35.
Lake Oku Clawed Frog
(Xenopus longipes)
CR
Overview
The African clawed frogs have undergone drastic changes in chromosome number during their evolution, making them some of the most genetically unusual creatures in the world. They exhibit polyploidy, meaning they have more than the two sets of chromosomes found in most other organisms (known as diploid). The Lake Oku clawed frog has 12 sets of chromosomes, so is dodecaploid.

The only known habitat is Lake Oku, a freshwater lake completely surrounded by montane rainforest found 2,219 metres above sea level on Mount Oku, the second highest peak in mainland West Africa. Local people respect this lake as a sacred place and no fish have been introduced. Invasive predators could prove catastrophic to the clawed frog population.
Urgent Conservation Actions
Support the ongoing BirdLife International project in the Mount Oku region; ensure predatory fish are not introduced to Lake Oku.
Distribution
Western Cameroon.
Fact
Uniquely among vertebrates (except for the Uganda clawed frog, Xenopus ruwenzoriensis), this is a dodecaploid species, and it is therefore of considerable conservation interest. It was probably formed by both hybridisation and polyploidisation (chromosome doubling) processes.

The tiny aquatic Lake Oku clawed frog (formally discovered by scientists in 1991) is found only in Lake Oku in the Northwest Province of Cameroon, on Mount Oku (the second highest peak in mainland West Africa). Frogs of the genus Xenopus are usually difficult to catch because they move very fast in the water propelled by their large webbed feet. Xenopus longipes, on the other hand, is very easy to catch, probably because it has been isolated in a remote lake with few predators.

African clawed frogs get their name from the fact that their fingers generally end in claw-like tips. These are not claws in the same sense as mammal or bird claws.

A close relative of the Lake Oku clawed frog called the African clawed frog (Xenopus laevis) is a popular laboratory animal and was used in an early pregnancy test. When injected with human chorionic gonadotrophin (a hormone found in the urine of pregnant women), the African clawed toad spontaneously produced eggs and many hospitals held large stocks of them for this purpose. In fact, many species of frogs and toads would react in a similar way to being injected with the urine of a pregnant woman, but the African clawed frog was used because it was common, cheap and easily maintained in captivity. Thankfully, the technique of using frogs in pregnancy tests has been rendered obsolete by the introduction of sophisticated chemical methods, but the African clawed frog is still widely used in laboratory research.
Associated Blog Posts
14th Jan 09
Here is an update from Thomas Doherty-Bone, who is working in Cameroon on EDGE Amphibian number 34, the Lake Oku clawed frog. Arrival in Cameroon After...  Read

17th Dec 08
Here is the second blog from Thomas Doherty-Bone, who is carrying out research on little know amphibians in Cameroon, including a number of EDGE and highly E...  Read

12th Nov 08
This is the first blog from Thomas Doherty-Bone, who is carryng out conservation research on amphibians unique to Cameroon. Thomas is being supported by a gr...  Read

11th Nov 08
Two years ago, for the first time in its 180-year history, the Zoological Society of London launched the Erasmus Darwin Barlow Conservation Expeditions. ...  Read

Media from ARKive
ARKive image - Savannah clawed frog, dorsal view
ARKive image - Savannah clawed frog on lake bottom
ARKive image - Savannah clawed frog in water
ARKive image - Savannah clawed frogs on lake bottom
ARKive image - Savannah clawed frog habitat
ARKive image - Savannah clawed frog in bag
Evolutionary Distinctiveness
Order: Anura
Family: Pipidae
The family Pipidae is also referred to as the “tongueless frogs” or “clawed frogs”. Pipids are highly aquatic frogs that rarely, if ever, venture out of water. They have several adaptations to aquatic life, including the loss of the tongue (tongues are not generally useful for feeding in water), and the presence of lateral line organs, which are used to detect wave motion in water and are present in most groups of fishes. The group is sometimes also referred to as the “Aglossa”, which means “without a tongue”. Many species in this family possess a bony voice box with two cartilaginous rods that produce a clicking sound, signaling their presence to their neighbours and / or the male’s will to mate.

