AXOLOTL BIOLOGY

The Axolotl (Ambystoma Mexicanum) is an unusual animal because it retains its larval form into adulthood. In fact, axolotls reach sexual maturity in this state. This stall in development, also known as neoteny, is often referred to as a step backwards in evolution because it prevents the axolotl from living on land. Incidentally, this also prevents them from spreading to new habitats. While axolotls do maintain their larval morphology, it is interesting to note that they do develop rudimentary lungs. It is not uncommon to see your axolotl swim to the surface, take a quick gulp of air, and then dive back to the bottom of the aquarium.

Axolotls are carnivorous and have the internal anatomy of a carnivore, with the exception of teeth. Axolotls teeth are cone shaped and are used for gripping its food, rather than tearing, thus they swallow their meal whole. Axolotls have a three-chambered heart, unlike that of a mammal which has a four-chambered heart.

One of the most notable and fascinating aspects of axolotl biology is their regeneration ability. Axolotls are used frequently in medical research due to this ability. Research has shown that not only can axolotls regrow limbs when they are lost or damaged, but they are capable of regrowing those limb perfectly an infinite number of times. This regeneration occurs via the formation of whats called a "bud" at the end of the damaged appendage, followed by growth of the new limb or foot. Entire limbs can be regenerated and even portions of the brain and spine.

Below, you can see an example of what happens when a minor mutation occurs during regrowth. While axolotls generally regrow limbs perfectly, sometimes mutations do happen.

Male and female axolotls differ in a number of ways. There are really only two reliable methods of distinguishing males from females.The first is that mature females tend to have very rounded bodies, while males tend to have slimmer, more elongated bodies with longer tails. Second, is that a sexually mature male's cloacal region is swollen, while that of the sexually mature female is significantly less so.

One interesting characteristic is that mature leucistic, golden, and albino axolotls will have dark brown or black tips to their toes, while wild type and melanoid axolotls' toe tips become slightly paler than the rest of their body.

Genetics

The Axolotl is studied extensively all over the world for a variety of reasons. All of the traits which make it so popular for study are dependent upon its genes.

Axolotls have 28 chromosomes per cell, in fourteen pairs. Humans have 46 chromosomes in 23 pairs. A chromosome is a thread-like structure composed of DNA and protein. The length of a chromosome is made up of many units of DNA called genes. Each gene has a special place on a chromosome and the position which it occupies is called the locus of that gene.

The sperm from the male contains 14 chromosomes, as does the egg from the female. When axolotls reproduce, the sperm and the egg fuse to form what's called the zygote, the first cell of the new axolotl which contains 28 chromosomes. During the production of the sperm and egg cells, via a process known as meiosis, small exchanges of parts of the chromosomes take place, as well as a random allotment of chromosomes from the mother or father's own parents. Basically, what this means is that the sperm or egg cells could have 4 maternal chromosomes and 10 paternal, or any other combination. This results in larvae that are genetically distinct in every way, even different from its siblings and its parents.

Color Variants

Axolotls come in many color varieties, some more rare than others. There are five basic colors of Axolotls including, Wild, Leucistic, Albino, Golden Albino, and Melanoid. These five are the most common colors.

Wild Type
  • A combination of greens, grays, blacks and olive coloring. These usually have speckles all over their body

  • Most are very dark, almost black, when they are younger, but get lighter as they reach maturity

  • They have golden or iridescent speckling making them look almost shiny

  • Their gills are usually purple or grey in color

  • Their eyes are black with a golden or shiny ring around them

Leucistic
  • This color morph is a very light or pale pink color.

  • They have very dark brown or black eyes.

  • Their gills are red or very dark pink.

  • Leucisitc axolotls will sometime develop speckling around their face and sometimes along their bodies. Leucistics born with face speckling are often referred to as "dirty lucys.

  • Toe tips will darken as they reach sexual maturity

 

Leucisitc axolotls can carry the melanoid gene, know as a "white melanoid" or "leucistic melanoid" depending on who you ask. This means they lack any iridescent pigmentation. The easiest way to tell if the axolotl is a normal leucistic or a white melanoid is to look at the eye. Normal leucisitc will have a shiny ring around their eye, a white melanoid will not.

Albino
  • Very similar to the leucistic axolotl, however the white albino lacks all dark pigmentation.They have a white body with no specs or freckling. These freckles will also never develop over time.

  • Red gills

  • Toe tips will darken as they reach maturity

Golden Albino
  • Due to the albinism, their eyes are clear. They cannot have black eyes.

  • Golden or peach colored skin, sometimes with shiny speckling or patches of color

  • The gills are a peachy pink in color

Black Melanoid
  • Due to an increased amount of pigmentation, melanoids are very dark brown or black in color.

  • Unlike the wild type, they have no iridescence or shiny speckling

  • Melanoids have black eyes and black gills

 

It is imporatnt to note that "melanoid" means the animal has more dark pigment and no iridophores (shiny or iridescent specs). The melanoid gene can also be present in leucisitc and albino axolotls.

Special Color Variants and Mutations

Copper

The copper color variant can only be found in certain countries such as the United States, Australia, and Germany.

  • Their body is a very pale copper color and is covered in dark copper freckles.

  • They have eyes that are tinted red, but appear darker than traditional albino axolots

  • They have dark copper-colored gills.

  • Females will lay white or light brown eggs.

GFP - Green Fluorescent Protein

GFP is a mutation in the axolotls DNA. This mutation allows them to glow a flourescent green under UV or Blacklight, thus earning the name “Green Fluorescent Protein”. Any color variants can have this mutation.

 

This gene was first introduced to the Axolotl species in a cancer research lab, but since, the gene has been passed down from generation to generation and is now a commonly available trait.

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