African clawed frog

Description

These frogs are plentiful in ponds and rivers within the south-eastern portion of Sub-Saharan Africa. They are aquatic and are often greenish-grey in color. African clawed frogs are also frequently sold as pets, and sometimes incorrectly misidentified as African dwarf frogs. Albino clawed frogs are common and sold as pets or for laboratories.

They reproduce by fertilizing eggs outside of the female's body (see frog reproduction). Of the seven amplexus modes (positions in which frogs mate), these frogs are found breeding in inguinal amplexus, where the male clasps the female in front of the female's back legs and squeezes until eggs come out. The male then sprays sperm over the eggs to fertilize them.

African clawed frogs are highly adaptable and will lay their eggs whenever conditions allow it. During wet rainy seasons they will travel to other ponds or puddles of water to search for food. During times of drought, the clawed frogs can burrow themselves into the mud, becoming dormant for up to a year.

Xenopus laevis have been known to survive 15 or more years in the wild and 25–30 years in captivity. They shed their skin every season, and eat their own shed skin.

Although lacking a vocal sac, the males make a mating call of alternating long and short trills, by contracting the intrinsic laryngeal muscles. Females also answer vocally, signaling either acceptance (a rapping sound) or rejection (slow ticking) of the male. This frog has smooth slippery skin which is multicolored on its back with blotches of olive gray or brown. The underside is creamy white with a yellow tinge.

Male and female frogs can be easily distinguished through the following differences. Male frogs are small and slim, while females are larger and more rotund. Males have black patches on their hands and arms which aid in grabbing onto females during amplexus. Females have a more pronounced cloaca and have hip-like bulges above their rear legs where their eggs are internally located.

Both males and females have a cloaca, which is a chamber through which digestive and urinary wastes pass and through which the reproductive systems also empty. The cloaca empties by way of the vent which in reptiles and amphibians is a single opening for all three systems.

Behavior

African clawed frogs are fully aquatic and will rarely leave the water except to migrate to new water bodies during droughts or other disturbances. Clawed frogs have powerful legs that help them move quickly both underwater and on land. Feral clawed frogs in South Wales have been found to travel up to 2 km (1.24 mi) between locations. The feet of Xenopus species have three black claws on the last three digits. These claws are used to rip apart food and scratch predators.

Clawed frogs are carnivores and will eat both living and dead prey including fish, tadpoles, crustaceans, annelids, arthropods, and more. Clawed frogs will try to consume anything that is able to fit into their mouths. Being aquatic, clawed frogs use their sense of smell and their lateral line to detect prey rather than eyesight like other frogs. However, clawed frogs can still see using their eyes and will stalk prey or watch predators by sticking their heads out of the water. Clawed frogs will dig through substrate to unearth worms and other food. Their tongue is unable to extend like other frogs, so clawed frogs use their hands to grab food and shovel it into their mouths.

These frogs are particularly cannibalistic; the stomach contents of feral clawed frogs in California have revealed large amounts of the frog's larvae. Clawed frog larvae are filter feeders and collect nutrients from plankton, allowing adult frogs that consume the tadpoles to have access to these nutrients. This allows clawed frogs to survive in areas that have little to no other food sources.

Clawed frogs are nocturnal and most reproductive activity and feeding occurs after dark. Male clawed frogs are very promiscuous and will grab onto other males and even other species of frogs. Male frogs that are grasped will make release calls and attempt to break free.

If not feeding, clawed frogs will just sit motionless on top of the substrate or floating at the top with their heads sticking out.

In the wild

In the wild, Xenopus laevis are native to wetlands, ponds, and lakes across arid/semiarid regions of Sub-Saharan Africa. Xenopus laevis and Xenopus muelleri occur along the western boundary of the Great African Rift. The people of the sub-Saharan are generally very familiar with this frog, and some cultures use it as a source of protein, an aphrodisiac, or as fertility medicine. Two historic outbreaks of priapism have been linked to consumption of frog legs from frogs that ate insects containing cantharidin.

Xenopus laevis in the wild are commonly infected by various parasites, including monogeneans in the urinary bladder.

Use in research

Xenopus embryos and eggs are a popular model system for a wide variety of biological studies, in part because they have the potential to lay eggs throughout the year. This animal is widely used because of its powerful combination of experimental tractability and close evolutionary relationship with humans, at least compared to many model organisms. For a more comprehensive discussion of the use of these frogs in biomedical research, see Xenopus.

Xenopus laevis is also notable for its use in the first widely used method of pregnancy testing. In the 1930s, two South African researchers, Hillel Shapiro and Harry Zwarenstein, students of Lancelot Hogben at Cape Town University, discovered that the urine from pregnant women would induce oocyte production in X. laevis within 8–12 hours of injection. This was used as a simple and reliable test up through to the 1960s. In the late 1940s, Carlos Galli Mainini found in separate studies that male specimens of Xenopus and Bufo could be used to indicate pregnancy Today, commercially available HCG is injected into Xenopus males and females to induce mating behavior and to breed these frogs in captivity at any time of the year.

Xenopus has long been an important tool for in vivo studies in molecular, cell, and developmental biology of vertebrate animals. However, the wide breadth of Xenopus research stems from the additional fact that cell-free extracts made from Xenopus are a premier in vitro system for studies of fundamental aspects of cell and molecular biology. Thus, Xenopus is the only vertebrate model system that allows for high-throughput in vivo analyses of gene function and high-throughput biochemistry. Finally, Xenopus oocytes are a leading system for studies of ion transport and channel physiology.

