I agree. Unfortunately, the gas giants are generally HORIFFIC radiation environments, so you'll need something VERY tough.

But it's not impossible. However, you need to think what Darwinian advantage this may offer, and how it couldevolve from some intermediate form.

I can envisage the latter - that there are good reasons for a resistant spore stage in many organisms' life cycle, and radiation resistance is important for anything that might get cycled through shallow near-surface ice.

The former is more tricky. Sure, you get to colonise a whole new moon, but once that's been done once for each moon, the game is over. And I'm not sure there's much "benefit" for the parent that does it the first time, either. But I need to think on this - it's an interesting idea.

While you're on Niven, how about "Integral Trees"?