There haven’t been enough lissamphibians on Tet Zoo lately. So here’s a recycled section of text on axolotls, originally from a 2008 ver 2 article. I haven’t updated it properly, but I have added a new section of text at the end.

Thanks to its perennial use in the pet and laboratory industries, the Axolotl Ambystoma mexicanum is one of the world’s most familiar lissamphibians. It’s certainly the most familiar member of its specific salamander group, the Ambystomatidae or mole salamanders [photo above by LoKiLeCh]. As everyone knows, the Axolotl is neotenous: it retains juvenile characters into sexual maturity, and even an adult is essentially an unmetamorphosed larva, exhibiting a perennibranchiate morphology (meaning that external gills are obvious). Unlike a metamorphosed salamander, a neotenous one retains external gills and a tail and body fin, lacks moveable eyelids, and of course is obligatorily aquatic.

Little known is that the wild Axolotl population is restricted entirely to Mexico’s Lake Xochimilco and Lake Chalco, and here they are critically endangered thanks to the pollution, habitat degradation and water extraction and resulting desiccation that has occurred. Use of the animals for food and medicinal purposes, and predation from introduced fish have also contributed to Axolotl decline, and a 2004 survey failed to capture any in areas where they had formerly occurred. They aren’t thought to be wholly extinct, however, as specimens continue to show up in local markets [UPDATE: this was written in 2008 and may not be true as of 2012. Anybody know of more recent Axolotl captures or sightings?]. A. mexicanum is one of the larger Ambystoma species, with some individuals reaching 45 cm in total length (although this is exceptional and half this length is more normal). [Adjacent photo (c) Richard Griffiths, from the Axolotl page at EDGE Amphibians. Image below by ZeWrestler.]

The word ‘axolotl’ (‘ajolote’ in Spanish*) is usually thought to mean ‘water dog’ (it is probably not coincidental that other, non-ambystomatid perennibranchiate salamanders are today termed ‘mudpuppies’ and ‘waterdogs’). It also has links to the concept of ugliness or monstrousness and is also often translated as ‘water monster’ (Smith 1969). All neotenous mole salamanders were originally termed 'axolotls', as were the larvae of metamorphosing species like the Tiger salamander A. tigrinum. Indeed, this is still the case among laypeople (some people even, apparently, use ‘axolotl’ as the name for any larval salamander). Herpetologists, however, have restricted the name to the perennibranchiate species A. mexicanum. Individuals of this species have been well known to scientists since 1864 when a shipment was sent to Paris. Lake Xochimilco animals were again sent to Paris in 1868, and these Parisian Axolotls were the ancestors of virtually all of the laboratory Axolotls bred forever afterwards.

* Which confuses things, because this name is today used for Bipes, the limbed amphisbaenians.

Early confusion as to whether perennibranchiate individuals should be given their own species or genus relative to metamorphosed individuals, and arguments as to whether A. mexicanum should be regarded as conspecific with the metamorphosing Tiger salamander, mean that the Axolotl has a complex nomenclatural history involving several appeals to, and actions of, the ICZN. Oh, and, remember that the ‘Tiger salamander’ of tradition isn’t one species, but a species complex, the members of which are not all close relatives (Shaffer 1993, Shaffer & McKnight 1996). Some of the animals involved may be “among the most recently derived vertebrate species known” (Shaffer & McKnight 1996, p. 430). Incidentally, the ‘Tiger salamanders’ most closely related to A. mexicanum are not part of the lineage associated with the name A. tigrinum: instead, the Plateau tiger salamander A. velasci is the Axolotl’s closest living relative.

The Axolotl’s distinctive morphology and highly endangered status mean that it’s among the top 100 amphibians on the EDGE list: EDGE species are Evolutionarily Distinct and Globally Endangered. The EDGE website on globally endangered amphibians can be found here, and here you can find a huge amount of information on endangered mole salamanders and other species. Nine mole salamanders are on the EDGE list, and several are poorly known, neotenous, Axolotl-like species.

A big surprise from the world of unisexual mole salamanders

While I’m here, it's worth discussing something new on mole salamanders that’s been discovered since I wrote the text above. Several Ambystoma species are unisexual – that is, they consist only of parthenogenetic females. Unisexuals always seem to be of hybrid origin; in these cases, combining mtDNA of one species (A. barbouri) with nuclear material from four bisexual ancestors (A. laterale, A. jeffersonianum, A. texanum and A. tigrinum). Some of the unisexuals are diploid, others are triploid, others tetraploid, and others pentaploid!

Because unisexual species are lower in genetic diversity than bisexual ones, and because deleterious mutations accumulate in their genomes (these don’t get purged every time meiosis occurs), a fair and apparently correct assumption is that unisexuals work on a risky, ‘boom and bust’ evolutionary strategy of short-term success, thriving in habitat edges where bisexual species don’t have an ecological advantage.

Based both on this logic, and on the genetic history of unisexual populations themselves, it has near-universally been thought that unisexual populations are short-lived and unable to persist as long as bisexual ones (Tet Zoo trivia: you may remember me using this argument when responding to the hilarious suggestion that the Loch Ness Monster might be parthenogenetic [it's here]). However, genetic data from unisexual mole salamander species has challenged this view: the amount of sequence divergence between unisexuals and their closest relatives indicates that the unisexuals have been in existence for 2, 3, or even as long as 5 million years or so (Bi & Bogart 2010). In other words, they've been around for a long time and cannot be considered short-lived entities.

How, then, are these animals able to avoid the problems otherwise associated with unisexual reproduction? The answer seemingly lies in the fact that they are not truly parthenogenetic – despite being unisexual, members of the salamander species concerned still mate with males of other species: note to fertilise their eggs, but to trigger the start of embryonic development. This is thus a form of parthenogenesis, but it’s a ‘leaky parthenogenesis’ (Lampert & Schartl 2010).

The biology and evolution of unisexual mole salamanders has become a contentious and much-debated subject in lissamphibian research and my coverage of it here is but fleeting and cursory – note that several key papers on the subject are open-access.

For previous Tet Zoo articles on salamanders, see…

Refs –

Bi, K. & Bogart, J. P. 2010. Time and time again: unisexual salamanders (genus Ambystoma) are the oldest unisexual vertebrates. BMC Evolutionary Biology 2010; 10: 238 doi: 10.1186/1471-2148-10-238

Lampert, K. P. & Schartl, M. 2010. A little bit is better than nothing: the incomplete parthenogenesis of salamanders, frogs and fish. BMC Evolutionary Biology 2010; 8: 78 doi: 10.1186/1741-7007-8-78

Shaffer, H. B. 1993. Phylogenetics of model organisms: the laboratory axolotl, Ambystoma mexicanum. Systematic Biology 42, 508-522.

- . & McKnight, M. L. 1996. The polytypic species revisited: genetic differentiation and molecular phylogenetics of the tiger salamander Ambystoma tigrinum (Amphibia: Caudata) complex. Evolution 50, 417-433.

Smith, H. M. 1969. The Mexican axolotl: some misconceptions and problems. BioScience 19, 593-597, 615.