp up yummy worms and other tasty titbits.
Sole eating worm
The flatfish is a fish that swims on its side. Indeed when it first hatches, a flatfish larva looks like the larva of other ‘normal’ fish, with eyes on both sides of the head. As the larva matures, it starts to swim on one side of its body. One eye moves to what becomes the upper side. There are also changes in the fin arrangement, skeleton and digestive system.

While being lopsided certain serves the flatfish well, scientists have long pondered how this came about.

Opponents of evolution have insisted that adult flatfishes could not have evolved gradually. A slightly asymmetrical skull offers no advantage.

Until this discovery of these fossil fishes! ...

Flatfish Fossils Fill In Evolutionary Missing Link
ScienceDaily 10 Jul 08

Hidden away in museums for more that 100 years, some recently rediscovered flatfish fossils have filled a puzzling gap in the story of evolution and answered a question that initially stumped even Charles Darwin.

Opponents of evolution have insisted that adult flatfishes, which have both eyes on one side of the head, could not have evolved gradually. A slightly asymmetrical skull offers no advantage. No such fish -- fossil or living -- had ever been discovered, until now.

All adult flatfishes--including the gastronomically familiar flounder, plaice, sole, turbot, and halibut--have asymmetrical skulls, with both eyes located on one side of the head. Because these fish lay on their sides at the ocean bottom, this arrangement enhances their vision, with both eyes constantly in play, peering up into the water.

This remarkable arrangement arises during the youth of every flatfish, where the symmetrical larva undergoes a metamorphosis to produce an asymmetrical juvenile. One eye 'migrates' up and over the top of the head before coming to rest in the adult position on the opposite side of the skull.

Opponents of evolution, however, insisted that this curious anatomy could not have evolved gradually through natural selection because there would be no apparent evolutionary advantage to a fish with a slightly asymmetrical skull but which retained eyes on opposite sides of the head. No fish--fossil or living--had ever been discovered with such an intermediate condition.

But in the 10 July 2008 issue of Nature, Matt Friedman, graduate student in the Committee on Evolutionary Biology at the University of Chicago and a member of the Department of Geology at the Field Museum, draws attention to several examples of such transitional forms that he uncovered in museum collections of underwater fossilized creatures from the Eocene epoch--about 50 million years ago.

"We owe this discovery, in part, to the European fondness for limestone," said Friedman. The fossils, which he found in museums in England, France, Italy, and Austria, came from limestone quarries in Northern Italy and underneath modern-day Paris.

Friedman examined multiple adult fossil remains of two primitive flatfishes, Amphistium and a new genus that he named Heteronectes.

"Amphistium has been known for quite some time," he said. "The first specimen was described more than 200 years ago, but its placement in the fish evolutionary tree has been uncertain ever since. Close examination of these fossils yield clues that they are indeed early flatfishes."

The most primitive flatfishes known, both Amphistium and Heteronectes have many characteristics that are no longer found in modern flatfish. But the one that caught Friedman's attention was the partial displacement of one eye, evident even in the first Amphistium fossil discovered over two centuries ago.

"Most remarkably," he said, "orbital migration, the movement of one eye from one side of the skull to the other during the larval stage, was present but incomplete in both of these primitive flatfishes." For both sets of fossils, the eye had begun the journey but had not crossed the midline from one side of the fish to the other.

"What we found was an intermediate stage between living flatfishes and the arrangement found in other fishes," he said. These two fossil fishes "indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature."

The Amphistium fossils were known and previously analyzed but not definitively linked to flatfish. Previous studies, relying on conventional techniques, did not detect the oddly shaped skull, but by performing CT scans on the fossils Friedman "unequivocally" demonstrated the cranial asymmetry.

Careful study of the Heteronectes chaneti fossil found that it represents a new genus. The genus name is derived from the Greek Heteros (different) and nectri (swimmer). The species, chaneti, honors Bruno Chanet, a pioneer in the study of fossil flatfish.

The two fossil sets "deliver the first clear picture of flatfish origins," said Friedman, "a hotly contested issue in debates on the mode and tempo of evolution."

