The Olm, often called the “human fish,” is a pale, blind salamander native to the deep, dark cave waters of Central and Southeastern Europe. In contrast, the Axolotl, the “Mexican walking fish,” is known for its powerful regeneration abilities and striking external gills. Despite their vastly different appearances and habitats, these two aquatic animals are related, belonging to the same broad group of amphibians. Their shared ancestry led to highly specialized and unique evolutionary paths of divergence. Both species have retained features of their larval past while adapting to entirely separate ecological worlds.
Shared Ancestry and Taxonomic Divergence
Both the Olm (Proteus anguinus) and the Axolotl (Ambystoma mexicanum) share the highest levels of biological classification with all other salamanders. They are members of the Class Amphibia and belong to the Order Caudata. This order encompasses all tailed amphibians, confirming their fundamental relationship as salamanders that evolved from a shared ancestor.
The divergence in their evolutionary paths becomes clear at the family level. The Olm belongs to the family Proteidae, commonly known as the mudpuppies and olms, a small group of fully aquatic salamanders. This family is characterized by permanent external gills and small, paddle-like limbs.
Conversely, the Axolotl is classified under the family Ambystomatidae, the mole salamanders, a group primarily found across North America. While many mole salamanders undergo complete metamorphosis, the Axolotl is unique within its family for typically remaining in its larval, gilled state. Their membership in different families confirms that their last common ancestor was a very ancient one, preceding the diversification of modern salamander lineages.
Obligate vs. Facultative Neoteny
A primary biological difference between these two species lies in their approach to neoteny, the retention of juvenile features like external gills and aquatic habits into adulthood. The Olm exhibits obligate neoteny, meaning it is biologically committed to its larval form. Its endocrine system prevents it from undergoing the metamorphosis necessary to transition into a terrestrial adult, regardless of environmental changes.
This permanent aquatic state represents an evolutionary commitment to the stable, underwater world of the caves it inhabits. The Olm’s entire existence is spent in the subterranean realm, where the larval form is the optimal one for survival.
The Axolotl, however, displays facultative neoteny, a condition where the animal retains its larval features but retains the capacity to transform. In its natural habitat, the Axolotl rarely metamorphoses due to the specific conditions of its high-altitude lake environment.
If exposed to environmental stressors, such as the drying of its water source, or if treated with thyroid hormones in a laboratory setting, the Axolotl can be induced to change. This transformation results in a terrestrial, lung-breathing salamander that loses its external gills and dorsal fin. This demonstrates that the genetic blueprint for metamorphosis still exists, unlike in the Olm.
Ecological Niches and Sensory Adaptations
The distinct environments the Olm and Axolotl inhabit have driven the evolution of specialized physical features.
Olm Adaptations
The Olm is an aquatic troglobite, living in the perpetually dark, nutrient-poor cave systems of the Dinaric Karst. The total absence of light has rendered its eyes vestigial, meaning they are undeveloped and covered by a layer of skin. To navigate and hunt, the Olm has developed heightened sensory systems, relying heavily on chemoreception to detect prey through minute chemical changes in the water. Its lateral line system allows it to detect water vibrations and movement with extreme sensitivity. The Olm’s skin is also depigmented, giving it its pale or pinkish color, an adaptation common among cave-dwelling organisms.
Axolotl Adaptations
The Axolotl evolved in the open, lacustrine environment of Lake Xochimilco and its canals near Mexico City, where light is present. Consequently, it possesses functional, though small, eyes and typically exhibits pigmentation, ranging from dark brown to albino variations. Its large, feathery external gills are highly developed, adapted to efficiently extract oxygen from the water in its natural lake habitat. While the Olm is adapted for physical stability in a constant cave environment, the Axolotl’s features are adapted for a more variable, open-water ecosystem, reflecting their separate evolutionary responses to their surroundings.