An Amusing Amphibian: The African Clawed Frog Xenopus Laevis

AN AMUSING AMPHIBIAN:  THE AFRICAN CLAWED FROG XENOPUS LAEVIS
By:  Suzanne Royer, Ph.D., CAS
Published in The Daphnian, February 1991

One of the amphibians commonly offered for sale in aquarium shops is Xenopus laevis, otherwise known as the African Clawed Frog.  Young frogs, and occasional adults, of both wild type and albino strains are frequently available, usually at a modest price.  These frogs make excellent aquarium inhabitants, as long as they are not kept with any fish small enough to be eaten.  Over a span of about ten years, I myself have maintained, bred, and raised Xenopus, both as a research scientist and as a hobbyist.  These hardy, undemanding, fascinating and often comical creatures have won a permanent place in my affections and aquaria.

Xenopus is a member of the Class Amphibia, Order Anura, and Suborder Aglossa.  The Order Anura includes tailless amphibians, such as frogs and toads and their relatives, while the Suborder name Aglossa refers to the lack of a tongue in Xenopus and other suborder members.  Xenopus belongs to the Family Pipidae, which also contains the genera Pipa, Protopipa, Hymenochirus and Pseudhymenochirus.  Pipa and Protopipa are native so South American, which Xenopus, Hymenochirus and Pseudhymenochirus come from Africa.  Of these three genera, Xenopus is considered the most primitive and is the most widely distributed.  The genus includes about eight to ten species, of which Xenopus laevis and Xenopus borealis are most commonly available.  Frogs of the genus Hymenochirus are often sold as Dwarf African Frogs.  While Xenopus has teeth and lacks webbing on the front feet, Hymenochirus is toothless and has half-webbed on the front toes.  Hymenochirus lives in the rain forest and lower Congo; Pseudhymenochirus comes from the northern rain forest region.

There is disagreement over whether Xenopus should properly be referred to as a “frog” of a “toad”.  Some experts insist that the word “frog” may be applied only to members of the family Ranidae, while “toad” originally referred to members of the Bufonidae.  However, both terms are often used for members of other families, and one authority I consulted asserted that there is now no taxonomic significance in the use of either term.  The application of the label “frog” to Xenopus seems appropriate in view of its totally aquatic habit, and I have adhered to this common usage.

Xenopus reaches a maximum length (nose-to-rump, excluding the legs) of about 5” (13 cm), although large females may be a bit bigger.  In comparison with the common frogs we are most used to seeing, Xenopus appears to be “flattened”.  Because of the way in which the hinde legs are articulated, Xenopus frogs are unable to squat and hop like “normal” frogs.  Instead, the muscular hind legs are spayed out to the side and well adapted for powerful swimming strokes.  The feet are quite large, with long, webbed toes.  The inner three toes on each foot bear hard, black claw-like caps that are responsible for the animal’s common name.  The genus name, Xenopus, means “Strange foot”.  It has been suggested that these claws may be useful in digging in the mud for insects.  Having received some painful scratches from frogs I was trying to hold, I suspect that these claws may also help to induce potential predators to turn loose.  The front legs, which are comparatively small and weak, each bear four slender toes and are used for conveying food to the mouth rather than for locomotion.

The body is streamlined, and head is relatively small, broad and flat.  The wide mouth basically extends across the whole front of the head.  Two round nostrils are located on the short snout.  The small eyes are directed upward.  The visual system of Xenopus frogs is much less well developed than that of frogs such a Rana that use their eyes to locate and orient toward prey.  However, casual observations suggest that the eyes of Xenopus are of some use in locating potential food items.  Xenopus frogs vary in color from buff to brownish to olive green to greenish grey or black, with a variable amount of mottling.  They are able to alter the body color and pattern for camouflage, and their color may change with lighting conditions.  When I was using Xenopus for research, we had one blind breeding male (Samson) and remained black all of the time.  The ventral surfaces of the body and legs are lighter, sometimes almost white.  The skin is smooth, except for a few rows of bumps resembling stitches that indicate the location of sensory organs related to the lateral line system.  Xenopus is one of the few animals I have ever encountered that is truly slimy, a fact that also make them very difficult to catch or hold.  The species name, laevis, probably derives from the Latin word levis, meaning smooth or slippery.

