The mystery of eels

Moray eel with fish hook caught in its mouth

By BBC News Online's Helen Briggs

A long-held belief that eels from all over the world migrate to the Sargasso Sea to breed in one big melting pot has been exposed as a myth.

Once they have made their long journey across the Atlantic Ocean, the snakelike fish prefer to breed with others from their native regions, say researchers who have carried out genetic studies on the creatures.

The Sargasso Sea is known as an international breeding ground for eels. The fish come from Europe, the Mediterranean and the United States to mate, spawn and die.

Stick together

Since the 1920s, it had always been thought that eels mingled and mated randomly in the Sargasso Sea. But now a detailed genetic analysis shows that European eels, at least, tend to stick together.

Using DNA techniques used in forensic investigations, the scientists, based in Canada, spotted genetic differences between the slippery creatures. They say eels that live relatively close to each other, such as in Mediterranean waters, are more alike than their Baltic and North Sea cousins.

Dr Thierry Wirth from the Universite Laval, Ste-Foy, Quebec, said the team's work overturned a popular theory.

"The European eel [was] believed to consist of a single, large population [panmictic]," he told BBC News Online.

Epic journey

"In our letter to Nature, we show that the European eel is not truly panmictic because we detected genetic differentiation between different samples."

However, some mysteries remain. The scientists still do not know how the eels' larvae find their way back to European coasts.

"Salmon migrate back to a place they have been at birth and could have registered environmental parameters specific to this place," said Dr Wirth.

"But in eels, the leptocephaly [larvae] have to find a place where they never were, thousands of miles away from their nursery."