Although X. laevis does not have the short generation time and genetic simplicity generally desired in genetic model organisms, it is an important model organism in developmental biology, cell biology, toxicology and neurobiology. X. laevis takes 1 to 2 years to reach sexual maturity and, like most of its genus, it is tetraploid. It does have a large and easily manipulated embryo, however. The ease of manipulation in amphibian embryos has given them an important place in historical and modern developmental biology. A related species, Xenopus tropicalis, is now being promoted as a more viable model for genetics.

Roger Wolcott Sperry used X. laevis for his famous experiments describing the development of the visual system. These experiments led to the formulation of the Chemoaffinity hypothesis.

Xenopus oocytes provide an important expression system for molecular biology. By injecting DNA or mRNA into the oocyte or developing embryo, scientists can study the protein products in a controlled system. This allows rapid functional expression of manipulated DNAs (or mRNA). This is particularly useful in electrophysiology, where the ease of recording from the oocyte makes expression of membrane channels attractive. One challenge of oocyte work is eliminating native proteins that might confound results, such as membrane channels native to the oocyte. Translation of proteins can be blocked or splicing of pre-mRNA can be modified by injection of Morpholino antisense oligos into the oocyte (for distribution throughout the embryo) or early embryo (for distribution only into daughter cells of the injected cell).

Extracts from the eggs of X. laevis frogs are also commonly used for biochemical studies of DNA replication and repair, as these extracts fully support DNA replication and other related processes in a cell-free environment which allows easier manipulation.

The first vertebrate ever to be cloned was an African clawed frog in 1962, an experiment for which Sir John Gurdon was awarded the Nobel Prize in Physiology or Medicine in 2012 "for the discovery that mature cells can be reprogrammed to become pluripotent".

Additionally, several African clawed frogs were present on the Space Shuttle Endeavour (which was launched into space on September 12, 1992) so that scientists could test whether reproduction and development could occur normally in zero gravity.

Xenopus laevis is also notable for its use in the first widely used method of pregnancy testing, after Lancelot Hogben discovered that the urine from pregnant women induced X. laevis oocyte production. Human chorionic gonadotropin (HCG) is a hormone found in substantial quantities in the urine of pregnant women. Today, commercially available HCG is injected into Xenopus males and females to induce mating behavior and to breed these frogs in captivity at any time of the year.

Xenopus laevis also serves as an ideal model system for the study of the mechanisms of apoptosis. In fact, iodine and thyroxine stimulate the spectacular apoptosis of the cells of the larval gills, tail and fins in amphibians metamorphosis, and stimulate the evolution of their nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog.

Genome sequencing

Early work on sequencing of the X. laevis genome was started when the Wallingford and Marcotte labs obtained funding from the Texas Institute for Drug and Diagnostic Development (TI3D), in conjunction with projects funded by the National Institutes of Health. The work rapidly expanded to include de novo reconstruction of X. laevis transcripts, in collaboration with groups around the world donating Illumina Hi-Seq RNA sequencing datasets. Genome sequencing by the Rokhsar and Harland groups (UC Berkeley) and by Taira and collaborators (University of Tokyo, Japan) gave a major boost to the project, which, with additional contributions from investigators in the Netherlands, Korea, Canada and Australia, led to publication of the genome sequence and its characterization in 2016.

Online Model Organism Database

Xenbase is the Model Organism Database (MOD) for both Xenopus laevis and Xenopus tropicalis. Xenbase hosts the full details and release information regarding the current Xenopus laevis genome (9.1).

As pets

Xenopus laevis have been kept as pets and research subjects since as early as the 1950s. They are extremely hardy and long lived, having been known to live up to 20 or even 30 years in captivity.

African clawed frogs are frequently mislabeled as African dwarf frogs in pet stores. Identifiable differences are:

• Dwarf frogs have four webbed feet. African clawed frogs have webbed hind feet while their front feet have autonomous digits.
• African dwarf frogs have eyes positioned on the side of their head, while African clawed frogs have eyes on the top of their heads.
• African clawed frogs have curved, flat snouts. The snout of an African dwarf frog is pointed.

As a pest

African clawed frogs are voracious predators and easily adapt to many habitats. For this reason, they can easily become a harmful invasive species. They can travel short distances to other bodies of water, and some have even been documented to survive mild freezes. They have been shown to devastate native populations of frogs and other creatures by eating their young.

In 2003, Xenopus laevis frogs were discovered in a pond at San Francisco's Golden Gate Park. Much debate now exists in the area on how to exterminate these creatures and keep them from spreading. It is unknown if these frogs entered the San Francisco ecosystem through intentional release or escape into the wild. San Francisco officials drained Lily Pond and fenced off the area to prevent the frogs from escaping to other ponds in the hopes they starve to death.

Due to incidents in which these frogs were released and allowed to escape into the wild, African clawed frogs are illegal to own, transport or sell without a permit in the following US states: Arizona, California, Kentucky, Louisiana, New Jersey, North Carolina, Oregon, Vermont, Virginia, Hawaii, Nevada, and Washington state. However, it is legal to own Xenopus laevis in New Brunswick (Canada) and Ohio.

Feral colonies of Xenopus laevis exist in South Wales, United Kingdom. In Yunnan, China there is a population of albino clawed frogs in Lake Kunming, along with another invasive: the American bullfrog. Because this population is albino, it suggests that the clawed frogs originated from the pet trade or a laboratory.

The African clawed frog may be an important vector and the initial source of Batrachochytrium dendrobatidis, a chytrid fungus that has been implicated in the drastic decline in amphibian populations in many parts of the world. Unlike in many other amphibian species (including the closely related western clawed frog) where this chytrid fungus causes the disease Chytridiomycosis, it does not appear to affect the African clawed frog, making it an effective carrier.