Even though Manatees lead a beautifully simple and satisfied existence, they ain't really the toughest or exciting of creatures. That medal goes to a creature that most people have probably never heard of.
You Probably think that an animal that at its biggest reaches less than 2cm probably wouldn't be very tough even in the arena of our back-boned colleagues but the Tardigrade or water bear is by all accounts fucking tough. I'll give you a list of its abilities....
Where can they live? - Pretty much anywhere, and by that i mean they have been found in hot springs or frozen ice, they have been found on top of the Himalayas or 6km below sea level, and of-coarse lakes, ponds and meadows, stone walls you name it.
What about temperature ranges?- Well they can survive being boiled at 151c for a few minutes down to 1c above absolute Zero (that's colder than space). They can be chilled for days at -200c, nearly 4 times colder than either of our poles.
Don't they get crushed under pressure or explode without it?- In a word...no. They can survive in the vacuum of space or 6,000 atmospheres of pressure, try swimming to the floor of a diving pool and you will get an appreciation of withstanding pressure (ocean divers are crazy).
Will dehydration kill them?- You can guess where I'm going with this. They can survive a decade without any water and be fine, you try and go without a drink for a day and see what its like.
And Radiation? - A lot of people are scared by radiation poisoning what with the uproar against nuclear power and all, 1-2,000 rads of radiation is enough to kill you. Our tiny, tough little friends can take up to 570,000 rads.
All these facts clearly lead to one logical conclusion, they are some sort of interstellar race of fighting beasts. They have been waiting round since the Cambrian for their time to strike out against a worthy opponent. They are still biding their time....