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|Example showing reduced pigmentation|
| Ambystoma mexicanum|
The Axolotl, Ambystoma mexicanum, is the best known of the Mexican mole salamanders: it belongs to the Tiger Salamander complex. The Axolotl shows neoteny: the larvae do not undergo metamorphosis, so the adults stay aquatic, with external gills.
Axolotls are closely related to waterdogs, the larval stage of the closely related Tiger Salamanders Ambystoma tigrinum and Ambystoma mavortium. These are common in much of North America and also sometimes become neotenic. The mudpuppies, Necturus, are fully-aquatic salamanders which are not closely related to the axolotl but bear a superficial resemblance.
Wild axolotls are now near extinction  due to population growth in Mexico City, and the polluted waters of the lake. Non-native fish, such as African tilapia and Asian carp, have also recently been introduced to the waters. These new fish have been eating the axolotls' young, as well as its primary source of food. The axolotl is currently on the IUCN Red List of threatened species.
Genetics and development
Compared with land-dwelling salamanders, the sexually mature adult Axolotl is a chimera (~ mixture) of larval and adult tissues. A mutation in hormone production slows the development of the non-sexual tissues compared to the gonads. So, it ends up as an adult which looks like a larva, except that it is sexually mature. This is an example of neoteny, a type of heterochrony.
- BBC - Earth News - Axolotl verges on wild extinction
- Weird creatures, with Nick Baker [Television series]. Dartmoor, England, U.K.: The Science Channel. Event occurs at 00:25.
- "Mexico City's 'water monster' nears extinction", David Koop, Yahoo News, November 2, 2008
- King R.C. Stansfield W.D. & Mulligan P.K. 2006. A dictionary of genetics. 7th ed, Oxford University Press. p203