The axolotl ( ; from Classical Nahuatl: āxōlōtl [aːˈʃoːloːtɬ] ) (Ambystoma mexicanum) is a species of paedomorphic mole salamander. They mature into the terrestrial adult form without undergoing metamorphosis; the adults remain fully aquatic with obvious external gills. This trait, although somewhat unusual among the majority of amphibians, is not unique. Axolotls may be difficult to distinguish from the larval stage of other neotenic adult mole salamanders, such as mudpuppies (Necturus spp.) or the occasionally paedomorphic tiger salamander (A. tigrinum). Axolotls originally inhabited a system of interconnected wetlands and lakes in the Mexican highlands. They were known to inhabit the smaller lakes of Xochimilco and Chalco, and are also presumed to have inhabited the larger lakes of Texcoco and Zumpango. These waterways were mostly drained by Spanish settlers after the conquest of the Aztec Empire, leading to the destruction of much of the axolotl's natural habitat, which is now largely occupied by Mexico City. Despite this, they remained abundant enough to form part of the staple in the diet of native Mexica during the colonial era. Today, due to continued urbanization in Mexico City, which causes water pollution in the remaining waterways, as well as the introduction of invasive species such as tilapia and carp, the axolotls are now near extinction. The species has been listed as critically endangered in the wild, with a decreasing population of around 50 to 1,000 adult individuals, by the International Union for Conservation of Nature (IUCN), and is listed under Appendix II of the Convention on International Trade in Endangered Species (CITES). A large captive population of axolotls currently exists, with the specimens being used extensively in scientific research for their remarkable ability to regenerate parts of their body, including limbs, gills and parts of their eyes and brains. In general, they are model organisms that are also used in other research matters, and as aquarium technology has developed, they have become a common exhibit in zoos and aquariums, and as an occasional pet in home aquaria. Axolotls are also a popular subject in contemporary culture, inspiring a number of works and characters in the media.