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From Ancient Greece to the 20th century, 
Aristotle, Sigmund Freud,

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and numerous other scholars were all 
looking for the same thing: eel testicles.

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

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but no one had ever seen them mate.

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And despite countless dissections,

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

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Devoid of data, naturalists proposed 
various eel origin stories.

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Aristotle suggested that eels spontaneously
emerged from mud.

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Pliny the Elder argued eels rubbed 
themselves against rocks,

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and the subsequent scrapings 
came to life.

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

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and even emerge from the 
bodies of beetles.

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

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And to solve this slippery mystery, 
scholars

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would have to rethink 
centuries of research.

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

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larval leptocepheli, miniscule glass 
eels, adolescent elvers,

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older yellow eels, and adult silver eels.

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Given the radical physical differences 
between these phases,

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

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

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Researchers were aware of leptocepheli
and glass eels,

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

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living hundreds of kilometers upstream.

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

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And the entirety of their time in the
rivers of Europe

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is essentially eel adolescence.

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

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

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in the salty waters of 
the Bermuda Triangle.

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At the height of the annual cyclone
season,

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thousands of three-millimeter eel larvae

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drift out of the Sargasso Sea.

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

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continents that were much 
closer when eels

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established these routes 
40 million years ago.

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

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6,500 km to the coast of Europe––

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making one of the longest 
known marine migrations.

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

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and transformed into semi-transparent 
glass eels.

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

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If most marine fish entered brackish
costal waters,

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their cells would swell with freshwater
in a lethal explosion.

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But when glass eels reach the coast,

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their kidneys shift to retain more salt

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and maintain their blood’s 
salinity levels.

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

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

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Those that make it upstream develop
into opaque elvers.

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Having finally arrived in their hunting
grounds,

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

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These omnivores grow in proportion
to their diets,

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

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

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and finally develop sexual organs.

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

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remains mysterious.

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In 1896, researchers identified 
leptocepheli as larval eels,

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

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However, to find this mysterious breeding
ground,

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someone would have to perform an 
unthinkable survey of the ocean

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for larvae no larger than 30mm.

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Enter Johannes Schmidt.

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For the next 18 years, this Danish 
oceanographer

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trawled the coasts of four continents,

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hunting down increasingly 
tiny leptocepheli.

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

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on the southern edge 
of the Sargasso Sea.

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Despite knowledge of their round trip
migration,

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scientists still haven’t observed 
mating in the wild,

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or found a single eel egg.

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Leading theories suggest that eels
reproduce

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in a flurry of external fertilization,

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in which clouds of sperm fertilize
free-floating eggs.

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But the powerful currents and 
tangling seaweed of the Sargasso Sea

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have made this theory 
difficult to confirm.

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Researchers don’t even know where to look,

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since they’ve yet to successfully track 
an eel

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over the course of its return migration.

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Until these challenges can be met,

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

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