Issue: Apr 2016
Author: Joshua Wiegert
My love for aquariums and aquatics extends well beyond the world of fish. Whenever I come across a natural body of water, such as a stream or pond, I can’t resist looking into it to see what types of fish I can find while also looking for frogs, turtles, snakes, heron, and whatever else I can spot.
Unfortunately, this interest doesn’t translate very well to the aquarium, as very few of the animals you’ll discover around a natural body of water are fully aquatic. Sure, you can keep a turtle in a fish tank, and it’ll probably even live, but it certainly won’t be happy—and the fish that cohabitate with it definitely won’t be either. However, there are a small handful of aquatic organisms that actually do quite well in an aquarium setting. One of the most unusual of these animals is the axolotl (Ambystoma mexicanum), also known as the “walking fish” or “water monster.” This popularity of this species is greatly increasing in the hobby and can be found in many aquarium stores around the country.
The axolotl appears otherworldly, like a cartoon dragon lacking wings. It has six feathery external gills that stick out around the head like a crown. A broad mouth and small, lidless eyes on an otherwise featureless face give A. mexicanum an appearance of perpetual contentment. In short, they’re appealing little creatures.
While they require slightly specialized husbandry requirements, these amphibians are hardy and easy to keep. They’re not just novelty pets but adorable animals to care for and enjoy.
What’s an Axolotl?
To describe what an axolotl is, it helps to be familiar with how amphibious life develops. For example, many aquarists have attempted to keep a tadpole in the aquarium and are quite familiar with the life cycle of frogs. Starting out as a jelly-like egg, a small tadpole emerges with a round body and a long tail. Tadpoles are fully aquatic, employing only gills for breathing, and would die if removed from the water, much like fish.
Over time, the shape becomes increasingly froglike, and legs develop. As it matures, the tadpole becomes progressively more dependent upon gulps of surface air until its legs are fully developed and the tail recedes and is no longer distinguishable. The tadpole then leaves the water, either staying around its margins as a miniature frog or wandering farther away as a juvenile toad. Besides obvious morphological changes, there are distinct physiological changes that occur as well. A tadpole’s gills vanish and lungs develop. Its entire digestive system also changes, from a long coil system capable of breaking down and subsisting on algae to a more expansive system that is able to digest meat and insects.
Salamanders and newts are also amphibians, and they follow a similar metamorphosis. In fact, the genus Ambystoma contains the familiar tiger salamanders (A. tigrinum), which are very similar to the axolotl, particularly in their juvenile stage. However, salamander larvae look superficially more like adults, and their metamorphosis is less extreme. Many salamander larvae have full legs, though some species develop these later. One significant difference between frog tadpoles and salamander larvae comes in the form of large, visible, external gills. Looking at a tadpole, the gills are not evident, but in most species of salamander larvae they are. As the salamander larvae mature, these gills vanish and other physiological changes take place, including development of the reproductive system.
In some species of salamander, the larvae can remain in the water for many years if conditions on land are not hospitable. In a handful of these species, they can also enter a stage called neoteny, which means “extended youth.” In this case, individuals retain their “youthful” features—specifically, gills—yet develop the ability to reproduce. This is partial neoteny, in which individuals look fairly similar to the adults but also retain some of their larval characteristics (e.g., a salamander with small gills), or complete neoteny, in which the individual is essentially suspended in a larval form but capable of reproduction. The exact triggers that induce neoteny in salamander larvae are not fully understood; some individuals will enter into this state while others will not.
A handful of exceptions are known to exist in which no adult forms ever emerge. In these cases, juveniles never lose their gills or any other juvenile characteristics but simply mature into a reproductive adult that is physically identical to the juveniles, though bigger. The best known of these completely neotenic creatures is the axolotl (A. mexicanum).
Caring for Water Monsters
In the aquarium, axolotls reach a size of about a foot (30 cm). They typically live more than ten years, with some rare individuals living twice as long. While a 20-gallon (76-liter) tank is an acceptable temporary home, I don’t recommend maintaining them in anything smaller than a 40-gallon (151-liter) breeder tank for a lengthy period. Axolotls tend to wander around the bottom of their aquariums, and though depth of water is unimportant, providing them with enough space to comfortably explore is key to maintaining their health in captivity.
Lake Xochimilco, the type locality for A. mexicanum, is a high-elevation mountain lake. Despite its location in a tropical region, the water is quite cool. Axolotls do not require a heater and should be kept at a cool temperature; 72°F (22°C) is at the high end of their range. Their tank should be located in a cool part of the house, such as a basement or garage—though be sure summer temperatures don’t cook them! They can handle temperatures down to the low 40s F (single digits C) without issue.
Feeding and Tank Conditions
Feeding is simple, as axolotls will eat any large, sinking pellet. These carnivorous amphibians will also devour earthworms, shrimp, and other meaty foods. Many will gladly eat Spirulina sinking tablets, but a high-quality carnivore pellet should be the bulk of their diet.
Axolotls have a tendency to ingest gravel when they eat and are better kept over fine sand. A bare-bottom tank is also suitable and allows them to easily locate their food. Decoration should include several large hiding places in which they will occasionally shelter. However, they are generally quite bold and will remain outside of these hiding places; shelters are needed to help them feel more secure in the tank.
The aquarium may also benefit from the addition of either artificial or live plants. In an axolotl tank, live plants are best maintained in pots; this arrangement also limits the amount of substrate required. Alternatively, plants—such as Anubias spp. or Java fern (Microsorum pteropus)—can be anchored to structures or mounted on wood, rock, and other decorations.
