AXOLOTL, the aquatic larval form, which may become sexually mature, of the salamander Amblystoma tigrinum. This larva was for long held to be a species of perenni branchiate urodele (see AMPHIBIA) ; but in 1865, some axolotls in the Jardin des Plantes, Paris, metamorphosed into the sala mander form. This phenomenon of larval sexual maturity is called neoteny (see METAMORPHOSIS). The axolotl, long known as an article of food in Mexico, inhabits the U.S.A. and the cooler parts of Mexico. In form it resembles the larva of the common newt but reaches a length of about a foot. A semi albino variety is known. The adult A. tigrinum is dark brown in colour, with yellow spots, and differs little in appearance from European salamanders. In Lakes Chalco and Xochimilco, near Mexico City, the axolotl apparently never metamorphoses; these lakes contain abundant food and good shelter, while the sur rounding country is dry and comparatively barren. Thus it is an advantage for the species to become mature in the aquatic form. Further north, another subspecies occurs which inhabits smaller pools, and metamorpho sis is much more readily induced, e.g., by the drying of the pools.
The permanently aquatic condi tion is in this species brought about by a suppression of the activity of the thyroid gland, the secretion of which is responsible for metamorphosis in Amphibia. Administration of thyroid will cause axolotls of any age to transform into the terrestrial form.