They live in environments that change drastically throughout the day, like the droplets of water on moss. The moss will sometimes dry out, and to survive this they enter their tun state - think hibernation combined with freeze-drying yourself. They end up a shriveled little ball a fraction of their original size, and then expand and return to normal when the water comes back. They also can enter their tun state for things like temperature changes they'd otherwise die from. They don't seem to age much during this process and have lots of resistance to radiation in their tun state, which is a lot more useful when you're that small and radiation damaging a single cell could mean you lose an entire eye.