Charles Darwin was baffled by what he referred to as the "remarkable peculiarity" of flatfish anatomy. "During early youth," he noted, the eyes "stand opposite to each other...Soon the eye proper to the lower side begins to glide slowly round the head to the upper side...The chief advantages thus gained seem to be protection from their enemies, and a facility for feeding on the ground."

Although the survival advantages of such asymmetry were clear, Darwin, when challenged, was unable to explain the mechanism of what appeared to be a rather sudden and radical change in morphology and suggested a Lamarckian adaptation in which the fish, through "muscular action," slowly pulled the down-side eye toward the upper side. The resulting distortion, he suggested, "would no doubt be increased through the principle of inheritance."

Darwin's explanation, which relied on the inheritance of acquired traits, preceded the discovery of genes, but geneticist Robert Goldschmidt, tackling the same flatfish issue in the 1930s, came up with a genetic explanation. He argued that such a sudden drastic change could be triggered by a single fortuitous mutation that triggered a deformity, which in some environments would prove beneficial--and then get passed on. He termed these sudden accidental evolutionary leaps "hopeful monsters," and made the mysterious origin of flatfishes the centerpiece of his argument.

Friedman's discovery eliminates the need for such optimistic accidents. It "refutes these claims of radical sudden change" he said, "and demonstrates that the assembly of the flatfish body plan occurred in a gradual, stepwise fashion."
Skull of heteronectes chaneti, showing incomplete orbital migration intermediate between generalized fishes and living flatfishes. (Credit: Matt Friedman, University of Chicago)

The research was supported by grants from the Lerner-Grey Fund for Marine Research, the Hinds Fund, the Evolving Earth Fund, the National Science Foundation and the Environmental Protection Agency.
New fossil tells twisted tale of how flatfishes ended up with two eyes on one side of head
Eurekalert 9 Jul 08;

Study eliminates a long-lingering challenge to the theory of evolution through natural selection

CHICAGO—A newly identified fossil and the reinterpretation of previously known fossils, all from Europe and about 50 million years old, fill in a "missing link" in the evolution of flatfishes and explain one of nature's most extraordinary phenomena.

All living flatfishes, which include halibut, flounder and sole, have a bizarre structural adaptation: both eyes are on one side of their head. What is even more remarkable is that every flatfish is born symmetrical, with one eye on each side of its skull. However, as it develops from a larva to a juvenile, it undergoes a metamorphosis where one eye moves (or "migrates") gradually up and over the top of the head, coming to rest in its adult position on the opposite side of the skull.

This unique specialization provides a clear survival advantage: it allows flatfishes to use both of their eyes to look up when they are lying on the seafloor. But scientists have had no idea how the forces of evolution gave rise to this curious structural adaptation because no fishes—living or fossil—with intermediate characteristics of this adaptation have ever been identified.

What might have led to this adaptation? What evolutionary advantage might intermediates have possessed? Many famous evolutionary thinkers, including Jean-Baptiste Lamarck, Charles Darwin, Alfred Russel Wallace, and Richard Goldschmidt, have had their say on this issue, but the new discoveries to be published in the journal Nature July 10, 2008, settle the matter.

"This problem of the evolution of asymmetrical flatfishes was particularly puzzling to biologists because it was very hard to explain what evolutionary forces might have led to this transition," said Matt Friedman, a research associate at The Field Museum, a graduate student at the University of Chicago and author of the study. "How can you arrive at the pattern seen in living flatfishes via gradual evolution? There seems to be no adaptive reason to start down the gradual evolutionary path toward the flatfish condition, because surely these intermediates would not have any kind of evolutionary advantage."

For this reason, flatfishes became a key example for a novel evolutionary argument that went like this: sometimes groups arise instantaneously from large-scale mutations that are usually deleterious but, under unusual conditions, might be adaptive. This "hopeful monster" scenario was invoked for flatfishes in the 1930s, and has been the popular perception of their origins ever since. But the new study in Nature, called "The evolutionary origin of flatfish asymmetry," determines that flatfishes are not "monsters" or mistakes, nor did their unusual body plan evolve suddenly.

