Gills are an immediate tip for you to know that this animal lives in an aquatic biome, where it will be using its gills to breathe underwater without the need to surface for air. Usually, the animals with gills are fish, because they filter out oxygen from the wanter they inhale.
If it is the abdomen and not the stomach area i would say its from the heat. if you've ever seen a dead animal on the side of the road over the course of a few days, they start to swell from the sun beating down on them all day.
