Our visceral reaction to eels, understandable but misinformed, is to see them as snakes. Eels on a plane would be frightening, but not dangerous. In fact eels are simply elongated fish, with small teeth like trout. They are more related to the guppy than the cobra. The misunderstanding began early on— Anguilla, the genus name, comes from anguis, Latin for serpent. *
Perverse creatures that we are, many of us find the sight of a piece of broiled eel on a bed of rice extremely attractive. The appetite for unagi, the sweetest sushi, is causing big trouble for the eels. With nets and dams, we're messing up the most significant event in their lives, an odyssey we know amazingly little about. Their migration to spawn—from freshwater to the ocean depths—is a feat of navigation and endurance that makes the march of the penguins look like the proverbial day at the beach.
The eels we're most likely to encounter at a sushi restaurant, a tapas bar, or a London jellied eel store are the heroic traveling anguillas. They are born and spend their infant stage—as a rather pretty 2-inch, leaf-shaped larva called a leptocephalus—in the ocean. As they mature, they find a river and make the climb inland to fresh water, where they spend their adulthood before returning to the ocean to spawn and die. Two much bigger, toothier, and less palatable eels, the moray and the conger, also have a leptocephalus stage, but they stay in salt water. (The famous electric eel of the Amazon basin isn't a close relative. In fact, it's a knife fish, not an eel.)
Salmon get a lot of good press for their journey from ocean to fresh water—those shimmering bodies leaping up waterfalls are a documentary staple. The Atlantic eels Anguilla anguilla and Anguilla rostrata make a journey at least as difficult, but in reverse. Salmon are anadromous (from the Greek ana, up), which means they live in the ocean and breed in fresh water. Eels are catadromous (from kata, down). They glide downriver when they're adults and make the much more difficult upriver climb when they're young.
Some autumn or winter day, something triggers in the eel an urge to leave its pond, lake, canal, or river and return to the sea. This comes after a decade or more of peaceful nocturnal feeding on insects and fish. (The eels will also happily do some carrion cleanup.) If there's a dam or a log in the way as the mature eels travel downstream, or the young ones come upstream, they'll leave the water to get around it. They've evolved to absorb some oxygen through their skin.
Here's the impressive part of the story: Do the adult eels perform their one-shot spawn-a-thon in the first convenient body of salt water they find? No, they swim through open ocean to the Sargasso Sea, between Bermuda and the Bahamas—3,500 miles for an eel coming from Lake Ontario, 600 miles from Florida. 4,000 or more for the Europeans. One scientist reckons nine miles of swimming a day. It takes months. How do they find their way? No one knows.
On that long, slow trip toward the spawning ground the adult eels may wish they had the fins their ancestors had 40 million years ago. Eel forefathers looked like tarpon fish. Along the evolutionary road, these enterprising animals jettisoned their appendages, keeping only a subtle ruffle along the belly. Fins were a disadvantage in tight spaces, and the newly streamlined creatures literally filled a niche. They could fit into small spaces between rocks and under logs to find food and evade predators. As they dropped fins, they added vertebrae, for a total of more than 100. (We have 33.)
Up until the 20th century, almost nothing was known about how eels reproduce. Aristotle thought they emerged from mud, without procreation. In 1922, a Danish professor, Johannes Schmidt, tracked the eels' passage as he sailed across the ocean, scooping up larvae on the way. The farther into the Atlantic he went, the smaller the larvae became. Finally, in the Sargasso Sea, he found the smallest eel larvae ever seen and deduced their birthplace. (If he'd kept going, he would have found the larvae getting bigger as he approached North America.)
Having a single spawning event, even at such a daunting distance, works for eels and salmon, as long as the water is unpolluted and the obstacles surmountable. The idea is to put every bit of energy into that one slog. The eels eat nothing on their journey, using stored fat not only to swim, but also to develop their sexual organs, which are almost invisible before they start the trek. As a student, Sigmund Freud spent eight years in a physiology lab where his primary task was to find the sexual organs of male eels. This must have been extremely frustrating work.
Why the Sargasso Sea? The location seems at first not worth the trip. It's a 2-million-square-mile, oval body of warm water floating southwest of Bermuda in the middle of a colder Atlantic. Mats of sargassum weed, kelp, and sea grass, along with old bleach jugs, plastic bottles, six-pack rings, and other nonbiodegradable junk swirl on the surface. Until the last century, scientists believed the area underneath the accumulated greenery and garbage was essentially a desert; the water there, three miles deep, is extraordinarily clear, still, and empty-looking. Now we know the place supports plenty of plankton and other tiny creatures. It appears uninhabited because it lacks the nutrients necessary to support larger fish, and this is what makes it a safe place for eels to reproduce. If eels spawned on the Grand Banks, or another of the world's great fisheries, the emerging young would be gobbled up.