1
00:00:06,843 --> 00:00:10,144
From Ancient Greece to the 20th century,

2
00:00:10,144 --> 00:00:14,244
Aristotle, Sigmund Freud,
and numerous other scholars

3
00:00:14,244 --> 00:00:17,334
were all looking for the same thing:

4
00:00:17,334 --> 00:00:19,784
eel testicles.

5
00:00:19,784 --> 00:00:25,916
Freshwater eels, or Anguilla Anguilla,
could be found in rivers across Europe,

6
00:00:25,916 --> 00:00:28,896
but no one had ever seen them mate.

7
00:00:28,896 --> 00:00:31,256
And despite countless dissections,

8
00:00:31,256 --> 00:00:37,726
no researcher could find eel eggs
or identify their reproductive organs.

9
00:00:37,726 --> 00:00:43,506
Devoid of data, naturalists proposed 
various eel origin stories.

10
00:00:43,506 --> 00:00:48,757
Aristotle suggested that eels 
spontaneously emerged from mud.

11
00:00:48,757 --> 00:00:53,561
Pliny the Elder argued eels 
rubbed themselves against rocks,

12
00:00:53,561 --> 00:00:57,391
and the subsequent scrapings 
came to life.

13
00:00:57,391 --> 00:01:02,748
Eels were said to hatch on rooftops,
manifest from the gills of other fish,

14
00:01:02,748 --> 00:01:06,736
and even emerge 
from the bodies of beetles.

15
00:01:06,736 --> 00:01:12,398
But the true story of eel reproduction
is even more difficult to imagine.

16
00:01:12,398 --> 00:01:15,098
And to solve this slippery mystery,

17
00:01:15,098 --> 00:01:19,268
scholars would have to rethink 
centuries of research.

18
00:01:19,268 --> 00:01:25,683
Today, we know the freshwater eel 
lifecycle has five distinct stages:

19
00:01:25,683 --> 00:01:32,893
larval leptocepheli, miniscule glass eels,
adolescent elvers,

20
00:01:32,893 --> 00:01:37,400
older yellow eels, and adult silver eels.

21
00:01:37,400 --> 00:01:41,320
Given the radical physical differences 
between these phases,

22
00:01:41,320 --> 00:01:45,200
you’d be forgiven for assuming 
these are different animals.

23
00:01:45,200 --> 00:01:49,770
In fact, that’s exactly what 
European naturalists thought.

24
00:01:49,770 --> 00:01:54,184
Researchers were aware of leptocepheli
and glass eels,

25
00:01:54,184 --> 00:01:58,284
but no one guessed they were related
to the elvers and yellow eels

26
00:01:58,284 --> 00:02:01,144
living hundreds of kilometers upstream.

27
00:02:01,144 --> 00:02:06,854
Confusing matters more, eels don’t 
develop sex organs until late in life.

28
00:02:06,854 --> 00:02:09,814
And the entirety of their time 
in the rivers of Europe

29
00:02:09,814 --> 00:02:12,754
is essentially eel adolescence.

30
00:02:12,754 --> 00:02:17,879
So when do eels reproduce, 
and where do they do it?

31
00:02:17,879 --> 00:02:23,085
Despite its name, the life 
of a freshwater eel actually begins

32
00:02:23,085 --> 00:02:26,605
in the salty waters 
of the Bermuda Triangle.

33
00:02:26,605 --> 00:02:29,305
At the height of the annual 
cyclone season,

34
00:02:29,305 --> 00:02:32,415
thousands of three-millimeter eel larvae

35
00:02:32,415 --> 00:02:34,945
drift out of the Sargasso Sea.

36
00:02:34,945 --> 00:02:39,440
From here, they follow migration 
paths to North America and Europe—

37
00:02:39,440 --> 00:02:41,520
continents that were 
much closer

38
00:02:41,520 --> 00:02:46,231
when eels established these routes 
40 million years ago.

39
00:02:46,231 --> 00:02:52,329
Over the next 300 days, Anguilla Anguilla
larvae ride the ocean currents

40
00:02:52,329 --> 00:02:56,521
6,500 km to the coast of Europe—

41
00:02:56,521 --> 00:03:00,669
making one of the longest 
known marine migrations.

