Throughout history, we humans have been drawn
to the biggest of questions. Where do we come from? Is there a God? Do we as a species have the capacity to overcome
our differences and work together to build a better future for us all? And of course, perhaps the most profound of
these big questions: do eels have testicles? OK, so maybe I’m slightly overstating the
importance of that last one - the hunt for eel gonads might not have quite the same scientific
glamour as the search for dark matter or a theory of everything. But, oddly enough, it’s a question that’s
had a hell of a lot of attention over the years in the scientific community. Because, as weird as it sounds, eels lack
balls. And I don’t mean that in some vague idiomatic
way, either - they don’t get all weak at the fins at the thought of extreme sports
or routinely back down from fights. No. They literally don’t have testicles. But if that’s the case, how on God’s green
earth do they reproduce? Well, before we get onto that, what exactly
are eels? While they kind of resemble snakes that have
gone into hiding underwater on account of being so monumentally ugly, they’re actually
a kind of fish. But for multitudinous reasons they’re remarkably
different to pretty much any other fish species you care to mention. Take European freshwater eels, for example. They lack scales, for a start, and can breathe
air through their skin. That ability allows them to survive out of
water for up to 48 hours, and they’re surprisingly nifty movers on land – they can even climb. They hibernate in the winter if it gets too
cold by burying themselves in mud, a trick they also employ if the pond they’re living
in happens to dry out in the summer. They can live to almost 100 years of age,
their blood is poisonous, and they’re also remarkably difficult to kill, and will keep
on swimming for hours even after you’ve cut off their heads thanks to a decidedly
stubborn nervous system. All in all, eels are pretty bloody odd. And that’s before we even get to their troubling
lack of testes. So, how does an animal without reproductive
organs create offspring? Believe it or not, this conundrum has occupied
some of humankind’s most brilliant thinkers for at least a couple of millennia. Aristotle was probably the first to grapple
with it, but the great Greek philosopher and thinker was as perplexed as anyone else by
the eels’ lack of Crown Jewels. Still, as the father of western logic, he
did manage to come up with a fittingly logical explanation. Eels don’t have reproductive organs, according
to Aristotle, because they don’t reproduce. And if they don’t reproduce, he reasoned,
they must come to life through some other mechanism. Ultimately, he concluded eels spontaneously
come into being when rainwater mixes with mud. Not one of his finest moments, that, but considering
he basically invented science, I think we can give him a pass on this one. As you’re no doubt aware, Aristotle’s
influence on science loomed large long after his death. And for that reason, the belief eels came
to life out of nothing was widely accepted as scientific fact for some two thousand years,
though others did come along to put their own hypotheses forward as to exactly how the
process worked. Roman author Pliny the Elder, who died in
the aftermath of the famous eruption of Mount Vesuvius that buried Pompei in 79 AD, believed
eels came into being from the partially shed skins of other eels. Others felt certain eels sprouted from sea
foam or burst into life when sunlight hit open stretches of water in a certain way. The Scots thought eels started out life as
beetles, and the English were convinced they were born when hairs from horse tails fell
into river water. I know I know; nobody comes out of this with
much credit, but hey - those were simpler times. It took a while, but eventually people began
to question whether it was really possible for an animal to just come to life from nothing. After all, the very idea seemed to violate
the laws of nature mankind had slowly been uncovering in the millennia since Aristotle
met his maker. One man who could firmly be considered a sceptic
was Sigmund Freud. And yes, I do mean that Sigmund Freud. You might be wondering what the most famous
psychoanalyst in history has to do with mankind’s hunt for eel goolies, and I’ll admit it
is a little weird to see him crop up in this story. But then again, if you know anything about
Freud, you’ll know the guy was absolutely obsessed with genitals, so the fact he was
once interested in eel nuts perhaps shouldn’t come as much of a surprise. In 1876, when Freud was still an enthusiastic
medical student at the University of Vienna, he traveled to the city of Trieste in north
east Italy, a stone’s throw away from the Slovenian border. The purpose of the trip was simple - Freud
wanted to do something nobody else ever had: get his hands on a nice set of eel balls. To that end, he took himself down to the city’s
port each morning to meet the fisherman coming home with their catch, from whom he would
buy as many eels as he could afford. Then he went back home to begin the dissections. Freud spent several months in Trieste, and
it’s thought he worked his way through as many as 400 eels during that time. The gory task seems to have gotten to him,
too, because he sounded on the verge of losing his marbles in one letter to a friend, where
he confessed: “All I see when I close my eyes is the shimmering dead tissue, which
haunts my dreams." It’s no wonder he decided to take his career
in another direction not long afterwards. Especially when you consider all that hard
work eviscerating eels hadn’t brought about the discovery Freud was hoping for. He’d found some interesting looking bits
and pieces inside the abdominal cavities of several large specimens, but there was nothing
he could definitely, conclusively claim to be an eel testicle. Freud had failed. Still, he wasn’t the only one sceptical
of the stories of miraculous eel origins, and ten years later in 1886, a breakthrough
came. French zoologist Yves Delage had been keeping
an odd, leaf-shaped marine creature known as a leptocephalus in a tank in his lab for
several months when, quite unexpectedly, the strange little beast turned into something
else entirely. Yves had observed something that would change
our understanding of eels forever. You see, eels are metamorphic creatures, and
they go through 6 distinct stages in what can only be described as one hell of an odd
lifecycle. They start out as leptocephali much like the
one Yves kept in his fish tank, before turning into transparent glass eels. Glass eels become elvers, elvers become yellow
eels, and yellow eels become silver eels, which are basically the eel’s endgame form. The funny thing is, science had known about
all of these individual animals for a long time - fisherman had been catching them and
