Where Do Eels Come From? The Sargasso Sea Mystery

Aristotle and the eels born from mud

For most of history, eels were a puzzle. People caught huge numbers of them in rivers and lakes, yet nobody ever found an eel with eggs or milt, and nobody saw a baby eel. In the 4th century BC the Greek philosopher Aristotle concluded that eels did not breed at all. He believed they arose from what he called the guts of the earth, worm-like bodies that formed spontaneously in mud and wet soil.

Other explanations followed. The Roman writer Pliny the Elder suggested that eels rubbed themselves against rocks and that the scraps of skin came to life. In parts of Britain, people long believed that horsehairs falling into ponds turned into eels. These ideas sound odd today, but they reflect a real mystery: the European eel does not mature until it leaves fresh water, so its breeding organs stay undeveloped as long as it lives in rivers.

A search for eggs and sex organs

Scientists gradually filled in the gaps. In 1777 the Italian anatomist Carlo Mondini described the ovaries of a female eel, proving that eels were ordinary fish that produce eggs. Males were harder. In 1876 a young medical student in Trieste named Sigmund Freud dissected hundreds of eels looking for their testes, without a clear result, and he soon turned to other subjects.

The missing link was the larva. For decades naturalists had been catching small, flat, transparent sea creatures shaped like willow leaves. They were classed as a separate fish, Leptocephalus brevirostris, meaning “short-snouted thin head”. In the 1890s the Italian zoologists Giovanni Battista Grassi and Salvatore Calandruccio watched these larvae, caught in the Strait of Messina, shrink and change into young eels. The leptocephalus was the eel’s baby, which meant eels must breed somewhere in the sea.

Johannes Schmidt follows the smallest larvae

The man who found the birthplace was the Danish biologist Johannes Schmidt. In 1904, aboard the research ship Thor, he caught a European eel larva west of the Faroe Islands, the first ever found in the open Atlantic. At the time, many experts thought eels bred in the Mediterranean.

Schmidt realized that the size of the larvae could act as a map. He spent almost two decades towing fine nets across the Atlantic, from Europe to the Caribbean, recording where each larva was caught and how long it was. The farther west and south he went, the smaller the larvae became. On his Dana expeditions in the early 1920s he caught the tiniest larvae of all, less than 1 cm (0.4 in) long, in the southern Sargasso Sea. In 1922 and 1923 he published his conclusion: this was where European eels were born.

Schmidt never saw an eel egg or a spawning adult, but his larval map has held up for a century. Later surveys have repeatedly found the smallest larvae in the same region.

Tracking adult eels to the Sargasso Sea

The next challenge was to follow the parents. Adult silver eels leave European rivers in autumn and swim out into the deep ocean, where they are almost impossible to observe. From the 2000s, researchers began fitting migrating eels with satellite tags that record depth, temperature and light, then pop up to the surface and transmit their data.

Studies of hundreds of tagged eels released from several European countries, including Ireland, France and Sweden, showed that the fish swim deep by day and rise toward the surface at night, and that they head southwest toward the Azores. Many tags were lost along the way, however, often to predators.

In 2022, an international team of scientists published the first direct evidence that adult eels reach the Sargasso Sea. They attached tags to 26 large eels from rivers in the Azores, which lie partway along the route, and tracked them for 40 to 366 days. The eels swam at only about 3–12 km (2–7.5 mi) a day, and some were tracked all the way into the presumed spawning area, a trip that took them many months.

The long drift back to Europe

After hatching, the leaf-shaped larvae rise into the upper layers of the ocean and are carried northeast by the Gulf Stream and the North Atlantic Drift. They seem to feed on marine snow, the tiny particles of organic matter that sink through the sea, and on small jellyfish-like animals. Estimates of how long the drift takes vary widely, from under a year to about two years.

When the larvae reach the edge of the European continental shelf, they transform into glass eels, small transparent eels about 6–8 cm (2.5–3 in) long that swim into estuaries along the coasts from Morocco to Norway. In 2017 scientists showed that young eels can sense Earth’s magnetic field and respond to it in ways that would help them reach the Gulf Stream, a clue to how such small animals find their way. The stages that follow are described in What are glass eels?

What we still do not know

Even after a century of research, nobody has seen European eels spawn in the wild, and no eel eggs or spawning adults have ever been collected in the Sargasso Sea. Scientists still do not know exactly how deep or when the eels spawn, how they navigate across the open ocean, or why they travel so far to breed. The American eel shares the same breeding area, spawning a little farther west, and its larvae drift to North America instead.

These open questions matter because the European eel is now Critically Endangered, and conditions in the ocean may affect how many larvae survive. Its journey is one of the most remarkable on the list of longest animal migrations, and the threats it faces are covered in Why are European eels endangered?