42
00:03:00,669 --> 00:03:05,599
By the time they arrive, they’ve grown 
approximately 45 mm,

43
00:03:05,599 --> 00:03:09,982
and transformed into semi-transparent 
glass eels.

44
00:03:09,982 --> 00:03:12,412
It’s not just their appearance 
that’s changed.

45
00:03:12,412 --> 00:03:16,162
If most marine fish entered 
brackish coastal waters,

46
00:03:16,162 --> 00:03:21,180
their cells would swell with freshwater
in a lethal explosion.

47
00:03:21,180 --> 00:03:23,562
But when glass eels reach the coast,

48
00:03:23,562 --> 00:03:26,440
their kidneys shift to retain more salt

49
00:03:26,440 --> 00:03:29,190
and maintain their blood’s 
salinity levels.

50
00:03:29,190 --> 00:03:34,469
Swarms of these newly freshwater 
fish migrate up streams and rivers,

51
00:03:34,469 --> 00:03:39,893
sometimes piling on top of each other
to clear obstacles and predators.

52
00:03:39,893 --> 00:03:44,620
Those that make it upstream develop
into opaque elvers.

53
00:03:44,620 --> 00:03:47,430
Having finally arrived 
in their hunting grounds,

54
00:03:47,430 --> 00:03:51,430
elvers begin to eat everything 
they can fit into their mouths.

55
00:03:51,430 --> 00:03:55,030
These omnivores grow in proportion
to their diets,

56
00:03:55,030 --> 00:03:59,863
and over the next decade they develop
into larger yellow eels.

57
00:03:59,863 --> 00:04:04,388
In this stage, they grow 
to be roughly 80 cm,

58
00:04:04,388 --> 00:04:07,898
and finally develop sexual organs.

59
00:04:07,898 --> 00:04:13,481
But the last phase of eel life— 
and the secret of their reproduction—

60
00:04:13,481 --> 00:04:15,701
remains mysterious.

61
00:04:15,701 --> 00:04:21,850
In 1896, researchers identified 
leptocepheli as larval eels,

62
00:04:21,850 --> 00:04:26,089
and deduced that they had come 
to Europe from somewhere in the Atlantic.

63
00:04:26,089 --> 00:04:29,239
However, to find this mysterious 
breeding ground,

64
00:04:29,239 --> 00:04:32,839
someone would have to perform 
an unthinkable survey of the ocean

65
00:04:32,839 --> 00:04:36,759
for larvae no larger than 30mm.

66
00:04:36,759 --> 00:04:39,095
Enter Johannes Schmidt.

67
00:04:39,095 --> 00:04:42,699
For the next 18 years, 
this Danish oceanographer

68
00:04:42,699 --> 00:04:45,339
trawled the coasts of four continents,

69
00:04:45,339 --> 00:04:49,269
hunting down increasingly 
tiny leptocepheli.

70
00:04:49,269 --> 00:04:53,770
Finally, in 1921, he found 
the smallest larvae yet,

71
00:04:53,770 --> 00:04:58,010
on the southern edge 
of the Sargasso Sea.

72
00:04:58,010 --> 00:05:00,950
Despite knowledge 
of their round trip migration,

73
00:05:00,950 --> 00:05:04,340
scientists still haven’t observed 
mating in the wild,

74
00:05:04,340 --> 00:05:06,893
or found a single eel egg.

75
00:05:06,893 --> 00:05:09,843
Leading theories suggest 
that eels reproduce

76
00:05:09,843 --> 00:05:12,913
in a flurry of external fertilization,

77
00:05:12,913 --> 00:05:17,368
in which clouds of sperm 
fertilize free-floating eggs.

78
00:05:17,368 --> 00:05:21,588
But the powerful currents 
and tangling seaweed of the Sargasso Sea

79
00:05:21,588 --> 00:05:24,798
have made this theory 
difficult to confirm.

80
00:05:24,798 --> 00:05:27,468
Researchers don’t even know where to look,

81
00:05:27,468 --> 00:05:29,918
since they’ve yet to successfully
track an eel

82
00:05:29,918 --> 00:05:32,588
over the course of its return migration.

83
00:05:32,588 --> 00:05:34,998
Until these challenges can be met,

84
00:05:34,998 --> 00:05:40,502
the eel’s ancient secret will continue
to slip through our fingers.