I'm trying to find a science fiction book from many years ago but I don't remember the name or author. I borrowed it from the library in the late '80s or early '90s.
The plot was generally about an alien race that invaded planets by first transplanting species from other planets that would destroy the inhabitants of the planet they were interested in, then once they were weakened they would take over.
The book starts with an exploration team on a planet that is dead, finding an animal that shouldn't be there and ultimately realized the species was responsible for the demise of the native inhabitants. They return to Earth to find an unusual species popping up, I think in the oceans, that eventually starts to kill off humanity.
The book ends with one of the characters asking the other something to the effect of "will we survive" and the scientist responding "we'll see". Unfortunately I do recall that the book had a very common sounding name, something like "across the stars" or something, so trying to search for it on Google has been impossible.