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The worldwide spread of invasive species is considered to be one of the main causes of global amphibian declines and the loss of biodiversity in general. The African Clawed Frog, Xenopus laevis, shows a strong ability to establish populations and invade various geographic regions. In 2004 X. laevis was found in Sicily for the first time. The Sicilian population is probably the largest in Europe with a range of about 225 km2 in an area characterized by numerous agricultural ponds. This high density of ponds has potentially facilitated the dispersal of X. laevis. The frogs can move far from rivers or watercourses by utilizing the ponds as suitable "islands". The analysis of their diet shows that the aquatic larvae of nektonic insects comprise the major portion in terms of mass while small planktonic crustaceans are the most numerous component. There is a significant difference between the diet of adults and juveniles.
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Amphibia-Reptilia 29 (2008): 405-412
The large invasive population of Xenopus laevis in Sicily, Italy
Francesco Paolo Faraone, Francesco Lillo, Gabriele Giacalone, Mario Lo Valvo
Abstract. The worldwide spread of invasive species is considered to be one of the main causes of global amphibian declines
and the loss of biodiversity in general. The African Clawed Frog, Xenopus laevis, shows a strong ability to establish
populations and invade various geographic regions. In 2004 X. laevis was found in Sicily for the first time. The Sicilian
population is probably the largest in Europe with a range of about 225 km2in an area characterized by numerous agricultural
ponds. This high density of ponds has potentially facilitated the dispersal of X. laevis. The frogs can move far from rivers or
watercourses by utilizing the ponds as suitable “islands”. The analysis of their diet shows that the aquatic larvae of nektonic
insects comprise the major portion in terms of mass while small planktonic crustaceans are the most numerous component.
There is a significant difference between the diet of adults and juveniles.
Keywords: Amphibians, diet, distribution, invasive species, Sicily, stomach flushing, Xenopus laevis.
Introduction
It is well-known that the introduction of non-
native anurans in natural ecosystems can have
deleterious effects on native species. One of
the most notorious examples is the frog, Rana
catesbeiana, a native of Eastern USA but nat-
uralized across North America (Pearl et al.,
2004), Mexico, the Caribbean, Hawaii, Japan
(Toshiaki, 2004) and Europe, including Italy
(Lanza, 1962; Ficetola et al., 2007). R. cates-
beiana can cause the decline of autochthonous
amphibian populations through competition for
resources or through direct predation (Kats and
Ferrer, 2003). Other invasive species, such as
Bufo marinus, can be a cause of mortality to
predators due to the presence of deadly tox-
ins (Phillips et al., 2004) or non-native anurans
can spread diseases like Batrachochytrium den-
drobatidis as in the case of R. catesbeiana and
Xenopus laevis (Weldon et al., 2004; Garner et
al., 2006).
Because of the high demand for X. laevis as
a laboratory model, it is one of the most wide-
spread amphibians in captivity (Gurdon, 1996).
This use has contributed to the establishment
Dipartimento di Biologia Animale “G. Reverberi”, Uni-
versità degli Studi di Palermo, Via Archirafi 18, I-90123
Palermo, Italy
*Corresponding author; e-mail: mlovalvo@unipa.it
of several allochthonous populations around the
world. These have been particularly successful
in areas with a Mediterranean climate. X. laevis
is a pipid frog native to sub-Saharan Africa. The
species shows many adaptations to aquatic life,
including the retention of the lateral line sys-
tem after metamorphosis, aquatic chemorecep-
tors (Elepfandt, 1996a, 1996b; Elepfandt et al.,
2000) and a body structure particularly adapted
for swimming (Videler and Jorna, 1985). In its
natural habitat X. laevis lives in stagnant or
still waters in ponds or sluggish streams, but
may also inhabit fast-flowing water (Tinsley et
al., 1996). Non-native populations of X. laevis
are known in Arizona and California (Crayon,
2005), Ascension Island (Tinsley and McCoid,
1996), Chile (Lobos and Measey, 2002), France
(Fouquet, 2001) and Wales (Measey and Tins-
ley, 1998). Recently X. laevis has been found
in an area of north-western Sicily (Lillo et al.,
2005).
Since the discovery of the Italian population
of X. laevis, the need for an in-depth investi-
gation on the possible consequences of this in-
troduction has become evident. Although the
ecological relationship between this pipid frog
and the native amphibians is not clear, its bi-
ological features cause marked concern since
X. laevis has biological features very similar to
predatory fish, and predatory fish can, at least,
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406 F.P. Faraone et al.
strongly affect amphibian populations (Hecnar
and M’Closkey, 1997). Therefore the large area
occupied by X. laevis in Sicily and its potential
to disperse further are a cause of great concern.
