Short guide to Axolotl Husbandry
by Susan T. Duhon
The Indiana University Axolotl Colony was founded in 1957 by Rufus R. Humphrey, who brought his small research colony to Bloomington when he retired from teaching at the University of Buffalo . The small colony gradually grew and became a genetic resource center as Humphrey pursued his research interests in mutant genes of the axolotl. Since 1969 the colony has been supported by the National Science Foundation as a living stock center to provide research material for research and instruction to laboratories and schools throughout the United States. Today we breed axolotls about ten months out of the year and send out tens of thousands of embryos, larvae, and adults to developmental biologists, neurobiologists, and other amphibian research scientists, as well as to primary, secondary, and undergraduate-level teachers. This guide is a partial response to the frequent questions and requests for advice and information on the care and feeding of axolotls that the colony receives.
Water is the most important component of the axolotls' environment. Never house them in extremely soft or distilled water. They need hard water to help them maintain the integrity of their skin, their most important defense against infection. Remove any chlorine, chloramines, or ammonia that may have been added as part of municipal water treatment. Commercial preparations (e.g., Amquel ) are available for this purpose. We pass our water through an exchange resin to remove heavy metals and past a sterilizing UV light as well. We also add salts to the water to make a modified Holtfreter’s solution. The recipe we use to make 40% Holtfreter's in a 44 gallon barrel is:
KCl: 1 teaspoon CaCl2: 2.5 teaspoons MgSO4.7H2O: 2 tablespoons NaCl: 240 ml (dry but measured in a liquid beaker)
The salts restore hardness after water treatment and help us maintain the animals' health by discouraging parasites and fungus. Extra salts are not essential, however, if you are attentive to good husbandry practices and the water is hard and free of chemicals and heavy metals. Keep the pH between about 6.5 and 8. If pH is at the high end of this range, monitor ammonia carefully because its toxicity will be greater than at neutral pH.
We house adult axolotls either individually in one gallon bowls (either squat, glass fish bowls or plastic ice cream buckets) or in groups of three to four in plastic tubs, approximately 10" x 18" x 6" deep (25 cm x 46 cm x 15 cm). A half gallon (2 liters) of water per axolotl is adequate, provided that the animal is completely submerged and the water is changed frequently. We prefer to keep male axolotls in individual bowls when they are not being mated because they breed better for us when housed singly between breedings than they do housed in groups. Female axolotls spawn well for us whether housed singly or in groups.
Although we use individual containers or small-group containers, others have successfully housed axolotls in filtered aquaria. As a rule of thumb, house two adults in a ten-gallon aquarium. Gravel, if used, should ideally be coarse. The axolotls will ingest smaller gravel, especially pea-sized. Although they regurgitate the gravel eventually, it's easier to avoid the problem. Be careful, especially if you use a power filter, not to circulate the water too fast (see below) and to maintain a biological filter that can convert ammonia and nitrites. Do partial water changes regularly and keep the water cool and out of direct sunlight.
You may find it difficult to raise small larvae in an aquarium because they will be very aggressive and cannibalistic toward one another. Juveniles and adults, however, can thrive in a properly maintained aquarium.
Axolotls in nonfiltered systems don't need continuously circulating water if the water is replaced regularly. In fact rapidly circulating water is stressful to them. If your system circulates water continuously, keep the rate of circulation as slow as possible. Nor do they seem to need a large volume in which to swim and move around. Even when placed in an aquarium, they spend the bulk of their time lying nearly motionless at the bottom.
They are extraordinarily resourceful when it comes to gas exchange. If you watch an axolotl for several minutes you will see it flick its gills periodically. Moving the fronds back and forth quickly stirs up the surrounding water and disperses any carbon dioxide that may have accumulated about the gills and mixes in fresh, oxygenated water. You may see the axolotl swim to the surface and gulp air as well because, although outwardly the neotenic axolotl has a larval form, it still undergoes "cryptic" metamorphosis and develops lungs.
Although we successfully use plastic containers, they must be kept very clean. Plastic tends to support the growth of a bacterial scum along the bottom and sides of the container. We have found that if this scum is not cleaned off periodically, the axolotls will develop sores on their toes and feet that will not heal. Instead the skin retreats up the leg and the toes eventually die and are sloughed off. If this happens put the axolotl temporarily into a glass bowl. Add a few drops of mercurochrome (enough to tint the water a pale orange) to the water as a disinfectant, and change the water frequently. The axolotl will heal nicely, and the toes will probably regenerate.
We keep young, larval axolotls in shallow glass bowls about 8" (~20 cm) in diameter. When they first hatch, fifty or more larvae may occupy the same bowl, but as they grow, reduce the number per bowl progressively. The larvae's rate of growth depends upon temperature, frequency and amount of food, and the number of animals per bowl. Larvae should be about an inch long by the time they are 1 1/2 to 2 months old.
Axolotl larvae never all grow at the same rate, so when it is time to divide them up, be sure to put similarly sized animals together. If mixed sizes are housed together, larger ones will try to eat the smaller ones. In any case, any young axolotls housed together will tend to lack toes or feet, because during this phase of rapid growth, the larvae will snap at anything that moves. For this reason, we try to put all young axolotls 2" (5 cm) or longer by themselves. We use small plastic bowls containing one liter of water for this purpose. Make sure the sides are tall enough to keep the axolotls from jumping out. If any larvae have lost toes to their fellows, they will regenerate them quickly.
