Axolotl

## Description

The axolotl (Ambystoma mexicanum) is a paedomorphic salamander related to the Tiger salamander. Axolotls are unusual among amphibians in that they reach adulthood without undergoing metamorphosis. Instead of taking to the land, adults remain aquatic and gilled. They are used extensively in scientific research due to their ability to regenerate limbs, gills and parts of their eyes and brains.

## Appearance

Axolotls possess features typical of salamander larvae, including external gills and a caudal fin extending from behind the head to the vent. External gills are usually lost when salamander species mature into adulthood, although the axolotl maintains this feature. This is because axolotls are much more aquatic than other salamander species. 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, which prevent food from entering and allow particles to filter through. Axolotls have barely visible vestigial teeth, which develop during metamorphosis. The primary method of feeding is by suction, during which their rakers interlock to close the gill slits. Axolotls have four pigmentation genes; when mutated they create different color variants. The normal wild-type animal is brown/tan with gold speckles and an olive undertone. The five more common mutant colors are leucistic (pale pink with black eyes), golden albino (golden with gold eyes), xanthic (grey with black eyes), albino (pale pink/white with red eyes) which is more common in axolotls than some other creatures, and melanoid (all black/dark blue 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. Axolotls also have some limited ability to alter their color to provide better camouflage by changing the relative size and thickness of their melanophores.

## Distribution

Axolotls are native only to the freshwater of Lake Xochimilco and Lake Chalco in the Valley of Mexico. Lake Chalco no longer exists, having been drained as a flood control measure, 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), although it may fall to 6-7 °C (43-45 °F) in the winter, and perhaps lower.

## Lifestyle

Axolotls may be active at any time of the day and lead a solitary lifestyle. They locate food by smell, and will "snap" at any potential meal, sucking the food into their stomachs with vacuum force.

## Diet

Axolotls are carnivores (piscivores, molluscivores, vermivores, insectivores). They eat small prey such as mollusks, worms, insects, other arthropods, and small fish.

## Reproduction

Axolotls are polygunandrpus (promiscuous) meaning both the males and the females of this species mate with multiple partners during the breeding season. Axolotls usually breed from March to June. The female lays 100-300 eggs in the water attached to substrates. Eggs hatch after 10-14 days and the young (larvae) swim away on their own. They become reproductively mature at 18-27 months of age.

## Population

### Population threats

The main threat to this species is the loss of its native habitat. Axolotls occur only in the Mexican Central Valley and their native habitat is now limited to Lake Xochimilco as a result of the expansion of Mexico City. Lake Xochimilco is not a large body of water, but rather a small series of artificial channels, small lakes, and temporary wetlands. Lake Xochimilco has poor water quality, caused by the region’s aquaculture and agriculture demands. It is also maintained by inputs of only partially treated wastewater. In addition, the intensive use of pesticides from agriculture around Lake Xochimilco causes runoff into the lake and a reduction of habitat quality for axolotls. The pesticides used contain chemical compounds that studies show to sharply increase mortality in axolotl embryos and larvae. Of the surviving embryo and larvae, there is also an increase in morphological, behavioral, and activity abnormalities. Another factor that threatens the native axolotl population is the introduction of invasive species such as the Nile tilapia and Common carp. These invasive fish species threaten axolotl populations by eating their eggs or young and by out-competing them for natural resources. The presence of these species has also been shown to change the behavior of axolotls, causing them to be less active to avoid predation. This reduction in activity greatly impacts the axolotls foraging and mating opportunities.

### Population number

According to IUCN Red List, the total population size of the axolotl is between 500 and 1,000 mature individuals. Currently, this species is classified as Critically Endangered (CR) on the IUCN Red List, and its numbers today are decreasing.