I don't remember the title or author.
It was about an expedition in Jupiter's atmosphere, and the people in their spacecraft crash onto a life form that resembled a giant whale or something. The creature turned out to be intelligent, and had 2 brains- one slept while the other kept the body going. They can't launch without killing the creature and something happens in the end where an eruption on the planet surface far below sends some kind of mouth-like creatures flying into the atmosphere eating everything in their path.
I read the story probably 20 years ago, so some of the details may be a bit fuzzy.