Amphibians

The family Ambystomatidae or “the mole salamanders” is included within the four earliest or most primitive family lineages of the order “Caudata” (the salamanders), diverging from all other salamanders in the Early Cretaceous period over 140 million years ago, around five million years before the koala and dolphin lineages diverged from their common ancestor. The small number of species that represent the genus Ambystomaare highly evolutionarily distinct members of both the salamanders and the amphibians as a whole.

The Lake Lerma salamander is capable of reaching sexual maturity in its neotenous form, retaining its aquatic larval characteristics such as fins and gills throughout its life. However, it is also able to metamorphose into the adult form and live a terrestrial life. Although this species has been little-studied, there are a couple of theories that may explain why some populations of the Lake Lerma salamander do not metamorphose. One idea is that the production or effectiveness of the hormone thyroxine is compromised, either by the species living in water bodies containing insufficient iodine (which is required in the manufacture of thyroxine by the body) or in water temperatures that are too cold for the thyroxine to be effective. This impacts upon the development of the species and sexually mature adults never develop adult characteristics but remain in the larval form. A second theory suggests that species evolving in pools surrounded by hostile terrestrial environments develop aquatic lives to obviate the need to exit the relative safety of their watery home. This is a common trait in species that inhabit high-elevation ponds. Since the Lake Lerma salamander inhabits various different water bodies across its range, and is able to disperse between them in its metamorphosed form, it is possible that the conditions in some of these water bodies are not conducive to metamorphosis.

An Ambystomatid or mole salamander known only from the central highlands of the State of Mexico, near the city of Toluca, in the Río Lerma, Lake Lerma, and around Almolya, at an elevation of 2,800–3,000m above sea level. Mole salamanders are medium to large, stocky salamanders, measuring between 90 to 350mm from the tip of the nose to the end of the tail, which salamanders retain throughout their life. Males are often larger than females, partly due to their longer tails. Ambystomatids generally exhibit both aquatic “neotenic” larval (or aquatic and permanently juvenile in form with external, feathery gills) and terrestrial “metamorphosed” (or ground-dwelling, fully developed adult in form with reduced gills) stages in their wild populations. Ambystomatids are often boldly patterned as adults, with well-developed costal grooves (successive vertical grooves along the sides of the body), especially the metamorphosing varieties. They have a rather flattened body with a wide, flattened head, a large mouth and smooth skin with many glands. The tail is roundish or laterally compressed. During the breeding season males have a very swollen cloacal zone (the region around the reproductory and excretory opening in amphibians located underneath the base of the tail).

The Lake Lerma salamander has established populations of both metamorphosing individuals (which develop from an aquatic juvenile form with larval characteristics to a terrestrial physical form with adult features) and neotenic individuals (which retain their aquatic larval characters throughout their life). This is a large species of salamander, reaching a total length of 240 mm or more, about 115 mm of this measurement being accounted for by the tail. The skin is generally smooth, although it may appear slightly granular especially on the sides of the body. Eleven distinct costal grooves in the skin are visible along either side of the body. Where present, the dorsal fin (running from the back onwards) is low, rising to its maximum height along the tail. The toes are flattened, pointed and slightly webbed. The colouration of the skin in metamorphosed adults is uniform grey-black, with the tips of the digits being grey-cream. A closer inspection of the skin reveals that it is peppered with minute cream dots. In the larval form (or neotenes) this species is a deep purplish-black.

Recorded only in the central highlands of the State of Mexico, near the city of Toluca, in the Río Lerma and Lake Lerma, and around Almolya, at 2,800–3,000m above sea level. It is possible that the species is more widespread in this general region, but more work is needed to investigate this prediction.