The axolotl (/ˈæksəlɒtəl/; from Classical Nahuatl: āxōlōtl [aːˈʃoːloːtɬ] (About this soundlisten); plural axolotls or rarely axolomeh;[3] Spanish: ajolote), Ambystoma mexicanum,[2] also known as the Mexican walking fish, is a neotenic salamander related to the tiger salamander.[2][4][5] Although colloquially known as a "walking fish",[4][5] the axolotl is not a fish but an amphibian.[2] The species was originally found in several lakes, such as Lake Xochimilco underlying Mexico City.[1] Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. Instead of developing lungs and taking to the land, adults remain aquatic and gilled.
Axolotls should not be confused with waterdogs, the larval stage of the closely related tiger salamanders (A. tigrinum and A. mavortium), which are widespread in much of North America and occasionally become neotenic. Neither should they be confused with mudpuppies (Necturus spp.), fully aquatic salamanders from a different family that are not closely related to the axolotl but bear a superficial resemblance.
As of 2010, wild axolotls were near extinction[7] due to urbanization in Mexico City and consequent water pollution, as well as the introduction of invasive species such as tilapia and perch. They are listed as critically endangered in the wild, with a decreasing population, by the International Union for Conservation of Nature and Natural Resources (IUCN) and as an endangered species by the IUCN's CITES treaty. Axolotls are used extensively in scientific research due to their ability to regenerate limbs.[8] Axolotls were also sold as food in Mexican markets and were a staple in the Aztec diet
A sexually mature adult axolotl, at age 18–24 months, ranges in length from 15 to 45 cm (6 to 18 in), although a size close to 23 cm (9 in) is most common and greater than 30 cm (12 in) is rare. Axolotls possess features typical of salamander larvae, including external gills and a caudal fin extending from behind the head to the vent.
Their heads are wide, and their eyes are lidless. Their limbs are underdeveloped and possess long, thin digits. Males are identified by their swollen cloacae lined with papillae, while females are noticeable for their wider bodies full of eggs. Three pairs of external gill stalks (rami) originate behind their heads and are used to move oxygenated water. The external gill rami are lined with filaments (fimbriae) to increase surface area for gas exchange. Four gill slits lined with gill rakers are hidden underneath the external gills.
Axolotls have barely visible vestigial teeth, which would have developed during metamorphosis. The primary method of feeding is by suction, during which their rakers interlock to close the gill slits. External gills are used for respiration, although buccal pumping (gulping air from the surface) may also be used to provide oxygen to their lungs.
