The Mexican walking fish -- more commonly referred to as the axolotl -- is not a fish, but rather an amphibian, native to Central Mexico. As Aztec legend goes, the sun god Xolotl took the form of this salamander to escape sacrifice, but he eventually was boiled in a kettle. Modern axolotls, however, are alive and well in private collections and research facilities, thanks in large part to cartoonishly cute features and a remarkable ability to regenerate limbs and skin. While these captive populations flourish, the fate of wild axolotls appears as ill-fated as their mythical predecessor.
Living the Permanently Larval Life
Most amphibians grow like this: Eggs hatch into larvae -- for frogs, these would be the tadpoles -- which metamorphose into adults; swapping gills for lungs along the way. But the axolotl, formally known as Ambystoma mexicanum, stays in its larval form throughout its life, a feature known as neoteny. In this preadolescent state, the axolotl ultimately grows 9 inches long and reaches sexual maturity, developing rudimentary lungs, along with the ability to absorb oxygen through their skin. But respiration primarily occurs through retained external gills, frilly structures that resemble a feathered headdress sitting atop their oddly curious face, complete with a permanent grin fixed beneath small, lidless eyes.
Neoteny can be seen in other salamanders, but often results from stressful environmental conditions, including extreme cold temperatures. But in axolotls, this indefinite postponement of metamorphosis is entirely genetic.
Unlocking the Secrets of Tissue Regeneration
Many kinds of lizards can lose a tail and regrow it. But this new appendage is missing bones and nerves. Newts and salamanders, including the axolotl, can regenerate not only a missing tail, but also limbs, jaws and spinal cords. And these newly formed parts are perfect replicas, complete with bones, nerves, muscles and skin, all formed without scar tissue. They can even regenerate the same limb, dozens if not hundreds of times, each time perfectly. Adding to their natural regenerative abilities, axolotls have shown a remarkable resistance to cancer being 100 times more resistant to carcinogens than mammals.
Flourishing, If Only in Captivity
At one time, axolotls lived in mountain lakes southeast of Mexico City. With the city's expansion, all that remains of these once bountiful wetlands is a network of heavily polluted canals, teaming with introduced predatory fishes. Loss and degradation of their only home has caused a precipitous drop in axolotl numbers, from 6,000 in 1998 to just 100 animals in 2008. By 2014, the species was feared extinct until scientists from Mexico's National Autonomous University spotted two.
Though critically endangered in the wild, axolotls are quite common in captivity, thanks in large part to unique biological attributes that have endeared them to scientists for over a century. In recent years, their appeal has extended to a cultlike following among private collectors, who say they're hardy, relatively easy to breed, and with proper care will live 10 to 15 years.
The "Cup Mouth"
Axolotls are carnivores; strict meat eaters that in the wild, slither along muddy bottoms, gobbling up any bugs, small fish, snails, crustaceans and worms that will fit into their large, wide mouths. The Greek portion of their scientific name -- Ambystoma -- fittingly translates as cup mouth.
Captive axolotls require a high-quality, nutritionally balanced diet. They'll eat both live and dead foods, but live foods carry a higher risk of introducing parasites. Earthworms are highly nutritious and a perennial axolotl favorite, but best if sourced from organic soils. Other types of worms -- tubifex and white worms notably -- contain too much oil and fat, which can lead to liver issues. At the Abystoma Genetic Stock Center, a breeding colony of axolotls housed at the University of Kentucky, they feed their charges brine shrimp, California blackworms and salmon pellets -- a high protein, vitamin-fortified feed developed for cultured fish.
To reduce mess, many hobbyists choose to feed their animals by hand, a task made easy as axolotls can recognize shapes -- including the hand that feeds them -- from a distance. This level of recognition is not unique to axolotls. In a study published in Animal Cognition, researchers found their cousin -- the red-back salamander -- can distinguish one number as being larger than another, up to three.
Cool and a Little Hard
Scientists at the now closed Indiana University Axolotl Colony -- the country's first axolotl stock center -- characterized water as the "most important component of the axolotls' environment." Axolotls prefer slightly hard water maintained at a stable, cool temperature between 50 and 68 degrees F. Anything colder and they'll slow or stop eating. Higher temperatures bring on stress and disease. And where hard water helps axolotls maintain healthy skin, their first line of defense against infection, soft water can lead to anemia. Salt can be added to water to maintain hardness and deter fungus and parasites. Chlorine and/or chloramine, added by municipalities to kill bacteria in water supplies, must be removed with a commercially available dechlorinator.
Wild axolotls are generally some shade of green or brown, with black, gold or shiny speckles, a color pattern now referred to as wild type. A number of mutations have been identified in axolotl genes that control color and pigmentation. By selectively cross-breeding animals with these mutations, ambitious hobbyists and researchers have produced other color types, including:
- Melanoid: very dark if not black axolotls that have an unusually large number of dark pigment cells.
- Leucistic: white or pink axolotls with dark eyes and a smattering of dark pigment cells on the top of their head and back.
- Albino: golden, yellow or white axolotls with red, pink or pale eyes, depending on the presence or absence of other types of pigment cells.
Slithering Towards Extinction
In an effort to avert the extinction of wild axolotls, scientists are exploring the possibility of releasing laboratory-raised axolotls back into Mexico's canals. Those working to conserve the species, including scientists at the National Autonomous University of Mexico, fear captive animals may introduce fungal infections or other diseases. They also worry these inbred captive animals will reduce the genetic diversity of wild axolotls. Their reservations may not be unfounded as virtually all captive axolotls can trace their ancestry to just two of the wild specimens shipped from Mexico to Paris in the 1860s.