Family Pipidae

Xenopus gilli Rose and Hewitt, 1927

Cape Platanna, Sago-belly Platanna, Sago-tummy, Gill’s Platanna, Cape Clawed Toad, Kaapse Platanna (A)

By A.L. de Villiers

Currently accepted name: Xenopus gilli Rose & Hewitt, 1927
Red listing status: Endangered (IUCN 2017)

Photo by Grundlingh Felicity, 2012. URL: FrogMAP: 854

RED LIST SPECIES

Status: Endangered (EN) Criteria: B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v)

Identification

X. gilli is a typical member of the genus. The upper body is light to yellow-brown with elongated, dark brown patches, sometimes paired, that begin between the eyes and extend backwards, breaking up into smaller patches on the lower back and upper surfaces of the hind limbs. The underside usually has clear blackish and yellow mottling, but this may be pale and indistinct in some individuals, especially in populations near Kleinmond (3419AC; J.A. Harrison in litt.).

X. gilli can be distinguished from X. laevis by the following morphological features: its smaller size (<60 mm in body length); a narrower, more acutely pointed head; the absence of a subocular tentacle (present but inconspicuous in laevis); a poorly developed inner metatarsal tubercle (a distinct ridge in laevis); and less conspicuous lateral line sense organs (Poynton 1964; Picker and De Villiers 1988; Passmore and Carruthers, 1995; Kobel et al. 1996; Channing 2001).

The advertisement call consists of a series of short, rapidly pulsed, metallic buzzes emitted under water at a rate of about two per second (for further details of call structure see Passmore and Carruthers 1995; Picker et al. 1996; Channing 2001).

Distribution

This species is endemic to the winter rainfall region of the Western Cape, generally occurring in relatively low-lying areas (10–140 m a.s.l.) within 10 km of the coastline. Its distribution is correlated with the presence of nearby mountain ranges and an annual rainfall exceeding 500 mm p.a. Confirmed records span a distance of about 160 km, from the Cape Peninsula southeastward towards the Agulhas district (Picker and De Villiers 1988, 1989).

For about 40 years following its description in 1927, the species was known only from the Cape Peninsula and adjoining Cape Flats (3418AB, BA; 3318CD, DC). However, in the late 1960s it was discovered further south on the Cape Peninsula in the Cape of Good Hope Nature Reserve (3418AD), and in the period from 1973 to 1988 it was collected along the coastal forelands to the southeast of the Cape Peninsula and Cape Flats, at Hangklip and Betty’s Bay (3418BD), Kleinmond (3419AC), midway between Gansbaai and Agulhas (3419DA), and northwest of Agulhas at the eastern base of Soetanysberg (3419DB, DD).

Two inland records from Nieuwoudtville (3119AC) in 1898 and Citrusdal (3219CA) dating from about 1937 (Rau 1978), have not been subsequently confirmed despite several field trips to these areas. The validity of these records is in doubt and they are excluded from the map.

During the 1980s a study of the distribution and habitat requirements of X. gilli was undertaken to assess its future survival prospects. The study showed that, although the species still occurred in at least 24 localities in seven quarter-degree grid cells, habitat destruction and degradation had led to extinction at 60% of its known localities (Picker and De Villiers 1989).

Thus, X. gilli has been recorded from a total of 10 quarter-degree grid cells (excluding the two inland localities), but since 1995 it has been found in only five of these cells (3418AB, AD, BD; 3419AC, DA). The atlas data are reliable.

Habitat

X. gilli inhabits blackwater wetlands in low-lying coastal areas. These are permanent and seasonal seepages, marshes, ponds, pans, vleis and coastal lakelets, in a variety of fynbos vegetation types and, in places, a mixture of fynbos and dune thicket. The vegetation types include mostly Mountain Fynbos, Sand Plain Fynbos (on the Cape Flats), or Mountain Fynbos mixed with either Limestone Fynbos or Dune Thicket. The substrate has a predominantly sandy base and varies, depending on the humic content, from white or grey to a dark brown or blackish soil.

