Lake Oku Clawed Frog

Xenopus longipes is a small sized frog with a snout-vent length ranging from 32 – 36 mm in females and 28 – 31 mm in males. It is one of only two described frog species that are dodecaploid (the other being X. ruwenzoriensis), resulting in a chromosome number of 108 (Tinsley and Kobel 1996), and is one of the two smallest species in the genus Xenopus (the other being X. itombwensis; Evans et al. 2008). Xenopus longipes has a pear-shaped body with relatively long, thin limbs. The feet are conspicuously large with long and slender toes; this characteristic contributes to its scientific name which means “long foot” in Latin. As in other species of the Xenopus fraseri subgroup, the metatarsal tubercle has a prominent keratinous claw and the first three toes terminate in keratinous claws. It has large, distinctive eyes and the lower eyelid only covers about 1/3 of the eye. The subocular tentacles are relatively short. There are 8 – 10 lateral line plaques around each eye and 15 – 17 extending from the eye to the cloaca. Xenopus longipes has rough skin due to its many small keratinized spinules, which tend to be more densely packed in males than females (Loumont and Kobel 1991). Xenopus longipes was described as having an “emaciated” appearance in its initial scientific description and wild-caught animals since then look similar. However, animals in captivity readily put on weight suggesting that this appearance is not an intrinsic part of their biology and may be due to environmental conditions in Lake Oku (Loumont and Kobel 1991; Blackburn et al. 2010).Xenopus longipes is distinguished from other small 4-clawed Xenopus species (including X. fraseri and X. pygmaeus) by its small size, ploidy (dodecaploid), large feet with elongate toes, and a distinctive orange or yellow coloration on the ventral surface in living specimens. It is further differentiated from other Xenopus species by osteological features including paired nasal bones (Loumont and Kobel 1991).In life, Xenopus longipes has a golden-brown to chocolate-brown dorsum and a ventral side that is orangey or yellow. Both surfaces are covered in small black spots (melanophores) and some have larger, irregularly shaped black spots on the dorsal side. The ventral surface of the thighs and throat is amber (Loumont and Kobel 1991). Coloration of captive animals may differ from those in the wild, especially by having less bright ventral coloration (Blackburn, personal communication).The color of the dorsum can vary highly between specimens. Some lack the irregularly shaped black spots or may only have several on the shoulders and head. Some specimens have so many melanophores on the ventral surface and throat that they appear black with a slightly gray-orange backdrop (Loumont and Kobel 1991).The species authority is: Loumont C., and Kobel H.R. 1991. Xenopus longipes sp. nov., a new polyploid pipid from western Cameroon. Revue Suisse de Zoologie 98(4): 731-738.Based on its distinct genetic and osteological features, Xenopus longipes was initially placed in its own subgroup (Loumont and Kobel 1991). However, recent phylogenetic work places X. longipes as a member of the larger X. fraseri subgroup that contains several other allopolyploid species in Cameroon, including X. amieti and X. boumbaensis (Evans et al. 2004, 2005, 2011).Xenopus longipes has feet that are conspicuously large with long and slender toes; this characteristic contributes to its scientific name which means “long foot” in Latin (Loumont and Kobel 1991; Blackburn et al. 2010).

Xenopus longipes is known only from Lake Oku, a crater lake at ~2219 m on Mount Oku, the second tallest mountain in Cameroon. This and other mountains in the Cameroon Volcanic Line receive high rain fall and have patches of mountain forest remaining at higher elevations. Lake Oku is surrounded by steep slopes and montane forest that is part of the Kilum-Ijum Reserve (Loumont and Kobel 1991). Despite field surveys elsewhere on Mount Oku and other highland areas in both Cameroon and Nigeria, this species remains known only from Lake Oku. Lake Oku is a relatively shallow lake approximately 0.2 km2 (or 0.9 square miles) with an estimated maximum depth of 52 m (Kling 1988), but the way in which this species uses this depth as either tadpoles or adults remains unknown (Blackburn, personal communication).

Hundreds of specimens of Xenopus longipes have been seen in Lake Oku after rain. The dodecaploidization found in this species is thought to have arisen from a combination of hybridization and polyploidization. It is the most abundant species found in Lake Oku. Although its feet are long, they are not very wide and do not give the frog a large surface area to use for swimming; consequently, Xenopus longipes are not very fast swimmers. The evolution of slender feet in this species may be due to its location in an isolated lake with very little predation (Loumont and Kobel 1991; Project Exploration 2006).

Lake Oku is the only known locality for Xenopus longipes. It is listed as Critically Endangered by the IUCN because its extent of occurrence is less than 100 km2 and its area of occupancy is less than 10 km2. Because it is found only at Lake Oku, it is feared that an increase in predation or an introduction of a new species to the lake may lead to a decline (Tinsley and Measey 2004). Beginning in 2006, dead and dying frogs were found at the lake edge, suggesting a significant mortality event (Doherty-Bone et al. 2013); large numbers of dead frogs had not been reported during previous trips to Lake Oku in the 1990s and 2000s. Independent studies of dead frogs collected in 2006 did not find evidence of the chytrid fungus Batrachochytrium dendrobatidis (Doherty-Bone et al. 2008; Blackburn et al. 2010), however recent surveys reveal Bd in these frogs (Blackburn, unpublished data). Similarly, there is no strong evidence for infection by iridoviruses (including ranavirus) that have also been linked with amphibian declines (Doherty-Bone et al. 2013; Blackburn, unpublished data). To date, no single event has been implicated as the main cause of the decline in the Xenopus longipes population (Blackburn et al. 2010; Doherty-Bone et al. 2013).

The Lake Oku clawed frog (Xenopus longipes) is a species of frog in the family Pipidae, endemic to Lake Oku, a small crater lake in northwest Cameroon. It is a small, dark-coloured, fully aquatic frog with a length of 28 to 36 mm (1.1 to 1.4 in), males being slightly smaller than females. The International Union for Conservation of Nature has rated this frog as "critically endangered" on the basis of its small area of occurrence at a single location, and the possibility that introduction of non-native fish into the lake could cause the frog to be wiped out.