How flatfishes evolved


This video is called Alaska Halibut: Underwater Footage.

From the Field Museum in the USA:

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.

See also here. And here.

Big Dutch sole caught: here.

3 thoughts on “How flatfishes evolved

  1. 149-kg halibut caught in western N.L.

    Last Updated: Friday, July 10, 2009 | 4:20 PM NT

    CBC News

    Sam LaCosta, Blair Payne, Roger LaCosta and Tracy LaCosta show off the 149-kilogram halibut they caught in the waters of Bay St. George on June 24.Sam LaCosta, Blair Payne, Roger LaCosta and Tracy LaCosta show off the 149-kilogram halibut they caught in the waters of Bay St. George on June 24. (Photo courtesy of Tracy LaCosta)Times may be gloomy for the fishing industry, but a crew from Campbell’s Creek, N.L., recently found one very big reason to smile — they landed a 149-kilogram halibut.

    Roger LaCosta, his brother Sam, niece Tracy and her boyfriend Blair Payne were out at dawn in the waters of Bay St. George on June 24, the second day of a commercial fishing trip.

    After trawling the waters for most of the day, in the afternoon the four encountered particularly strong resistance on one net.

    Suspecting they had caught a large fish, they started to reel the net in by hand. When it became clear how massive the ensnared fish was, they let it swim to the bottom of the bay to tire itself out.

    After bringing the fish back up to the surface, the four managed to get a rope around its mouth and up through the gill.

    It took the combined strength of all four to haul the 149-kilogram fish over the stern of their boat.

    Halibut caught in Newfoundland typically weigh about nine kilograms. Sam LaCosta, who has been fishing for 36 years, says it’s the biggest fish he’s caught.

    The four brought the fearsome fish to shore at Little Port Harmon and later sold it.

    Surprisingly, they would have made more, pound-for-pound, if it was smaller — bigger fish are of lower quality. They finally ended up selling the halibut for $492.

    http://www.cbc.ca/canada/newfoundland-labrador/story/2009/07/10/nl-big-halibut-catch511.html

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  2. Those delicious flatfishes, like halibut and sole, are also evolutionary puzzles. Their profoundly asymmetrical heads have one of the most unusual body plans among all backboned animals (vertebrates) but the evolution of their bizarre anatomy has long been a mystery. How did flatfishes, with both of their eyes on one side of their head, evolve? So puzzling was the anatomy of flounders and their kin that they were used in early arguments against Darwin and his theory of natural selection. Skeptics wondered how such unusual features could have slowly evolved whilst remaining advantageous for the fishes’ survival.

    A new fossil discovery described in the latest issue of the Journal of Vertebrate Paleontology by Oxford University researcher Dr Matt Friedman finally solves the mystery. Friedman’s fossil fish, named Heteronectes (meaning ‘different swimmer’), was found in 50 million year old marine rocks from northern Italy. This study provides the first detailed description of a primitive flatfish, revealing that the migrated eye had not yet crossed to the opposite side of the skull in early members of this group. Heteronectes, with its flattened form, shows the perfect intermediate stage between most fish with eyes on each side of the head and specialized flatfishes where both eyes are on the same side.

    “This fossil comes from Bolca in northern Italy, a site that has literally been mined for hundreds of years for its fossil fishes. This remarkable site provides a snapshot of an early coral reef assemblage. Reefs are well known as biodiversity hotspots, so it is perhaps not surprising that Bolca provides us with the first evidence of many modern fish groups,” said Friedman. “Our understanding of the relationships of some of these groups is in a state of change with the increasing influx of molecular genetic studies. Fossils have not contributed very much to this debate, but specimens like that of Heteronectes reveal the superb level of detail that can be extracted from extinct species.”

    http://www.eurekalert.org/pub_releases/2012-06/foas-mot061912.php

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  3. Pingback: Dinosaurs extinct, fish survived | Dear Kitty. Some blog

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