That Reptile Blog – Reptile, Amphibian and Exotic Pet Care and Information

As a herpetologist and New York City native, I’m thrilled by the prospect of finding wildlife, endangered or otherwise, within big cities.  I was, therefore, very happy to read about the recent discovery of a population of critically endangered salamanders (Mexican Axolotls, Ambystoma mexicanum) in the heart of one of the world’s most densely populated cities. Read More »

Check out: Captive Care of the Mexican Axolotl Ambystoma mexicanum – Part 1, for the first part of this article.

Feeding

Repto-min, trout chow or salmon chow serves well as a mainstay; alternate this with Hikari Massivore Delight or a pelleted cichlid food, freeze dried prawn, live blackworms, earthworms, minnows, shiners and guppies.  Axolotls will also take frozen clams, mussels and similar foods, but marine-derived items should not be used as a major part of the diet.

Although normally bottom feeders, well-habituated axolotls will rise to the surface to eat, and take readily to hand- feeding.  A finger waved before one will be grabbed and “swallowed”.

Finely-chopped blackworms (use a razor) are the best food to start off with when raising larvae; brine shrimp are also accepted, but growth with be faster for those consuming blackworms (and each other!).  Be aware that blackworms, even chopped pieces, clump together; larvae may choke trying to take down a large ball.  Be sure to swish the worms about the tank, and watch that they do not “find each other” after awhile.  A surface worm feeder that releases the worms individually will help relieve this problem.  I’ve not been able to induce axolotls under 2 inches in length to accept Repto-min or other dry foods, but others report success with very small individuals.

Social Groups

While not overly aggressive toward each other when un-crowded, the feeding of live food seems to stimulate a near frenzy, during which animals may bite the gills and toes of their tank-mates. Generally these grow back without problems, but a treatment with Aquarium Pharmaceuticals Stresscoat is a useful safety measure.

Axolotls housed together should be nearly the same size – even a difference of 25% is too great, as the larger animal will eventually bite off the gills and limbs of the smaller, possibly killing it.  Feeding injuries among similarly-sized animals are less severe – the loss of a toe or gill tip.

Losses are usually high when young animals are reared together, but can be lessened somewhat by the provision of dense cover (please see “Physical Environment”, above).

Captive Longevity

Captive longevity approaches 25 years.  Animals in my collection are still breeding at age 17.

Handling

It is best to usher an axolotl into a plastic container, as they may damage their delicate skins when thrashing about in a nylon net.  Axolotls may also be picked up by hand, but use caution as they tend to lie very still and then suddenly explode, and can easily propel themselves to the floor.  Handle only when necessary to move or examine an animal, and use wet hands and great care as the skin is quite injury-prone.

Miscellaneous

The axolotl’s long history as a laboratory animal has given rise to a wide variety of attractive color phases, including leucistic, black, albino, piebald and others.  The genetics of color inheritance in this species is quite interesting, and seems not to follow the “normal” rules…at least not as I learned them!  Recently, bio-engineered axolotls that glow fluorescent green have appeared in the trade.

Rearing axolotls in the lab differs in some respects from home care, but much of value is contained in the protocols of institutions maintaining large research colonies.  You can read about how it’s done at laboratories and universities all over the world at:

http://www.ambystoma.org/AGSC/suppleguide.pdf

Please see Natural History of the Axolotl for information on axolotls in their natural habitat.

I highly recommend the Mexican axolotl as an aquarium animal for both beginning and advanced hobbyists, and include them whenever possible in the zoo exhibits that I design.  Assuming that attention is paid to temperature and water quality (please see below), axolotls are hearty, long-lived and quite simple to breed.  They are also extremely responsive pets, taking readily to hand feeding and perfectly content to be out and about by day.

Enclosure

Axolotls are entirely aquatic, with an adult requiring an aquarium of approximately 10 gallons in size; 5-10 gallons more should be provided for each additional animal.

Axolotls also fare well in plastic terrariums or sweater boxes that are emptied and cleaned as opposed to filtered; this method is very useful when rearing large numbers of young.

