I read this book maybe 8-10 years ago, and it was in the well-worn sci-fi section of my library so I'd put it in the 80's-early 90's. It was in English and I feel like I remember a giant stingray(?) on the cover?
All I can remember about the plot is that a human was reborn as one of the space stingrays that swim around this gas giant planet. I can't remember why he was doing it, just that a middle-aged guy was basically turned into one of these giant stingray things.
I also remember the twist ending, which was that he ends up figuring out that
when the oldest space stingrays die their bodies sink to the bottom of the atmosphere and block the energy coming off the planet, which is killing their species.
I don't remember any names or other helpful info, and googling
space stingrays dying killing planet book is, unsurprisingly, not turning much up...
Here's a couple more points from the comments;
I may not have made it clear enough; I'm not 100% sure they were stingrays. I am fairly sure there were references to the aliens being like some kind of sea creature, except existing in the air around the gas giant rather than water. My memory says stingray, but it may have been a shark or manta ray or something else.
The guy didn't swap bodies into a 'ray, he was definitely "born" as one. I am fairly sure I remember a graphic-ish "birth scene" as his conscious, in an alien body, was birthed by a female/birther of the alien species.
In some way their existence required the energy/waves that were radiating off their planet,
which was starting to disappear because all of their dead bodies were covering the surface, blocking the energy. Loss of energy = their species/way of life was ending.
The more I think about it the more likely it seems that there was some kind of military subplot. I seem to remember it reading a lot like Ender's Game where the sci-fi plot was interspersed with a military, more "behind-the-scenes" story with political drama. I think I skimmed those sections pretty heavily as a kid because I found the space alien parts way more interesting, but I'm remembering that the guy had to get something from the aliens, which is why he'd been put in one of their bodies.
Thinking about it this hard is bringing back some weird memories. I think when the guy was only a couple hours/days/weeks old he got to meet one of the "elder" rays. The elder had to be "pushed" up to the higher levels of atmosphere and it was this whole effort to get the guy low enough (weighed down by a larger-than-him ray) and the elder high enough (pushed up by smaller-than-him rays) that they could actually exist in the same space to have a chat.