Axolotls are paedomorphic: they reach sexual maturity while retaining larval features, including three pairs of external gill stalks, a dorsal fin running along the tail, and an aquatic lifestyle. In most ambystomatid salamanders, a surge of thyroid hormone triggered by the thyroid gland reshapes the animal for land. In the axolotl the tissues still respond to that hormone, but the signal never arrives in normal development.
The demonstration is old and reliable. Injecting thyroxine, or adding iodine to the water, induces metamorphosis: the gills resorb, the tail fin shrinks, the skin thickens, and a terrestrial salamander resembling a tiger salamander emerges. Metamorphosed axolotls are generally shorter-lived and are not used for the regeneration research that makes the species valuable.
A persistent misreading treats the axolotl as an animal that cannot metamorphose. It can, and rare spontaneous cases occur. The species is also widely assumed to be secure because so many live in tanks; the wild Xochimilco population is separately assessed as Critically Endangered.