The Living Force
I think extremophiles are very interesting organisms. These are organisms that tolerate or require extreme ranges of variability of environmental factors to live. In most cases, extremophiles are unicellular organisms belonging to the Archaea domain, but other groups are also among them.
Some extremophiles are specialized, they like acidic or alkaline environments, frosts or heat, but there are also organisms that get used to various kinds of variability. These are the so-called polyextremophiles.
An example of extremophiles are tardigrades:
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Or, less scary looking, almost friendly, nereis sandersi:
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Video of the little fellow. I once read that they can even temporarily survive in the vacuum of space.