the breeding places of the eel

In 1922 a Danish biologist named Johannes Schmidt published a paper in the Philosophical Transactions of the Royal Society. He called it The Breeding Places of the Eel. He had never seen an eel breed. He had never seen a breeding eel, or an eel egg, or anything resembling the act his title set out to locate. More than a century later, nobody has.

This is the strange fact at the center of one of the longest-running puzzles in natural history. The European eel, Anguilla anguilla, is not rare. It has lived in the rivers and ponds and ditches of Europe for as long as there have been rivers and ponds and people to notice them. You can catch one, eat one, keep one alive in a barrel for years. What no one can do — what no one has ever done — is watch one reproduce. The eel’s central act takes place somewhere out in the Atlantic, in the dark, and it has never once been observed.

For roughly two thousand years the working explanation was that eels did not reproduce at all. Aristotle, who dissected more animals than anyone before him and was wrong about this one with great confidence, concluded that eels arose spontaneously from mud — from what he called the earth’s guts. Ponds filled with eels after the rain; the eels had no eggs anyone could find and no sex anyone could locate; therefore they came from the ground itself, the way he believed certain insects came from dew. Later naturalists, working hard to improve on him, proposed that eels grew from horsehairs dropped in water, from the scrapings of rocks, from rooftiles, from rotting matter, from the bodies of a particular small beetle. These were not stupid people. They were looking at the animal honestly, and the animal was hiding the only part that mattered.

In 1876 the marine station at Trieste took on a young researcher, not yet twenty, sent to settle one corner of the question. The eel’s testes had never been identified. If eels reproduced sexually there had to be males, and males had to have testes, and someone simply needed to find them. The young man dissected something like four hundred eels over a few weeks, searching. He found nothing he could confidently call a testis. He wrote it up, inconclusive, and soon left marine biology for other work. His name was Sigmund Freud, and the eel was the first thing he failed to get to the bottom of.

He failed for a reason no one understood at the time. The eels he was opening did not have testes because eels in fresh water are not yet sexually mature — they will not be for years, sometimes for decades. The reproductive organs develop only later, only once the animal turns back toward the ocean and begins the journey out. Freud was searching for an organ the eel does not grow until it leaves. The part he wanted was not missing. It was simply not there yet.

There was another animal in the sea that nobody had connected to the eel: a tiny, transparent, leaf-shaped thing, flat as a willow leaf and clear as glass, that drifted in the open Atlantic. It had been caught, described, and given a scientific name of its own — its own genus, its own species, a fish in its own right. For about forty years it sat in the books as a separate creature. Then, in 1896, two Italian researchers kept some alive and watched them change — watched the glassy leaf shorten and thicken and darken into the unmistakable body of a young eel. The larva and the adult were the same animal. They had been filed under two different names because nothing about the one suggested the other.

This was the thread Schmidt picked up. If the leaf-larvae were infant eels, then they hatched somewhere, and the smallest ones would be found nearest to wherever that was. So he went looking for small larvae. From 1904 he trawled the Atlantic, year after year, through the First World War and around it, measuring every larva his nets brought up and marking where it was caught. The larvae got smaller toward the southwest. He followed the gradient the way you would follow a smell, drawing on the chart a set of rings — bands of diminishing larval size — that closed, finally, on a patch of warm still water in the western Atlantic, a sea with no shores, held in place by the currents that ring it: the Sargasso. The smallest larvae of all, the freshly hatched, were there and nowhere else. He had found the nursery by following the children home.

He never found the parents. Eighteen years of trawling produced the birthplace and not one breeding adult, not a single egg. He reasoned the center from its rim and published the inference, honestly titled, and it has held up. Every survey since has found the youngest larvae in the same warm water.

The eel makes the crossing on a body rebuilt for it. As it leaves fresh water it stops eating and never eats again; its gut shrinks and shuts down; its eyes enlarge for the deep dark ocean; its flanks turn silver. It swims somewhere between five and ten thousand kilometers on the fat it stored, ripening as it goes, and — it is presumed — spawns once at the end and dies. Presumed, because even now the act stays out of sight. In 2022 researchers fixed satellite tags to twenty-six of these silver eels and tracked them across the ocean; a handful reached the edge of the Sargasso, one to within the presumed spawning ground. Direct evidence, at last, that the adults finish the journey. Still no one has seen what they do when they arrive.

And there may not be much time left to see it. Since the 1980s the number of young eels returning to European rivers has fallen by around ninety-five percent. The species is now listed as critically endangered. It is entirely possible that the European eel will be gone before any human being witnesses it do the one thing its whole life is shaped around.

What we know about the breeding of the eel, we know the way Schmidt knew it: from the edges. From the larvae, the rings, the tags that pop loose at the boundary, the body that dismantles its own stomach for a one-way swim. The center — the act itself, the eggs, the meeting in the dark water — has been reasoned toward from every side for twenty-five centuries and never once seen plainly. It is one of the few things left on a thoroughly mapped planet that we understand almost entirely by its consequences and hardly at all by looking.

Aristotle was wrong about the mud. But he was right about the shape of the difficulty, which is that some things give themselves away only by what they leave behind — the pond suddenly full, the smallest larva in the warm far water — and never by being caught in the act. The eel has kept its one secret through spontaneous generation and horsehairs and Freud’s scalpel and Schmidt’s eighteen years and the whole satellite age. It is keeping it still.