people had been eating them since time immemorial. Jellied eel, anyone? But it had always been assumed they were different
species entirely. Nobody had ever realised they were all the
same animal at different stages of its lifecycle. Although Yves witnessed the metamorphosis
first hand, it took a decade or so for his discovery to be widely accepted. That might be because it went against conventional
wisdom, or it may just be that people didn’t take poor old Yves too seriously. You see, he had a habit of making outlandish
statements, and in later life would famously claim to have ‘calculated’ that the Turin
Shroud - which some people believe is the fabric Christ was buried in after the crucifixion
– had a 1 in 10 billion chance of being a fake. I’d really love to see your working there
Yves. Anyway, it was with the discovery of this
highly unusual life cycle that the missing piece fell into place to help science finally
understand why eels lack testicles. Not because they spontaneously come to life
from mud, not because they’re remarkably energetic horse tail hairs, but because they
grow reproductive organs only when they need them in their final, mature form. And you have to hand it to them; it does sound
like a sensible system. Because, as any guy will tell you, when you
aren’t using them genitals don’t half get in the way. So, one of the oldest questions in the animal
kingdom had finally been put to bed. But, rather annoyingly, the answer brought
with it a question of its own. Because the first stage of the eel’s life
cycle - the leptocephalus - was a marine creature - it was found only in the ocean. Which neatly explained why poor old Sigmund
had never stumbled across so much as a sniff of testicle during his 400 dissections - by
the time eels reach their mature stage – which can take anything from 7 to 70 years - they’ve
left our shores far behind. So, that being the case - where exactly do
all these millions of eels bugger off to each year? In 1904, Danish scientist Johannes Schmidt
was asking himself the very same question. Luckily, Schmidt was a doer. When he came across a question that left him
scratching his head, he didn’t just shrug his shoulders and go make a sandwich. He went out and found the answer - even if
it meant dedicating his life to the task. Which is why, over a period of some 19 years,
Schmidt carried out the biggest survey of leptocephali ever completed before or since. He was particularly interested in their size
- and for good reason. Because, as he slowly built up more and more
data, he began to realise something - the closer he got to a certain point in the Atlantic
Ocean, the smaller the leptocephali he netted were, until eventually, they couldn’t get
any smaller. He’d found the eel’s spawning grounds
– smack in the middle of the Sargasso Sea. Not only was that rather fitting - because
much like the creatures who go there for a bit of ‘sexual eeling’, the Sargasso itself
is a bit of an enigma - it was also pretty staggering. Because it meant European freshwater eels
were undertaking a previously unknown migration of some 6000 kilometres to have their young. The Sargasso is the only sea in the world
that doesn’t have a single land border. Instead, it’s surrounded by a vortex of
ocean currents that completely encircle it on all sides. It’s a sea within a sea, if you like, and
its border currents act a bit like a plug hole for the Atlantic. That’s why, as well as millions of randy
eels, it’s also home to the North Atlantic garbage patch - hundreds of square kilometres
of ocean filled with rubbish. Nice one, humans. Despite Schmidt’s conclusions, which are
still accepted today, nobody has ever actually seen eels mating in the Sargasso Sea. In fact, even with all our fancy technology,
nobody has ever seen eels in the Sargasso at all. Despite the fact we’re pretty confident
millions of them make the journey there every year. And in keeping with that somewhat patchy understanding
of the behaviour of our slippery friends, we don’t really know how the tiny eel larvae
born there make it the 6000 kilometres back to Europe, either. Their strange, leaf-like shape is thought
to help with drifting, but some researchers believe active swimming plays at least some
part in the trip. We also don’t know how long the journey
takes, or even how long eels stay in their larval state - estimates range from as little
as 6 months to as many as 3 years. So yeah, we may have found their balls, but
eels still seem to have plenty of other secrets hidden up their sleeves. Well, sleeve, I guess. We’ve mostly been covering freshwater eels
in this video, but eels don’t get any less weird no matter which bit of the family tree
you’re looking at. There are snipe eels that look a bit like
nightmarish lassos, slender moray eels that can grow to be 4 metres long, and European
congas that can weigh as much as 110 kilos. Rarely seen gulper eels live at depths of
up to 3000 metres and are a confusing mix of disturbing and cute - kind of like what
might have happened if Nintendo had decided to turn Kirby into a horror franchise. And then there are parasitic snubnosed eels. These disturbing little living horrors use
their powerful jaws to burrow into the bodies of larger fish, where they just… make themselves
at home. Typically they can be found snuggled up alongside
the spines of species like cod and halibut, but they’ve also been known to burrow directly
into the abdomen of gravid fish where they promptly devour thousands of unhatched eggs
within the space of just a few minutes. In 1992, two parasitic eels were even found
to have made a cosy residence in the still-beating heart of a 400kg Mako shark caught near Montauk
in New York State. I should probably mention electric eels, too. Not because they can deliver an electric shock
of close to a thousand volts - although they can - but because they aren’t actually eels
at all - they’re a kind of knifefish. Bit of an embarrassing slip there, hey science
- although not an especially uncommon one. Killer whales aren’t whales - they’re
members of the dolphin family. American buffalo are actually bison, honey
badgers are most closely related to martens, and horny toads are in fact lizards - and,
I might add, no more desperate for sex than any other animal out there. Knifefish imposters aside, eels are some of
the weirdest animals ever to have existed on planet earth. But while there are still plenty of mysteries
left to uncover about these strange and elusive creatures, we do at least finally know for
sure that, yes, they do have balls. I’m sure you’ll all sleep easier tonight
now that’s been cleared up. Which should give you plenty of time to vividly
imagine a parasitic snubnosed eel eating its way through your chest and taking up residence
in your heart. Thanks for watching.
