In a bookstore in the 1980s, I read the first pages of a hard science fiction novel that began with the memorable description of some creature or ship or intelligence that was like a shark swimming through space with the singular goal of turning anything it encountered into "shark shit".
For some reason I think it might have been by Frederik Pohl. Or maybe Harry Harrison or... I don't know.
Edit: I just read the guide... I can't remember what it looked like but a few other details I can think of are that:
1) this thing was headed for our solar system, which would be Bad News for us...
2) I am almost sure that I put the book down because I hadn't read anything else by the author. Up to that point I am pretty sure that I'd only read Isaac Asimov, Arthur C. Clarke, Robert A. Heinlein, Frank Herbert, Larry Niven. So probably not by one of them... although I could be wrong.
Edit #2: I just tried a more thorough Google Books search than I have before and came across the passage from an old issue of Analog:
was largely untranslatable, but you might think of it manifesting as a kind of giant space-going shark, a moving appetite, a vast, fast, terrible eating- machine which saw its purpose to be turning everything edible in the universe into shark shit.
...apparently by John Wood Campbell in Analog Science Fiction/science Fact - Volume 106 - Page 162
So maybe a short story... trying to figure out how to see more of it.