The Incredible Axolotl - Nature’s Smiling Salamander

The axolotl (Ambystoma mexicanum) is a salamander from the lake and canal system of Xochimilco, on the southern edge of Mexico City, that keeps its larval body for life: feathery external gills, a finned tail, and a fully aquatic habit. The word for it is paedomorphosis, and it is not the same as staying a baby. An axolotl reaches full adult size and breeds normally. Nor is it strictly permanent. The tissues still respond to thyroid hormone, and given the right dose an axolotl can be pushed through metamorphosis into a land-dwelling adult. What is underactive is the hormonal axis that would normally trigger it.

How an axolotl regrows a limb

Axolotls regenerate entire limbs, the tail, parts of the eye, substantial parts of the brain and spinal cord, and parts of the heart. The limb is the best understood, and it runs in a fixed order.

1. The wound closes without a scar. Skin covers the stump and forms a thin wound epidermis instead of scar tissue.
2. A blastema forms. Cells under that epidermis gather into a bud of growing tissue called the blastema. In the axolotl most of it comes from stem and progenitor cells already in the limb, while newts get more of theirs by turning mature muscle back into dividing cells.
3. Nerves drive the growth. The cut nerves at the stump are required. A limb stripped of its nerve supply forms no growing blastema at all.
4. Only the missing part comes back. Blastema cells carry positional memory, so a limb cut at the wrist grows a hand and a limb cut at the shoulder grows the whole arm.

There are limits. Lens regeneration works in the first two weeks after hatching and is lost after that, which is one of the ways the axolotl differs from newts.

Axolotls also carry the tools researchers use to watch this happen. In 2006 a research team put the jellyfish gene for green fluorescent protein into the axolotl germline. Their offspring carried the glow in cells all over the body, so a scientist can follow which cells rebuild a limb. GFP is therefore not a natural axolotl color: the glow comes from that jellyfish gene.

Life cycle

Females lay their eggs 12 to 20 hours after mating, spread over one or two days. Embryos at room temperature hatch in two to three weeks as tiny larvae that live on live food, and a larva should be about an inch long by 6 to 8 weeks old. Growth is fast, and axolotls reach sexual maturity at about one year. Until then they cannot be reliably sexed. Because they skip metamorphosis, a breeding axolotl looks much like a large juvenile, with the gills and finned tail of the larva still in place.

Color morphs

The dark olive, speckled wild type is the color of every axolotl in Xochimilco. The pale and gold animals in the pet trade are single-gene mutants, most of them recessive, which means an animal needs two copies to show the color and two wild-type parents can both carry one hidden.

• Leucistic (the lab "white" strain): fewer black and yellow pigment cells develop and most of them fail to migrate into the skin, leaving a mostly white animal. The cause is a defect in the edn3 gene.
• Albino: the tyrosinase enzyme that makes melanin is broken, so the body has no black pigment at all. The laboratory albino allele did not arise in axolotls: it came from a tiger salamander collected in Minnesota in 1962 and crossed into the lab colony. By 2000 every axolotl in that collection descended from the cross, and 88 percent of wild-type-looking adults carried the albino gene hidden.
• Melanoid: large numbers of black pigment cells, fewer yellow ones, and no reflective iridophores, which gives a solid dark animal. It is recessive and traces to the ltk gene.
• GFP: the transgenic line described above, which can be combined with any of the colors.

That tiger salamander ancestry is the same cross that ties the axolotl to the tiger salamander, its closest relative in the pet trade and the one that does metamorphose on land.

Conservation

The species is listed as Critically Endangered on the IUCN Red List. Surveys in the Xochimilco canals counted about 6,000 axolotls per square kilometer in 1998 and 36 per square kilometer in 2014, a fall of 99 percent. The drivers are pesticide and wastewater pollution, and introduced carp and tilapia that eat axolotl eggs and young and compete for food and shelter. The most hopeful work is on the chinampas, the traditional floating farm plots, where 21 refuges now filter canal water through wood, gravel and native plants and screen out the invasive fish.

The result is an animal that is common in laboratories and home aquariums and close to gone from the one lake system it evolved in.

What that biology means in a tank

Coming from a cool, high-altitude lake, axolotls do best at 15.6 to 17.8°C (60 to 64°F) and should never be kept above about 22°C (72°F). They also need hard water: the stock center restores minerals to treated water with a modified Holtfreter's salt mix, because soft or distilled water harms their skin. The hardness targets and dechlorinating steps apply to any aquatic amphibian, and a summer heat wave is the main threat to that temperature ceiling, which is why cooling a tank without a chiller matters more for this species than for almost any fish.

Staying larval for life is the root of the rest of it, and the axolotl Beastfile sets that out.

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