The axolotl (Ambystoma mexicanum) is a neotenic salamander known for retaining its larval features throughout its adult life, and it exhibits a wide array of colors. The appearance of this unique amphibian is determined by genetic factors that control pigment cells in its skin. These color variations, known as morphs, range significantly from the animal’s original natural coloration. Understanding axolotl color involves distinguishing between the wild-type appearance and the numerous selectively bred colors resulting from genetic mutations.
The Native Appearance
The coloration of the axolotl in its native environment is referred to as the “Wild Type,” which closely resembles its natural ancestors from the lakes and canals near Mexico City. This natural color is typically a mottled brown or tan, often presenting with an olive or dark gray undertone. The varied pattern is interspersed with darker spots of black or gray, which provides excellent camouflage against the murky, muddy lakebeds of its original habitat.
Wild Type axolotls possess all three types of pigment cells, contributing to their complex coloration. They frequently display subtle speckles of shiny gold or silver pigment across their bodies. Their eyes are usually dark, often featuring a distinct metallic gold ring around the pupil.
The Biology of Axolotl Color
The varied colors of the axolotl are entirely dependent on specialized pigment cells called chromatophores, which reside in the amphibian’s skin. These cells are responsible for synthesizing and storing the different types of pigment molecules. The relative presence, absence, or functionality of three primary chromatophore types determines the final color of any individual axolotl.
Melanophores
One type is the melanophore, which contains eumelanin, the pigment responsible for dark brown and black coloration. Melanophores are generally the most prevalent pigment cells throughout the axolotl’s development.
Xanthophores
Another class is the xanthophore, which synthesizes pteridines, molecules that produce yellow, orange, and sometimes reddish hues. These yellow pigments are also believed to play a role in the axolotl’s immune function.
Iridophores
The third type of pigment cell is the iridophore, which does not contain color pigment but instead holds crystallized purines that reflect light. These reflective platelets create the iridescent or shiny appearance seen as gold or silver speckles on the skin. Genetic mutations affecting any one of these cell types result in the distinct color morphs seen in captivity.
The Four Core Color Morphs
Selective breeding and natural mutations have established four core color morphs, each resulting from a recessive genetic change that affects one or more chromatophore types.
Leucistic
The Leucistic morph is characterized by a pale pink or white body, resulting from dark pigment cells failing to migrate across the skin during embryonic development. This morph still has dark, pigmented eyes because the melanophores remain present in the ocular tissue. The external gills often appear bright red or pink due to the visible blood vessels beneath the translucent skin.
Albino
The Albino morph, which includes White Albino and Golden Albino variations, lacks functional melanin pigment completely. While melanophores are present, they are unable to produce eumelanin, leading to a body color that is white, pale pink, or yellow. The eyes of an albino axolotl appear clear or red due to the visible blood vessels in the retina. Golden Albino specifically retains xanthophores, giving it a prominent yellowish-gold color.
Melanoid
The Melanoid morph is a solid black or dark gray color, often lacking the olive or brown tones of the Wild Type. This deep, uniform darkness results from the recessive melanoid gene, which causes a complete absence of iridophores. This lack of reflective pigment, combined with a potential increase in the number of melanophores, creates a non-shiny, intensely dark appearance.
Axanthic
The Axanthic morph, often called the Gold morph, results from a mutation that removes both xanthophores and iridophores. The resulting color is typically a pale, yellowish gold or a light gray. The Copper morph is a variation of albinism that displays a brownish-orange or copper hue because it produces an alternate red-brown pigment called pheomelanin instead of eumelanin.
Complex and Unique Color Variations
Beyond the four foundational morphs, complex variations emerge from the combination of different color genes or from unique biological events.
Mosaic
One striking example is the Mosaic, which features distinct patches of two different color morphs on a single animal. This coloration is not a heritable trait but rather a random developmental mutation that occurs early in the embryonic stage. Mosaics can display patterns like half-white and half-dark, resulting from two cell lines developing simultaneously.
Green Fluorescent Protein (GFP)
Another unique variation is the Green Fluorescent Protein (GFP) trait, which is not a natural color but a genetic modification introduced in a laboratory setting. Axolotls with the GFP trait have a gene, originally sourced from jellyfish, that causes their cells to produce a fluorescent protein. This protein appears bright yellow in normal light but glows a vivid neon green when the animal is viewed under a UV or black light. This trait is often bred into Leucistic or Wild Type morphs and can be passed down to offspring.
Speckled and Dirty Morphs
The Speckled or “Dirty” morphs are variations where a base color, such as Leucistic or Albino, shows dark spotting or freckling. For example, a “Dirty Leucistic” has the pale body color but retains scattered melanophores, often resulting in dark freckles on the face and back. The Silver Dalmatian, also called Lavender, is another such combination, presenting a pale, silvery lavender base color with numerous dark speckles.