Mexican Axolotl (Ambystoma mexicanum)

The Mexican axolotl is an aquatic salamander that has the distinction of being both a common laboratory animal and a species whose wild populations are in serious decline. This unique amphibian is native only to Lake Xochimilco and Lake Chalco, southeast of Mexico City. Both lakes have been drained and channelized, and although recovery projects are underway, it is feared that most or all of the native population is gone. The axolotl's plight is further exacerbated by the fact that local people find them quite tasty. It is said that some may survive in garden ponds in the area, but the majority today are to be found in laboratories and as pet or zoo animals.

The Mexican axolotl makes a fascinating aquarium animal. It is a neotenic species, meaning that the adult form of the salamander retains certain larval characteristics — in this case external lungs and an aquatic lifestyle. Adults have lungs as well and also breathe through the highly porous skin. Normally colored olive brown, with branching red gills, captive strains are now available in the leucistic, albino, black, yellow, and piebald patterns.

Axolotls require fairly cool temperatures, preferably less than 70°F (19°C), and soft water, with a pH of 6.9 to 7.2 (hard or acidic water dam-ages the gills). They become quite bold in captivity, swimming to the surface like fish for floating food items. Although they will use shelters, these are not required, and they seem quite content to remain in view all day. Axolotls grow to a length of about 8 inches and are quite stocky but do not require a good deal of space. A pair can be comfortably housed in a 15 gallon aquarium.

The axolotl has long been a valuable laboratory animal, used in the study of human developmental biology and embryology. Animals that were originally collected, and that form the basis of today's captive populations, likely hybridized with closely related tiger salamanders collected at the same time. It is therefore unlikely that genetically pure specimens can be found in today's captive populations.

Axolotls breed quite readily if exposed to a period of cooler water temperatures (approximately 52°F, 10°C). Males are easily identified by their swollen cloacas, and the females are generally stouter in build. The several hundred eggs are laid on aquarium plants or other surfaces. The eggs should be removed to a mildly aerated aquarium as soon as possible, as the adults feed upon them quite readily. Newly hatched young require live food in the form of black worms. They are cannibalistic, but this can be prevented somewhat by giving them lots of hiding places. Eventually the young can be weaned onto commercial foods such as trout chow or other pellets designed for carnivorous fish. Such can form the basis of the diet, although they do appreciate the frequent meals of earthworms, black worms, and fish.

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