The axolotl, Ambystoma mexicanum, is a unique amphibian known for retaining larval features throughout its life. Native to ancient lake systems near Mexico City, particularly Lake Xochimilco, this fully aquatic creature retains prominent external gills. Despite its popularity in captivity, the axolotl faces a challenging existence in its natural habitat, where it is classified as critically endangered.
The Wild’s Palette
Wild axolotls display subdued, natural colors for camouflage. These “wild type” axolotls are often a mottled olive-brown, dark grey, or black, sometimes with subtle gold speckles. This allows them to blend seamlessly with murky waters and substrate, protecting them from predators.
A Spectrum of Captive Hues
Selective breeding in captivity has produced a wide array of axolotl color morphs, differing significantly from their wild counterparts. Leucistic axolotls are a common morph, characterized by a pale pink or white body with dark eyes and bright red gills. Unlike albinos, leucistics possess some pigment cells, but their distribution is limited.
Albino axolotls lack melanin, resulting in a white or yellowish body with red or clear eyes and pink gills. Golden albinos are a variation with a golden-yellow body, often with shiny patches, and clear or peachy eyes. Melanoid axolotls are solid black or very dark grey, lacking shiny iridophores and often having entirely dark eyes without a golden ring.
Other captive morphs include copper axolotls, with a brown to copper-colored body, light or dark spots, and red-tinted eyes. Mosaic axolotls display chaotic patterns of different pigments, often mottled with white, black, or gold. Piebald morphs are typically white with distinct dark or black patches. Genetically modified Green Fluorescent Protein (GFP) variants can glow vibrant green under UV or blue light, a trait introduced for research.
Understanding the Pigment
Axolotl coloration is determined by specialized pigment cells called chromatophores, which reside in their skin. Three primary types of chromatophores contribute to their diverse appearances. Melanophores contain eumelanin, a pigment responsible for black and brown hues.
Xanthophores produce yellow and reddish pigments, containing carotenoids and pteridines. Iridophores contain crystallized purines that reflect light, creating a shiny, iridescent, or metallic sheen, often appearing as gold or silver speckles. The specific color morphs observed in axolotls result from genetic variations that affect the presence, absence, or distribution of these pigment cell types. For instance, a lack of melanophores leads to albino traits, while an abundance of them without iridophores results in melanoid forms.