Axolotls thrive at cool temperatures. We keep our axolotls at 15-18°C (60-65°F). They should never be kept above about 22°C (72°F). Too warm temperatures are dangerous for axolotls. To prevent overheating never house them where they are exposed to direct sun.
Feeding and Routine Care
In their native habitat, axolotls ate of the abundant small fauna, including snails, worms, crustaceans, various small invertebrates such as Daphnia, and small fish and amphibia ( Shaffer, 1989). In the laboratory we feed our young larvae brine shrimp and juveniles and adults pelleted food.
We always change the water in the axolotls' bowls before feeding. To change the water of axolotl larvae, carefully pour the axolotls from their bowl into a net. Clean the bowl with scrub mixture (baking soda and salt mixed together in a 2:1 ratio). Put one liter of clean water in the bowl, then invert the net and release the larvae into the clean water.
Feed newly hatched, live_brine shrimp to young larvae daily. We strain the shrimp out of the brine, resuspend them in axolotl water, and deliver them to the axolotls using a large glass pipette. Feed enough shrimp so that all of the young larvae have orange bellies afterward, but few shrimp are left uneaten in the bowl. Change the water again within 24 hours after feeding, because shrimp cause a rapid deterioration of water quality if many are left uneaten.
When the larvae get to be about 1 1/2" (4 cm) long, we begin to supplement the shrimp diet with pellets. We use soft-moist salmon pellets, a vitamin and mineral fortified, fishmeal-based sinking pellet, 1/8" (3 mm) in diameter ( Rangen, Inc. ). We gradually wean the larvae off shrimp and increase the number of pellets being offered until the young axolotls are 2-3" (5-8 cm) long, when they no longer need any shrimp.
As the larvae grow, we increase the number of pellets, always seeking to fill the larvae up without leaving a lot of extra food to foul the water.
Young larvae are very vulnerable to disease, but difficult to treat. Therefore, we have found the most effective strategy is to feed them generously, pay particular attention to water quality, guard against overcrowding, and grow them up out of the vulnerable stage as quickly as possible.
Always change the water of pellet-fed axolotls before feeding. We change young animals' water daily, and adult animals' water at least every other day. Frequent changes prevent the buildup of ammonia and other metabolic wastes and keep a large population of bacteria from getting established.
We change the water of axolotls housed in individual bowls by carefully pouring it, axolotl and all, into a plastic colander. We clean the bowl with scrub mixture (baking soda and salt mixed together in a 2:1 ratio), put in a fresh pitcher of water, and return the axolotl to its home. The axolotls will tolerate short periods of time out of the water very well. They are able to continue gas exchange as long as their skin is moist, but never leave them out of the water long enough for their skin to dry out or become "tacky."
We pump water from a large tank to flush the tubs of the axolotls housed in groups. Prior to flushing, we vacuum up (by aspiration) uneaten food and feces from the bottom of the tub. Once a month we transfer the animals from their tubs to a temporary holding tank and clean off any scum that has accumulated on the sides and bottoms of the tubs.
When axolotls reach about 6" (15 cm) in length, we begin feeding them 3/16" (5 mm) diameter pellets. When they are about one year old, decrease the number of feedings to three or four times per week, no more than five pellets at a time.
Axolotls may snatch pellets dropped in front of them out of the water, using their lateral line to detect the falling pellets. They also quickly learn to locate those pellets that fall to the bottom of the bowl. Often you can see them arching their necks as they move about their bowls seeking food that is on the bottom.
Juvenile and adult axolotls will accept a wide variety of foods. Besides salmon pellets, other feeding possibilities include Daphnia or water fleas, mosquito larvae, earth worms (a favorite), "feeder" guppies or goldfish, trout pellets, and beef liver or heart. Any live fish or amphibia used as food should be aquarium bred in order to avoid the introduction of harmful parasites. Never, for example, feed fresh water minnows obtained from bait shops because they are usually heavily infested with parasites.
You can distinguish adult male axolotls from females by their relatively straight bodies and large glands about the cloaca (vent). Mature females have round, plump bodies because they are filled with eggs, and they lack the conspicuous glands. Axolotls cannot be sexed until they are sexually mature, at about one year.
Choose healthy, mature animals for mating and put them together in a suitable container. We use large plastic tubs. Add enough water to cover the animals. The bottom of the container must be textured (that is, not smooth glass), or contain rocks to which the male can attach his spermatophores, small cones of clear jelly with a white, sperm-containing cap. Cover the container and leave them alone.
The male will begin to court the female by nudging her with his snout. This begins a short mating dance, during which the male deposits his spermatophores on the rocks. The female follows behind and picks up the spermatophores with her cloaca and stores the sperm inside in a special structure called the spermatotheca ( Eisthen, 1989).
The next day, or at least a few hours later, look for spermatophores on the rocks or loose in the water (The female does not usually pick them all up). Return the male to his home.
