Aksolotl

A. mexicanum, also known as the Mexican Axolotl, is a long, cylindrical salamander, reaching lengths of about 30 centimeters (12 inches). A neotenic salamander, its most notable physical feature is its gills, which protrude from the back of its wide head and remain there throughout adulthood. Its legs are short. It has four fingers on each of its front legs and five toes on each of its back legs. In the wild, its coloration is dark, but an albino variety has been bred in captivity (Utah's Hogle Zoo 2003) (http://hoglezoo.org/animals/view.php?id=20). This species was featured in News of the Week on 19 February 2018.The axolotl is the most common salamander used in biological research; they are easily bred, and thousands live in home aquariums and labs. Its long association with humans is fascinating. In the 13th century, the indigenous Mexica people built an island city in Lake Texcoco in the Central Valley of Mexico. They also built floating gardens and canals, which the native axolotls invaded. Eventually, the lakes were drained and the salamanders were cut off. Their numbers declined; a 1998 census found 6,000 axolotls per square kilometer. In 2000, Luis Zambrano, a biologist at the Universidad Nacional Autónoma de Mexico, found only 1,000 animals/km2. By 2008, the census registered 100/km2, and currently the estimate is only 35/km2. With isolation, reduction in numbers, invasive predators, and environmental contaminants, the axolotl is almost extinct in the wild. (David Cannatella)

Ambystoma mexicanum lives only in Lake Xochimilco and Lake Chalco, both adjacent to Mexico City. Xochimilco and Chalco are part of a complex of five lakes, among which the Aztecs built Mexico City and around which the city has since expanded. The lakes were once highly productive, many of their species being economically and nutritionally valuable. Today a great deal of the area has been drained, or compromised by development of other kinds. Lake Xochimilco is known for its “floating gardens,” or “chinampas,” strips of land between drainage channels where locals grow vegetables and flowers for market. A. mexicanum can be found in these channels and in remaining lake areas. It may have previously lived in the channels joining two of the other lakes in the complex (Griffiths et al. 1988). In the wild, the species lives underwater. Outside of the wild, it is kept as an aquarium pet and is used widely in laboratory experiments.

The most notable feature of the life history of A. mexicanum is that the species exhibits an extreme form of neoteny: it remains in its aquatic larval form its entire life, meaning that when it reaches sexual maturity at the approximate age of one and a half years, it remains in other regards a larva. Paedomorphic features include the maintenance of gills into adulthood. While young, A. mexicanum feeds on algae, but as it grows older, it takes to eating aquatic insects. If a locality where it lives dries up, A. mexicanum metamorphoses into the Mexican salamander. As for reproduction, the male releases sperm packets, which are taken up by the female for internal fertilization. Incubation lasts 2-3 weeks. In the wild, A. mexicanum lives ten to twelve years. Its major predators are predatory birds such as herons (Utah's Hogle Zoo 2003) (http://hoglezoo.org/animals/view.php?id=20).

A. mexicanum has become vulnerable in the wild. The major threats to its continued existence are land drainage and the growth of Mexico City. Various efforts at flood control and sewage disposal starting in the seventeenth century have led to serious damage to the lake complex. The digging of wells for the burgeoning population of Mexico City has also caused drying of the valley in which the lakes are located. The largest of the lakes, Texcoco, has been greatly diminished in size, while Lake Chalco has all but disappeared. Xochimilco has likewise suffered a decline in size and water quality (Griffiths et al. 1988). A further threat specific to A. mexicanum is its commercial sale as food in the markets of Mexico (Utah's Hogle Zoo 2003) (http://hoglezoo.org/animals/view.php?id=20).

In the wild, the species lives underwater and is not commonly seen. Outside of the wild, it is a popular aquarium species around the world and is used widely in laboratory experiments (Griffiths et al. 1988).