The Enigma of the Eel: The Elusive Science of Earth’s Most Mysterious Creature

“Science has come up against many mysteries, but few have proven as intractable and difficult to solve as the eel.”

The Enigma of the Eel: The Elusive Science of Earth’s Most Mysterious Creature

It is impossible to know why people travel in the same way. This is the ideal place to spawn eels. Anguilla anguillaThese creatures are mysteries that transcend magic.

Eels can live almost a century without any signs of aging. You can see it swimming thousands of miles each hour, and then settle in one spot for years. It is capable of surviving for many years without food, and it can walk through the dry desert for hours to search out its next riverine leg. Nobody knows how or why the eel chooses the location where it will spend the best part of its lives before it goes back to its home to spawn again. It is the Sargasso Ocean that it starts and ends its alternate worldly existence.

Art from the world’s first color encyclopedia of marine life (1719), partway between natural history and myth. Prints are available to benefit The Nature Conservancy.

“As they passed through the surf and out to sea,” Rachel Carson wrote of the eels completing their return journey, “so also they passed from human sight and almost from human knowledge.” Her lyrical 1941 book Under the Sea-Wind, which always remained her favorite piece of writing, brought that otherworldly life of the eel to the human imagination for the first time in a systematic chronicle of its science and its enduring mystery. In the near-century since, humanity has made little progress in solving the enigma of the eel, so that Carson’s poetic prose reads today as resonant with wonder as ever:

No one knows how the eels traveled to their common destination… But somehow they came to the continent’s edge, where the muddy slopes of the sea’s wall fell away steeply, and so they passed to the deepest abyss of the Atlantic. The young eels would be born in the depths of the sea, while the older eels would die to make way for the new sea.

Billions of small specks suspended below the sea’s surface, of protoplasm floating in darkness at the beginning of February. They were the newly hatched larvae — the only testament that remained of the parent eels. They were the first to experience life on the transition zone between surface water and the depths of the abyss. The sun’s rays were blocked by the thousand-foot high water that hung above the young eels. Only the longest and strongest of the rays filtered down to the level where the eels drifted in the sea — a cold and sterile residue of blue and ultraviolet, shorn of all its warmth of reds and yellows and greens. A strange and unarthly light, a bright and vivid blue from up above, displaced the darkness for a twenty-year period. But only the straight, long rays of the sun when it passed the zenith had power to dispel the blackness, and the deep sea’s hour of dawn light was merged in its hour of twilight. Soon, the blue light disappeared and the eels survived again in the long, dark night.

The tiny larvae are half-blind, translucent, and hatched once they have been identified. Willow leavesThey begin to travel toward land immediately, crossing thousands of miles on the Gulf Stream until landing ashore. There, they undergo their first metamorphosis and become glass eels — slippery transparencies the length of a pinky — adapting instantly to this freshwater chapter of their lives.

As they make their way up the riverine lungs of the land, another metamorphosis takes place and they become yellow eels — small but mighty serpentine swimmers who now have jaws, tiny scales and tiny gills, and a tender mohawk of soft fins along the back and belly. This is the eel as an adolescent — but it is an epochal adolescence that can last for decades. It spends its whole life hunting and eating at night, in a mysteriously chosen habitat, while hibernating and eating for extended periods of time.

The one-and-only yellow eel, now a single, is suddenly overcome by the need to reproduce. It enters its last metamorphosis and begins the return journey to the Sargasso. This tender leaf of willow now transforms into a big, strong silver eel. The yellowish-brown body is transformed into a black eel with silvery sides and dark stripes. The eel’s reproductive organs blossom into existence. In preparation to navigate the shallow-wavelength nocturnes of deep sea, its eyes become large and bright. Its stomach dissolves, its digestive system shuts down, and the eel becomes a metabolic monastic — from this point on, throughout the thousands of miles down rivers and across open ocean, the female will only use her fat reserves for fuel as her body fills with roe. She somehow makes it back to the Sargasso sea, almost like magic.

Eel, An Essay Towards a Natural History of Serpents (1742). This book is also available as a printed copy, which will benefit The Nature Conservancy.

