What is the unusual feature of the axolotl among amphibians?

The axolotl is a paedomorphic salamander related to the tiger salamander. The species was originally found in several lakes, such as Lake Xochimilco beneath Mexico City. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. Instead of taking to the land, adults remain aquatic with gills. This is due to their neoteny (retention of juvenile features) evolution, where axolotls are much more aquatic than other salamander species.

When most amphibians are young, they live in water and use gills that can breathe in the water. After becoming adults, they go through a process called metamorphosis, in which they lose their gills and start living on land. The unusual fact about the axolotl is that it lacks the thyroid-stimulating hormone, which is needed to produce thyroxine for the axolotl to go through metamorphosis. So it just retains its gills and lives in the water all its life, even after it becomes an adult and can reproduce.