Eels make remarkable migrations. Adults from both sides of the Atlantic Ocean swim to the Sargasso Sea to reproduce, and their offspring take a few years to return the 6000 kilometers to their respective coastlines. How they get there has been an enduring mystery. Many biologists have assumed these newborns drift aimlessly until swept up into the Gulf Stream, but like a few other marine creatures, they may actually use Earth’s magnetic field to guide their course, marine biologists have now discovered.
Some researchers question whether this sixth sense exists in the very youngest eels found in the Sargasso Sea, but if it does, “this study adds to the growing body of evidence that the magnetic sense may be an important component of fishes that make long migrations in the ocean,” says Michael Miller, an eel biologist at Nihon University in Fujisawa, Japan, who was not involved in the work. And if it holds up that newborn eels do more than drift aimlessly, the study paints a new picture: These young eels “may be the ultimate swimming machines,” he adds.
Over the years, researchers have found that a magnetic sixth sense exists, perhaps even in people, and is important in salmon, sea turtle, and trout migrations. And some work indicates that adult eels also sense magnetic fields. But proving the same was true of juvenile eels was quite tricky, as these fish can be quite erratic in their behavior. That makes it hard to discern any patterns in their orientation, and thus they are difficult to test.
To better quantify whether eels did tend to swim in a particular direction in response to a magnetic field, Lewis Naisbett-Jones, a marine biologist at University of North Carolina in Chapel Hill, and Nathan Putman, a marine biologist from the National Oceanic and Atmospheric Administration Atlantic Oceanographic and Meteorological Laboratory at the University of Miami in Florida, designed a 25-centimeter-wide underwater arena: a central chamber connected to a dozen outer sections, each representing 30° on a compass. They tested so-called glass eels, which is what the juvenile eels transform into when they arrive in Europe. A movable wall kept the eel in the center until the magnetic field was turned on and testing began.
The researchers simulated magnetic fields from different parts of the migration route, and the fish responded as if they knew where they were going. In the magnetic field that was the same as in the Sargasso Sea, most of the eels headed southwest, showing they did head in a particular direction in response to the magnetic field. In the test using the magnetic field found in the Atlantic off North America, the eels turned northeast, showing that slightly different magnetic fields caused them to orient in a different way, the researchers report today in Current Biology.
“We were not surprised to find eels have a magnetic map, but we were surprised to discover how well they can detect subtle differences in magnetic fields” Naisbett-Jones says.
At first, the orientations were confusing—the eels weren't headed the most direct way home. Yet a computer model showed that the headings taken by the juvenile eels in the experiment would in fact make their migration more efficient, as they got the fish into the Gulf Stream faster and kept them in this current so that they headed to Europe.
“Swimming the 'wrong' direction for a bit gets them into the Antilles Current, which connects them to the Gulf Stream,” Putman explains. “So, with a little bit of work they can get a mostly free ride to Europe.”
The oceanographic model "strengthens the overall findings,” says Miguel Baltazar-Soares, a marine biologist at Bournemouth University in Poole, U.K., who was not involved with the work.
However, the study has a major flaw, several experts say. Eels undergo several major transformations. Newborns are not very eellike at all, being more like flattened gelatinous ribbons than rounded elongated fish, so it’s not clear that these newborns would have the same sensory capabilities of the juveniles tested, says Caroline Durif, a marine biologist at the Institute of Marine Research in Bergen, Norway. “I would also like to point out that none of the authors are recognized eel experts—the paper’s main authors have not focused on eels before in their research. Otherwise, they would have realized how absurd this study is.”
But Putman doesn’t think the use of older eels is a problem and assumes even newborns have this magnetic sense. Instead, he thinks that the novelty of finding a magnetic compass in eels “might rub some people the wrong way” because they had not discovered it before.
Next, he hopes to demonstrate that adult eels also use magnetism to find their way to the Sargasso Sea.