Mole salamander

The mole salamanders, belonging to the genus Ambystoma, are a group of advanced salamanders found only in North America. They are best known for two members: the axolotl (A. mexicanum), which is a key subject in studies of paedomorphosis, and the tiger salamander (A. tigrinum and A. mavortium), a common pet and the official amphibian of four U.S. states.

General description

Terrestrial mole salamanders have wide, bulging eyes, noticeable costal grooves, and thick arms. Their dark bodies are often covered with vivid patterns, ranging from deep blue spots to large yellow bars depending on the species.

Adults live mostly underground in burrows, either dug themselves or taken from other animals; some northern species hibernate there through winter. They are solitary and eat any invertebrates they can find. Adults rarely enter water, returning only to the ponds where they hatched to breed.

All mole salamanders lay eggs, which are large and deposited in clumps in water. Their larvae are fully aquatic, with three pairs of external gills behind the head and above the gill slits.

Larvae also have large fins running from the back of the head to the tail and cloaca. Soon after hatching, they grow limbs—four toes on the front feet and five on the back. Their eyes are wide-set and lack true eyelids.

In some species, especially southern ones and tiger salamanders, larvae can reach adult size before metamorphosis. During this change, the gills and fins disappear. The tail, skin, and limbs become thicker, eyelids develop, and the lungs fully mature, allowing a completely terrestrial life.

Some mole salamander species, and even certain populations of normally terrestrial species, are neotenic—they keep their larval form into adulthood. The axolotl is the most famous example. It cannot produce thyroxine, so metamorphosis only happens if the hormone is injected from outside, which usually shortens the salamander’s lifespan.

Tiger salamander complex

Tiger salamanders, part of the Ambystoma tigrinum complex, have large heads, small eyes, and thick bodies. This basic shape is common across most mole salamanders, but tiger salamanders are among the largest and have relatively big larvae. They live in a wide range of ecosystems across North America. Because of this geographic spread, different subpopulations show varied body forms and behaviors.

Whether these groups are separate species or subspecies, and what drives their differences, is still being studied as of 2024. Several former subspecies have been reclassified as independent species. For example, Ambystoma mavortium (barred tiger salamander) now includes what were once A. t. diaboli, A. t. mavortium, A. t. melanostictum, A. t. nebulosum, and A. t. stebbinsi. Other examples are Ambystoma californiense (California tiger salamander) and Ambystoma velasci (Plateau tiger salamander), which may be paraphyletic and shares habitats with the axolotl.

Hybrid all-female populations

Unisexual, all-female populations of mole salamanders are widespread across the Great Lakes region and northeastern North America. These females need sperm from a related species to fertilize their eggs and start development. Usually, the eggs then discard the sperm’s genome and develop asexually (a process called gynogenesis, with premeiotic doubling).

However, they may sometimes keep the sperm genome, either adding it (raising the ploidy of the offspring) or replacing one of the maternal genomes. This unique reproductive method is called kleptogenesis, a term coined by Bogart and colleagues. It differs from hybridogenesis, where the maternal genome is passed on hemiclonally and the paternal genome is discarded each generation before the egg matures, then reacquired from another species’ sperm.

The nuclear DNA of these unisexuals can include genomes from up to five species: the blue-spotted salamander (A. laterale), Jefferson salamander (A. jeffersonianum), small-mouthed salamander (A. texanum), streamside salamander (A. barbouri), and tiger salamander (A. tigrinum), labeled L, J, Tx, B, and Ti. This flexibility creates many possible genome combinations, or biotypes. For instance, an LJJ individual is a triploid with one A. laterale genome and two A. jeffersonianum genomes, while an LTxJTi is a tetraploid with genomes from four species. Because they have hybrid genomes, these unisexual salamanders are a cryptic species, looking similar to the coexisting species.

For example, LLJs resemble blue-spotted salamanders, and LJJs look like Jefferson salamanders. The silvery salamander (LJJ, formerly A. platineum), Tremblay’s salamander (LLJ, formerly A. tremblayi), and Kelly’s Island salamander (LTxTx and LTxTi, formerly A. nothagenes) were once described as separate species, but those names were dropped due to the complexity of their genomes. Offspring from a single mother can have different genome combinations; a single egg mass might contain both LLJJ and LJJ larvae.

Despite the complexity of their nuclear genomes, all unisexual salamanders form a single group based on their mitochondrial DNA. The maternal ancestor of these unisexuals was most closely related to the streamside salamander, with the original hybridization likely occurring between 2.4 and 3.9 million years ago.