The Axolotl also known as a Mexican Salamander (Ambystoma mexicanum) or a Mexican walking fish, is a neotenic salamander, closely related to the tiger salamander (2)(3).
Although the axolotl is colloquially known as a "walking fish", it is not a fish, but an amphibian. The species originates from numerous lakes, such as Lake Xochimilco underlying Mexico City (4). Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. Instead of developing lungs and taking to land, the 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 which are not closely related to the axolotl but bear a superficial resemblance (5).
As of 2010, wild axolotls were near extinction (6) due to urbanization in Mexico City and consequent water pollution. They are currently listed by CITIES as an endangered species and by IUCN as critically endangered the wild, with a decreasing population. Axolotls are used extensively in scientific research due to their ability to regenerate limbs (7). Axolotls were also sold as food in Mexican markets and were a staple in the Aztec diet (8).
A four-month-long search in 2013 turned up no surviving individuals in the wild. Previous surveys in 1998, 2003 and 2008 had found 6,000, 1,000 and 100 axolotls per square kilometer in its LakeXochimilco habitat, respectively (9).
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.
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 gills 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.
Axolotls have four pigmentation genes which when mutated create different color variants. The normal Wild Type animal is brown/tan with gold speckles and an olive undertone. The four mutant colors are leucistic (pale pink with black eyes), albino (golden with gold eyes), axanthic (gray with black eyes) and melanoid (all black with no gold speckling or olive tone). In addition, there is wide individual variability in the size, frequency, and intensity of the gold speckling and at least one variant that develops a black and white piebald appearance on reaching maturity. Because pet breeders frequently cross the variant colors, animals that are double recessive mutants are common in the pet trade, especially white/pink animals with pink eyes that are double homozygous mutants for both the albino and leucistic trait(10).Axolotls also have some limited ability to alter their color to provide better camouflage by changing the relative size and thickness of their melanophores(11).
Habitat and ecology
The axolotl is only native toLake Xochimilco and Lake Chalco in central Mexico. Unfortunately for the axolotl, Lake Chalco no longer exists, as it was artificially drained to avoid periodic flooding, and Lake Xochimilco remains a remnant of its former self, existing mainly as canals. The water temperature in Xochimilco rarely rises above 20 °C (68 °F), though it may fall to 6 to 7 °C in the winter, and perhaps lower.
The wild population has been put under heavy pressure by the growth ofMexico City. The axolotl is currently on the International Union for Conservation of Nature's annual Red List of threatened species. 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(12).
Axolotls are members of the Ambystoma tigrinum (tiger salamander) complex, along with all other Mexican species of Ambystoma. Their habitat is like that of most neotenic species—a high altitude body of water surrounded by a risky terrestrial environment. These conditions are thought to favor neoteny. However, a terrestrial population of Mexican tiger salamanders occupies and breeds in the axolotl's habitat
The axolotl is carnivorous, consuming small prey such as worms, insects, and small fish in the wild. Axolotls locate food by smell, and will "snap" at any potential meal, sucking the food into their stomachs with vacuum force(13).
(3) "Axolotls (Walking Fish)". Aquarium Online. Retrieved 2013-09-12.
(4) "Ambystoma mexicanum". Retrieved July 10, 2011
(5) Malacinski, George M. (Spring 1978). "The Mexican Axolotl, Ambystoma mexicanum: Its Biology and Developmental Genetics, and Its Autonomous Cell-Lethal Genes". American Zoologist. Oxford University Press. 18: 195–206. doi:10.1093/icb/18.2.195.
(6) Matt Walker (2009-08-26). "Axolotl verges on wild extinction". BBC. Retrieved 2010-06-28.
(7) Weird Creatures with Nick Baker (Television series). Dartmoor, England, U.K.: The Science Channel. 2009-11-11. Event occurs at 00:25.
(8)"Mythic Salamander Faces Crucial Test: Survival in the Wild". The New York Times. Retrieved 30 July 2015.
(9) Stevenson, M. (2014-01-28). "Mexico's 'water monster' may have disappeared". Associated Press. Retrieved 2014-01-29.
(10) Frost et al., A color atlas of pigment genes in the Mexican axolotl (Ambystoma mexicanum) Differentiation Volume 26, Issue 1-3, pages 182–188, June 1984
(11) Pietsch & Schneider Vision and the skin camouflage reactions of Ambystoma larvae: the effects of eye transplants and brain lesions Brain Research Volume 340, Issue 1, 5 August 1985, Pages 37–60
(12) "Mexico City's 'water monster' nears extinction". November 2008. Archived from the original on 2011-07-23. Retrieved 2010-06-28.
(13) Wainwright, P. C., et al. (1989). "Evolution of motor patterns: aquatic feeding in salamanders and ray-finned fishes." Brain, behavior and evolution 34(6): 329-341.