冰海中的生物 Hints: re-form food-krill centimeter individual

In the summer, when the sea ice melts, they can reach the islands that were trapped in the ice and eventually the continent itself. But when the ice re-forms, they have to retreat north. So now, in winter, with the sea ice at its fullest extent, it's in the sea that we must look for life.
The southern ocean is extremely rich in food. Millions of penguins and seals and thousands of whales feed here. The majority of them rely on just one source of food-krill. Krill are small shrimp-like creatures about six centimeters long. In winter, they are dispersed widely, mostly under the ice, but in summer they assemble in vast swarms, some of which may contain a billion individuals. They are the most numerous animals on earth. Their total weight far exceeds that of the total human population.

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