1. The Eel Question

Joseph Stromberg
How science still knows close to nothing about these slithering sea creatures.

A drab, greenish-brown, foot-long creature, it looks, more or less, like the head of a fish stuck on the body of a meaty snake. During the day, it mostly hides in the mud, motionless for hours at a time. At night, it prowls, feeding on virtually anything it finds: frogs, worms, insects, fish, crustaceans, and plants.  

But unlike every other fish in those waters, the eel is a long-distance traveler. It has many lives, some still unknown.  

The whole eel population of North America, the entire Anguilla rostrata species, is born in one faraway place: the lightless bottom of an ocean gyre, thousands of miles off the Atlantic coast, called the Sargasso Sea. 1 At the bottom of a trench four miles beneath the surface, tiny, clear, leaf-shaped eel larvae hatch by the millions. After spending months drifting upward, they approach the coast and transform, elongating until they resemble clear worms.  

The horde of glass eels swims up the estuaries and bays of the Atlantic coast before migrating further inland via rivers and creeks. They darken as they take on the pigmentation of the freshwater creatures they prey upon. At times, these elvers wriggle across mud or wet grass for days to go from one pond to another, breathing through their slimy skin. Eventually, one by one, the eels abandon their voyage, settle down, and live for years in a single pond or riverbed.  

But one day, decades from now, each eel will respond as if to an invisible signal. They will transform one final time, turning a deeper black, with a silver underbelly, growing longer fins and larger eyes. They will swim downstream and then across the open ocean for months at a time, their stomachs dissolving as they rely on stored fat reserves for fuel. Eventually, hundreds of thousands of eels from across North America, and even cousins from Europe, will return to the Sargasso Sea. There, each will fulfill its last mission: laying or fertilizing a clutch of eggs before its long life finally comes to an end. 2  

This, at least, is what scientists believe. Somehow, despite centuries of attempts, no one has ever observed eels mating or seen mature eels in the Sargasso Sea. The everyday eel is one of the oldest, most unlikely unsolved mysteries of the natural world.

More than two thousand years ago, Aristotle puzzled over the eel. As Swedish author, Patrik Svensson writes in his recent natural history-cum-memoir The Book of Eels, the polymath dissected eels as part of his zoological research and found no eggs, testicles, or other sex organs, as other fish have. Perplexed, he fell back on the theory of spontaneous generation. “Eels are derived from the so-called ‘earth’s guts’ that grow spontaneously in mud and in humid ground,” he wrote 3. Pliny the Elder, meanwhile, believed they reproduced by rubbing against rocks and throwing off minuscule fragments of themselves. For centuries, folklore held that eels had all manner of magical origins: they were born from morning dew, or from “electrical disturbances,” or hatched from eggs on rooftops.  

Well into the 1800s, long after spontaneous generation had been disproven, scientists struggled to answer what had become known as the “eel question:” where do they come from? In 1850, a female eel with eggs was finally found, but in 1876 — when a young Sigmund Freud began dissecting eels in Trieste, Italy as a graduate student —a mature male with testes still hadn’t been identified. Freud spent a month slicing open eel after eel, frustrated he couldn’t find the testes that would represent a real scientific breakthrough. Empiricism and scientific rigor had evolved in the years since Aristotle, but Freud was just as blind to the truth in trying to define the eels as male or female. They were both and neither.  

It turns out eels live out almost their entire lives in a juvenile form, only developing sex organs as they make their final journey back to the Sargasso. Most of what we know about this migration is due to the painstaking work of the Danish biologist Johannes Schmidt, who tracked the European eel back to its source. Starting off the coasts of Europe, he spent years searching for ever-smaller Leptocephalus larvae, reasoning that the smallest would be found closest to their breeding grounds. Over the course of two decades, he followed the trail of tiny, translucent larvae across the Atlantic — and found the very smallest, about the size of a grain of rice, in the Sargasso Sea, along with those of the nearly identical American eel. Both species, Schmidt determined in 1921, hatch in the Sargasso, migrate to freshwater habitats in their respective continents, and return to the deepwater breeding grounds in their final days  4.  

We now know that eels use a remarkable range of adaptations to live out sucha long, meandering life. They produce a thick slime that coats their entire bodies, allowing them to survive out of water and slip out of a predator’s grasp. They navigate using the Earth’s magnetic field 5. They survive temperatures close to freezing by going into a deep hibernation at the bottom of a lake for weeks at a time 6. If prevented from returning to the Sargasso, they can suspend their normal life cycle almost indefinitely, essentially postponing death: the oldest verified eel lived for 88 years, almost all of it in captivity, though others report eels living well over a century.  

But in the century since Schmidt, we still haven’t really, fully answered the eel question. Scientists have still never seen mature eels in the Sargasso Sea or even spotted a dead eel in the waters. In the most exhaustive attempt to date, a European research team attached tracking devices to 707 eels released from Europe. Every single sensor fell off or stopped working long before they reached the Sargasso 7. The eel is so slick we have yet to invent a device that can stick to it. 