Physical Environment

Axolotls are unusual among salamanders in not requiring a shelter, but the provision of such will help to alleviate aggression when groups are kept together.  They readily take up residence in plastic reptile caves, clay flower pots or PVC pipes.

The young enjoy consuming their brethren, and should be afforded as much opportunity to avoid each other as is possible.  Individual shelters are not effective in preventing this (among young animals) in group situations.  Rather, fill most of the aquarium’s water column with Penn Plax Baby Hideouts and other plastic aquarium plants.  Weight these down with plant ties to cover the base of the aquarium, and position tall plants so that they fill the vertical space as well.

Axolotls can be kept in bare-bottomed or planted tanks.  If gravel is used, it should be of a size that cannot be swallowed.  Small rocks ingested during feeding are often passed, but impactions can occur.  Live aquarium plants, if used, should be sturdy and well-rooted, or floating.

Light and Heat

A florescent bulb is best used for illumination, as it will not add significantly to water temperature.  Axolotls do not need a source of UVB light.

Axolotls are native to cool, high-altitude lakes, and do best at water temperatures of 62-70 F, with a dip to 50-52 F in winter, if possible.  People do keep them at warmer temperatures, but such leaves the salamanders susceptible to illness and fungal problems.  At temperatures above 75 F, Saprolegnia infection is not infrequent (this and certain bacterial infections usually cause the animals to float to the surface).  A cool basement is ideal for year-round maintenance.

Water Quality and Filtration

Axolotls are fairly tolerant of a wide variety of conditions, but should ideally be kept in soft water with a pH of 6.9-7.6.  Hard or acidic water can damage the gills.

Adequate filtration and frequent water changes are essential.  Axolotls have large appetites and excrete copious amounts of nitrogenous wastes (which are largely dissolved in water and not visible), and can quickly succumb to ammonia toxicity.  An ammonia test kit should always be on hand.

Canister, hanging or in-tank filters all have their place in axolotl aquariums, but each must be coupled with regular water changes.  Larvae are best kept in aquariums filtered with sponge or corner filters.  In all cases, filter outflow should be adjusted so as not to disturb the salamanders, as they are not strong swimmers and prefer still water.

Check out: Captive Care of the Mexican Axolotl Ambystoma mexicanum – Part 2, to read the rest of this article.

Click here: The Natural History and Captive Care of the Mexican Axolotl, Ambystoma mexicanum: Natural History – Part 1, to read the first part of this article.

Diet
Mexican axolotls feed upon shrimp, aquatic worms, copepods, insects and other invertebrates and small fish. Adults are major predators upon small axolotls and axolotl eggs. The larvae are largely carnivorous, but may consume some algae as well.

Their main predators are herons and other wading birds, fishes and larger axolotls.

Reproduction
Males deposit spermatophores (sperm-filled capsules) on the lake bottom from March through June. After nudging the cloacal region of a gravid female – and being nudged in return – the male undulates his tail in front of her, most likely releasing pheromones (chemical attractants) in the process. He then leads the female over the spermatophore, which is taken into her cloaca. Fertilization occurs internally. The eggs, 100-300 in number, are laid after a gestation period of 2 weeks. They are attached to aquatic vegetation and hatch in 10-14 days.

The young reach sexual maturity in approximately 18 months. The Mexican axolotl is a paedormorphic (or neotenic) salamander, meaning that it retains a number of larval characteristics, including external gills (adults have lungs as well) and an aquatic lifestyle. Animals injected with the thyroid gland hormone thyroxin will transform into terrestrial adults, but do not survive for long.

Economic Importance
The Mexican axolotl is an ideal laboratory subject, and quite important in medical research. Studies of it have led to important advances many fields, including gene expression, neurobiology and limb/organ regeneration.

Axolotls possess a greater facility for regeneration than do most amphibians, and can re-grow limbs, digits, gills, tails and portions of the liver, spleen and eye. This ability may have important implications for people, and has been under investigation for some time.

