Carl Sagan, the American astrophysicist and author, speculated on what life that could arise on a gas giant like Jupiter may be like, where it is all atmosphere and no solid ground (at least a pressure that any life could conceivably survive).
He described creatures that resembled vast, living balloons. They stay buoyant by pumping heavy gasses from their interiors, or by keeping a warm internal temperature via a kind of metabolism, fed by the sun or by consuming microorganisms. These creatures, which he called 'floaters' could be kilometers across, or even the size of cities.
Carl Sagan, Cosmos
We could see them arrayed in great lazy herds as far as the eye could see, concentrated in the great updrafts in the enormous sea of cloud.
But these creatures may be just one part of a flourishing ecosystem. There may also be 'hunters', who eat the floaters for their organic molecules and their stores of energy, and are fast and maneuverable.
-Adolf Schaller, Hunters, Floaters, and Sinkers