In his fascinating Book of Eels (public library) — a homily on science and on mystery, synthesizing everything we know so far about the eel, how we came to know it, and the vast everythingness of the remaining enigma — Patrik Svensson reflects on the eel as “a fish that transcends the piscine condition” and writes:

While science may have encountered many mysteries, none are as hard to unravel as the mystery of the eel. Eels have turned out to be not only uncommonly difficult to observe — due to their strange life cycle, their shyness, their metamorphoses, and their roundabout approach to reproduction — but also secretive in a way that comes across as deliberate and preordained. Even when observation succeeds, even close inspection can cause the eel to withdraw.


We often feel a distance from them because of our uncertainty regarding the fundamental nature and origins of the eel. Eels have been viewed as either frightening or offensive by many people. They’re slimy and slithery, look like snakes and are said to eat human bodies; they move surreptitiously, in the dark and the mud. The eel, unlike all other animals, is an alien and has remained so despite being present in many of our rivers, lakes, and tables.

In millennia humans have not encountered male eels. We now know that the male eels are only found in the Sargasso Sea, but the females travel up streams and rivers, while the males wait at the river mouths, awaiting their return. Aristotle was so baffled by this, and despite his many contributions to Zoology and general scientific bent, concluded that the eel was born from mud. According to the Egyptians, the eel was believed to have emerged from infancy when the Nile’s waters warmed. Eels are believed to have been brought alive by the drop of horse hairs into the water. This belief was echoed in the British Isles ancient cultures. The Middle Ages saw the discovery of a male eel. By then, it was considered a hermaphrodite. It could perform both genders within one body in order to fertilize itself. In the eighteenth century, it was classed with serpents and illustrated alongside mermaids in the world’s first color encyclopedia of marine life.

From the 1862 book, engraving Silver Eel: The OriginAccording to, the eel was born from a beetle.

The nineteenth century saw the birth of gendered eels in Germany. Newspapers announced that fifty marks would be given to those who send in female eels with roe. A renowned professor had to inspect them. Postage would be paid for by the government fishing agent. After hundreds of eels turning up from every corner of the country — half-rotten eels, half-eaten eels, eels full not of roe but of parasites — not a single sexually mature eel was found and the government agency nearly went bankrupt. Although the discovery of the first roe-laden egg-laden eel in 1850 was a success, it left unsolved the mystery about the ghostly male.

The ancient mystery was solved by a nineteen-year old medical student who had a passion in zoology. After learning that a Polish zoologist had claimed that he had discovered a mature male eel inside, Sigmund Freud decided to investigate the organ’s testicles. It was not possible to determine if the organ was capable of producing semen, as dissections of dead animals cannot reveal the inner workings of living organisms. Patient, you may be doomed.

Freud’s teenage son with his mother.

It was so that Freud, a teenage Freud, ended up living in an unfinished laboratory at the Mediterranean coast. The teenage Freud took quarters in Trieste’s smallest room and placed his worktable in front of the window. He stationed his microscope in one corner of the desk, his dissection dish in the other, his notepad and four sharpened pencils at the center, and his arsenal of specimens in the front — various glass vessels, pans, and bowls containing whole “beasts” or eel bits in seawater.

“In between stand or lie test tubes, instruments, needles, cover slips, microscope slides,” he wrote to his best friend, “so that when I am busy working there is not a spot left on which I can rest my hand.” Each morning at daybreak, he made his way to the docks to buy fresh eels from the local fishermen, then returned to his desk to work for four hours, taking a lunch break of precisely one hour before resuming for another five hours until dusk, then heading out for his thinking-walk, noting the uncommon beauty of the local women.

Freud, despite his devotion and diligence, failed to discover the male gonads. Others scientists tried, but failed. Svensson wrote:

Freud, but not Freud. [other scientists of his time]It was well-known that the sex organs of eels are not visible until they require them. Metamorphoses of eels are more than just adaptations to changing life circumstances. They’re existential. When the right time comes, an eel can become what it needs.

Twenty years after Freud’s failed efforts, a sexually mature male silver eel was finally found off the coast of Messina in Sicily. The eel was now a fish. An animal not too different from the rest.

And yet this creature remains Earth’s greatest living mystery — a mystery the natural and cultural history of which Svensson traces in the wholly wondrous Book of Eels, folded into which is a tender memoir of his father, who shaped his own relationship with the eel. Complement it with this excellent Gastropod episode about reinventing the edible eel, then pair it with the equally and differently wondrous evolutionary-cultural history of the oyster.

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