Axolotl (Ambystoma mexicanum)
|Also known as:||Mexican axolotl|
|Size||Length: up to 30 cm (2)|
Weight of male: 125 – 130 g (3)
Weight of female: 170 – 180 g (3)
- The axolotl is an unusual species of salamander which retains its larval features, such as gills, and remains aquatic throughout its life.
- Most axolotl fail to undergo metamorphosis, but if its habitat dries up this species can metamorphose into an adult form.
- Instead of forming scar tissue when wounded, the axolotl can regenerate tissue at the wound site and even re-grow missing limbs.
- Despite remaining in a larval form, the axolotl does become sexually mature and reproduces at around 12 to 18 months old.
The axolotl is classified as Critically Endangered (CR) on the IUCN Red List (1) and is listed on Appendix II of CITES (4).
The remarkable axolotl (Ambystoma mexicanum) is part of the family of ‘mole salamanders’, but exhibits an unusual and extreme trait known as neoteny, or paedomorphosis. This is the retention of larval stage characteristics throughout life, so axolotls usually never fully resemble an adult salamander. Unlike other amphibians, most axolotl fail to metamorphose, living permanently in water (3). Although it does develop lungs, the axolotl’s most bizarre feature is its retention of its branch-like gills. These are external projections from the neck on each side of the head. Each side has three branches covered with feathery filaments which increase the surface area for gas exchange. The axolotl has a long, slim and darkly coloured body, and short legs, with four digits on the front feet and five digits on the hind feet. Albino individuals have been bred in captivity, but are not known to live in the wild (2).
The axolotl previously occupied Lakes Xochimilca and Chalco and the surrounding water channels on the southern edge of Mexico City, but has been lost from most of its range (1).
The axolotl is native to the ancient water channel system of Mexico City, preferring deep brackish water with plenty of vegetation (1).
Although the axolotl remains in larval form throughout its life, it becomes sexually mature between 12 and 18 months of age. Males ‘dance’ to initiate courtship, nudging the female before depositing several cone-shaped packets of sperm known as spermatophores onto rocks and plants. These are taken up by the female’s cloaca, for internal fertilisation of her eggs. She lays the eggs 24 hours later, each one becoming enveloped in mucus as it emerges. They become glued to each other and to the substrate (3) where they incubate for two to three weeks (2). A single female axolotl can produce up to 400 eggs in a day, averaging 175 – 200 (3).
The axolotl is inactive during the day, resting on the substrate with the gills splayed. They move slowly and may surface occasionally to take a breath of air (3). Young axolotl feed on algae, but older individuals will take aquatic invertebrates. The axolotl is primarily preyed upon by herons (2).
A species of fascination to scientists the world over, the axolotl has many study-worthy traits. Whilst able to remain in larval form throughout its life, the axolotl can metamorphose into a fully-adult Mexican salamander if its habitat dries up. Additionally, rather than forming scar tissue when wounded, tissues at the wound site convert to a stem cell-like state, meaning that they are able to re-grow missing tissue in its entirety, even a whole limb (3).
Whilst there are large numbers of axolotls in captivity around the world, particularly in biomedical and physiological research laboratories, numbers of wild axolotls are very low. Previously, capture of this species for the international pet and research trade contributed to population declines, but the axolotl now breeds well in captivity, alleviating this threat. It was also captured for consumption by local people, although numbers are now too low for this. The most significant threat to the axolotl is the increasing pollution of the lakes and canal system as Mexico City continues to grow (1). Land drainage, flood control and sewage disposal methods from the 17th century to the present have all contributed to the destruction of the water system of Mexico City (3).
Recent stabilisation of axolotl numbers may be attributable to the restoration of the Parco Ecologico Xochimilco over the last 20 years. This protected area requires continued restoration to support the axolotl population, which, it is hoped, may begin to increase with the introduction of captive-bred axolotls. The axolotl is protected under category Pr (special protection) by the government of Mexico (1).
For further information on the axolotl see:
EDGE of Existence:
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- Albinism: heritable (passed on from one generation to the next) genetic condition in which the pigment melanin is not produced in the skin or hair resulting in white, or partially white individuals.
- Alga: a collection of taxonomically unrelated groups that share some common features but are grouped together for historical reasons and for convenience. They are of simple construction, and are mainly photoautotrophic, obtaining all their energy from light and carbon dioxide, and possess the photosynthetic pigment, chlorophyll A. They range in complexity from microscopic single cells to very complex plant-like forms, such as kelps. Algal groups include blue-green algae (cyanobacteria), red algae (rhodophyta), green algae (chlorophyta), brown algae and diatoms (chromista) as well as euglenophyta.
- Cloaca: a common opening into which the reproductive, alimentary and urinary systems open.
- Larval: of the stage in an animal’s lifecycle after it hatches from the egg. Larvae are typically very different in appearance to adults; they are able to feed and move around but usually are unable to reproduce.
- Metamorphosis: an abrupt physical change from the larval to the adult form.
- Spermatophore: gelatinous jelly cone with a sperm cap deposited by a male during courtship and picked up by the cloacal lips of the female.
IUCN Red List (May, 2006)
AmphibiaWeb - Axolotl (May, 2005)
Lab Animal - Axolotl (May, 2005)
CITES (May, 2005)