英語(yǔ)時(shí)差:垂直遷移

00:00

00:00

MP3下載 文本下載

https://online2.tingclass.net/lesson/shi0529/10000/10183/986.mp3

https://image.tingclass.net/statics/js/2012

Imagine if the only way for you to eat were to trek twenty-five miles on foot to a restaurant and then trek all the way back home afterwards.   Imagine that this restaurant is a buffet, so you can't carry anything home in a doggie bag--you carry only what you can pack into your stomach. You burn so much energy making this trip that you have to make the same trek every single day of your life or else you starve. How well would you fare if this were your life?

For many of the ocean's smallest creatures, a meal is a very long journey away--all the way to the surface waters of the ocean. Millions of deep-sea creatures, such as krill, survive on plant material called phytoplankton. Because plants require sunlight in order to live, they're absent deep in the ocean where the light of day never reaches.

Yet the darkness of the deep is precisely why so many of the ocean's smallest creatures make it their home. It's safer down there where there isn't any light to make them visible to their predators.

So these animals have to travel from the ocean's depths toward its upper layers to feast on the phytoplankton growing there. We call this the vertical migration. Among these migrators are creatures so tiny we can't see them without a microscope. Larger migrators include the shrimp-like krill, animals that resemble small shell-less snails and jellyfish, and other fascinating creatures.

Not only do they have to make their journey quickly before the sun returns and exposes them as potential meals to larger animals, but they make this journey night after night after night.