The pipids are present in a small sub group of the Anura or “frogs and toads” which contains all of the most ancient families. This sub group (also called the Archaeobatrachia) comprises less than 7% of all of the frogs and toads, including some of the most evolutionarily distinct amphibian species. The pipids diverged from all other amphibians over 130 million years ago in the Creataceous period, which is around the same time that the first evidence of flowering plants dates from.

The genus Xenopus (the “African clawed frogs”) diverged from the other pipids just under 50 million years ago, meaning this group is the most recently evolved within the tongueless frogs family. The African clawed frogs have undergone drastic changes in chromosome number during their evolution, making them some of the most genetically unusual creatures in the world. They exhibit a trait known as polyploidy, which is when an organism has more than two sets of chromosomes (the structures found in the nucleus of a cell that are each composed of a very long, continuous strand of DNA, along which all the genes of the organism are found). Ordinarily, an animal will have just two sets of chromosomes, half inherited during reproduction from the mother and half from the father. Polyploidy occurs during the process of genome doubling, where a species’ entire set of chromosomes is replicated and retained within each bodily cell of the organism during reproduction, or hybridization, where two species with different numbers of chromosomes manage to breed. This is very common in plants, with examples of polyploidy occurring in a number of important agricultural crops such as wheat, rye, potatoes, bananas and apples. Some species may even have eight sets of chromosomes, which is termed octaploidy and may be found in strawberries and sugar cane. Polyploidy is less frequent in the animal kingdom, although it is found in “lower” forms of life such as leeches and flatworms. It may also occur in reptiles and amphibians, but is almost unheard of in mammals where any cases of polyploidy will usually result in prenatal death. Humans therefore (like all mammals, with the one known exception of Red Viscacha-Rat (Tympanoctomys barrerae) from Argentina) have just two sets of chromosomes, although polyploidy does occur in mammalian liver cells.

The Lake Oku clawed frog is dodecaploid, which means it has 12 sets of chromosomes. This is a very high number even in plants, and is unique in animals (with the exception of its close relative, the Uganda clawed frog, Xenopus ruwenzoriensis). This extreme level of polyploidy is thought to have resulted from both hybridisation (or inter-breeding) between different species and chromosome doubling. The Lake Oku clawed frog has a total of 108 chromosomes in each of its somatic (body) cells – well over twice the number found in the somatic cells of humans (totaling just 46 chromosomes). There are 17 species of African clawed frogs, currently divided into two subgroups on the basis of their chromosomal make-up: the tropicalis subgroup (where the genome is in sets of ten chromosomes) and the second group composed of four further subgroups, where the genome is in sets of 18 chromosomes. The four subgroups of the second group include the fraseri group; the laevis group; the muelleri group; and the longipes group, of which the Lake Oku clawed frog is the only living representative.
Description
The Lake Oku clawed toad is a small species, with a total length of 28-31 mm in males and 32-36 mm in females. The body is pyriform (or pear-shaped) with long, slender limbs. The feet are particularly well-developed and possess long, thin, webbed toes. The head is short and broad with a rounded snout. The eyes are large and protrude form the top of the head, pointing upwards to allow them to peer out of their watery habitat. They have a small lower eyelid which covers just a third of the eye, barely reaching the pupil. The back is golden-brown in colour and is heavily speckled with melanophores (dark spots) and irregularly shaped large blotches. The stomach is whitish and also covered in melanophores, with the throat and thighs being yellow-orange. The skin is not entirely smooth, but is covered in tiny spinules that are strengthened with keratin (the same substance that composes human hair and nails). These spinules can reach a very high density in males.

Tadpoles of this species are dark in colour, lack keratinised beaks and posses long barbels either side of their mouths, causing them to have a slight resemblance to tiny catfish.
Ecology
The Lake Oku clawed frog is entirely water-dependent and is the main aquatic vertebrate in the lake, filling the ecological niche of a fish. Surprisingly few frogs and toads are found in large bodies of water throughout their life cycle without moving onto the land at times, but the Lake Oku clawed frog is a definite exception. They are quite inept on land, since their bodies are entirely adapted to an aquatic (or water-dwelling) existence.

