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From Ancient Greece to the 20th century,

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Aristotle, Sigmund Freud,[br]and numerous other scholars

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were all looking for the same thing:

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eel testicles.

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Freshwater eels, or Anguilla Anguilla,[br]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[br]or identify their reproductive organs.

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Devoid of data, naturalists proposed [br]various eel origin stories.

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Aristotle suggested that eels [br]spontaneously emerged from mud.

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Pliny the Elder argued eels [br]rubbed themselves against rocks,

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and the subsequent scrapings [br]came to life.

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

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and even emerge [br]from the bodies of beetles.

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

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And to solve this slippery mystery,

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scholars would have to rethink [br]centuries of research.

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

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larval leptocepheli, miniscule glass eels,[br]adolescent elvers,

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older yellow eels, and adult silver eels.

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Given the radical physical differences [br]between these phases,

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

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

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Researchers were aware of leptocepheli[br]and glass eels,

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but no one guessed they were related[br]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 [br]develop sex organs until late in life.

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And the entirety of their time [br]in the rivers of Europe

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is essentially eel adolescence.

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So when do eels reproduce, [br]and where do they do it?

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

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in the salty waters [br]of the Bermuda Triangle.

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At the height of the annual [br]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 [br]paths to North America and Europe—

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continents that were [br]much closer

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when eels established these routes [br]40 million years ago.

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Over the next 300 days, Anguilla Anguilla[br]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 [br]known marine migrations.

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

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and transformed into semi-transparent [br]glass eels.

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It’s not just their appearance [br]that’s changed.

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If most marine fish entered [br]brackish coastal waters,

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their cells would swell with freshwater[br]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 [br]salinity levels.

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

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

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Those that make it upstream develop[br]into opaque elvers.

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Having finally arrived [br]in their hunting grounds,

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

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These omnivores grow in proportion[br]to their diets,

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

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In this stage, they grow [br]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— [br]and the secret of their reproduction—

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remains mysterious.

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In 1896, researchers identified [br]leptocepheli as larval eels,

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

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However, to find this mysterious [br]breeding ground,

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someone would have to perform [br]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, [br]this Danish oceanographer

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trawled the coasts of four continents,

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hunting down increasingly [br]tiny leptocepheli.

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Finally, in 1921, he found [br]the smallest larvae yet,

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on the southern edge [br]of the Sargasso Sea.

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Despite knowledge [br]of their round trip migration,

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scientists still haven’t observed [br]mating in the wild,

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or found a single eel egg.

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Leading theories suggest [br]that eels reproduce

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in a flurry of external fertilization,

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in which clouds of sperm [br]fertilize free-floating eggs.

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But the powerful currents [br]and tangling seaweed of the Sargasso Sea

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have made this theory [br]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[br]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[br]to slip through our fingers.