I am trying to remember a science fiction story I read that involved a civilization of sentient manta rays living in Jupiter. The book starts with the main character, a human, being given a body to join the manta rays, and ends with the humans and manta rays realizing that they can use the core of Jupiter to travel to other solar systems. I would have read this in the late 90s, earlier 2000s.
This is probably Timothy Zahn's Manta's Gift.
From the Goodreads summary:
When Matt Raimey had his accident, he thought his life was over. He never dreamed, in his wildest fantasies, that he'd end up in a spot like this. In the toxic atmosphere of Jupiter, born into the body of an enormous creature that looked like a cross between a manta ray and a dolphin, he is living a new life, unlike any humankind had previously experienced.
Also at the Science Fiction Review with more details about the story (although without revealing the ending).
When I searched Google for
science fiction manta rays jupiter, the top hits were all about this book. Then again, Google search results are different for everyone, and I've done a lot of searching for science fiction stories to answer people's story-ID questions; maybe you'd get completely different results with the same search.