In their native southern Africa, where they are known by the common name Platanna, Xenopus frogs inhabit rivers, lakes, ponds, and swamps.  They are truly aquatic animals, under ordinary conditions never needing to leave the water.  They do, however, breathe atmospheric oxygen and must periodically come to the surface to fill their lungs with air.  They are often found in shallow ponds that may dry up during the long African summers.  If this happens, the frogs are able to bury themselves in the mud and aestivate until more hospitable conditions return.  They are also capable of moving short distances over dry land from one body of water to another, in response to seasonal conditions or overpopulation.  Although they are strong and agile swimmers, with the ability to move backward as well as forward, out of the water Xenopus can only be described as clumsy.  Their locomotors movements on land suggest a parody of swimming, as they move in a series of ungainly flops.

Xenopus frogs are voracious carnivores that will eat any animal matter, living or dead, they can get the “hands” on.  In the wild, their diet consists largely of fish, freshwater crustaceans, aquatic insects and their larvae, and Xenopus tadpoles and young.  The frogs are valued for mosquito control in their native habitat because they consume large numbers of the eggs and larvae of these insects.  They feed by using their short front legs to stuff food into their mouths.  I have often watched a group of young Xenopus go into a “feeding frenzy” when food was dropped into their tank.  As soon as they detect the presence of food (probably through olfaction), they begin to swim around the tank, performing jerky stuffing movements toward the mouth with their front legs.  When they touch a piece of food, it is immediately seized and crammed (no other word will do) into the mouth.  If the morsel is too big to go in the first time, the frog will use the “hands” to take it back out, turn it around, and try it again.  A very small frog can be very persistent about trying to put a very big piece of food into its mouth.  I have also had to disentangle a couple of small frogs when one wound up with another’s leg in its mouth up to the hip during a feeding frenzy.

Although an article in Tropical Fish Hobbyist (“Spawning the African Clawed Frog, Xenopus laevis,” April, 1985) asserted that Xenopus has not often been bred in captivity, these frogs have routinely been bred in research laboratories for many years.  However, hormone injections are commonly used to induce mating in these frogs.  I suspect that the author was referring to “natural” spawning accomplished without the aid of hormones, which are indeed not very common in captivity.  I would refer you to this article for advice if you want to try a natural spawning of these animals yourself.

While it is not possible to determine the sex of young Xenopus frogs from external characteristics, as the frogs mature the sexes are more readily identified.  Mature female frogs are larger than the males (sometimes much larger) and are typically very broad across the abdomen, giving the body an oval shape when viewed from above.  Males are more slender.  At the rear, the female also possesses genital papillae (or cloacae lips), which swell and redden when she is ready to spawn.  Just prior to mating, the male may develop dark brown or black pads on the insides of the forelimbs and paws.  These assist him in holding the slippery female.  During mating, the male grasps the female from behind and clasps his forelegs around her in front of her hind limbs.  This joined state is referred to as am plexus. The male may be carried around on the female’s back for quite some time, during which she expels single eggs which rolls along the male’s belly and across his cloaca, where they are fertilized.  The jelly-coated eggs then roll off and, in nature, come to rest on a plant, where they stick.  In captivity, eggs should be removed from the spawning container to prevent the adults from eating them.  A spawning may produce from a few hundred to a few thousand eggs, and a single female may produce as many as 15,000 eggs a year,  Females begin to produce eggs at an age of about ten to eighteen months, although captive frogs may mature in a shorter time with heavy feeding.

