One of the natural world’s longest-running mysteries spanned thousands of years before it was finally solved: where did eels come from?
European eels (Anguilla anguilla) have been eaten since at least the Roman times and experienced a particular boom in popularity in Britain the 17th and 18th centuries where they were caught in the River Thames.
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But beyond being found in freshwater environments, no one seemed to know where these snake-like creatures came from.
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The origin of eels
Scientists couldn’t find the reproductive organs of eels, leaving them with no explanation for their seemingly sudden appearance in adult form.
The mystery began around the 4th century BCE, with the Greek philosopher Aristotle widely quoted as believing that eels came from “the entrails of the earth” – meaning that they spontaneously appeared.
This echoes the Ancient Egyptian belief that eels sprang up from the River Nile when the sun warmed its waters.
The theory of spontaneous spawning was widely believed for a long time. Similar beliefs also circulated, such as new eels generating from the dead skin of existing eels, or that the creatures grew from the hairs of horses drinking from pools of water.
Then finally, in the late 18th century, anatomist and physician Carol Mondini conducted a detailed study of the reproductive organs of a mature female European eel. The race to find gonads was now on.
One person that took on this challenge was a young Sigmund Freud, as part of his early university studies. In 1876, Freud worked at a marine biology research station in Trieste, northeast Italy, for one month. There, he was tasked with dissecting 400 male eels to search for testes – with no luck.
He later moved away from biology and medicine and become one of the most pioneering psychoanalysts of his time.
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Metamorphic lifecycle
Unbeknownst to the researchers, they were all were missing a key piece of information: eels don’t become sexually mature until they have to. When they’re ready to reproduce, eels begin a one-way migration thousands of miles across the Atlantic. After they breed, they die.
The life cycle of an eel also includes several metamorphoses, with these different stages previously being mistaken for different species. The European eel spends most of its life as a yellow eel, which is the most familiar life stage that people would recognise.
However, when they first turn up on European shores, the larvae are in the form of a glass eel.
Leptocephalus brevirostris was initially described as an individual species, but studies in the 19th century, which kept them in laboratory tanks, revealed that they metamorphosed into larger glass eels.
From 1904, the Danish professor Johannes Schmidt led a series of expeditions into the Mediterranean Sea and the North Atlantic to investigate eels. Schmidt discovered that the further west the eel larvae were caught, the smaller they were.
He concluded that they must travel from European freshwaters to reproduce in the Sargasso Sea in the far west of the North Atlantic – although without any direct evidence, he could not prove his hypothesis.
When he first identified the presumed spawning area, he stated: “Years of research rich in excitement and suspense: disappointment alternating with encouraging discoveries and periods of rapid progress with others during which the solution of the problem seemed wrapped in deeper darkness than ever.”
Although his theory has been widely accepted, there was no direct evidence for this until as recently as 2022 – almost 100 years since Schmidt’s proposal.
An international team of researchers, which included those working for the UK’s Environment Agency, fitted eels with satellite tags. Previous studies had shown tracked eels making their way to the Azores archipelago, in the North Atlantic Ocean around 1,500km west of Portugal, but no further.
The researchers tagged eels already in the Azores and discovered that they were making their way to the Sargasso Sea.
While they were once eaten in abundance, the European eel is now classified as critically endangered. The researchers hoped that by finally solving the mystery of their migration, the results can be used to inform conservation efforts.
Top image: glass eels, the juvenile stage of the European eel. Credit: Getty