Miscellaneous
Nearly all axolotls in captivity today can trace their origins to specimens collected from Lake Xochimilco and shipped to Paris in 1864. Three aquatic salamanders in the Genus Ambystoma have historically shared this species’ habitat, and it is believed that hybridization with these occurred at the time of that original collection. Current lab and pet populations may, therefore, differ genetically from wild axolotls.

The axolotl is closely related to the tiger salamander (A. tigrinum), and hybrids are fertile. Tiger salamanders also exist in neotenic form in certain populations. There is evidence that the animals forming the basis of today’s captive stock have interbred with neotenic tiger salamanders.

The name “axolotl” is taken from that of the Aztec god Xolotl, who was believed to have taken on the animal’s form, and loosely translates to terms such as “water dog”, water sprite”, “water slave” and “water monstrosity”.

I have written articles about axolotl relatives as well. Please see:
The Spotted Salamander, Ambystoma maculatum – Part I and Part II
Algae and Salamander Eggs – an odd partnership

The abstract of an International Zoo Yearbook article detailing the use of the axolotl as a “flagship species” for the conservation of its habitat is posted at:
http://www.ingentaconnect.com/content/bpl/izy/2008/00000042/00000001/art00014;jsessionid=1hpc0rtn02l2l.alexandra?format=print

Relatively scarce in the pet trade when I first began in the field (admittedly quite awhile ago!), interest in the Mexican axolotl has exploded in recent years, and today it is arguably the most commonly-kept salamander in the USA. Its popularity has soared in other countries as well, and oddly enough, it has become somewhat of a pop culture figure in Japan.

The Mexican axolotl has the rare distinction of being simultaneously one of the world’s most highly endangered amphibians (in the wild) and a common pet and laboratory subject.

Please note: the aquatic larvae of all species of salamanders are termed “axolotls” in some references, i.e. as in “tiger salamander axolotls”. The name as used here refers to young and adult Ambystoma mexicanum only.

Classification
Class: Amphibia
Order: Caudata
Family: Ambystomatidae

A complex of a dozen or more related aquatic salamanders dwell in mountainous lakes in the vicinity of this species’ range. The waters inhabited by one of these, the Lake Patzucuaro salamander (A. dumrelii), are high in dissolved salts. Perhaps as an adaptation to its brackish environment, this salamander sheds its outer skin layer continuously throughout its life.

The family Ambystomatidae, limited to North America, contains approximately 40 terrestrial and aquatic members, including such well known species as the tiger salamander (A. tigrinum), marbled salamander (A. opacum), spotted salamander (A. maculatum) and Pacific giant salamander (Dicamptodon ensatus).

Physical Description
The Mexican axolotl is stoutly built and reaches 9-12 inches in length. The head bears large, bushy red gills and the laterally compressed tail is equipped with a swimming fin. Wild specimens are dark brown in color; albino, leucistic, black and piebald strains are common in laboratories and the pet trade.

Range
This species’ entire natural range is limited to an area of 6.2 square miles in size. It is native only to Lakes Chalco and Xochimilco, adjacent to the southern edge of Mexico City, in central Mexico.

It appears now to survive only in a small fragment of that tiny habitat – the southern remnants of Lake Xochimilco, and perhaps within associated canals and private garden ponds.

Habitat
The Mexican axolotl is entirely aquatic. The high-altitude lakes it inhabits are deep, heavily-vegetated, mud-bottomed and cool in temperature. Axolotls can extract oxygen from air or water, utilizing gills, lungs and skin.

Status in the Wild
This salamander is extremely rare in the wild, and is considered to be among the world’s most endangered amphibians. Recent surveys in its tiny natural range have turned up only 0-42 specimens, despite 1,800 net casts.

Lake Chalco has been largely drained, and no axolotls survive there. Lake Xochimilco has been severely polluted and impacted by flood control measures, and is extensively channelized to accommodate “floating gardens” of flowers and vegetables.

Protected by the Mexican government and listed on Appendix 2 of CITES, Mexican axolotls are none-the-less still collected for the food and medicinal trade, and are also threatened by large populations of introduced fish (Tilapia and carp).

Click: The Natural History and Captive Care of the Mexican Axolotl: Part 2, To read the 2nd half of this article.