Narrator: Listen to part of a lecture in a marine biology class.

旁白: 请听一段海洋生物学课上的讲座。

Female professor: And the sea is teeming with tiny organisms, but they don't get as much popular attention as, say, whales.

女教授：海洋充满微小的有机物，但它们并没有像一些别的生物，比如鲸鱼那样得到很多关注。

Microscopic algae just aren't as exciting, I suppose, and yet those organisms are the foundation of the bulk of the marine food chain.

我想微型藻类并没有那么令人兴奋，不过这些有机物是大量海洋生物食物链的基础。

Without plankton, which is the global term for these tiny organisms, there would be no whales.

没有这些浮游生物，浮游生物是这些微生物的全球术语，鲸类也就不复存在了。

Plankton is found both in freshwater and marine environments.

浮游生物在淡水和海洋环境中都存在。

Again, it's a term we use for any small organisms that float along with the current, either because they're too small or weak to swim against it, or because they don't have any capacity at all to move by themselves.

再说一下，这个术语被我们用来形容所有漂浮在水流中的小型生物，或因为它们太弱小无法逆流游动，或因为它们没有能力靠自身移动。

Plants and plantlike plankton are called phytoplankton, while animals and animal-like plankton are called zooplankton.

植物型或类植物型的浮游生物被称作浮游植物，而动物型或类动物型的浮游生物叫做浮游动物。

For over a century now, researchers have been trying to solve a mystery about zooplankton.

一百多年以来，研究者一直在尝试解开浮游动物的谜团。

You see, some species of zooplankton migrate, er, n-n-not the way birds do when the seasons change, but daily, in a phenomenon we call diel vertical migration, or DVM.

你们看到，有些浮游动物的移动，并不像鸟类在季节变换时迁徙那样，而是每天都在进行，通过一种我们称之为昼夜垂直移动或DVM的现象。

In their diel vertical migration, zooplankton swim up near the surface of the water during the night and swim down to deeper water during the day.

在昼夜垂直移动过程中，浮游动物在夜里浮上水表，而在白天潜入水底。

Depending on the species and region, this can be a round trip of between one hundred and four hundred meters.

根据种类和地区，这个往返旅程会在100到400米不等。

For a tiny microscopic organism, that's a huge distance. Remember now, zooplankton can't swim very well, and DVM requires a lot of energy, so there must be an important benefit to this daily up and down commuting.

对于一个小型微生物来说，这是一段很长的距离。要记住，浮游动物不能很好地游动且昼夜垂直移动需要很多能量，所以每日沉沉浮浮的移动过程一定有重要的益处。

We're not exactly sure what this benefit is, though there are several compelling theories.

我们并不确定这种益处是什么，尽管有一些令人信服的理论。

I'll talk about them in a moment, but first, I want to talk about what we do know, or rather, what we're pretty sure we know.

我一会儿讲它们，但我想先说说我们知道，或者更准确地说，我们相当确定的东西。

So, researchers generally agree that the stimulus for zooplankton DVM is light.

研究者大都同意这种昼夜垂直移动的刺激因素是光。

Zooplankton tend to swim away from sunlight into deeper water where the Sun's rays barely penetrate.

浮游动物倾向于远离阳光游向太阳光几乎无法穿透的深层水域。

At night, when the Sun no longer illuminates shallower water, zooplankton head back toward the surface.

夜晚的时候，太阳不再照耀浅层水域，浮游动物重回水的表面。

Now, why would light cause zooplankton to expend all that energy and migrate?

现在为什么光会使浮游生物将所有能量花费在迁徙中呢?

One popular theory is that zooplankton are hiding during the day from visual predators, uh, those animals that hunt by sight.

一种常见的理论认为浮游动物在白天躲避视觉捕食者，呃……也就是用视力捕食的动物。

The darkness provides safety during the day.

而在白天，黑暗确保了安全。

Then at night, after migrating upward, they have an opportunity to feed on phytoplankton that float at the surface.

而后到了夜晚，它们在向上迁徙之后，就有机会捕食浮在表层的浮游植物了。

Makes sense, doesn't it?

很有道理不是吗?

But what do we do with the data showing that many kinds of zooplankton don't dive deep enough during the day to become invisible to predators?

但是我们该如何看待这些数据呢，这些数据表明有许多类浮游动物在白天并没有潜到深到让捕食者看不见的地方。

Or that others dive deeper than is necessary to escape hunters' eyes?

或有些浮游动物潜到了比逃避捕食者所需要的距离更深的地方，这类的数据又怎样解释呢？

And that some zooplankton are bioluminescent?

并且有些浮游动物是发光的，这类数据呢？

Which means they have special organs that light up and make them visible even at great depths.

这意味着它们有着特殊的器官能发光，让它们即使是在很深的海洋中也能被看见。

Well, despite all this, we believe predator avoidance is a possible explanation because of studies done in freshwater lakes.

嗯，尽管如此，因为一些淡水湖中完成的研究，我们仍然相信逃避捕食者是个合理的解释。

It turns out there's a correlation between the presence or absence of vertical migration and the presence or absence of fish that find their prey by sight.

实验结果表明了垂直移动的出现和消失与通过视觉来捕食的鱼类的出现和消失之间是有关联的。

But what are some other possible explanations?

但其他的一些合理的解释是什么呢？

Some researchers suggest that zooplankton migrate to avoid the Sun's ultraviolet light.

有些研究者认为浮游动物迁徙来躲避太阳的紫外线。

That would explain why some zooplankton are found at such great depths-visible light may not penetrate very far down, but ultraviolet light can.

这就解释了为何有些浮游动物被发现于特别深的地方--可见光或许不能穿透这么远，但紫外线可以。

And we know that some zooplankton have special pigments that protect them from the damage ultraviolet light can cause.

而且我们知道有些浮游动物有些特别的色素来保护它们免受紫外线可能带来的伤害。

That could be why some zooplankton are able to stay closer to the surface during daylight hours. And there's a third theory.

这也是为何有些浮游动物能在白天时待在离水面更近的地方。然后还有第三个理论。

Although it takes a lot of energy for the zooplankton to migrate, they conserve energy while floating in deeper, colder water.

尽管浮游动物需要很多能量来迁徙，它们在更深层、更冷的水中漂浮时会保存能量。

So, while they're not feeding, they're quietly digesting in cooler water.

所以当它们没有进食的时候，它们安静地在更冷的水里消化吸收。

But remember, zooplankton consist of any number of different organisms, from microscopic worms to crab larvae to tiny fish, and they are found in a large range of marine habitats-cold water, warm water, shallow water, deep water...

但要记得，浮游动物由任意数量的不同有机体组成，从微型蠕虫到幼蟹到小鱼，它们也存在于很大范围的海洋生物栖息地，包括冷水，温水，浅水，深水中都存在……

So, there may be different reasons for different species.

所以可能不同种类有不同的原因。