The water is humic and dark in colour, low in nutrients, high in dissolved solids, and typically has a low pH (minimum 3.4; Picker 1985). It has been demonstrated that the tadpoles of X. gilli can tolerate pH as low as 3.6, whereas X. laevis tadpoles have a reduced rate of survival below pH 5–6. This accounts for the observed habitat segregation between these species on the Cape Peninsula where X. gilli occurs in acidic blackwater seepages and ponds, while X. laevis prefers clear water bodies with elevated pH, for example, artificial impoundments (Picker et al. 1996). Disturbances of X. gilli water bodies which alter the humic content and nutrient levels, cause an increase in pH levels and often result in the colonisation of these water bodies by X. laevis, providing an opportunity for hybridization between the species (Simmonds 1985; Picker et al. 1996).

While X. gilli avoids habitats that have been disturbed by urban development or agriculture, or that contain invasive plants and animals (Picker 1985; Picker and De Villiers 1989), it is interesting to note that healthy populations of this frog inhabit certain seepages that were excavated to form waterholes in the Cape of Good Hope area of the Cape Peninsula National Park (CPNP). However, no other habitat disturbances or threats are evident in these pools, the water quality meets the necessary requirements, and consequently X. gilli populations have flourished in them for well over two decades.

Life history

When its wetland habitat dries up during the summer months, X. gilli survives by aestivating below the surface. Rau (1978) found several specimens encapsulated in the mud of dried-up vleis on the Cape Flats, and Rose (1962: 33) found one individual at a depth of 15 cm amongst the roots of a large “weed” growing on the site of a small dried-up vlei. During the rainy season, overland migrations between ponds have been observed in Cape Peninsula National Park (Picker 1985).

Breeding commences during the wet winter months (July), and continues until late October (Rau 1978). Three to four hundred dark brown eggs are laid over a period of a day, each surrounded by a jelly capsule 1.3 mm in diameter (Channing 2001). The nektonic tadpoles feed on phytoplankton in the water and complete their metamorphosis by the end of summer. Rau (1978) recorded spawning activity over a four-month period and found metamorphosis to take about 120 days. The breeding season of X. gilli overlaps that of X. laevis, which increases the opportunity for hybridization (Rau 1978; Picker et al. 1996).

Adult frogs feed on living and dead animal material in their wetland habitat, including aquatic invertebrates and the eggs, tadpoles and smaller frogs of their own kind and other species (Picker and De Villiers 1988). Predation of the immature stages of X. gilli by the larger X. laevis is presumably intense in disturbed habitats that have been invaded by the latter. Other predators include herons, cormorants and water mongoose.

Conservation

Status

Development and general habitat degradation have severely impacted on the extent of occurrence and area of occupancy of this species, resulting in a loss of more than 50% of its habitat, and severe fragmentation of its populations (Harrison et al. 2001). This is particularly serious on the Cape Flats and adjoining Cape Peninsula where extensive urban development has taken place. By now, X. gilli is possibly extinct in both of these areas except for populations in the Cape Peninsula National Park. Similar threats are escalating between Rooiels and Kleinmond on the south coast, leading to further habitat loss and fragmentation.

About 70% of all currently known X. gilli habitat is situated in the Cape Peninsula National Park. This is the stronghold of the species with healthy populations in the Cape of Good Hope area. The populations at some sites in this area have been found to vary from 121–591 frogs (Picker and De Villiers 1989; Picker et al. 1996). It is nevertheless of concern that X. laevis and/or hybrids have been reported from most of the wetlands in this area, and the situation needs to be monitored. It is also of concern that the only X. gilli habitat protected within a conservation area, besides Cape Peninsula National Park and Greater Betty’s Bay Nature Reserve, is a small remnant of habitat in Agulhas National Park.

X. gilli was included in the first South African Red Data book for amphibians, in the Rare category (McLachlan 1978). In the revision (Branch 1988), it was classified Endangered. Endangered status was retained in Harrison et al. (2001), based on an extent of occurrence <5000 km², an area of occupancy <500 km², a severely fragmented habitat, continuing decline in the extent of occurrence, area of occupancy, extent and quality of habitat and the number of locations/subpopulations and mature individuals. The species is legally protected by Nature Conservation Ordinance 19 of 1974, but is not listed by CITES.

