What Are Axolotls?
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 isn’t a fish at all — it’s an amphibian. The species originates from a small number of lakes, including Lake Xochimilco, beneath Mexico City (4). Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis: instead of developing lungs and moving onto land, adults remain fully aquatic and keep their gills for life.
Axolotls shouldn’t be confused with waterdogs, the larval stage of the closely related tiger salamander (A. tigrinum and A. mavortium), which is widespread across much of North America and occasionally becomes neotenic itself. Nor should they be confused with mudpuppies (Necturus spp.), fully aquatic salamanders that bear a superficial resemblance but aren’t closely related to the axolotl (5).
As of 2010, wild axolotls were nearing extinction (6) due to urbanisation in Mexico City and the water pollution that came with it. They’re currently listed by CITES as an endangered species, and by the IUCN as critically endangered in the wild, with a declining population. Axolotls are used extensively in scientific research for their ability to regenerate limbs (7), and historically were also sold as food in Mexican markets, forming part of the Aztec diet (8).
A four-month search in 2013 turned up no surviving individuals in the wild. Earlier surveys in 1998, 2003, and 2008 had found 6,000, 1,000, and 100 axolotls per square kilometre respectively in the Lake Xochimilco habitat (9).
Description
A sexually mature adult axolotl, at 18–24 months old, typically ranges from 15–45cm (6–18in) in length, though around 23cm (9in) is most common, and anything over 30cm (12in) is rare. Axolotls retain features typical of salamander larvae, including external gills and a caudal fin running from behind the head to the vent.
Their heads are wide and their eyes are lidless. Their limbs are underdeveloped, with long, thin digits. Males are identified by a swollen cloaca lined with papillae, while females are noticeable for their wider, egg-filled bodies. Three pairs of external gill stalks (rami) extend from behind the head and are used to move oxygenated water; these rami are lined with filaments (fimbriae) that increase surface area for gas exchange. Four gill slits lined with gill rakers sit hidden beneath the external gills.
Axolotls have barely visible vestigial teeth, which would otherwise develop during metamorphosis. They feed primarily by suction, interlocking their rakers to close the gill slits as they do. External gills handle most respiration, though buccal pumping (gulping air at the surface) can also supply oxygen to the lungs.
Axolotls carry four pigmentation genes that, when mutated, produce different colour variants. The normal wild-type animal is brown or tan with gold speckling and an olive undertone. The four mutant colours are leucistic (pale pink with black eyes), albino (golden with gold eyes), axanthic (grey with black eyes), and melanoid (solid black, with no gold speckling or olive tone). There’s also considerable individual variation in the size, frequency, and intensity of gold speckling, along with at least one variant that develops a black-and-white piebald appearance at maturity. Because breeders frequently cross these colour variants, double-recessive mutants are common in the pet trade — particularly white or pink animals with pink eyes, which carry both the albino and leucistic traits (10). Axolotls also have a limited ability to adjust their colour for camouflage, by changing the relative size and thickness of their melanophores (11).
Habitat and Ecology
The axolotl is native only to Lake Xochimilco and Lake Chalco in central Mexico. Lake Chalco no longer exists — it was artificially drained to prevent periodic flooding — and Lake Xochimilco survives today mainly as a network of canals. Water temperature there rarely rises above 20°C (68°F), and can fall to 6–7°C, or lower, in winter.
The wild population has come under heavy pressure from the growth of Mexico City, and the axolotl now sits on the IUCN’s Red List of threatened species. Non-native fish, including African tilapia and Asian carp, have also been introduced to these waters and prey on axolotl young as well as their primary food sources (12).
Axolotls belong to the Ambystoma tigrinum (tiger salamander) species complex, along with all other Mexican species of Ambystoma. Their habitat resembles that of most neotenic species — a high-altitude body of water surrounded by a comparatively risky terrestrial environment — conditions thought to favour neoteny. That said, a fully terrestrial population of Mexican tiger salamanders does occupy and breed within the axolotl’s own habitat.
Axolotls are carnivorous, feeding on small prey such as worms, insects, and small fish in the wild. They locate food by smell and “snap” at anything edible within range, sucking it into their stomachs with considerable vacuum force (13).
Portions of this article are adapted from the Wikipedia article “Axolotl”, used under the CC BY-SA licence.
(2) aquariumindustries.com.au — Mexican Walking Fish
(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.