Something old, something new

More than 500 species of modern flatfishes live in fresh and salt water. All have an unusual flattened body form that is well adapted to life at the bottom. Some families of flatfishes have both eyes on the right side of their head while other families have both eyes on the left side.

Typically, the undersides of flatfishes are white or pale, but their uppersides are camouflaged to fit in with the surroundings. Some species are able to change the color of their upperside. Weighing up to 720 pounds, these carnivorous bottom-feeders vary considerably in size from 4 inches to 7 feet. Many are important game and food fishes.

The Nature study examined several specimens of two kinds of fossil fishes from the Eocene (about 50 million years ago) of northern Italy. One of these is a newly described genus that Friedman has named Heteronectes (meaning "different swimmer"). He discovered it in a museum drawer at the Naturhistorisches Museum in Vienna, Austria.

The other fossil, Amphistium, is known from several specimens from the same Italian site as Heteronectes and a single fossil from France. It has been known to science but incorrectly classified for more than 100 years. All previous studies of Amphistium mistakenly concluded that it had a symmetrical skull.

Examining the anatomy of Amphistium and Heteronectes with a diverse range of techniques (including CAT scanning and chemical preparation, which dissolves the rock surrounding the fossil skeleton), Friedman discovered that both represent primitive flatfishes with a somewhat asymmetrical skull. Nevertheless, in both cases the eyes remain on opposite sides of the head in adults. In other words, the fossils show incomplete asymmetry, displaying an intermediate condition between what is found in ordinary symmetrical fishes and extraordinary asymmetrical flatfishes.

This discovery rejects the notion that flatfishes must have arisen suddenly as "hopeful monsters" and documents two steps in the gradual assembly of one of the most bizarre body plans found among vertebrates. The position of Amphistium and Heteronectes within the fish evolutionary tree is confirmed by many aspects of their anatomy. Features unrelated to asymmetry link these fossils with flatfishes, but the specimens also show characters more primitive than those found in any living form.

Adding a further twist to the story, the right eye migrated in some specimens of Amphistium, while the left eye moved in other specimens. This is unlike most living species of flatfishes, where individuals are always right- or left-eyed. It also indicates that that mixed 'handedness' is primitive for flatfishes.

"There is a broad implication for the tempo and mode of evolution here," Friedman concluded. "Scientists had long assumed flatfishes must have arisen suddenly because they could not imagine the adaptive significance of intermediates, but this work delivers clear evidence that such intermediates did exist, and therefore, that flatfish asymmetry arose gradually."

The fossils described in Nature give clues to the lifestyle of these primitive flatfishes and the possible evolutionary forces that might have led to their bizarre anatomy. One specimen of Amphistium preserves the remains its last meal in its stomach: a fish nearly half its own length.

"It is certain that these extinct fishes were predators," Friedman said. "Many flatfishes lie in wait on their sides to ambush unsuspecting prey, but they don't always lie flat—they often prop themselves above the seafloor with their fins. It's possible that Amphistium and Heteronectes did the same, and that even incomplete asymmetry would have given them a better view of things above and around them than no asymmetry at all."


MORE about flatfishes!

Why are flounder funny looking?
PZ Myers explains on Science Blogs

With this really snazzy diagram of the change in larval flatfishes.


Alex Schreiber studies the genes that cause this transformation with these amazing photos on his website.
A study of the swim posture changes of a larval flatfish as it undergoes metamorphosis.
A study on the remodeling of skull bone
A study on other aspects of the flatfish metamorphosis.

As I was looking around for photos of flatfish metamorphosis, I came across this one that I just had to include.
On the US National Marine Fisheries Services website was this humungous flatfish. The caption says "Pacific halibut are the largest flatfish in the ocean. The International Game Fishing Association lists a 459-pound giant taken near Dutch Harbor, Alaska, in 1996 as its all-tackle record". If I saw something like that I too would be flat on my back.

So odd, cock-eyed creatures can be successful. Those of us who don't quite fit into the definition of 'normal' should take heart.

No comments:

Post a Comment

LinkWithin

Related Posts with Thumbnails