Eels are guided by Earth’s magnetic field

FROM ARISTOTLE to Sigmund Freud, eels’ reproductive habits have puzzled observers of the natural world. In a life-cycle the opposite of a salmon’s, they grow from youth to maturity in rivers and ponds and then go to sea to spawn. Exactly where they do this spawning, though, was a mystery—until, a century ago, a Danish marine biologist called Johannes Schmidt painstakingly trawled eel larvae from the depths of the Atlantic Ocean and found that they got smaller and smaller until he arrived at the Sargasso Sea, which thus seemed to be European eels’ .Subsequent work traced American eels to the Sargasso, too, while those from East Asia and some other parts of the Pacific spawn near Guam. Even with all this to go on, however, no one has yet captured a sexually mature eel from the wild, nor observed the species in the act of spawning. And there also remains the question of how adult eels, having left their riparian homes, find their way to these spawning grounds in the first place. To investigate that, Caroline Durif of the Institute of Marine Research in Bergen, Norway, has been studying eels’ magnetic sense.