Official 54 Set 2

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Migration of Zooplankton

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What does the professor mainly discuss?
  • A. The importance of zooplankton in the marine food chain

  • B. The interdependence of two types of tiny marine organisms

  • C. A physical feature of zooplankton that makes them well adapted for swimming

  • D. A phenomenon observed in some species of zooplankton

显示答案 正确答案: D

我的笔记 编辑笔记

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

    FEMALE PROFESSOR: And the sea is teeming with tiny organisms, but they don’t get as much popular attention as, say, whales.Microscopic algae just aren’t as exciting, I suppose, and yet those organisms are the foundation of the bulk of the marine food chain.Without plankton, which is the global term for these tiny organisms, there would be no whales.

    Plankton is found both in freshwater and marine environments.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.Plants and plantlike plankton are called phytoplankton, while animals and animal-like plankton are called zooplankton.

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

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

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

    So, researchers generally agree that the stimulus for zooplankton DVM is light.Zooplankton tend to swim away from sunlight into deeper water where the Sun’s rays barely penetrate.At night, when the Sun no longer illuminates shallower water, zooplankton head back toward the surface.

    Now, why would light cause zooplankton to expend all that energy and migrate?One popular theory is that zooplankton are hiding during the day from visual predators, uh, those animals that hunt by sight.The darkness provides safety during the day.Then at night, after migrating upward, they have an opportunity to feed on phytoplankton that float at the surface.Makes sense, doesn’t it?

    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?Or that others dive deeper than is necessary to escape hunters’ eyes?And that some zooplankton are bioluminescent?Which means they have special organs that light up and make them visible even at great depths.

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

    But what are some other possible explanations?Some researchers suggest that zooplankton migrate to avoid the Sun’s ultraviolet light.That would explain why some zooplankton are found at such great depths—visible light may not penetrate very far down, but ultraviolet light can.

    And we know that some zooplankton have special pigments that protect them from the damage ultraviolet light can cause.That could be why some zooplankton are able to stay closer to the surface during daylight hours. And there’s a third theory.Although it takes a lot of energy for the zooplankton to migrate, they conserve energy while floating in deeper, colder water.

    So, while they’re not feeding, they’re quietly digesting in cooler water.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...so, there may be different reasons for different species.

  • 请听一段海洋生物学课上的讲座片段。

    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。我猜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。没有了这些浮游生物,也就是这些微生物的统称,鲸类也就不复存在了。

    浮游生物在淡水和海洋环境中都存在。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。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。

    一百多年以来,研究者一直在尝试解开浮游动物的谜团。你们看到,有些种类的浮游动物,它们的移动…并不像鸟类在季节变换时迁徙那样,而是每日都会进行,通过一种我们称之为昼夜垂直移动或DVM的现象。

    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。根据种类和地区,这个往返旅程会在100到400米不等。对于一个小型微生物来说,这是一个很长的距离。要记住,浮游动物不能很好的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。

    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。我一会儿讲它们,但我想先说说我们知道或相当确定的东西。

    调查者大都同意这种昼夜垂直移动的刺激因素是光。浮游动物倾向于远离阳光游向太阳光几乎照不到的深层水域。在晚上,当太阳不再照耀浅层水域,浮游动物重回水的表面。

    现在为什么光会使浮游生物将所有能量花费在迁徙中呢?一种流行理论说浮游动物在白天躲避视觉捕食者,呃……这些动物通常用视力捕食而在白天,黑暗提供了安全保障。而后在夜晚向上迁徙之后,它们有了捕食浮于表层的浮游植物的机会。很有道理,不是吗?

    但是我们该如何看待这些数据呢,这些数据显示有许多种类的浮游动物在白天并没有潜到深到让捕食者看不见的地方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。并且有些浮游动物是发光的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。

    嗯,尽管如此,我们相信逃避捕食者是个合理的解释,因为其得到淡水湖中实验的支持。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有关联。

    但其他的一些合理的解释是什么呢?有些研究者认为浮游动物迁徙来躲避太阳的紫外线。这能够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。

    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的伤害。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。然后是第三个理论。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。

    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食物。但要记得,浮游动物由无数的不同有机体组成,从微型蠕虫到幼蟹到小鱼,它们在很大范围的海洋生物栖息地被发现,冷水,温水,浅水,深水中都存在。所以可能不同种类有不同的理由。

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  • 本题对应音频:
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    题型分类:内容主旨题

    题干分析:教授讨论对主旨。

    原文定位:

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

    选项分析:教授从whale引入Plankton,明确讨论主题:zooplankton,继而在定位句明确关于zooplankton的特定DVM迁徙现象,后文给出针对该现象的几个解释。D选项最准确。

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