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From Ancient Greece to the 20th century, Aristotle, Sigmund Freud, and numerous other scholars were all looking for the same thing: eel testicles.
从古希腊到20世纪，亚里士多德、西格蒙德·弗洛伊德以及许多其他学者都在寻找同样的东西：鳗鱼睾丸。
Freshwater eels, or Anguilla Anguilla, could be found in rivers across Europe, but no one had ever seen them mate.
淡水鳗鱼，又称安圭拉鳗，在欧洲各地河流中都能见到，但没人见过它们交配。
And despite countless dissections, no researcher could find eel eggs or identify their reproductive organs.
尽管进行了无数次解剖，但没有研究人员能找到鳗鱼卵或识别出它们的生殖器官。
Devoid of data, naturalists proposed various eel origin stories.
由于缺乏数据，博物学家提出了各种鳗鱼起源的故事。
Aristotle suggested that eels spontaneously emerged from mud.
亚里士多德认为鳗鱼会自发地从泥里钻出来。
Pliny the Elder argued eels rubbed themselves against rocks, and the subsequent scrapings came to life.
老普林尼则认为，鳗鱼会在岩石上摩擦自己，随后刮下的碎屑就有了生命。
Eels were said to hatch on rooftops, manifest from the gills of other fish, and even emerge from the bodies of beetles.
据说鳗鱼会在屋顶孵化、从其他鱼类的鳃中钻出来、甚至从甲虫的身体中出现。
But the true story of eel reproduction is even more difficult to imagine.
但鳗鱼繁殖的真相更难想象。
And to solve this slippery mystery, scholars would have to rethink centuries of research.
为了解决这个棘手的谜团，学者们必须重新思考几个世纪以来的研究。
Today, we know the freshwater eel lifecycle has five distinct stages:
如今，已知淡水鳗的生命周期有五个不同的阶段：
larval leptocepheli, miniscule glass eels, adolescent elvers, older yellow eels, and adult silver eels.
细鳞鳗鱼小鱼、微型玻璃鳗鱼、青少年鳗鱼、老年黄鳗鱼和成年银鳗鱼。
Given the radical physical differences between these phases, you'd be forgiven for assuming these are different animals.
考虑到鳗鱼在这些阶段存在根本的物理差异，把它们当做不同物种是可以理解的。
In fact, that's exactly what European naturalists thought.
其实，这正是欧洲博物学者的看法。
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.
研究人员知道细叶鳗和玻璃鳗，但没有人猜到它们与白叶鳗和黄鳗有关，后者生活在上游数百公里处。
Confusing matters more, eels don't develop sex organs until late in life.
更令人困惑的是，鳗鱼到晚年才发育出性器官。
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?
它们在欧洲河流中的全部时间基本上都是鳗鱼的青春期。那么鳗鱼什么时候繁殖，又在哪里繁殖呢？
Despite its name, the life of a freshwater eel actually begins in the salty waters of the Bermuda Triangle.
尽管名字叫淡水鳗鱼，但它的生命其实起源于百慕大三角的咸水海域。
At the height of the annual cyclone season, thousands of three-millimeter eel larvae drift out of the Sargasso Sea.
在每年气旋季节的高峰期，成千上万条3毫米长的鳗鱼鱼苗会从马尾藻海漂流而出。
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.
从这里出发，它们沿迁徙路线到达北美洲和欧洲--4千万年前鳗鱼建立这些路线时，这两大洲的距离要近得多。
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.
在接下来的300天里，安圭拉鱼苗顺着洋流漂流6500公里到达欧洲海岸--这是已知最长的海洋迁徙之一。
By the time they arrive, they've grown approximately 45 mm, and transformed into semi-transparent glass eels.
它们到达的时候，已经长到大约45毫米长，变成了半透明的玻璃鳗鱼。
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.
改变的不仅仅是他们的外表。大多数海鱼如果进入含盐的沿海水域，淡水会令它们的细胞膨胀，从而导致致命的爆炸。
But when glass eels reach the coast, their kidneys shift to retain more salt and maintain their blood's salinity levels.
但是当玻璃鳗鱼到达海岸时，它们的肾脏转而会保留更多盐分，并保持血液中的盐分水平。
Swarms of these newly freshwater fish migrate up streams and rivers, sometimes piling on top of each other to clear obstacles and predators.
这些新出现的淡水鱼群向溪流和河流上游迁徙，有时会在彼此身上堆积起来，以清除障碍物和捕食者。
Those that make it upstream develop into opaque elvers.
那些成功抵达上游的鱼变成了不透明的鳗鱼。
Having finally arrived in their hunting grounds, elvers begin to eat everything they can fit into their mouths.
在终于抵达狩猎场后，它们开始吞食任何能吃东西。
These omnivores grow in proportion to their diets, and over the next decade they develop into larger yellow eels.
这些杂食动物的生长速度与其食量成比例，在接下来的十年里，它们会长成更大的黄鳝。
In this stage, they grow to be roughly 80 cm, and finally develop sexual organs.
这个阶段，它们大约会长到80厘米长，并终于发育出性器官。
But the last phase of eel life -- and the secret of their reproduction -- remains mysterious.
但鳗鱼生命的最后阶段以及它们繁殖的秘密，仍然是一个谜。
In 1896, researchers identified leptocepheli as larval eels, and deduced that they had come to Europe from somewhere in the Atlantic.
1896年，研究人员确认细鳞鱼是幼鳗，并推断出它们是从大西洋某处来到了欧洲。
However, to find this mysterious breeding ground, someone would have to perform an unthinkable survey of the ocean for larvae no larger than 30mm.
然而，要找到这个神秘繁殖地，就必须有人对海洋进行难度极大的搜寻，来寻找这些不足30毫米的幼虫。
Enter Johannes Schmidt. For the next 18 years, this Danish oceanographer trawled the coasts of four continents, hunting down increasingly tiny leptocepheli.
约翰内斯·施密特就是真正采取行动的那个人。在接下来的18年里，这位丹麦海洋学家在四大洲的海岸进行了拖网捕鱼，捕捉到越来越小的细鳞鱼。
Finally, in 1921, he found the smallest larvae yet, on the southern edge of the Sargasso Sea.
终于，在1921年，他在马尾藻海的南部边缘发现了迄今为止最小的鱼苗。
Despite knowledge of their round trip migration, scientists still haven't observed mating in the wild, or found a single eel egg.
尽管科学家们已经了解了鳗鱼的往返迁徙，但还从未在野外观察到交配现象，也没有发现过一颗鳗鱼卵。
Leading theories suggest that eels reproduce in a flurry of external fertilization, in which clouds of sperm fertilize free-floating eggs.
主流理论认为，鳗鱼繁殖是一连串外部受精过程，在这个过程中，大量的精子为自由漂浮的卵子受精。
But the powerful currents and tangling seaweed of the Sargasso Sea have made this theory difficult to confirm.
但马尾藻海强大的海流和缠结在一起的海藻使得这个理论难以被证实。
Researchers don't even know where to look, since they've yet to successfully track an eel over the course of its return migration.
研究人员甚至不知该去哪里观察，因为他们还没有成功地追踪到鳗鱼的洄游过程。
Until these challenges can be met, the eel's ancient secret will continue to slip through our fingers.
在这些挑战得以解决之前，鳗鱼的古老秘密将继续从我们指缝中溜走。
