For more than two thousand years, nobody knew where eels came from.
This was not a minor gap in zoological knowledge. Eels were one of the most common animals in Europe. They filled every river, lake, estuary, and ditch from Scandinavia to North Africa. People caught them in willow traps, smoked them, salted them, dried them, ate them in vast quantities, and paid their rent with them. By the end of the eleventh century, over 540,000 eels were used as currency annually in England alone. The Domesday Book records hundreds of eel-rent payments. The town of Ely, in Cambridgeshire, may take its name from the Old Northumbrian word for "district of eels." In 1392, King Richard II adjusted tariffs on eels in London because they were that important to the economy.
And yet nobody had ever seen one mate, or produce eggs, or give birth. Nobody had found reproductive organs inside one. Nobody had found a small eel that appeared to be the offspring of a larger one. They simply appeared — fully formed, finger-length, in rivers across the continent, every spring. Where they came from was, by any reasonable measure, one of the oldest unsolved questions in natural history.
Aristotle was the first to investigate seriously, and he got it spectacularly wrong. In the fourth century BCE, he dissected eel after eel and found nothing — no eggs, no milt, no ovaries, no testes. In History of Animals, he concluded that eels were "spontaneously generated within the entrails of the earth." They emerged, he proposed, from mud itself, animated by pneuma — the breath of life — which carried a kind of vital heat capable of making matter quicken. This was not sloppy thinking by the standards of the time. Aristotle was rigorous. He simply confronted a genuine mystery and reached for the best theory available.
Others were less rigorous. Pliny the Elder, writing in the first century CE, suggested that eels rubbed themselves against rocks and the particles of dead skin that fell off "came to life" as baby eels. Ancient Egyptians associated eels with the sun god Atum and believed they sprang to life when sunlight warmed the Nile. Later writers attributed eel origins to horses' tails dropped in water, to dew on riverbanks, to the slime that coated stones in streams.
Spontaneous generation — the idea that living organisms could arise from non-living matter — was not definitively debunked until Pasteur's experiments in the 1860s. But even as the broader theory fell, the eel remained its last holdout. Every other animal had been shown to reproduce sexually. The eel, as far as anyone could demonstrate, still came from mud.
The mystery had economic and theological consequences. Medieval Christians were required to abstain from meat during Lent and other fasting periods, and the Church's definition of "meat" turned on reproduction: food from animals that copulated could "excite sexual appetites," as Thomas Aquinas put it, and was therefore forbidden. Fish were generally exempted. But eels held a privileged position. Since they appeared to reproduce without sex — since nobody had ever witnessed eel copulation or found eel gonads — they were considered "particularly sexless." Eating an eel was eating something that existed outside the carnal order entirely. This made them ideal Lenten food, which further drove demand, which made them even more economically important, which made the question of their origins even more pressing.
Monastic estates collected eel-rents from tenants living on their land. A "stick" of eels was 25; a "bind" was 10 sticks. People ate more eels than all other freshwater and marine fish combined. Prominent families incorporated eels into their heraldry. The eel was, in medieval England, simultaneously one of the most familiar animals in daily life and one of the least understood.
The first crack in the mystery came in 1777, when an Italian anatomist named Carlo Mondini dissected eels from the Comacchio lagoons, on the Adriatic coast, and identified a structure he believed to be an ovary. The scientific community was skeptical. It was not until 1824 that Martin Rathke, a German anatomist, independently confirmed Mondini's identification — yes, female eels had ovaries. They were fish after all. They reproduced sexually. Somewhere.
But if females existed, where were the males?
In 1874, a Polish zoologist named Simone de Syrski, working in Trieste, claimed to have found the male eel's testes — tiny, ribbon-like organs buried in the abdominal cavity, easy to overlook. Confirmation was needed. In 1876, the University of Vienna sent a nineteen-year-old medical student to Trieste to verify Syrski's claim. His name was Sigmund Freud.
Freud spent four weeks at Professor Carl Claus's marine biology station, dissecting eels. Hundreds of them. He cut them open, examined them under the microscope, searched for the structures Syrski had described. "I have been tormenting myself and the eels," he wrote to his friend Eduard Silberstein, "in a vain effort to rediscover the male eels, but all the eels I cut open are of the gentler sex." By around eel number 400, he found structures that might be gonads — small, indeterminate, buried in the abdominal cavity. His results were inconclusive. He published a paper in 1877 that amounted to a shrug: maybe these were testes, maybe not.
The problem, which no one understood at the time, was that male eels do not develop mature sexual organs until they leave fresh water and begin migrating out to sea. Freud was dissecting river eels — animals that had not yet entered their reproductive phase. He was looking for something that did not yet exist in the animals available to him. A mature male eel was not identified until almost two decades later, near Sicily.
Freud abandoned zoology and turned to neurology, then to the unconscious. His first published scientific paper was about eel gonads.
