Feral Xenopus laevis in South Wales, UK

Accidental and deliberate introductions of X. laevis to alien environments are associated with its use in human pregnancy diagnosis, as a laboratory animal, and in the pet trade in the 1950s and 1960s.  A recent review of feral populations (Tinsley & McCoid, 1996) has shown this species to have colonised a variety of lentic habitats world-wide, but principally in Mediterranean climates similar to that of its native southern Africa.

The most  intensively studied populations are those in California where X. laevis invaded and colonised several river drainages.  California has a Mediterranean climate, and McCoid and Fritts (1995) found that X. laevis grows all year round in optimal conditions, matures in 8 months, and has an extended breeding season. 

The climate of the UK appears to be ill-suited to this southern African species.  However, feral populations of X. laevis in the UK include a number of isolated reports as well as two established populations in the Isle of Wight (now possibly extinct) and South Wales. This is a brief review of a long term study of the Welsh population of X. laevis from 1982 to 1996 and a shorter more intensive study of one site within the area (1994 to 1996).

Population estimates of X. laevis in both habitats in South Wales suggest that numbers have declined sharply from those at the start of the study period.  It is thought that contributory factors include predation, particularly the lack of recruitment into the population due to cannibalism, and  loss of appropriate lentic habitats.  Skeletochronological results show conclusively that LAG formation is annual, enabling accurate demographic analysis of the Pond population.  This confirms the reproductive potential of X. laevis: more than 97 % of the population could be traced to metamorphosis during a single summer (1993).

Documentation of movements of individually-marked animals demonstrates that, although Xenopus is principally adapted to aquatic life, travel between habitats involved overland movement.  Direct distance between capture sites may not accurately reflect the actual distance and obstacles encountered during these journeys.  For the individual found to move 0.2 km in 48 hours, the route would have included an initial distance overland (150 m), crossing the river and then overland through woodland with dense undergrowth, over a metalled road and into a quarry, where the pond was situated.  For the animals marked in the Pond, the most likely route to the habitats in which they were found would be using the river as a corridor.  These ponds were both within 20 m of the river, but were not visible from the water surface, and in both cases travel would have involved movement up steep banks and walls.  Movement may possibly be stimulated by olfactory cues emanating from the pond, and Savage (1961) has suggested that for Rana temporaria the cues may come from phytoplankton.  True navigation, homing without landmarks, is known in amphibians and may involve magnetoreception.

The composition of stomach contents was very similar to that found in previous studies of X. laevis in an impoundment in Transkei (Schoonbee, Prinsloo & Nxiweni, 1992) and from a stream in California (McCoid & Fritts, 1980).  In common with the present study, tubificids found to be abundant in the environment in Transkei were completely absent from stomach contents.  This phenomenon has been observed in fish diets, and is attributed to the unavailability of tubificids which are found to move deeper into the sediment in the presence of predators.

In a recent review of diet in Xenopus, it was considered to be unlikely that the large amount of terrestrial prey previously reported in stomach contents originates solely from invertebrates which have fallen or been swept from overhanging vegetation into the water.  In the present study, items of terrestrial origin make up a quarter of the weight of all ingested prey items.  Many may have been ingested after falling inadvertently into the pond, where they may have become trapped in the surface tension or fallen to the bottom.  However, some may have been caught whilst on land: current laboratory studies have confirmed that X. laevis can prey terrestrially on invertebrates (Measey, 1998).

The record of 2 animals recaptured 14 years after first being marked with a Panjet represents the longest period reported for X. laevis, or any anuran, recaptured from the wild, as well as a record period of Panjet mark retention.  This also represents a record for the longevity of X. laevis, if it is assumed that each animal must have been at least 2 years old on first capture.

Text From;

Measey,G.J. and Tinsley, R.C. (1998) Feral Xenopus laevis in South Wales. Herpetological Journal 8:23-27