ambystoma maculatum

Mole salamander

Mole salamanders (genus Ambystoma) are a group of salamanders endemic to North America. The genus has become famous due to the presence of the Axolotl (Ambystoma mexicanum), widely used in research, and the Tiger Salamander (Ambystoma tigrinum, Ambystoma mavortium) which is the official amphibian of many states, and often sold as a pet.

General description

Terrestrial mole salamanders are identified by having wide, protruding eyes, prominent costal grooves, thick arms, and rounded tails. Most have vivid patterning on dark backgrounds, with marks ranging from deep blue spots to large yellow bars depending on the species. Terrestrial adults spend most of their life underground in burrows either of their own making or abandoned by other animals. Some Northern species may hibernate in these burrows throughout the winter. They live alone and feed on any available invertebrate. Adults spend little time in the water, only returning to the ponds of their birth to breed.

All mole salamanders are oviparous and lay large eggs in clumps in the water. Their fully aquatic larvae are branchiate, with 3 pairs of external gills behind their head and above their gill slits. Larvae have large caudal fins which extend from the back of their head to their tail and to their cloaca. Larvae grow limbs soon after hatching, with four toes on the forearms, and five toes on the hindlegs. Their eyes are wide-set and lack true eye-lids.

The larvae of some species (especially those in the South, and Tiger salamanders) can reach their adult size before undergoing metamorphosis. During metamorphosis, the gills of the larvae disappear, as does the fin. The tail, skin, and limbs become thicker, and the eyes develop lids. Their lungs become fully developed, allowing for a fully terrestrial existence.

Neoteny

The existence of large larvae has allowed some species to become semi or wholly neotenic (sometimes referred to as paedomorphosis). Neotenic adults of these species remain fully aquatic, retaining both their gills and fins, while developing their lungs to aid in breathing. They become sexually mature without ever undergoing metamorphosis. Neotenic Ambystoma species and populations are primarily found in mountainous regions of the United States, and in the central plateau of Mexico. The conditions that lead to neoteny are high-altitude, lack of predation in the water, and dry conditions outside the water. Most neotenic populations belong to the Tiger Salamander complex - Ambystoma tigrinum, Ambystoma velasci, Ambystoma mavortium, and their close relatives. Wholly neotenic Ambystoma species include the Axolotl, Ambystoma taylori, Ambystoma andersoni, and Ambystoma dumerilii. Neotenes retain the regenerative abilities of young larvae, able to regenerate limbs, tails, and nearly every organ in their body.

Tiger salamander complex

The presence of neotenic populations near those with large larvae has made it difficult to identify mole salamander species. The Tiger Salamander complex used to be considered a single species ranging from Canada to Mexico, falling under the name Ambystoma tigrinum. Despite differences in coloration and larvae, Tiger Salamanders were found throughout their unbroken range, which made it difficult to delineate subspecies, let-alone elevate any populations to species status. In morphological terms, Tiger salamanders are all very similar, with large heads, small eyes, and thick bodies. This is probably because Tiger Salamanders have the primitive morphology of mole salamanders. They are also the largest of the mole salamanders, and have very large larvae. All populations have similar lifestyles and life-cycles are identical. However, when one looks at Tiger Salamander populations that were distant from each other, it becomes apparent that there are different species within this complex. The problem is that the ranges of these potential species overlap, and hybridization occurs, blurring the lines between species. Several subspecies of Ambystoma tigrinum were named in order to deal with this problem. Recently, the Barred Tiger Salamander, Ambystoma mavortium, was elevated to species status - covering the Tiger salamander populations in the Western and Central United States. Several distinct subspecies still exist in Ambystoma mavortium which may be elevated to species status at some point in the future. The California Tiger Salamander,Ambystoma californiense, has also been elevated out of Ambystoma tigrinum, and is actually very distantly related to all other mole salamander species. The Mexican Tiger Salamander, Ambystoma velasci, was elevated out of Ambystoma tigrinum through genetic analysis in 1997. All accounts referring to the Axolotl as a close relative of Ambystoma tigrinum are now considered wrong, as they are now separated by both geography and many species between. Instead, it is Ambystoma velasci which shares the Axolotl's habitat, and is probably closely related to it. The Mexican Tiger Salamander was probably the parent of most of the neotenic species, which raises the possibility that Ambystoma velasci is paraphyletic, and may be broken up into more species in the future.

laterale-jeffersonianum complex and all-female hybrids

The most famous result of the tendency of mole salamander species to hybridize where they come in contact is the jeffersonianum-laterale complex of species. These two species were separated by ice age glaciation, but when the ice melted, they came into easy contact with each other and interbred. The Jefferson Salamander (jeffersonianum), and the Blue-spotted Salamander (laterale) have variously been put together as one species, and taken apart based on these hybrid populations. They are currently considered separate due to the genetic make up of isolated populations.

The hybridization of Ambystoma laterale and Ambystoma jeffersonianum has led to the development of two completely different populations of mole salamander. Both of these are polyploid, like the all-female species of Whiptail lizards. Ambystoma platineum, the Silvery Salamander, is technically a population of hybrid jeffersonianum-laterale salamanders. However, the Silvery Salamander is completely asexual and does not technically breed. Ambystoma platineum females go through mating rituals with Blue-spotted males in order to begin parthenogenesis, a process known as gynogenesis. They then lay eggs that contain only clones of the mother. Therefore all members of this populations are clonal. Silvery Salamanders may be considered a species, since there is no genetic exchange with any other salamander group, but it is usually classified within Ambystoma laterale. Genetically, they are all one Blue-Spotted-Jefferson hybrid salamander. Hybridization has also given rise to a second all-female population, Tremblay's salamander, Ambystoma tremblayi. These females pretend to breed with Jefferson's males in order to initiate cloning. It is also usually classified within Ambystoma laterale. Both these female populations strongly resemble the Blue-spotted salamander.

Genus Ambystoma and contained species

Ambystoma is traditionally translated as "cup-mouth," but is actually nonsense. Johann Jakob von Tschudi, who described the genus, intended to call it Amblystoma, "blunt-mouth." However, he misspelled the genus as Ambystoma at many points throughout his description. Under biological naming rules, the misspelling Ambystoma takes precedence and cannot be changed. This is especially true because it was widely used by other authors. Occasionally, old specimens and documents will bear the intended genus name.

This genus contains 32 species, listed below, the newest being A. bishopi. Terrestrial species are labeled with a "T," neotenic species are labeled with an "N," while species with established populations of both are labeled with a "V." Subspecies and hybrids are listed beneath their parent species.

Former genus Rhyacosiredon

Rhyacosiredon used to be considered a separate genus within the family Ambystomatidae. However, cladistic analysis of the mole salamanders found that the existence of Rhyacosiredon makes Ambystoma paraphyletic, since the species are more closely related to some Ambystoma species than those species are to others in Ambystoma. This genus previously included:

The stream-type morphology of these salamanders (which includes larvae and neotenes with short gills and thicker gular folds) may have led to their misclassification as a different genus.

Notes

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