Researchers have found that European eels can sense magnetic fields and may use this ability to navigate thousands of miles through the Atlantic Ocean.
Eels have always been mysterious animals. More than 2,000 years ago, Artistotle proposed that they were born spontaneously from mud.
"I think it's fascinating because as humans we've been pondering the life history of eels for a long time," says Lewis Naisbett-Jones, a graduate student in marine biology at the University of North Carolina, Chapel Hill.
He's one of the humans that, all these years later, is still pondering eels.
No one has ever seen European or American eels, as these two species are known, spawn or collected their eggs in the wild. But it's now known that the vast majority of eels in Europe and North America are born in the same place — deep in the Atlantic Ocean in a warm spot northeast of Cuba known as the Sargasso Sea.
After they're born, the baby eels make an epic journey, sometimes traveling thousands of miles to their new homes in rivers and lakes from North America to North Africa to Scandinavia.
"It's pretty crazy," says Naisbett-Jones, who at the time of the study was at Aberystwyth University in the U.K. He wanted to know how those little eels do it.
Other scientists had found that eels can somehow sense the Earth's magnetic field, an ability dubbed "magnetoreception" that sounds like something out of the movie X-Men. Naisbett-Jones wondered if it eels picked up on those magnetic signals to guide their long journey home.
To find out, he rounded up about a thousand young European eels (Anguilla anguilla), each about the length of a pinky finger and delicate as noodles. At that stage, they're called "glass eels," because they're transparent except for a dark spine down the middle.
He spent a summer wrangling those baby eels into contraptions he built that imitated the Earth's magnetic field at three points along the journey from the Sargasso Sea to Europe. The intensity and direction of that field changes with location.
All day every day for about a month, he would scoop up batches of 16 eels from a tank, and place each one into the center of a little container. Then, he would turn on a magnetic coil surrounding the containers and let the eels decide where they wanted to go.
They could choose between 12 chambers to swim into, like an eel version of a choose-your-own-adventure.
As Naisbett-Jones and his colleagues wrote in the journal Current Biology, they found that overall, the eels did indeed respond to the magnetic fields. And they did so in a way that, had they been in the Atlantic Ocean, would have directed them into the Gulf Stream.
"The eels oriented in a manner that would increase their entrainment into the Gulf Stream system," says Naisbett-Jones. When exposed to the field they'd find in the northern mid-Atlantic, and to the ambient one in Wales, where the experiment took place, they swam in random directions. But when exposed to a magnetic field like the one in the Sargasso Sea, the eels tended to orient themselves southwest. And when exposed to a field like the one they'd encounter in the northwest Atlantic, many eels pointed themselves northeast.
Getting into the Gulf Stream isn't a guaranteed ride to Europe, he says, "But it certainly increases their chances of eventually finding Europe, and if an eel has a magnetic map and it's able to detect where it is, then it can adjust accordingly during its migration."
"What we show here is that they're able to detect the magnetic field and use that to orient as a magnetic map. They're able to pick up on these predictable features of the magnetic field in order to gain positional information, kind of analogous to maybe a GPS positioning system," says Naisbett-Jones, who hopes to study whether adult eels use the same ability to find their way back to the Sargasso Sea years later to spawn.
If eels really do navigate this way, then they join a long list of animals including sea turtles, salmon and homing pigeons that rely on what biologist James Gould calls "innate magnetic coordinates."
"The magnetic-based 'map' has been found in amphibians, fish, reptiles, birds, and aquatic mammals," James Gould, a biologist affiliated with Princeton University, told NPR in an email. Some species use additional cues such as odors to supplement the magnetic ones.
"The use of a grid of total magnetic intensity vs. vertical intensity seems to be evolutionarily quite ancient," says Gould, who has investigated animal navigation in his own research and in the process of co-writing a book on the topic.
What's particularly interesting about eels, he says, is not necessarily their ability to sense magnetic fields, but how that ability may have affected them over the years.
"Eel migration eons ago was quite short — perhaps only a few tens of miles. But as the Atlantic basin has spread, their journey has gotten longer and longer," Gould says. "They are trapped in an absurdly long journey by their reliance on this otherwise wonderful 'map.' "
Now, each year, that magnetic sense launches every young eel on a slithery version of the Odyssey.
ARI SHAPIRO, HOST:
Eels have always been mysterious animals. Aristotle thought they were born spontaneously from mud. Researchers do know that many eels found in places like Europe and North America are born in the same place - deep in the Atlantic Ocean. And scientists now say the elusive animals spread from that point using the Earth's magnetic field. NPR's Rae Ellen Bichell reports.
RAE ELLEN BICHELL, BYLINE: Baby eels make an epic journey from that spot in the ocean, sometimes traveling thousands of miles before finding their homes in rivers and lakes all over the place, from North America to North Africa to Scandinavia.
LEWIS NAISBETT-JONES: It's pretty crazy, yeah.
BICHELL: That's Lewis Naisbett-Jones, a marine biologist with the University of North Carolina, Chapel Hill, who studies the species commonly known as American and European eels. He wanted to know how those little eels do it. Other scientists had found that eels can somehow sense the Earth's magnetic field, which has a slightly different strength and direction at different points on the globe. Naisbett-Jones wondered if that was what guided the young animals on their trek. So he got a bunch of baby European eels.
NAISBETT-JONES: Around about a thousand (laughter) in the end.
BICHELL: They're each about the length of a pinky finger and delicate as noodles.
NAISBETT-JONES: And just small and transparent really is the best way to describe them, almost jelly-like.
BICHELL: Then he spent a summer putting those baby eels one by one into contraptions that he built to imitate the Earth's magnetic field at points along the journey. He'd drop an eel in, turn on a magnetic coil and let the eel decide which direction it wanted to swim in.
NAISBETT-JONES: And then recorded the directions that they moved in response to those magnetic fields.
BICHELL: As they wrote in the journal Current Biology, Naisbett-Jones and his colleagues found that the eels did respond to the magnetic fields, and they did so in a way that, had they been in the Atlantic Ocean, would have led them roughly to the Gulf Stream, a powerful current that could carry them to Europe.
NAISBETT-JONES: What we show here is that they're able to detect the magnetic field and use that to orient as a magnetic map.
BICHELL: If eels really do navigate this way, says Naisbett-Jones, then they'd join sea turtles, salmon and homing pigeons on a growing list of animals that can navigate using the planet's magnetism. The next research question, he says, is how about a decade after that first journey adult eels find their way all the way back to that same spot in the ocean to begin the cycle again. Rae Ellen Bichell, NPR News. Transcript provided by NPR, Copyright NPR.