Natures wonders: eels migrate 5,000 kilometers to mate (Introduction)

by David Turell @, Wednesday, August 26, 2015, 01:00 (1246 days ago) @ David Turell

A very strange story of speciation and travelling go mate:

"The eel's life story is difficult to understand from an evolutionary perspective.

"Take an eel in a Danish stream -- when it is time to mate, they do not choose the easy option and find an eel in the same river to breed with.

"Instead, they embark on a perilous 5,000 kilometre journey to spawn in the Sargasso Sea.

"The same goes for eels from all across Europe, and in this way they comprise a single population of European eels that all go to breed on the other side of the Atlantic.

"After the breeding season is over, the new eel larvae float all the way back to Europe, following the Gulf Stream and other ocean currents. A few years later, they settle in coastal waters, rivers, and lakes. They can be found in Iceland and Norway in the north, all the way down to Morocco in the south, and this epic life cycle begins all over again.

"But the Sargasso Sea is not the reserve of European eels -- American eels also breed there, and the two breeding areas overlap. Sometimes the two species also breed with each other, which has been difficult to explain.


"In another study, scientists looked at the genetic differences between American and European eels that could mark them out as two separate species.

"They analysed more than 300,000 genetic markers throughout the entire genome of European and American eels.

"The genomes were virtually the same, except for a couple of differences in a few very specific parts -- those affecting growth and metabolism.

"An illustration to show how the American eels grow (starting at 10 mm and growing up to 45 mm) whilst floating north from their breeding grounds in the Sargasso Sea. (Illustration: Uwe Kils)

"This makes sense, says Hansen, as the American eel only exists as larvae for six to nine months, while the European eel's larval stage can last for a few years.

“'American eel larvae only float with the Gulf Stream 1,500 kilometres to reach the nursery areas along the North American coast. European eels, on the other hand, make a journey of more than 5,000 kilometres,” he says.


"Although the American eel and the European eel are two different species, they can still breed with one and other, which they do from time to time.

"Curiously, these offspring do not end up in mainland Europe or North America, but in Iceland where they make up over ten per cent of the Icelandic eel population.

"Hansen and his team believe they know why.

"'We believe that the larval phase of these hybrid offspring is somewhere between that of the European eel and the American eel. They come out of the larval stage as they float along with the Gulf Stream in the middle of the Atlantic Ocean. So there is only one place they can settle down and survive, and that is Iceland,” says Hansen."

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