In another study, wild females were caught and injected with hormones to induce sexual maturity, with a plan to tie them to buoys in the Sargasso to lure males. But most of the eels died in captivity, and the few that made it to the sea disappeared, buoys and all, before any data could be collected.   

Humans have still never observed eels breeding, even in a lab. In Japan, where the eel is widely eaten, industry efforts to breed eels in captivity have been dramatically unsuccessful. At times, it almost feels as if the eel wants to stay hidden.

Given all this, Svensson asks in The Book of Eels, do we really know all that much more than Johannes Schmidt did in 1921? “We know, then, that the old eels vanish from our ken into the sea, and that the sea sends us in return innumerable hosts of elvers,” Schmidt wrote in his seminal paper. “But whither have they wandered, these old eels, and whence have the elvers come?”  

And deeper questions remain. Why do eels travel around the world only to live in a single creek or pond for decades? The most intriguing hypothesis is that they are so ancient (they likely evolved into their current state some 50 million years ago) that the movement of the tectonic plates has distorted their journeys. As the North American and European plates have drifted apart over the millennia, the thinking goes, generations of eels have simply continued returning to the same spot for breeding, their migrations inadvertently growing longer and longer. Each journey is a brief history of geologic time, condensed into a single lifespan.  

But the most pressing question is why, after millions of years of stability, eel species are uniformly in rapid decline. We have extremely sparse data, but in some areas, recent censuses show as little as 1% of the number of eels present in the 1980s. Unsurprisingly, the causes are mostly of human origin: pollution, overfishing, physical barriers to migration like dams. The greatest danger, however, may be the way climate change is altering the temperature, salinity, and currents of the oceans. In this way, our inability to fully understand the eel — to trace its life cycle starting from a larva helplessly drifting for months at sea — prevents us from knowing the best way to protect it. Despite decades of research and millions of dollars, the eel remains a stranger.  In truth, we know little more than Schmidt did nearly a century ago. Not long after he died in 1933, Rachel Carson wrote about his discoveries for a broader audience in “Under the Sea-Wind,” her first book. Published decades before “Silent Spring” would inspire the modern environmental movement, the book tells the story of three sea creatures (a seabird, a mackerel, and an eel) in order to illustrate the interconnectedness of the marine ecosystem.  

But it is the eel section of the book that courses with a particular sense of mystery and haunting. Carson describes schools of young glass eels, their lives ahead of them, unknowingly passing by the ancient silver eels, returning to their death in the Sargasso. She relates the utter foreignness of this ocean depth, where so little sunlight penetrates that the days, seasons, and years of the surface have no meaning. 8  

In a letter to her editor from the time, Carson hints at how much the eel has to teach us — and the limits of what we can really know. “To see an eel is something like meeting a person who has traveled to the most remote and wonderful places of the earth,” she wrote. “In a flash I see a vivid picture of the strange places that eel has been—places which I, being merely human, can never visit.”

01. Svensson, Patrik. The Book of Eels. New York: Harper Collins, 2020.
02. Cooke, Lucy. The Truth about Animals: Stoned Sloths, Lovelorn Hippos, and Other Tales from the Wild Side of Wildlife. New York: Basic Books, 2018.
03. Aristoteles, and Jonathan Barnes. Complete Works of Aristotle, Volume 1: The Revised Oxford Translation. Princeton, NJ: Princeton University Press, 1984.
04. Schmidt Johs.  1923 IV.—The breeding places of the eel. Phil. Trans. R. Soc. Lond. B211179–208 http://doi.org/10.1098/rstb.1923.0004
05. Cresci, A., Durif, C.M., Paris, C.B. et al. Glass eels (Anguilla anguilla) imprint the magnetic direction of tidal currents from their juvenile estuaries. Commun Biol 2, 366 (2019). https://doi.org/10.1038/s42003-019-0619-8
06. Walsh, Patrick, Foster, Glen, and Thomas Moon. The Effects of Temperature on Metabolism of the American Eel Anguilla rostrata (LeSueur): Compensation in the Summer and Torpor in the Winter. Physiological Zoology 26,4 (1983) https://www.journals.uchicago.edu/doi/10.1086/physzool.56.4.30155876
07. David Righton, Håkan Westerberg, Eric Feunteun, Finn Økland, Patrick Gargan, Elsa Amilhat, Julian Metcalfe, Javier Lobon-Cervia, Niklas Sjöberg, Janek Simon, Anthony Acou, Marisa Vedor, Alan Walker, Thomas Trancart, Uwe Brämick and Kim Aarestrup. Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea. Science Advances  05 Oct 2016: Vol. 2, no. 10, e1501694. DOI: 10.1126/sciadv.1501694
08. Carson, Rachel. Under the Sea Wind: A Naturalist’s Picture of Ocean Life. Penguin Books, 1941.