The axolotl is the Peter Pan of the amphibian world, being able to reproduce its own kind while still in its aquatic larval stage. This is unlike the usual development of amphibians such as the common frog, toads and newts, which as larvae, or tadpoles, are confined to fresh water. In the adult form they can live in water and on land, reproducing in water in the breeding season. Certain amphibians, the Mexican axolotl being the most famous, are able to complete their life cycle without ever leaving the water, as sexual maturity is reached in the larval stage.
The axolotl is a newt-like creature, 4-7 in. long, usually black, or dark brown with black spots, but albinos are quite common. The legs and feet are small and weak, while the tail is long, with a fin running from the back of the head to the tail and along the underside of the tail. It breathes through the three pairs of feathery gills on the sides of the head.
Habits and habitat
Axolotls are quite often kept in aquaria, especially in schools. This is rather surprising as they are rather dull animals, spending most of their time at the bottom of the tank, occasionally swimming about lazily for a few seconds before sinking again. A probable reason is that the axolotl can reproduce its own kind without ever leaving the water. Newts and most salamanders, kept in captivity, need water, land and very careful keeping if they are to survive and breed successfully.
Axolotls cannot be kept together with complete safety as they are liable to bite off each other’s gills and feet, and bite pieces out of the tail. If this does happen, however, and they are then separated, the missing pieces will regenerate.
In the wild, axolotls are confined to certain lakes around Mexico City, where they are regarded as delicacies when roasted. The name axolotl is Mexican for «water sport».
Zoologists were unable to decide where to place axolotls in the classification of amphibians, until 1865 when, at the Jardin des Plantes in Paris, the problem was solved. Several specimens had bred successfully, when one day it was noticed that the young of one brood had lost their gills and tails, and had quite a different coloration. They had, in fact, turned into salamanders. This was the secret of the axolotl. It is one of several species of salamander, an amphibian which normally has an aquatic tadpole resembling the axolotl, that normally changes straight into the adult. The axolotl, however, usually becomes sexually mature while still a larva. This is because the axolotl fails to metamorphose.
In most frogs and toads, fertilisation of the eggs takes place externally. In other words, the female sheds the eggs into the water and the male simply releases his sperm near them, to make their own way to the eggs. The axolotls, related salamanders, and newts have a system of internal fertilisation but it is different from the normal method in which the male introduces the sperm into the female’s body to meet the eggs waiting there. Instead, the male axolotl sheds his sperm in a packet called a spermatophore. It sinks to the bottom and the female settles over it and picks it up with her cloaca.
The male attracts the female by a courtship dance, secreting a chemical from glands in his abdomen and swishing his tail, presumably to spread the chemical until a female detects it and swims towards him.
About a week later, 200 – 600 eggs are laid, in April or May. They are sticky, and the female attaches them to plants with her back legs. The young axolotls hatch out a fortnight to three weeks later, depending on the temperature of the water. At this stage they are only about ½ in. long and remain on the plant where the eggs are laid. After a week they start swimming in search of food and, if the water is warm and food plentiful, they will be 5 – 7 in. long by winter. They will then hibernate, taking no food, if the water temperature drops below 10°C/50°F.
The youngest axolotls feed on plankton, minute organisms that float in water. Later they eat water fleas such as daphnia, and when fully grown they hunt for worms, tad poles, insect larvae, crustaceans and wounded fish. Their prey has to move, however, and axolotls will ignore still, dead food given to them but will snap up a piece of food that is waved about in the water.
The axolotl’s habit of breeding while in the larval stage is known as neoteny, or the retention of juvenile characteristics in the adult form. By «adult» is meant a sexually mature animal. This habit is not restricted to the axolotl. Other amphibians, including some salamanders, sometimes exhibit neoteny, failing to emerge onto land, but continuing to grow in the larval form.
The basic cause of neoteny seems to be a lack of thyroxine, the hormone secreted by the thyroid gland, which controls metabolism. If the secretion is upset in humans, several bodily disorders occur, including the formation of goitres, swellings in the neck caused by the thyroid gland enlarging. Administration of thyroid gland extracted from cattle, for instance, can often cure the goitre, and axolotls will change into adult salamanders if given thyroid gland.
It would seem, then, that there is something lacking in the diet of both axolotls, and humans with goitres. In Wyoming and the Rocky Mountain area the tiger salamander regularly exhibits neoteny and humans are liable to get goitres. This has been traced to a lack of iodine in the water, for iodine is an essential component of thyroxine. In these cases the administration of iodine, rather than thyroxine, is all that is needed either to effect the metamorphosis of an amphibian or cure a goitre. However, iodine treatment is not the only way of making axolotls metamorphose. Sometimes a consignment sent to a dealer or a laboratory will change into adults shortly after being received. Apparently, the jolting during travel has been sufficient to start the change.
When faced with an odd occurrence like this, a zoologist asks whether it confers any advantage on the animal. In the discussion on amphiuma it was seen that a freshwater animal has an advantage over a land animal because it does not have to conserve its body water. This could well be the reason for the axolotl’s neoteny. The lakes where it lives do not dry up and there is an abundance of water, so it is an advantage to live and breed there, rather than risk life on the dry, barren land around. If the lakes dry up, then it can still change into a salamander, having the best of both worlds.