The aims of this study are: to assess the ef-
fective size of the Sicilian range of distribution
of this species; to investigate potential disper-
sal patterns with regard to landscape character-
istics; and to analyze its diet. This information
is particularly important in order to begin to un-
derstand the impact of X. laevis on the local
amphibian communities and to plan adequate
counter-measures against this biological inva-
sion.
Materials and methods
An investigation area to determine the Sicilian range size
and distribution of X. laevis was identified from the sites
where the presence of this species was already known (Lillo
et al., 2005). This area extends for about 300 km2around
the first site where X. laevis was reported. It is mainly
agricultural land cultivated with vineyards, olive groves and
cornfields. The area is situated in the catchment basins of
the Belice Destro and Jato Rivers where there is a large
reservoir (Poma Lake) and hundreds of agricultural ponds
with surface areas ranging between 100 and 2000 m2.There
are no agricultural channels or temporary flooded fields in
the area and therefore X. laevis does not have any obvious
dispersal corridors except along the rivers.
An agricultural pond near Poma Lake (375855N-
130721E) was selected for the diet study. It is located
at 240 m a.s.l. with maximum dimensions of 20 ×35 m
and a depth of about 3 m. The pond is partially encircled
by a thin cane-brake and the aquatic vegetation is domi-
nated by Characean algae and Potamogeton pusillus. We
detected Rana synklepton hispanica and X. laevis in this
pond whereas in the neighbouring ponds we also detected
Discoglossus pictus,Hyla intermedia,Bufo bufo and Bufo
siculus (see Stöck et al., 2008).
The study area was sampled from March to July 2005
to verify the presence of X. laevis. The search was carried
out by visual observation with the aid of dipnets (McCoid et
al., 1993), X. laevis was considered to be present if adults,
tadpoles or spawn was found. The ponds were located
with 1 : 10 000 maps (Cartografia Tecnica Regionale of
Regione Siciliana), aerial and satellite photos. The ponds
that were sampled, all located in private land, were selected
on the basis of their accessibility, taking care to cover
the widest possible area starting from the areas where the
presence of X. laevis was known. To estimate the size of
the distribution area we used the Minimum Convex Polygon
(MCP) method, given the fact that the investigated area is
homogeneous for physical and biotic characteristics. For
the diet study four bi-weekly samplings were carried out
in the sample pond between 4 March and 21 April 2006.
A plastic dredge measuring 100 ×50 ×100 cm attached
to a short rope was quickly dragged through the pond
to catch X. laevis. This method was preferred to the use
of funnel-traps in order to avoid the influence that these
could have on diet composition. The use of funnel-traps
can induce unnatural feeding behaviour in X. laevis, due
to the concentration of frogs and prey attracted by the bait
used (see Tinsley et al., 1996; Crayon, 2005). Nevertheless
dragging must be performed with caution as this method can
damage underwater vegetation and, possibly, the spawn of
autochthonous amphibians.
The snout-vent length (SVL) of captured specimens was
measured with a digital calliper (to the nearest 0.01 mm). To
avoid bias due to the conformation and functionality of the
pelvis of X. laevis (see Videler and Jorna, 1985) the mea-
surements were taken while keeping the frogs with their
femurs arranged perpendicularly to the sagittal plane and
gently pressing the frogs in a dorsoventral direction. The
adults of X. laevis were sexed through the secondary sex-
ual characteristics: a protruding cloaca in females and the
presence of nuptial pads on the forearms of males. We con-
sidered as juveniles any specimens with a SVL less than
the minimum length observed in males with nuptial pads,
considering that, in general. both sexes reach sexual matu-
rity at the same size (McCoid and Fritts, 1989). The stom-
ach contents were obtained by the stomach-flushing method
(Measey, 1998; Solè et al., 2005) using a 50 ml syringe with
a 2 mm diameter catheter and suitably filtered water from
the pond. The stomach contents were separated and imme-
diately preserved in 90% ethanol and subsequently analyzed
in the laboratory with the aid of a steremicroscope. The prey
mass was estimated as the mean of the humid weight in al-
cohol of a known number of specimens of potential prey
homogeneous for species, weighed using an electronic pre-
cision balance. The weighed invertebrates were captured in
the pond using a hand-net with a mesh of 500 μmfixedtoa
collection jar.
