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From Ancient Greece to the 20th century,
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Aristotle, Sigmund Freud, 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, 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,
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scholars 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
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when eels 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 coastal 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 into 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 to slip through our fingers.