The next breakthrough came from a different direction entirely. In 1896, the Italian zoologist Giovanni Battista Grassi, working with his colleague Salvatore Calandruccio in the Strait of Messina, demonstrated that a small transparent fish long classified as its own species — Leptocephalus brevirostris — was actually the larval form of the European eel. This was not a minor reclassification. For decades, Leptocephalus brevirostris had been catalogued in collections, described in textbooks, and treated as a distinct animal. Grassi showed that it metamorphosed into a glass eel, and from there into the familiar river eel. The eel did have a larval stage. It had been found. It had simply been misidentified as something else.
Grassi found these larvae in abundance in the western Mediterranean, which led him to conclude that eels spawned somewhere in that region. He was wrong about the location, but right about everything else. The four years of research he and Calandruccio conducted "dispelled the great mystery which had hitherto surrounded the reproduction and development of the Common Eel," as one contemporary put it. Except, of course, it hadn't. The real mystery — where did the larvae come from in the first place — remained.
Enter Johannes Schmidt, a Danish biologist with access to funding from the Carlsberg Foundation — the brewing company — and a methodical mind. Beginning in 1904, Schmidt collected eel larvae from across the Atlantic Ocean and the Mediterranean Sea, measuring their size and recording their location. His logic was simple: larvae grow as they drift on ocean currents, so the smallest larvae would be found closest to the spawning ground. If he kept sampling westward, eventually he would find the smallest ones of all.
It took him seventeen years. Expedition after expedition, aboard a series of research vessels, the last and most famous being the Dana. He trawled the deep Atlantic systematically, always moving toward where the larvae were smaller. Across the Mediterranean, past the Azores, past the mid-Atlantic ridge, into the central North Atlantic. In 1921, he found what he was looking for: the smallest leptocephali, just hatched, concentrated in an area of the western Atlantic south of Bermuda.
The Sargasso Sea.
The Sargasso Sea is one of the strangest places on Earth. It is the only sea with no land boundaries. Its borders are four ocean currents: the Gulf Stream to the west, the North Atlantic Current to the north, the Canary Current to the east, and the North Atlantic Equatorial Current to the south. These currents circulate clockwise around an area of roughly three million square kilometers, creating a vast, slow-rotating gyre. The water inside is calm, warm, extraordinarily clear, and nutrient-poor — the blue of deep ocean, not the green of productive coastal water. The name comes from Sargassum, a genus of brown seaweed that floats in thick mats on the surface, kept buoyant by small gas-filled bladders. It is the only seaweed in the world that reproduces vegetatively on the open ocean, never touching the seafloor. It creates a floating ecosystem — juvenile sea turtles, shrimp, crabs, and fish shelter in its tangles.
Into this improbable place, Schmidt proposed, every European eel traveled to breed. The distance from the rivers of Scandinavia to the Sargasso Sea is roughly 6,500 kilometers. From the Mediterranean, somewhat less — but still thousands of kilometers of open ocean. Every eel that had ever been caught in a European river, Schmidt argued, had been born there, crossed the Atlantic as a larva, spent years or decades in fresh water, and would eventually return to die.
The eel's full life cycle, as pieced together over the twentieth century, is one of the most dramatic in the animal kingdom.
It begins with eggs — billions of them — hatching in the Sargasso Sea. The larvae are flat, transparent, willow-leaf-shaped creatures that look nothing like eels. They drift on the Gulf Stream and the North Atlantic Drift for one to three years, crossing the Atlantic. As they approach the continental shelf of Europe, they metamorphose into glass eels — tiny, translucent, threadlike. They enter estuaries, rivers, and streams and become elvers, developing pigment. Over the next several years — sometimes four, sometimes twenty — they grow into yellow eels, the form most people recognize: dark-backed, yellowish-bellied, living in freshwater, feeding on insects, crustaceans, and smaller fish.
Then, when the time comes — triggered by factors still not fully understood — the yellow eel begins to change. The transformation is called silvering, and it is total. The belly turns silvery white. The pectoral fins lengthen. The skin thickens. The eyes grow to as much as ten times their original size, developing new visual pigments suited to the deep ocean. The musculature changes. Most remarkably, the digestive tract shuts down completely. The eel stops eating. It will never eat again. Everything it needs for the journey — 6,500 kilometers of swimming without food — must already be stored in its body. The skeleton begins to dissolve, its calcium redirected to fuel the trip.
The silver eel leaves its river and enters the sea. It swims westward, at depths of several hundred meters during the day, rising toward the surface at night. It navigates across the open Atlantic by mechanisms nobody fully understands — possibly the Earth's magnetic field, possibly ocean currents, possibly something else. It arrives in the Sargasso Sea. It spawns. It dies. Its larvae begin the drift eastward. The cycle takes a generation.