As mentioned previously, breeding may be induced by injection of hormones.  In the laboratory both the male and female are usually given one or more injections of a hormone called chorionic gonadotropin. The hormone is injected into the dorsal lymph sac on the back of the frog.  On the day following the injection, the frogs will usually spawn.  In the past, Xenopus frogs were used in pregnancy testing--physicians found that  if a female frog was injected with urine from a pregnant woman, the frog would lay eggs from five to twenty-hours later; an injected male frog would release sperm.  Other species of frogs and toads were also suitable for these tests.  Not surprisingly, chorionic gonadotropin used to induce spawning is commonly obtained from the urine of pregnant women.  Today, other kinds of pregnancy tests have replaced the frogs.

Xenopus eggs, exclusive of the jelly coating, are about a millimeter or so in diameter.  The dorsal part of the egg (and the early embryo), also called the “animal pole”, bears brownish pigment--except in albino strains.  The ventral part, or “vegetal pole”, is yellowish because it contains much yolky material.  To greatly simplify the embryology, the cells that develop from the pigmented, dorsal part of the egg will eventually form the skin, muscles, skeleton and nervous system of the frog, while the ventral yolky portion will contribute to the visceral structures.  The development of Xenopus embryos and tadpoles is fascinating to watch, although observation of the early stages requires a strong magnifying glass or a dissecting microscope.  The observation of development in Xenopus is especially easy because the tadpoles are transparent, a fact that also contributes to their popularity as research subjects.  The transparency of the skin and underlying tissues allows direct observation of the anatomy and development of the brain, nervous system, heart and blood vessels, and other internal organs in living specimens.

After about a day, the embryo has a recognizable head and tail, as well as rudimentary eyes and ears and a cement gland.  Spontaneous movements begin after about 1 day and 5 hours; the heart begins to beat after about 1 day and 20 hours.  About two days after fertilization, the embryo is approximately 5 to 6 mm long and has a rudimentary tail and gills.  The eyes are clearly visible, as is the cement gland on the ventral side of the head.  The mouth consists only of an indentation in the surface of the skin.  Hatching begins at about this time. Once they hatch the tadpoles spend most of the next day or two (at least in the laboratory) hanging tail-down at the surface of the water in tight clusters.  Alternatively, a tadpole may adhere to some surface by its cement gland (which disappears a bit later in development).  If disturbed, these young tadpoles will swim briefly in an apparently undirected manner.  At an age of just over four days the tadpoles begin to feed.  They are filter feeders able to take only very fine food.

The hind limb bud is first visible after about 4 days and 10 hours, when the tadpole is 9-12 mm long.  The forelimb bud appears after about 7.5 days,  About 17 days after fertilization the tadpole is about 28-36 mm in length, the limb buds are more distinct, and the tadpole also has well-developed tentacles on the upper jaw resembling the barbells of catfish.  At this point the head is flat and broader than the body, and the tadpole--with its tentacles and long tail--looks much more like an exotic catfish than a young frog.  Tadpoles typically adopt a nose=down swimming and feeding posture.  The distal third of the elevated tail undulates continuously.

At first the forelimbs develop under the skin, in a chamber called the atrium.  At about 44 days of age the forelimbs begin to erupt, and they are completely free by about a day later.  By then the hind limbs are well-developed, with long, webbed toes; the tentacles are beginning to shrivel.  At about 48 days, the head is narrower than the trunk.  Other changes taking place include continued development of the limbs, progressive appearance of opaque adult skin, movement of the corners of the mouth from in front of to well behind the eyes, movement of the heart from final burst of development takes place during “metamorphic climax”, which takes about a week.  During this time the tadpole is unable to feed because the filter apparatus degenerates and the internal organs are profoundly rearranged to equip the young frog for a new diet.  At the beginning of metamorphic climax, when the tadpole has four legs and still has a long tail, it resembles a small dragon.  Metamorphosis is completed by about 58 days of age.  The new frog will then be perhaps 15-25 mm in length and will have pigmented skin like an adult.