Water quality is important, and organic debris should not be allowed to build up. Substrate should be cleaned regularly, with any mulm removed from the tank. This is another advantage of a bare-bottom tank—gunk can simply be siphoned out as needed. Oxygen levels should be kept high because these animals are intolerant of low levels. Provided the water is clean (low in organic wastes), cool, and well aerated, A. mexicanum will thrive in the tank.
Many aquarists seek to keep axolotls with fish. This is a mistake. Their long gill filaments make a tempting target for fish to nip. Even relatively peaceful fish will find them irresistible. These filaments are effectively their lungs, so one can only imagine that having something biting them is not healthy. Axolotls will attempt to eat fish, which can often end in disaster. I had a friend attempt to put a bushynose pleco (Ancistrus sp.) in with his axolotl only to find that the “water monster” attempted to eat it. At the same time, the plecos’ spines became lodged in A. mexicanum and neither animal survived this encounter.
Some aquarists will maintain A. mexicanum with snails, which make fairly good tankmates for them, at least briefly. Sooner or later, an axolotl will figure out that the snail is edible, and they don’t seem to be deterred by shells.
Axolotls can be maintained with one another provided they are similar in size. They can be cannibalistic, particularly in the juvenile stage, though adults seem to be less so. If one specimen is more than an inch (2.5 cm) or so smaller than the other, expect to have only one axolotl! Additionally, juveniles may actually bite the legs off one another, although these appendages do regrow. This can be prevented by keeping them well fed. They seem to outgrow this bad habit by the time they reach 4 to 5 inches (10 to 13 cm) in length. Beyond the addition of another similarly sized axolotl (or several), these animals are best maintained alone.
Many aquarists are tempted to handle their pet axolotl. Don’t do it! Axolotls have a relatively delicate skeletal system, and removing an axolotl from the water can damage it. Even when netted at the dealer, they should be kept completely submerged if possible—that is, steered into a collection cup rather than netted. This is particularly true of young animals. They’re able to breathe atmospheric air, but their gills can be damaged by prolonged exposure.
A Colorful Selection
The axolotl has been selectively bred into a number of interesting morphs. While the natural form is a light gray, melanistic organisms were once found even in the wild. These individuals are much darker and lack speckling on the body. Albino individuals are very common in the trade, as are white or leucistic (partial pigmentation loss) specimens. Eye coloration also differs among morphs, with true albinos sporting red eyes.
While not commonly seen in the United States, a copper variety of A. mexicanum also exists, which is an albino with lots of dark spots on it. A golden variety is also sometimes seen; these individuals may be albino or leucistic. Additionally, a number of less common mutations show up, including mosaics, which feature large patches of white or black; chimeric, which display one type of coloration on the left and a different kind on the right; and many others. However, these types of color morphs are very rare, and only true collectors are likely to keep them.
As previously mentioned, the axolotl is native to Lake Xochimilco, which is a water source for Mexico City. However, as Mexico City has grown, wild A. mexicanum populations have plummeted. Despite being listed as a biological reserve by the Mexican government, Lake Xochimilco is heavily impacted by ongoing urban development. Much of the lake has been drained and replaced with a canal system. The water is heavily polluted, and invasive carp and tilapia compete with axolotls for food. The International Union for the Conservation of Nature lists this species as critically endangered. A four-month-long survey in 2013 failed to find a single axolotl in its habitat, and only two were spotted around Lake Xochimilco in early 2014. It is likely that A. mexicanum is nearly extinct in the wild.
Because of their relatively unique life cycle, axolotls have become popular laboratory animals. They are much easier to breed in large numbers in a lab setting than other salamanders because they do not require any terrestrial stages. They are also thoroughly studied for their amazing regenerative powers: An axolotl that suffers a non-life-threatening injury, including the loss of legs, tail, or other body parts, can regenerate those parts within two weeks. Remarkably, they also heal without scars.
Nearly all of the axolotls extant in the hobby today are a result of a small number of specimens that were collected and bred for laboratory work. Sadly, genetic research on captive members of the species indicates that the neoteny observed in our specimens may be the result of artificial selection and not a result of the same genetic deficiency present in wild populations. This means that our axolotls in the hobby may not be the same as the original, wild types.
The purchase or sale of axolotls is restricted in several municipalities out of fear that they may pose environmental risks should they be released. Although they pose very little risk, the same is not true of the various species of tiger salamander in genus Ambystoma. Tiger salamanders spread several diseases that decimate wild amphibian populations. As larvae of the various Ambystoma spp. are nearly indistinguishable from one another, many regions have enacted blanket bans on all species in the genus.
This problem is exacerbated by the occasional unscrupulous dealer who sells relatively inexpensive tiger salamander larvae as axolotls. Aquarists who purchase these fake axolotls are in for a surprise when they begin to transform into terrestrial salamanders, which live the bulk of their life underground! Often, this is not even the fault of the dealer. I once received a shipment of “axolotls” from a wholesaler that began transforming into tiger salamanders before they left quarantine. I certainly never would have known had they been a month or two younger.
Tiger salamander larvae are not typically seen in any color morph except the wild type, so you can rest assured that albino or gold axolotls are the real deal. In some areas, only these non-wild types may be sold. Check your local area’s laws, especially if you are purchasing specimens from a dealer in another state or region. In the United States, states that have laws restricting or limiting the sale of axolotls include Maine, New Jersey, and California, with the recent addition of Virginia. This is not a comprehensive list, and these laws change fairly frequently, so be sure to check before purchase—particularly local statutes, as towns and counties may enact restrictions of their own.
Loving a “Monster”
Axolotls are truly bizarre and unique animals, and it is only through the efforts of aquarists that they are likely to continue to survive at all. Although best maintained in a species-only setup, they are interesting enough to warrant such a tank. The next time you come across these fascinating “walking fish” for sale, bring one home. You may end up loving a monster.
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