<P> As of 2010, wild axolotls were near extinction 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 currently listed by CITES as an endangered species and by IUCN as critically endangered in the wild, with a decreasing population . Axolotls are used extensively in scientific research due to their ability to regenerate limbs . Axolotls were also sold as food in Mexican markets and were a staple in the Aztec diet . </P> <P> Surveys in 1998, 2003 and 2008 found 6,000, 1,000 and 100 axolotls per square kilometer in its Lake Xochimilco habitat, respectively . A four - month - long search in 2013, however, turned up no surviving individuals in the wild . Just a month later, two wild ones were spotted in a network of canals leading from Xochimilco . The city is currently working on conserving axolotls by building "axolotl shelters" and conserving remaining and potential habitats for the salamanders . </P> <P> A sexually mature adult axolotl, at age 18--24 months, ranges in length from 15--45 cm (6--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 . </P> <P> 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 . </P>

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