Axolotl
The Axolotl (Ambystoma mexicanum) colloquially
known as the "Mexican walking fish" is an aquatic salamander.
It is noted for its appearance and its demonstration of neoteny,
remaining in its aquatic larval form even as a sexually-mature
adult, and not undergoing metamorphosis into a terrestrial
form.
Axolotl,
Ambystoma mexicanum
A fully grown axolotl, at age 18-24 months, ranges in length from
150-450 mm (6-18 inches), although a size close to 230 mm (9
inches) is most common and greater than 300 mm (12 inches) is rare.
They have distinctive 'fern-like' gill structures that are not
covered (as are gills in fish and in frog tadpoles) - usually 3
stalks on each side of the head; axolotls also breathe through the
skin and possess lungs. In colour they range from albino or white
(leucistic) to black, through greys, tans and browns. Wild axolotls
are rarely white, though all leucistic (white with black eyes)
axolotls are decended from a single mutant male brought to Paris in
1863 in a shipment to the Jardin des Plantes in the city. The true
albino mutant common in labs and pet stores was created in an
American laboratory in the 1950s. No axolotls commercially
available today are wild caught (since doing so is strictly
forbidden), the majority being descended from these same research
specimens, even in the pet trade.
The axolotl is carnivorous, consuming small prey such as worms,
insects, and small fish in the wild. In captivity, they can be fed
a variety of readily available foods, including trout and salmon
pellets, frozen or live bloodworms, earthworms, and waxworms on
occasion. Beef Heart is sometimes recommended, but is not a good
choice for the animal as mammal proteins are not readily
digestible. Small juveniles can be fed with Daphnia or brine shrimp
until old enough for the larger foodstuffs. Axolotls locate food by
smell, and will "snap" at any potential meal, sucking the food into
their stomachs with vacuum force. This is why other animals may not
be housed with axolotls (excluding adults with adults) as anything
smaller will invariably end up in their mouths - even siblings.
Fish can also nibble an axolotl's gill stalks and appendages while
it is asleep, leading to infection.
Axolotls live at temperatures of 14-20 C (54-68 F). Lower
temperatures slow down their metabolism, although temperatures
around 10 C (50 F) are a panacea when it comes to treating disease.
Higher temperatures can lead to stress and increased appetite. In
the wild, the water temperature in Xochimilco rarely rises above
20°C (68°F), though it may fall to 6 or 7°C (43°F) in the winter,
and perhaps lower. Tapwater should be treated with a dechlorinator
before the animal is introduced into it, as chlorine is harmful. A
single typical axolotl should be kept in a normal 10 US gallon tank
with around 150 mm (6 inches) of water. A full tank without a lid
can lead to the animal "jumping ship" as it were. A power filter or
underwater filter run by an airpump can be used, though water flow
should be kept to a minimum to avoid stressing the animal. A 20%
water change should be done every week, with debris being siphoned
from the bottom. Small gravel should be avoided, as the axolotl can
mistakenly injest the rocks, leading to stress and gas. Floating
plants should be provided for juveniles as a hiding place, and
small caves should likewise be provided for adults. No light is
necessary as axolotls are nocturnal. Axolotl's will spend a
majority of the time on the bottom of the tank (floating is a sign
of stress and illness), so design accordingly.
Axolotls are very closely related to Tiger Salamanders
(Ambystoma tigrinum). Some
axolotls can metamorphose, although they do so rarely, and usually
have to be treated with iodine or shots of thyroxine hormone to
achieve the full transformation. The loss of the ability to produce
the hormones on their own is what results in the animal's neotenic
existence. Metamorphosis also dramatically shortens the axolotl's
lifespan, assuming they even survive the process. A neotenic
axolotl will live an average of 10-15 years (though an individual
in Paris is credited with achieving 25 years), while a metamophosed
speciman will scarcely live past the age of 5.
Native to Lake Xochimilco and Lake Chalco in central Mexico, they
also live in "axalapascos" (volcanic craters filled with water,
maare). The wild population has been put under heavy pressure by
the growth of Mexico City. Axolotls are also sold as food in
Mexican markets, and were a staple in the Aztec diet. It is
currently listed by CITES as an endangered species. In the Spanish
language it is called the ajolote.
The axolotl is used in research and large numbers are bred in
captivity. The feature of the salamander that attracts most
attention is its healing ability: the axolotl does not heal by
scarring and is capable of the regeneration of entire lost
appendages in a period of months, and, in certain cases, more vital
structures. Some have indeed been found restoring the less vital
parts of their brains. They can also readily accept transplants
from other individuals, including eyes and parts of the brain -
restoring these alien organs to full fuctionality. In metamorphosed
individuals, however, this ability is greatly diminished. Another
attractive feature, for research, is the large and robust embryos.
Axolotls are especially easy to breed compared to other salamanders
in their family, which are almost never captive bred.
The name is from the Aztec Nahuatl language, from "atl" meaning
water, and "xolotl" meaning dog. It also derives its name from the
Aztec God of Death Xolotl, who - according to myth - assumed the
amphibian's form in an effort to escape exile, and was killed while
such.
Pop cultural reference: The word "axolotl" is used frequently in
crossword puzzles. It was often used in the early years of Mad
Magazine as a running joke.
Axolotl tanks are frequently mentioned in Frank Herbert's Dune
series of books.
Conservation status: Vulnerable
Issue #6