The species feeds by a method called suction feeding, employing negative pressure to draw water and prey items into a suddenly opened mouth. African clawed frogs do not have tongues at all and rely heavily on their hands to assist them in making a capture. They habitually sit motionless in the water with their arms and fingers outstretched, waiting to detect motions in the water caused by another animal swimming close by. Lake Oku clawed frogs have retained the lateral line system (often found in fish) as a motion detection sensor to aid them in their sensitivity to movements in the water. Adults have been observed coming to the surface of the lake and jutting their heads above the water after the rain. This may be a particularly productive time to hunt for prey as the water becomes stirred up with nutrients flowing into the lake during precipitation.

Tadpoles of this species obtain their food by filtering suspended particles from the water. They filter feed by holding themselves in a characteristic head-down posture in mid-water and taking regular gulps. To improve the efficiency of this method they beat their long filament-like tails continually to create a flow of water and food (a kind of organic soup) towards their mouths. The water is then pumped through the mouth and back towards the gill chamber, but first passes through a filtration mechanism which traps suspended particles. These particles then swirl about until they come into contact with a patch of sticky, mucus secreting cells which catches them and transports them to the entrance of the digestive tract. As a result, particles as small as two micro-metres are consumed almost as a by-product of the respiration process, as the filtered water continues into the gill chambers where oxygen is removed.
Habitat
The only known habitat of the Lake Oku clawed frog is Lake Oku, a permanent, eutrophic (or nutrient-rich) freshwater lake completely surrounded by montane rainforest. This lake is found 2,219 metres above sea level on Mount Oku, situated in the middle of the “Dorsale Camerounaise”, a series of volcanic massifs, 2000-3000m high, that receive abundant rain. The higher parts of these massifs are covered by montane forests isolated by grassland and savanna-type vegetation. Lake Oku, at an altitude of 2,219 m lies close to the summit of Mount Oku (3,011 m). With montane forest covering the slopes down to the shore, this large crater lake appears rather undisturbed and oligotrophic (or nutrient poor). Local people respect this lake as a sacred place and no fish have been introduced. The Lake Oku clawed frog has been found only in this lake and represents the lake’s only Xenopus species. There nearest other Xenopus species is the volcano clawed frog (Xenopus amieti) and can be found at Tadu Sangere (2,000 metres above sea level), 15 km east of Lake Oku, on the eastern border of the Ndop plain.
Distribution
Endemic to Lake Oku at 2,219 metres above sea level on Mount Oku, in western Cameroon in the Northwest Province. Lake Oku lies on the watershed between the Sanaga and the Bénoué river systems. It might occur elsewhere in the Cameroon highlands, but there are few lakes with ecological characteristics similar to Lake Oku.
Population Estimate
Although no exact population estimate exists, it is considered to be abundant in Lake Oku.
Population Trend
Considered to have a stable population trend in the IUCN Red List of Threatened Species.
Status
Listed as Critically Endangered in the IUCN Red List of Threatened Species because its extent of occurrence is less than 100 km sq. and its area of occupancy is less than 10 km sq., all individuals are in a single location and there is a projected decline in the number of mature individuals due to the high likelihood of a fish introduction into Lake Oku.
Threats
The main threat to the Lake Oku clawed frog appears to be the risk of introduction of a predatory fish species into Lake Oku, which could wipe out this restricted-range species. Such a scenario is plausible given the high protein demands of surrounding communities. However Catherine Loumont and Hans Rudolf Kobel (the pair of scientists that originally described the Lake Oku clawed frog in 1991) observed that local people actually shun the lake as a sacred place and will not introduce fish into it for this reason. Whether this continues to be the case remains to be seen.

General habitat alteration or destruction is also an ongoing concern for this species because there are no known populations in other locations, meaning this species is dangerously dependent on just one habitat location.
Conservation Underway
A conservation project has been conducted on Mount Oku for several years by BirdLife International (since 1987). Seven species of endemic amphibian, including Xenopus longipes, have been identified in the montane forests of the Bamenda Highlands. Here, Birdlife projects build on the convergence of the interests of the conservation community with those of the local population. The project works with local communities to put in place systems whereby communities can manage the forests for themselves whilst conserving the biodiversity of a unique and important area.
Projects

This project works towards the conservation of Cameroon's amphibians through scientific research.

