Tag: Ambystoma

Common Name: Axolotls

A.K.A.: Ambystoma mexicanum

Vital Stats:

• Grow to a length of 15-45cm (6-18”)
• Can live up to 15 years
• Have no eyelids
• Usually black or brown in colour, but mutation occasionally produces pink skin
• Eat insects, worms, and small aquatic animals
• Commonly kept as pets and, in parts of Mexico, food

Found: In the Xochimilco lake system, near Mexico City

It Does What?!

Axolotls are the Lost Boys of the amphibian world… they never grow up. These bizarre little salamanders are found only in a single lake system near Mexico City and, if the city’s pollution gets much worse, may soon not be found there, either.

First, a little background on salamanders in general. These amphibious, lizard-like creatures begin life in a larval stage. While adult salamanders have lungs and spend much of their time out of the water, larvae have only gills and are completely aquatic. They commonly undergo a metamorphosis in which the gills are lost and the body changes shape, thinning out and losing its ‘tadpole with legs’ appearance. Many salamanders have displayed the ability to occasionally forego metamorphosis, remaining in their larval stage for life. This phenomenon of looking like a juvenile even during adulthood is called “neoteny.”

What makes axolotls special is that they’re what’s called “obligate neotenes,” meaning they simply never go through metamorphosis… every adult axolotl looks like the larval stage of other salamander species. At some point in their evolution, it became either more beneficial or downright necessary for them to remain aquatic. Biologists have speculated that this is because their smaller larval form requires less food, and because the lakes where they live are low in iodine, an element required for their transformation.

Interestingly, while axolotls almost never go through metamorphosis in the wild, in a certain percentage of them, the genetic instructions for doing so seem to still be intact. If you have a larval axolotl and you want an adult form, you can either give it an injection of iodine, or, for the more deranged among you, gradually deprive it of its pool of water. Either method of forced metamorphosis has a high mortality rate and, at best, causes a hugely decreased lifespan, but it does show they haven’t entirely lost that capacity.

An eternally youthful appearance isn’t even the axolotls’ only superpower. The creatures also possess a Wolverine-like ability to heal themselves. Not only can they – and other salamanders – regrow lost limbs, they can actually regenerate parts of vital organs, including sections of the brain, spinal cord, and, in one study, up to 50% of the heart ventricle. Axolotls can also receive organ transplants from other individuals without rejection or problems with lack of function in the new tissue. Obviously, these traits have made them of intense interest to a certain species which doesn’t regrow limbs, hearts, or spinal cords. Researchers hope that by studying the genetic and biochemical basis of these heightened healing abilities, they can create their own army of X-Men help amputees and victims of spinal cord injuries. But this research is still in its early stages. In the meantime, it would probably be in our best interests not to drive them to extinction.

Fun Facts:

• Axolotls have tiny vestigial teeth, which in other salamanders only grow during metamorphosis.
• Sometimes, an axolotl with a heavily damaged limb will both repair the old limb and regrow a new one, ending up with an extra leg (see above).
• Forced metamorphosis can be only half-successful, producing adult forms with juvenile characteristics, such as a thickened neck.
• Obligate neotenes like axolotls end up with a lot of extra “junk” DNA [biologists: via duplications of the pseudogenes created when their life history changed], which has actually resulted in their having larger cells than other salamanders.

  

Says Who?

• Chernoff (1996) International Journal of Developmental Biology 40: 823-831
• Martin & Gordon (1995) Journal of Evolutionary Biology 8: 339-354
• Neff et al. (1996) International Journal of Developmental Biology 40: 719-725
• Rosenkilde & Ussing (1996) International Journal of Developmental Biology 40: 665-673