axolotl [key], a salamander, Ambystoma mexicanum, found in certain lakes in the region of Mexico City, which reaches reproductive maturity without losing its larval characteristics. This phenomenon is called neoteny; in salamanders it is apparently caused by certain environmental conditions, particularly a low level of iodine in the water, which affects the functioning of the thyroid gland. The axolotl grows larger than ordinary larval salamanders and develops sexually, but it retains bushy external gills and a well-developed tail. It also has a broad head and stubby legs; its skin is a black-speckled dark brown. Permanently aquatic, never undergoing metamorphosis to a terrestrial form characteristic of amphibians, it may grow as long as 13 in. (33 cm). It is also known as the Mexican walking fish because of its appearance.
The axolotl was not recognized as a salamander until 1865, when several specimens at the Jardin des Plantes in Paris suddenly underwent metamorphosis. After some experimentation it was discovered that when their pools were dried up most of the animals changed into the adult form. Axolotls will also mature normally if fed thyroid gland extract. The related North American tiger salamander, A. tigrinum, often exhibits neoteny in the Rocky Mts., where the iodine content of the water is low. Since the late 1990s the axolotl has become increasingly endangered in its natural habitat due to pollution and competition from introduced fish.
The axolotl is classified in the phylum Chordata, subphylum Vertebrata, class Amphibia, order Urodela, family Ambystomidae.
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