Conservation Proposed
It is important to work with existing conservation projects in the area to encourage the conservation of the Lake Oku clawed frog. It is therefore an important first step to support the ongoing BirdLife International project in the Mount Oku region to take the conservation needs of this species into account. A high priority conservation consideration for this species is that predatory fish are not introduced into Lake Oku. It is therefore important to investigate land use option whereby the conservation of the Lake Oku clawed frog and the needs of the local communities are taken into account, which would ideally involve a means of sourcing sustainable protein for local people which does not destroy the delicately balanced ecosystem within Lake Oku.

In addition to conserving native habitat for this species, the IUCN Technical Guidelines for the Management of ex situ Populations, part of the IUCN Red List of Threatened Species, recommend that all Critically Endangered species should have an ex situ population managed to guard against the extinction of the species. An ex situ population is ideally a breeding colony of a species maintained outside of its natural habitat, giving rise to individuals from that species that are sheltered from problems associated with their situation in the wild. This can be located in the specie's range or in a foreign country that has the facilities to support a captive breeding programme for that species. Since the Lake Oku clawed frog is categorised as Critically Endangered, the possibility of a captive breeding programme for this species should be investigated in view of the risk of a catastrophic collapse of the population if a predatory fish species is introduced to the lake.
Associated EDGE Community members

Thomas is researching, among other things, EDGE amphibian number 34, the Lake Oku clawed toad.

Links
Zoo population

The Lake Oku clawed frog is resident at ZSL London Zoo

References
AmphibiaWeb: Information on amphibian biology and conservation [web application]. 2006. Berkeley, California: AmphibiaWeb. Available: amphibiaweb. Accessed: 08 December 2006.

Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. De Sá, A. Channing, M. Wilkinson, S. C. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green, and W. C. Wheeler. 2006. The Amphibian Tree of Life. Bulletin of the American Museum of Natural History 297: 1-370.

Frost, Darrel R. 2006. Amphibian Species of the World: an Online Reference. Version 4 (17 August 2006). Electronic Database accessible at: . American Museum of Natural History, New York, USA.

Gartshore, M.E. 1986. The status of the montane herpetofauna of the Cameroon highlands. In: S.N. Stuart (ed.), Conservation of Cameroon Montane Forests, pp. 204-240. International Council for Bird Preservation, Cambridge, U.K.

Halliday, T. and Adler, C. (eds.). 2002. The new encyclopedia of reptiles and amphibians. Oxford University Press, Oxford.

IUCN, Conservation International and NatureServe. 2006. Global Amphibian Assessment. Global Amphibian Assessment. Accessed on 08 December 2006.

Kobel, H.R., Barundun, B. and Thiebaud, C.H. 1998. Mitochondrial rDNA phylogeny in Xenopus. Herpetological Journal 8: 13-17.

Loumont, C. and Kobel, H.R. 1991. Xenopus longipes sp. nov., a new polyploid pipid from western Cameroon. Rev. Suisse Zool. 98: 731-738.

Mattison, C. 1987. Frogs and Toads of the World. Blandford Press, London.

Obst, F.J., Richter, K. and Jacob, U. 1984. The Completely Illustrated Atlas of Reptiles and Amphibians for the Terrarium. T.F.H. Publication Inc., N.J., U.S.A.

Roelants, K., Gower, D. J., Wilkinson, M., Loader, S. P., Biju, S. D., Guillaume, K., Moiau, L. and Bossuyt, F. 2007. Global patterns of diversification in the history of modern amphibians. Proceedings of the National Academy of Sciences 104: 887-892.

Tinsley, R. & Measey, J. 2004. Xenopus longipes. In: IUCN 2006. 2006 IUCN Red List of Threatened Species. IUCN Red List of Threatened Species. Downloaded on 24 August 2007.

Tinsley, R.C. and Kobel, H.R. (eds) 1996. The Biology of Xenopus. Zoological Society of London, Clarendon Press, London.

if you can provide new information to update this species account or to correct any errors, please email us at info@edgeofexistence.org


Forum comments
  1. wildam
    Member

    Lake Oku Clawed Frog

    http://scienceblogs.com/tetrapodzoology/Xenopus_longipes_Robert_Browne.jpg

    http://www.puk.ac.za/opencms/export/system/galleries/pics/puk-fak-nw-drk-aacrg/Xenopus_gilli.jpg

    Posted 5 years ago #

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