Careful observers discovered that what had long been taken for several different kinds of animals were in fact just one. The eel was a creature of metamorphosis, transforming itself over the course of its life into four distinct beings: a tiny gossamer larva with huge eyes, floating toward Europe in the open sea; a shimmering glass eel, known as an elver, a few inches in length with visible insides, making its way along coasts and up rivers; a yellow-brown eel, the kind you might catch in ponds, which can move across dry land, hibernate in mud until you’ve forgotten it was ever there, and live quietly for half a century in a single place; and, finally, the silver eel, a long, powerful muscle that ripples its way back to sea. When this last metamorphosis happens, the eel’s stomach dissolves—it will travel thousands of miles on its fat reserves alone—and its reproductive organs develop for the first time. In the eels of Europe, no one could find those organs because they did not yet exist.

The International Union for Conservation of Nature, a body charged with assessing the status of the natural world, has, unsurprisingly, had some trouble with eels. Ideally, to determine how the species is doing, the I.U.C.N. would like to know the number of “mature eels at their spawning grounds”—a number that is as knowable as the number of angels who can dance on the head of a pin.

Because ours is a world of making do, the I.U.C.N. instead counts the small glass eels that arrive in Europe each spring. For every hundred eels that showed up on the coasts in the nineteen-seventies, it’s believed that now only five come to wriggle their way inland. The formerly ordinary, everyday eel is classified as critically endangered, the last official designation on the road toward nonexistence. (Well, there is also “extinct in the wild,” but without their wild breeding grounds eels are nothing. No one has ever managed to breed them in captivity.) Svensson writes, “This is the latest and most urgent eel question: Why is it disappearing?”

– Where Do Eels Come From?