Threats

Loss of habitat and habitat fragmentation pose the most serious threats to the survival of X. gilli. In particular, urban development and agriculture have resulted in the filling in and drainage of its wetland habitat or have led to the pollution and eutrophication of breeding sites. Furthermore, the building of artificial reservoirs and irrigation systems has enabled the highly adaptable X. laevis to invade areas from which it was previously excluded, including disturbed blackwater wetlands containing populations of X. gilli. Predation by X. laevis on the eggs, tadpoles and froglets of X. gilli represents a further threat to the survival of X. gilli.

Urban expansion and human activities also accelerate the spread of invasive alien vegetation. At some localities, indigenous fynbos vegetation has been replaced by stands of exotic trees (Port Jackson Willow Acacia saligna and Rooikrans A. cyclops). This alters the water chemistry and results in unsuitable habitat for X. gilli. Introduced predatory fish may pose an additional threat, particularly in some of the larger, permanent wetlands.

The invasion of disturbed X. gilli habitat by X. laevis, and the subsequent hybridization of these two species, has been well documented (Rau 1978; Picker 1985; Simmonds 1985; Picker and De Villiers 1989; Picker et al. 1996; Evans et al. 1998). Hybridization threatens the gene pool of the smaller, less numerous X. gilli, through potential genetic swamping of populations. This threat is present throughout the distribution area of X. gilli (Picker et al. 1996) but appears not to be as serious as was previously thought (Evans et al. 1998).

Recommended conservation actions

The distribution and conservation status of X. gilli is monitored by the Western Cape Nature Conservation Board (WCNCB) as part of a threatened species monitoring programme (De Villiers 1997). Healthy populations in the Cape of Good Hope area were monitored mainly by the Zoology Department of the University of Cape Town, but monitoring ceased in 2000. It will be continued by the CPNP in conjunction with the WCNCB. Besides the monitoring of known localities, additional survey work is to be conducted in surrounding areas, including montane habitats.

Although X. gilli populations on the Cape Peninsula are well protected, it is important that other viable populations be included in statutory conservation areas and managed appropriately. This was strongly recommended by Evans et al. (1997), who found that populations to the east of False Bay showed significant genetic differences from the Cape Peninsula populations, although these were not considered to be taxonomically significant. In particular, they indicated that “protective measures within X. gilli habitat near Kleinmond would conserve much of the genetic diversity seen in this species”.

The main management activity is control of alien vegetation. This is undertaken on the Cape Peninsula by CPNP, in the Kleinmond area by the WCNCB, and in the Betty’s Bay area by the Overstrand Municipality. Alien vegetation clearing programmes are now underway in the Gansbaai to Agulhas area, but they need to be intensified in X. gilli habitat.

In 1985, a Cape Platanna Conservation Committee built a precast wall around Geps Dam, one of the pools in the Cape of Good Hope area, to protect its X. gilli population from contamination with X. laevis (Picker and De Villiers 1989). After the wall was built, X. laevis and hybrids were removed from the water body. Holes were regularly found under the wall, but no X. laevis or hybrids were observed in the water body when it was last examined in 2000. In fact, X. laevis and hybrids are apparently on the decline in at least this section of the Cape of Good Hope area (M.D. Picker pers. comm.). Although the situation requires continued monitoring, it seems that the wall now serves little purpose and might safely be removed.

In 1988, the Cape Platanna Conservation Committee translocated 154 juvenile X. gilli from the Cape of Good Hope area to four blackwater pools in the Silvermine nature area, both areas falling within the present CPNP. Although a survey produced no sign of X. gilli in the Silvermine nature area shortly before the translocation, about 16 specimens were collected in 1926 from somewhere in and next to the “Sylvermyn River” which drains this area (Rose and Hewitt 1927). The main reason for the translocation was to establish a separate breeding colony of X. gilli on the Cape Peninsula that would perhaps be free of the X. laevis threat. It would appear that this experiment has had some success: from one to six adults have been seen in one of the pools on about four occasions during the 10-year period following the translocation. Further recommendations are being formulated in this regard, and monitoring work is to continue.

Current distribution map

Undated records;  pre-1996;  1996 to 2002;  2003 to present