Schmidt's theory was accepted as scientific consensus almost immediately, but it came with a significant caveat: no one had ever found a single European eel egg in the Sargasso Sea, and no one had ever observed an adult European eel there. The evidence was entirely inferential — this is where the smallest larvae are, so this must be where they come from. For a century, that inference remained unconfirmed by direct observation.
Not for lack of trying. The Sargasso Sea is enormous. Research vessels cost $30,000 to $50,000 per day to operate. The German Thünen Institute of Fisheries Ecology, one of the primary research institutions, can afford to send a ship every three years. The eels spawn somewhere in those three million square kilometers of open ocean, at unknown depths, at unknown times. Finding the spawning act would require either extraordinary luck or sustained surveillance of an area roughly the size of India.
The Japanese, working with a different species — Anguilla japonica — have had more success, thanks to decades of sustained government funding. They identified spawning locations at 150 to 200 meters depth, collected fertilized eggs, tracked tagged eels to spawning areas, and determined that spawning occurs over several days around new moons, with temperature shifts as a trigger. Some individuals spawn more than once. This work, led by Katsumi Tsukamoto, demonstrates what is possible with enough money and institutional patience. The European eel has received neither.
The mystery also attracted its eccentrics. In 1959, Denys Tucker of the British Museum — the world's leading authority on eels and a principal scientific officer at one of the most prestigious natural history institutions on Earth — published a paper arguing that European and American eels were not actually two separate species but rather environmental variants of a single species, with differences in vertebral count caused by water temperature during development. This directly contradicted Schmidt and provoked, as one source put it, a controversy "that even included the Archbishop of Canterbury." Tucker was wrong — genetic analysis later confirmed the two species are distinct — but the episode became stranger still when Tucker publicly claimed to have seen the Loch Ness Monster during a holiday in Scotland. In 1960, the new director of the Natural History Museum fired him. Tucker sued the trustees. The world's foremost eel expert spent the rest of his career in obscurity, remembered more for Nessie than for Anguilla.
The first direct evidence of adult European eels reaching the Sargasso Sea came only in 2022. Researchers captured 26 female silver eels from rivers in the Azores — the archipelago in the mid-Atlantic, considerably closer to the Sargasso than mainland Europe — and fitted them with pop-up satellite archival tags. These small devices record depth, temperature, and light level, then detach at a pre-programmed time, float to the surface, and transmit their stored data to satellites.
The results were modest but historic. The tagged eels traveled straight-line distances of 272 to 2,275 kilometers, at speeds averaging 6.8 kilometers per day. Five of the 26 reached locations within the boundaries of the Sargasso Sea. One reached the presumed breeding zone. None was observed to spawn — the tags detached before that could happen. None traveled fast enough to reach the spawning area before the recognized spawning season ended. But for the first time in the nearly one hundred years since Schmidt proposed his theory, there was direct evidence of an adult European eel in the Sargasso Sea. The inference had a data point.
The European eel is now critically endangered. Recruitment of glass eels — the number arriving at European coasts each year — has fallen to roughly 1% of its 1960–1979 baseline. Some years it drops below that. The population decline exceeds 90% over the historical range. The causes are the usual litany: habitat destruction, dams that block migration, pollution, turbines that kill eels passing through hydroelectric facilities, overfishing, and a black market that illegally exports more than 90 tonnes of live European eels to Asia annually for aquaculture.
Captive breeding has never been achieved at commercial scale. The larvae refuse to eat in laboratory conditions — scientists believe they feed on marine snow, the constant slow rain of organic detritus in the open ocean, but replicating this in a tank has proved impossible. Without captive breeding, the species depends entirely on wild reproduction. And wild reproduction depends on a journey of thousands of kilometers to a spawning ground that has never been definitively located, to engage in an act of reproduction that has never been observed, producing eggs that have never been collected from the wild.
The European Union mandates a three-month winter fishing closure, but researchers have noted that the scientific basis for this specific intervention is thin. An international research consortium recently recommended closing all eel fisheries until glass eel stocks recover, but no consensus exists on what recovery would look like or how to measure it.
Reinhold Hanel, director of the Thünen Institute, has said that without a unified international effort and sustained funding for frequent expeditions to the Sargasso, the answers to fundamental questions about eel spawning will "stay hidden somewhere in the watery depths."
So: Aristotle was the first to ask where eels come from, in the fourth century BCE. Freud spent four weeks slicing them open in Trieste, found nothing conclusive, and became a psychoanalyst. Schmidt spent seventeen years chasing larvae across the Atlantic and identified the Sargasso Sea as the answer. Grassi proved that a fish nobody recognized was a baby eel. Tucker, the world's foremost eel expert, got fired for believing in the Loch Ness Monster. And as of 2026, the following remains true: no human being has ever witnessed European eels mating in the wild. No eel egg has been collected from the Sargasso Sea. The spawning act itself — the central event that makes every European eel that has ever existed — has never been observed by anyone.
Twenty-three centuries of looking, and the eel is still, in the way that matters most, keeping its secret.