The relatively short time of development from egg to frog (approximately 60 days) is another advantage of Xenopus as a laboratory animal; common Rana frogs take as much as twice that length of time to complete development and metamorphosis.  In addition, Xenopus are much easier to maintain and feed in a laboratory environment than Rana.  Because they are totally aquatic, metamorphosing Xenopus tadpoles and young frogs need not be provided with a place to get out of the water.  On the other hand, unless they can get out of the water, Rana tadpoles undergoing metamorphic climax will drown.  (I managed to kill quite a few Rana tadpoles before I got this through my head).  While Rana must be provided with live foods or force-fed, Xenopus will readily accept a variety of foods, including bits of liver or hamburger, earthworms, tubifex worms, insects, bits of fish, and prepared foods.  Xenopus tadpoles are fed pastes made from finely ground prepared food or nettle leaf powder.  They can also feed on green water, rotifers, or infusoria.

Xenopus frogs are hardy, undemanding aquarium residents.  Although they appreciate clean water, they can tolerate a variety of water conditions and do not necessarily need a heated tank if they are located in a reasonably warm room.  They do not require a place to be able to get out of the water, although they must be able to come to the surface for air.  They should have a tank in which they can move around freely and have room to grow.  The tank must also have a tightly-fitting cover, because these frogs are accomplished escape artists.  A tiny post-metamorphic frog can go straight up the side of a glass tank, clinging to it by suction from his little wet body.  I have found any number of frogs dried up on the floor.  They can move quite a distance away from the tank once they get out; how long they can survive out of the water depends upon the humidity in the spot where they decide to hide--usually under a heavy piece of furniture.  In an aquarium, Xenopus frogs spend much of the time either resting on the bottom or hanging in a nearly vertical position, trailing their enormous feet.  This inactivity is punctuated by occasional dashes to the surface to breathe or around the tank in pursuit of food.  They are very nimble in eluding predators or aquarists who want to catch them.

Although a young frog may be suitable for a community aquarium, remember that it will grow.  After only a few months, your little Xenopus can grow large enough to make a mean of many of your fish.  To illustrate, I one watched a Xenopus frog about 3 inches (75 mm) in length swallow four 1.5” (37 mm) goldfish in a period of five minutes.  Don’t plan to keep a frog in a community situation permanently, you would probably be better off with one of the dwarf frogs of the genus Hymenochirus.  The cannibalistic tendencies of Xenopus must also be considered--a larger frog will eat a smaller one if it can.  Tadpoles and young frogs must also be separated.

A further warning--do not under any circumstances release a Xenopus frog (or any other foreign species) into the wild.  In California, where Xenopus was banned many years ago, wild populations derived from escaped Xenopus frogs are a serious problem.  They are prolific and virtually indestructible.  They can adapt to a variety of environmental conditions, are resistant to many poisons and apparently have no natural predators.  Like many other frogs, Xenopus secretes a distasteful and potentially poisonous substance from its skin that seems to protect if from predation.  These voracious frogs have threatened to wipe out native species in many locations.  There is also potential for a problem with these frogs in Florida, Texas, and other states with mild climates.  In 1982 the state of Washington added Xenopus to its “Deleterious Species List” and banned their possession, fearing that the frogs might be able to gain a foothold there.  Apparently the frogs have adapted their technique of aestivation to survive periods of relatively mild cold.  Although it is unlikely that Xenopus could survive the harsher winters in Colorado, there is no excuse for taking the chance.

REFERENCES:

• The Audobon Society Encyclopedia of Animal Life, Crown Publishers, Inc., New York, 1982.
• Collier’s Encyclopedia, 1988 edition.
• “Frog Ware,” by Timothy Branning, National Wildlife Magazine, 1979. Macmillan Illustrated Animal Encyclopedia, Dr. Philip Whitfield, editor, Macmillan Publishing Co., New York, 1984.
• Normal Tables of Xenopus laevis (Daudin), by P.D. Nieuwkoop and J. Faber, North-Holland Publishing Company, Amsterdam, 1967.
• “Spawning the African Clawed Frog, Xenopus laevis,” by Ivan Schoeneck, Tropical Fish Hobbyist, April, 1985.

Reprinted from the Colorado Aquarist the official publication of the Colorado Aquarium Society, Autumn, 1990.