axolotl (ăkˈsəlŏtˌəl) [key], a salamander, Siredon 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 affect the functioning of the thyroid gland. Axolotls are permanently aquatic, never undergoing the metamorphosis to a terrestrial form characteristic of amphibians. They grow larger than ordinary larval salamanders and develop sexually, but they retain external gills and a well-developed tail. 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, Abystoma tigrinum, often exhibits neoteny in the Rocky Mts., where the iodine content of the water is low. The axolotl has a broad head and bushy gills; its skin is a black-speckled dark brown. It may grow as long as 13 in. (33 cm). In Mexico City, axolotls are sometimes cooked and eaten as delicacies. They are classified in the phylum Chordata, subphylum Vertebrata, class Amphibia, order Urodela, family Abystomidae.