Family Pipidae

Xenopus Wagler, 1827

platannas (A), clawed toads

By G.J. Measey

Currently accepted name: Xenopus indet.
Red listing status:

Description

The genus name is derived from Greek: xenos = strange; pous = a foot. This refers to a feature that is unique among frogs, namely the keratinized, clawlike tips of the inner three toes.

Adults are almost entirely aquatic, emerging only when migrating overland to other water bodies or when capturing prey at the water’s edge. They possess several morphological and sensory adaptations to their aquatic habitat. The body is dorsoventrally flattened and streamlined, with laterally projecting limbs. Tympanum, tongue, vocal cords and movable eyelids are lacking. The eyes and nostrils are positioned on top of the head in the same plane, allowing the frogs to suspend themselves with only these structures breaking the surface. The eyes are adapted for vision in air, while receptors in the nasal cavities allow the frogs to sense chemicals in both water and air.

They are powerful swimmers due to their muscular hind limbs and the extensive webbing between their toes. They can even project themselves out of the water to catch insects walking along the edge (Measey 1998a). The skin is smooth, with prominent lateral line organs that resemble rows of stitches along the sides of the body and around the eyes and mouth.

The tadpoles have a distinctive appearance and are sometimes mistaken for fish by the novice. They are translucent with occasional spots of pigment and a pair of long barbels at the sides of the mouth. The tapering tail is constantly in motion, keeping the tadpole suspended in the water column.

Distribution

Native populations are ubiquitous in and endemic to sub-Saharan Africa. Xenopus species occupy every type of freshwater body, seemingly regardless of altitude, pH, and water temperature. Of the 18 currently recognized species, only three occur in the atlas region (Kobel et al. 1996). An endemic species, X. gilli, is restricted to acidic, humic waters in the Cape Peninsula and adjoining areas, while X. laevis and X. muelleri are widely distributed, extending beyond the limits of the atlas region. While these and the other Xenopus species may be identified by means of published keys (e.g., Kobel et al. 1996) current taxonomic studies suggest that additional cryptic species may be present within the atlas region.

The presence of Xenopus is notoriously difficult to assess by traditional techniques because adults spend most of their time in deep, often turbid, water. Calls may be barely audible out of the water as a dull grating, although surfacing activity of both adults and tadpoles is easily spotted when they occur in high densities. The best method of sampling animals is with a submerged funnel trap containing pungent bait (usually meat). The atlas records consist mainly of visual or specimen records of adults or tadpoles. Adults are often conspicuous in torchlight at night, either at the surface or lying on the bottom of water bodies. In wet weather, adults and juveniles are sometimes seen crossing land.

The distribution maps are reliable and overall coverage is good. Gaps in distribution in arid areas such as Bushmanland and the Kalahari may reflect inadequate sampling rather than true absence. On the other hand, isolated records from arid areas may represent artificial introductions by man.

Habitat

Xenopus may be excluded from certain water bodies by predatory fish, but there seem to be many exceptions to this rule, and animals are commonly found in every type of water body, including rivers, streams, dams, flooded pits, ditches, drinking troughs and wells.

Vegetation does not seem to be a necessary requirement, either in or around aquatic habitats, although the presence of aquatic vegetation may increase the variety of food sources encountered. Certain species have been shown to survive without food for long periods, and this may be typical of the genus as a whole (Merkle 1990). Although Xenopus also inhabits streams and rivers, breeding takes place in stagnant or slow-moving water, including temporary pans and ponds. Only the endemic X. gilli has special habitat requirements, although X. muelleri and X. laevis may have different water temperature preferences.

The construction of dams, wells, irrigation channels and watering points for domestic and wild animals has probably allowed Xenopus to colonize arid areas from which it was previously excluded. The ability of these frogs to aestivate for long periods in dried mud, also enables them to survive in arid areas.

Life history

The life histories of indigenous Xenopus species are poorly reported and little understood, and the best studies have been based on invasive populations of X. laevis (Tinsley and McCoid 1996). However, the following generalizations may apply to the genus as a whole.

Xenopus are crepuscular and/or nocturnal in habit, detecting aquatic food items using their lateral line organs (Elepfandt 1996) and sense of smell, or terrestrial food items by sight (Measey 1998a). A wide variety of food sources from all microhabitats in water bodies are utilised, including carrion, which is shredded to size with the claws (Tinsley et al. 1996; Measey 1998b). In the absence of a tongue, the forelimbs assist in the ingestion of food. Adults are notorious cannibals of eggs, tadpoles, recent metamorphs and even small adults.

Laboratory experiments suggest a complex mating behaviour (Kelley 1996; Yager 1996), although most studies have relied heavily on hormonally induced behaviour in artificial environments (e.g., Picker 1980; Tobias et al. 1998). Complex courtship, involving both male and female calls, culminates in a successful inguinal amplexus, after which eggs are deposited singly throughout the water body. Tadpoles hatch after a few days, but remain temporarily attached to the substrate by a buccal mucus thread. Tadpoles are obligatory mid-water suspension feeders, adopting a characteristic “head-down” attitude in the water. They exhibit schooling behaviour and often congregate in shady, still areas of pools (Wassersug 1996).

Conservation

Only one species, X. gilli, is known to be threatened throughout its entire range. There are documented cases of displacement, over a period of years, of certain Xenopus species from their known habitats by other Xenopus species (Tinsley et al. 1996), which indicates that some species in the genus are highly invasive. Also X. laevis, a species commonly used in laboratories, has become a feral animal in a number of countries outside of Africa, and may pose a threat to the indigenous species of those countries.