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These are spider crabs.
这些是蜘蛛蟹
They spend most of their lives in deep water.
它们大半辈子都待在深海
But once a year, off the coast of Southern Australia,
但每年有一次在澳洲南部外海
a quarter of a million crabs
二十五万只蜘蛛蟹
set off on a long journey to the shallows.
开始长途跋涉前往浅水区
They are here because they all share a problem.
它们来这里是因为一个共同问题
Each crab has been wearing the same suit of armour for a year now.
每只蜘蛛蟹身上的盔甲 都已经穿了一年
And it's getting uncomfortably tight.
现在紧绷到不舒服的地步
So each crab eventually has to shed its shell
所有螃蟹终究必须褪下旧壳
and produce a bigger one.
制造更大的新壳
Replacing an old shell is understandably a tricky process.
可以想象替换旧壳的步骤很棘手
First the crab
首先螃蟹
grows an entirely new skin within the old shell.
必须先在旧壳内 长出一层新皮肤
It then flexes its body to force its shell
然后收缩身体 迫使外壳
to split along the back...
沿着背部裂开
..before gingerly backing itself out.
然后缓缓后退，从旧壳离开

重点解释

a quarter of a million crabs set off on a long journey to the shallows.二十五万只蜘蛛蟹开始长途跋涉前往浅水区

a quarter of ... 的1/4

It's a quarter of two now.

现在是差一刻两点。

A quarter of the area is covered with virgin forest.

四分之一的地区都是原始森林。

set off出发，动身

What time are you planning to set off tomorrow?

你打算明天几点钟启程?

They've set off on a journey round the world.

他们已经开始环游世界。

It then flexes its body to force its shell to split along the back.....before gingerly backing itself out.然后收缩身体，迫使外壳沿着背部裂开，然后缓缓后退，从旧壳离开

force somebody to do something迫使某人去做某事

No power on earth could force me to do it .

世上没人能强迫我做这事。

Circumstances force us to adopt this policy.

形势迫使我们采取这项政策。

back out退出

He backed out of the deal when he saw the risks.

当他看到有风险的时候,立即退出交易。

It is too late to back out of the deal.

现在打退堂鼓为时已晚。