Scientists find European eels adapt to different environments with physiological not genetic changes

European eels have long fascinated biologists because of their complex life cycle and diverse habitats, ranging from north of the Arctic Circle to North Africa in the south, the Azores in the west to the Black Sea in the east. 

It's been known for a century that European eels spawn in the Sargasso Sea, a large region of the north Atlantic Ocean, stretching roughly from Bermuda to the Canary Islands.

The question scientists have asked is whether European eels developed into genetically distinct subpopulations depending on their particular ecological conditions, such as freshwater or brackish coastal waters. Or could spawning in particular areas of the Sargasso Sea, which is 2,000 kilometers wide, at certain times of the year have produced subpopulations that migrated to particular destinations.

A group of researchers at Uppsala University in Sweden answered this question conclusively in a research paper published in the Proceedings of the National Academy of Sciences (PNAS), Jan. 26.

The research group used whole-genome analysis to determine that all European eels belong to the same genetic population, no matter where they spend their adult life. 

"Here we conclude, based on whole genome sequencing, that there is a complete absence of genetic differentiation among eels from different parts of Europe and North Africa," the group wrote. 

The European eel is a single panmictic population, the scientists concluded, randomly mating with each other regardless of where they spend their adult life. 

Instead of genetic variation according to very diverse ecological conditions, the authors proposed that European eels respond to the vastly different climates they inhabit by physiological changes without genetic changes or phenotypic plasticity. 

The authors contrast the European eel to the Atlantic herring, which has developed genetically different subpopulations in response to different geographic and climate conditions. They attribute this different development to the fact that all European eels spawn in the Sargasso Sea, while Atlantic herring spawn in different locations under diverse conditions and seasons.

Where the European eel and the Atlantic herring spawn may determine local genetic adaptations, the authors write, because the "most sensitive period of life for a fish is during embryonic development and early larval life." They suggest that for the European eel "constraints imposed by spawning and embryo development in the Sargasso Sea apparently preclude reproductive isolation between continental subpopulations."

In an Uppsala University news release, study leader professor Leif Andersson from Uppsala and Texas A&M University said: "An important topic for future studies is to explore how eels are able to cope with such diverse environmental conditions. It is likely that eels for millions of years have had a life history where spawning takes place under very similar conditions whereas most part of the lifecycle takes place under diverse environmental conditions. Mechanisms for handling this challenge may thus have evolved by natural selection."