From Ancient Greece to the 20th century, 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.
