What creature can regenerate limbs, is sought for research, and has the largest animal genome known?
The axolotl, or mbystoma mexicanum, of course!
Axolotls, known as “Mexican walking fish”, are not a fish at all, but a type of amphibious salamander. There are lots of peculiar things about this creature, including that it never matures into a terrestrial animal like other salamanders do.
Instead, it exhibits what we call “neoteny”; it retains its tadpole-like features. These include a rosy wreath of external gills arrayed around its neck in three pairs, though it also has functional lungs and can breathe through its skin. Axolotls have a caudal fin (or tail fin) running right down their backs, along with wide heads with lidless eyes and little smiley mouths that open when they yawn in an adorable display!
They are carnivorous, eating worms, insects and small fish in the wild, finding food by smell and ingesting it by sucking it in like a vacuum cleaner directly into their stomachs. Axolotls come in a variety of colors; in the wild, they are mostly brown or tan with gold speckles and an olive undertone, but they are domestically bred to become pink, gold, gray, white, black or dark blue. They become sexually mature in 18-24 months, live up to 10-15 years, and usually grow up to 9 inches in length, though they can reach double that size.
Roasted axolotls are considered a delicacy in Mexico, despite their agreeable smiles!
One of their most unusual characteristics – which is of great research interest – is their ability to regrow limbs (as well as some other organs) when lost or damaged. They are studied to analyze heart defects and neural tube closure, and the genetics of their color variation are also of interest. But their most important research value lies in their regenerative features. They do not scar, and thus are capable of perfectly restoring lost limbs in a couple of months, and sometimes even more vital structures such as tails, nervous systems, and certain tissues in their eyes, heart and brain. They also can readily accept transplants from other individuals and restore these organs to full functionality. In addition to all of the above, axolotls are observed to be 1000 times more cancer-resistant than any other species because of their large genome. Truly a trait to envy.
What was that about ‘Neoteny’?
Axolotls reach sexual maturity without undergoing metamorphosis, like frogs and most other amphibians, due to an apparent lack of thyroid-stimulating hormones. This phenomenon is called ‘neoteny’. Interestingly, the genes responsible for their neoteny are not found in wild populations, which suggests this characteristic is related to their domesticated breeding. Metamorphosis can be artificially induced in axolotls with injections of thyroid hormone or iodine, but left alone, the axolotl will retain its larval appearance. Though similar species don’t generally share this trait, food deprivation can produce neoteny in other types of salamander, because those smaller larval stages can survive on fewer and poorer nutrients. Ah, the joys of staying young!
Check out this short video about axolotls and their regenerative capacities:
What is their typical breeding behavior?
Axolotls in the wild breed once a year between March and June, depending on when the water temperature warms to a certain level, though in captivity they can be bred more often. They begin this process with an intimate dance to initiate the mating process. Then, after about a 30-second ritual of vigorous tail shaking, the male will drop a sperm-capped cone-shaped mass that the female picks up, beginning the fertilization process.
The female lays between 300 and 1000 individual eggs, each covered by a protective substance and planted on rocks and leaves to keep them safe. Once the eggs hatch in about two weeks, these young are left on their own to fend for themselves; there is no critter-cal period of parental love for these new wee young! Nature is not always easy…
An adored yet unappreciated creature
Before colonial settlers arrived in Mexico, the Aztec people coexisted with axolotls for thousands of years. The Aztecs respected axolotls so much that they believed there was a mythological connection between the creature and Xolotl, a dog-headed Aztec god who made himself to be a salamander in order to stay alive. Being associated with a god is quite the honor!
Unfortunately, once Spanish settlers arrived in Mexico and transformed the landscape, axolotls were no longer respected. This set the stage for today, as, centuries later, axolotls are viewed as human entertainment or objects of curiosity. The change in Mexico City’s lakes, and high demand for these animals in the pet trade and research has created a ‘conservation paradox,’ where more axolotls live in tanks than their natural habitat. Even the most admired animals are sadly inching closer to extinction in the wild.
How are human activities impacting axolotls?
Despite there being numerous axolotls in captivity, this doesn’t ensure survival for the species. Artificial selection for scientific research has led to decreased genetic diversity, reducing their overall gene pool which makes them more vulnerable to disease and extinction.
Although axolotls are adored by many, the first study on their native habitat and its conditions was not undertaken until 1998 by Mexican scientist Dr. Luis Zambrano. He noticed a severe reduction in axolotl population size over the course of 17 years. These dire results motivated Dr. Zambrano to begin a conservation program. His team is currently buying Xochimilco lake canals – the last area wild axolotls exist in – with the goal of cleaning the water and increasing natural vegetation. To pay for this project, Dr. Zambrano plans to grow organic foods and use the profits to buy the canals. After centuries of axolotls being used and their habitat being destroyed, one man took action and now there is hope for these creatures.
Maybe one day conservation and restoration efforts could help axolotl populations regenerate as effectively as their bodies.
Here’s to growth and regrowth,
Fred and Tania
Fred Jennings is from Ipswich, MA, where he has spent most of his life. He is an ecological economist with a B.A. from Harvard and a Ph.D. from Stanford, both in economics. Fred is also an avid conservationist and fly fisherman. He enjoys the outdoors, and has written about both natural processes and about economic theory. He has 40 years of teaching and research experience, first in academics and then in economic litigation. He also enjoys his seasonal practice as a saltwater fly fishing guide in Ipswich.
Tania graduated from Tufts University with a Master of Science in Animals and Public Policy. Her academic research projects focused on wildlife conservation efforts, and the impacts that human activities have on wild habitats. As a writer and activist, Tania emphasizes the connections between planet, human, and animal health. She loves hiking, snorkeling, and advocating for social justice.