Aristotle, Sigmund Freud, and numerous other scholars

were all looking for the same thing:

eel testicles.

Freshwater eels, or Anguilla Anguilla, could be found in rivers across Europe,

but no one had ever seen them mate.

And despite countless dissections,

no researcher could find eel eggs or identify their reproductive organs.

Devoid of data, naturalists proposed various eel origin stories.

Aristotle suggested that eels spontaneously emerged from mud.

Pliny the Elder argued eels rubbed themselves against rocks,

and the subsequent scrapings came to life.

Eels were said to hatch on rooftops, manifest from the gills of other fish,

and even emerge from the bodies of beetles.

But the true story of eel reproduction is even more difficult to imagine.

And to solve this slippery mystery,

scholars would have to rethink centuries of research.

Today, we know the freshwater eel lifecycle has five distinct stages:

larval leptocepheli, miniscule glass eels, adolescent elvers,

older yellow eels, and adult silver eels.

Given the radical physical differences between these phases,

you'd be forgiven for assuming these are different animals.

In fact, that's exactly what European naturalists thought.

Researchers were aware of leptocepheli and glass eels,

but no one guessed they were related to the elvers and yellow eels

living hundreds of kilometers upstream.

Confusing matters more, eels don't develop sex organs until late in life.

And the entirety of their time in the rivers of Europe

is essentially eel adolescence.

So when do eels reproduce, and where do they do it?

Despite its name, the life of a freshwater eel actually begins

in the salty waters of the Bermuda Triangle.

At the height of the annual cyclone season,

thousands of three-millimeter eel larvae

drift out of the Sargasso Sea.

From here, they follow migration paths to North America and Europe—

continents that were much closer

when eels established these routes 40 million years ago.

Over the next 300 days, Anguilla Anguilla larvae ride the ocean currents

6,500 km to the coast of Europe—

making one of the longest known marine migrations.

By the time they arrive, they've grown approximately 45 mm,

and transformed into semi-transparent glass eels.

It's not just their appearance that's changed.

If most marine fish entered brackish coastal waters,

their cells would swell with freshwater in a lethal explosion.

But when glass eels reach the coast,

their kidneys shift to retain more salt

and maintain their blood's salinity levels.

Swarms of these newly freshwater fish migrate up streams and rivers,

sometimes piling on top of each other to clear obstacles and predators.

Those that make it upstream develop into opaque elvers.

Having finally arrived in their hunting grounds,

elvers begin to eat everything they can fit into their mouths.

These omnivores grow in proportion to their diets,

and over the next decade they develop into larger yellow eels.

In this stage, they grow to be roughly 80 cm,

and finally develop sexual organs.

But the last phase of eel life— and the secret of their reproduction—

remains mysterious.

In 1896, researchers identified leptocepheli as larval eels,

and deduced that they had come to Europe from somewhere in the Atlantic.

However, to find this mysterious breeding ground,

someone would have to perform an unthinkable survey of the ocean

for larvae no larger than 30mm.

Enter Johannes Schmidt.

For the next 18 years, this Danish oceanographer

trawled the coasts of four continents,

hunting down increasingly tiny leptocepheli.

Finally, in 1921, he found the smallest larvae yet,

on the southern edge of the Sargasso Sea.

Despite knowledge of their round trip migration,

scientists still haven't observed mating in the wild,

or found a single eel egg.

Leading theories suggest that eels reproduce

in a flurry of external fertilization,

in which clouds of sperm fertilize free-floating eggs.

But the powerful currents and tangling seaweed of the Sargasso Sea

have made this theory difficult to confirm.

Researchers don't even know where to look,

since they've yet to successfully track an eel

over the course of its return migration.

Until these challenges can be met,

the eel's ancient secret will continue to slip through our fingers.