If you see spermatophores, place the female in a glass bowl. Cover her with a towel and wait. By the next morning, you will probably see eggs in the bowl if she is going to spawn. Usually the female will begin to lay her eggs 12 to 20 hours after mating. Rarely, she will wait several days to begin. She sheds her eggs over a period of one to two days. If the female spawns in a container that has rocks or plants, you will find the eggs spread about on them. She never lays her eggs in a single mass or clump. If you allow your axolotls to spawn in an aquarium, you will need to remove either the eggs or the adults before the eggs hatch. Otherwise the adults will eat the hatchlings. Usually they will not touch eggs while they are still in their jelly coats.
We allow males at least a week or two and females at least two, preferably three months, between spawns.
We never use hormones to induce spawnings, because healthy axolotls are not difficult to get to spawn. The success of a particular pairing is unpredictable however. Most pairings should produce spermatophores, and at least 1/3 to 1/2 of the ones with spermatophores should produce spawns.
Changes in lighting are critical for spawning. If we shorten the light period, we get fewer spawns. If we then lengthen it gradually over a period of a few weeks, spawns will again increase in number. The absolute length of the light period is not critical. Our axolotls get about 14 hours of light each day.
In their native habitat, axolotl spawn in February ( Gadow, 1903), but our captive-bred axolotls display only residual seasonality. We obtain spawns readily year round, although we have slightly less success from August through October.
Collect the eggs from the female's bowl with a wide-mouthed pipette. Remove eggs attached to rocks or plants with forceps. Axolotl eggs are enclosed in a clear jelly coat that protects them from physical injury and bacterial infection. Eggs that accidentally come out of the jelly usually die without special handling.
Place the eggs in a shallow bowl such as those in which young larvae are kept. Avoid overcrowding. Put no more than 50-100 eggs or developing embryos in each bowl. Discard any excess jelly, and do not allow the embryos to clump. Embryos in the interior of a clump may not receive enough oxygen. If you see embryos in the center of a cluster of eggs developing more slowly than those on the periphery, use forceps to pull the clump of embryos apart. Otherwise the embryos in the middle will die.
It is a good idea to sort the embryos when they are about a day old (at room temperature they will have reached blastula stage) to remove infertile and nonviable eggs.
You can control the embryos' rate of development by modifying the temperature at which they are kept. Embryos that have reached blastula or later stages may tolerate temperatures as low as 1-2°C for from one to three weeks ( Ginsburg et al., 1987), but earlier embryos may be damaged by temperatures below about 10°C. A lower temperature delays development, and raising the temperature (to a maximum of 25°C) accelerates development. We often hold embryos at cold temperatures for anywhere from a few days to a week or more so that we can ship them out to laboratories while they are still at an early stage of development.
Embryos kept at room temperature will hatch in two to three weeks. As the larvae hatch, carefully remove the discarded jelly capsules with a pipette. Some larvae may not hatch on their own. After the majority have hatched, remove the jelly coats from any unhatched larvae by gently puncturing the capsules with forceps.
Begin feeding the young larvae when you can no longer see yolk in their bellies. Usually they are ready to eat soon after hatching. If they are not fed right away, they may swallow air, and get air bubbles in their stomach. When they begin to eat, however, they will be able to expel the air.
Axolotls that are cared for regularly, fed properly, and kept in clean water at suitable temperatures are hardy animals that seldom get sick. Axolotls stressed by poor husbandry, adverse environmental conditions, or experimental procedures are vulnerable to infection by opportunistic bacterial pathogens. Common pathogens are Pseudomonas, Aeromonas, and other gram-negative organisms.
The first signs of illness are loss of appetite and deterioration of the gills. You may see some anemia. More severely ill axolotls may be jaundiced and have small open skin sores. Very ill animals may develop ascites or severe edema.
Aside from very young larvae, we have found that axolotls about one year old that are just becoming sexually mature are most prone to illness. Prompt treatment with antibiotics will often help. We use amikacin diluted with physiological saline to 5 mg/ml. We give the axolotls three intraperitoneal injections (5 mg/kg body weight) 48 hours apart. The injections are best given at the first sign of illness ( Duhon, 1989a; 1989b).
Axolotls are fascinating creatures and important laboratory animals. Their status as an endangered species underlines the importance of proper care and maintenance of health.
Duhon, Susan T. Diseases of axolotls. In: John B. Armstrong and George M. Malacinski, Eds. Developmental Biology of the Axolotl . Oxford University Press, New York, pp. 264-269, 1989.
Duhon, Susan T. Disease control in a large colony of axolotls. Herpetopathologia 1:105-108, 1989.
Eisthen, Heather L. Courtship and mating behavior in the axolotl. Axolotl Newsletter 18:18-19, 1989.
Gadow, H. The Mexican axolotl. Nature 1736:330-332, 1903.
Ginsburg, Mary F., Twersky, Laura H., and Cohen, William D. Ambystoma embryo development after cold storage. Axolotl Newsletter 16:3, 1987.
Shaffer, H. Bradley. Natural history, ecology, and evolution of the Mexican "axolotls." Axolotl Newsletter 18:5-11, 1989.