Why does this animal never age? - Raymond Adkins · TED-Ed
本视频探索水螅惊人的再生能力与生物学特性，揭秘其近乎“永生”的秘密。内容涵盖其发现历史、身体结构、捕食与繁殖方式，适合练习生物科普类听力与相关学术词汇。
In 1740, while Swiss naturalist Abraham Trembley was tutoring the sons of a nobleman, he had scooped up some pond water for them to study when he found something shocking inside.
1740年，瑞士博物学家Abraham Trembley在给一位贵族的儿子们当家庭教师时，从池塘里舀了些水供他们研究，结果发现里面有令人震惊的东西。
He couldn't identify the odd little thing or even tell if it was a plant or an animal.
他无法辨认这个奇怪的小东西，甚至分不清它是植物还是动物。
Its way of moving and consuming things seemed animal-like.
它的移动方式和进食方式似乎像动物。
But when he cut parts off, or even split it in two, it regenerated, which was decidedly plant-like, in keeping with its green hue.
但当他切掉它的部分身体，甚至把它一分为二时，它却能再生，这显然更像植物，与它绿色的外表相符。
Trembley noted the organism’s similarity to the Hydra, the monster from Greek mythology, and the name stuck.
Trembley注意到这种生物与希腊神话中的怪物Hydra相似，于是这个名字就沿用下来了。
Today, we know hydras are animals capable of some incredible feats, apparently including immortality.
如今，我们知道水螅是一种动物，拥有一些惊人的能力，显然包括永生。
They comprise a genus of tiny aquatic animals in the broader phylum Cnidaria.
它们属于刺胞动物门中一个微小的水生动物属。
Some cnidarians, like jellyfish, have a stationary polyp phase, then a free-swimming Medusa stage.
一些刺胞动物，比如水母，有一个固定的水螅体阶段，然后是一个自由游动的水母体阶段。
But hydras and some of their relatives stay polyps their whole lives.
但水螅和它们的一些亲戚终其一生都保持水螅体形态。
Hydras have relatively simple cylindrical body plans, one end forming the foot, the other the mouth, framed by fanning tentacles.
水螅的身体结构相对简单，呈圆柱形，一端形成足部，另一端是嘴巴，周围环绕着扇形的触手。
They’ll eat just about any critter they can snag.
它们几乎会吃掉任何能抓到的生物。
And although they can scoot around if needed, most of the time, they remain stuck to things using a gluey substance secreted from their foot.
虽然必要时它们可以移动，但大多数时候，它们会利用足部分泌的粘性物质附着在物体上。
They can reproduce sexually, but they more often do so asexually, budding new growths off their bodies.
它们可以进行有性繁殖，但更常见的是无性繁殖，通过身体出芽产生新个体。
Where exactly they hunker down in the freshwater ponds and slow-moving streams they populate varies by species, which likewise differ in shape, size, and hue.
它们栖息的淡水池塘和缓流溪流中的具体位置因物种而异，不同物种的形状、大小和颜色也各不相同。
Green hydras, for one, bask in sunny spots to feed the photosynthetic algae they host, which lend them their color and provide sugars in return for other nutrients.
例如，绿色水螅喜欢待在阳光充足的地方，以供养它们体内的光合藻类，这些藻类赋予了它们颜色，并提供糖分以换取其他营养物质。
This explains some of Trembley’s “animal-or-plant” confusion.
这解释了Trembley部分关于‘是动物还是植物’的困惑。
Meanwhile, other species, like brown hydras, stick to the shade and hunt more actively.
与此同时，其他物种，如棕色水螅，则喜欢待在阴凉处，并更积极地捕食。
Hydra tentacles are covered with stinging cells containing little capsules of pressurized, harpoon-like structures.
水螅的触手上覆盖着刺细胞，里面含有带压力、类似鱼叉结构的小囊。
When triggered, they fly out at explosive speeds, immobilizing small prey.
当被触发时，它们会以爆炸般的速度射出，使小型猎物无法动弹。
This adaptation doesn’t protect hydras from larger predators— but what they lack in defenses, they make up for in their unparalleled regenerative abilities.
这种适应性并不能保护水螅免受大型捕食者的侵害——但它们在防御方面的不足，被其无与伦比的再生能力所弥补。
Not only can hydras regrow lost appendages, they can also reassemble themselves after going through a blending process of sorts, where they're completely broken down into individual cells.
水螅不仅能重新长出失去的附肢，即使在经过某种混合过程被完全分解成单个细胞后，它们也能自我重组。
To do this, these cells first aggregate into a sphere, then they begin to sort themselves, gradually hollowing out and elongating as they group into external and internal layers.
为此，这些细胞首先聚集成一个球体，然后开始自我分类，随着它们分成外部和内部层，逐渐形成中空并拉长。
Soon, one end sprouts a few buds, which become the tentacles, while the other end develops into the foot.
很快，一端长出几个芽，变成触手，而另一端则发育成足部。
In a matter of days, the hydra is back in one piece, bopping around and catching worms like nothing happened.
几天之内，水螅就恢复了原状，四处游动，捕捉蠕虫，就像什么都没发生过一样。
In labs, this process has helped shed light on embryonic development, because some of the stages and proteins are common among all animals— though the ability to perform it willy-nilly, as hydras do,
在实验室里，这个过程有助于揭示胚胎发育的奥秘，因为其中一些阶段和蛋白质在所有动物中都是共通的——尽管像水螅这样随意进行这种过程的能力，
is decidedly not.
这绝对是做不到的。
Interestingly, if multiple hydras go through this shearing process, the resulting cells will form a single hydra.
有趣的是，如果多个水螅经历这种剪切过程，产生的细胞会形成一个单一的水螅。
This is probably because their immune systems don’t distinguish between self and others.
这可能是因为它们的免疫系统无法区分自我和他人。
And this even holds true for interspecies mixtures.
甚至不同物种的混合体也是如此。
However, hydras made from multiple species will fall apart because their cell types can’t adhere properly.
然而，由多个物种组成的水螅会解体，因为它们的细胞类型无法正常粘附。
But when they're all in one place, they're kind of unstoppable.
但当它们聚集在一起时，就有点势不可挡了。
All animals have stem cells, which can divide and differentiate into specialized cell types.
所有动物都有干细胞，可以分裂并分化成特化的细胞类型。
But for humans, they decline after embryonic development and become limited to specific parts of the body.
但对人类来说，它们在胚胎发育后会减少，并局限于身体的特定部位。
Meanwhile, hydra stem cell levels remain constant and can help replace any part of their body and seemingly divide forever.
而水螅的干细胞水平保持恒定，可以帮助替换身体的任何部分，并且似乎能无限分裂。
In a roughly 20-day span, a hydra has all new cells.
在大约20天的时间里，水螅会拥有全新的细胞。
This perpetual turnover prevents the buildup of age-related cellular damage, while enabling apparently boundless regeneration and reproduction.
这种永续的更新防止了与年龄相关的细胞损伤积累，同时实现了看似无限的再生和繁殖。
They aren’t indestructible.
它们并非坚不可摧。
But so long as nothing kills them, they can theoretically live forever, without aging.
但只要没有东西杀死它们，理论上它们可以永生，不会衰老。
And many have indeed been persisting in labs for years on end, teaching us about development, regeneration, mortality, and tiny, tentacled freshwater life.
事实上，许多水螅在实验室里已经持续存活了多年，教会我们关于发育、再生、死亡以及微小、有触手的淡水生物的知识。
All of this from some supposed pond scum.
所有这些都来自一些所谓的池塘浮渣。