Every animal needs a way to get oxygen. Most salamanders (including mudpuppies) have gills when they hatch from the egg. At metamorphosis, most species lose the gills and develop lungs. But some fully-aquatic species, such as mudpuppies, mature without losing their gills. They may also have rudimentary lungs, but they continue to use their gills all their life. Having gills all their life is a big advantage, because they never have to come to the surface to breathe (which would use energy, and expose them to predators). So, having gills in adulthood is an adaptation to their deep-water fully aquatic lifestyle. Does this help?
