Mudpuppies are neotenic: unlike most salamanders, they never lose their larval external gills, and they breed while retaining them. Those gill stalks are highly vascularised and their size responds to the surrounding water. Individuals from warm, stagnant or silty habitats typically show longer, bushier filaments, while animals from cold, oxygen-rich streams show short, compact gills.
The explanation is straightforward physiology: oxygen dissolves less readily in warm water, so a larger exchange surface is needed to extract the same amount. Mudpuppies also possess lungs, but as in hellbenders these contribute mainly to buoyancy rather than to substantial gas exchange.
Mudpuppies attract more folklore than most salamanders. They are neither venomous nor poisonous to handle, they do not bark despite the name, and they do not damage sport fisheries; anglers who catch them in winter can release them. They are, however, sensitive to the lampricides used in the Great Lakes.