systematically consuming all other biomatter to enable its own rapid evolution and reproduction."
I never understood why the tyranids would leave a world barren. Wouldn't it make more sense to turn each planet into a hostile (for non-tyranids) biosphere that would constantly produce harvestable biomatter, i.e. food, for the hive fleet? It's already established that the hive fleet is a long trailing tendril from beyond the galaxy, meaning they have a supply train.
I just reread a preview by the German magazine gamestar, summarizing some of the game's features as apparently encountered by one of the editors who got a chance to play it at Gamescom. He states (
http://www.gamestar.de/spiele/stellaris-/artikel/stellaris,52692,3234846,2.html):
"Zudem warten im All natürlich die üblichen Monster, etwa interstellare Riesenquallen." Translates to: "In addition, the usual monsters, e.g. interstellar giant squids, are waiting in space"; see 7th bullet point on "Phase 1: Erkundung und nicht-lineare Forschung" (Phase 1: Reconnnaissance and non-linear research), page 2 of article.
Guess I had forgotten I read that some time ago.
Heck yes, space monsters. So pumped. TO BECOME THE SPACE MONSTERS.
I can't really come up with a plausible evolutionary history for a biological organism that flies around in space, messing up inhabited planets.
Yeah, but who the heck would have thought up platypi or dodos or extremophile bacteria until we actually found them? Our limited understanding of exotic biologies doesn't mean it's not unreasonable.
Consider the following. A biological hivemind race rules a planet, but to ensure its survival, it wishes to expand. It proceeds to develop biological flight (already exists) and eventually spaceflight.
Consider the alternative, the atmosphere of a planet, upon which such a hivemind exists, is ripped away. Either the hivemind was prepared for this eventuality or it survived long enough to use direct evolution to adapt to its new conditions.
And lastly, even if the large bioforms aren't totally compatible with space, you can certainly make a large biological ship in which less space-tolerant substrains could survive.
Or maybe a planet with a super weak atmosphere gets broken up and the (now) spacewhales that lived there are sort of fine with that.
So many ideas! Giant photosynthetic space vines that float about asteroids!