The identified prey was separated into five major types
(see Measey, 1998; Lobos and Measey, 2002): Plankton,
Benthos, Necton, Terrestrial and Xenopus. For the analysis
of the diet we calculated: i) the percentage of X. laevis eaten
items, ii) the percentage of prey occurrence in stomach, iii)
the percentage of prey biomass, iv) the mean of items of
each taxon ingested from all specimens of X. laevis and v)
only from specimens eating a specific prey type.
The comparison between the percentages was carried out
through the χ2test. The comparison between the means was
carried out through the t-test.
Results
A total of 631 ponds were counted in the in-
vestigation area. Of these 96 were inspected
and X. laevis was found to be present in 52
ponds (54.2%), and absent in the remaining 44
Xenopus laevis in Italy 407
Figure 1. The distribution of Xenopus laevis in Sicily. The dotted line represents the species’ range estimated with Minimum
Convex Polygon (MCP).
(45.8%). The present range of distribution falls
in the catchment basins of the Belice Destro
and Jato Rivers with an extension of about 225
km2from the Jato River valley (North bound)
to Camporeale (South bound) (fig. 1). It seems
likely that the species has not (yet) colonized the
ponds to the south of the Belice Destro River.
We found X. laevis in ponds quite far from
flowing water (maximum distance =7.2 km).
The most isolated pond containing X. laevis is
about 480 m away from the nearest neighbour-
ing pond.
The use of the dragging method to capture
X. laevis proved to be very effective. In fact
we caught a maximum of 23 specimens with a
single dragging. Moreover, the use of dragging
in late winter and at the beginning of spring
appears to cause significantly less damage since
the aquatic vegetation is not fully developed.
During the four samplings at the focal pond,
80 specimens were captured, 22 adults (7 males
and 15 females) and 58 juveniles. The sex ratio
results showed a female bias (M:F=1:2.1).
Concerning the diet study, of the 80 speci-
mens captured, 59 (21 adults and 38 juveniles)
provided stomach contents. Tables 1 and 2 show
the list of the identified taxa with the frequen-
cies for both age classes. The percentage of di-
etary items shows that planktonic items are the
most numerically abundant prey.
As well as skin ingested during the shed,
cannibalism has been confirmed by the presence
of X. laevis eggs and larvae in stomach contents
(Tinsley et al., 1996). Eggs and larvae were
first found in stomach contents during the third
sampling (3 April). This was the first time in
408 F.P. Faraone et al.
Tab le 1. Analysis of prey items of 21 adults of Xenopus laevis caught between 4 March and 21 April 2006 at a pond near
Poma Lake (Province of Palermo, Sicily). * mean of items of each taxon ingested from specimens eating a specific prey type.
Prey categories Stage % of % of prey Mean ±SE of Mean ±SE of Range % of total
X. laevis occurrence ingestion* ingested items prey biomass
eaten items in stomachs per stomach (2.421 g)
(n=1849) (n=21) (n=21)
PLANKTON 82.5 76.2 95.3±49.072.6±38.1 1-780 3.896
Crustacea Calanoidea 10.6 23.8 39.2±21.89.3±6.1 2-109 0.211
Cyclopoidea 15.9 38.1 36.8±21.114.0±8.7 2-178 0.317
Cladocera 55.5 71.4 68.5±32.648.9±24.0 1-493 3.138
Insecta Chaoboridae larvae 0.4 28.6 1.3±0.20.4±0.2 1-2 0.231
BENTHOS 7.5 66.7 9.9 ±2.96.6±2.2 1-40 4.065
Insecta Chironomidae larvae 6.8 52.4 11.4±3.16.6±2.2 2-36 3.679
pupae 0.4 38.1 1.0±0.00.4±0.1 – 0.324
Ceratopogonidae larvae 0.3 14.3 2.0±0.60.3±0.2 1-3 0.062
NEKTON 9.5 85.7 9.7±1.78.3±1.6 1-27 89.387
Insecta Ephemeroptera larvae 5.4 66.7 7.1±1.94.8±1.5 1-25 13.900
Zygoptera larvae 2.2 71.4 2.7±0.42.0±0.4 1-6 38.603
Anisoptera larvae 1.2 33.3 3.1±1.11.1±0.5 1-9 36.408
Pleidae (Plea) 0.1 4.8 2.0 0.1±0.1 – 0.008
Leptoceridae larvae 0.4 23.8 1.4±0.20.3±0.1 1-2 0.466
Arachnida Acarina 0.2 14.3 1.0±0.00.1±0.1 – 0.002
TERRESTRIAL 0.3 9.5 2.5 ±1.50.2±0.2 1-4 1.701
Insecta Curculionidae 0.1 4.8 1.0 0.1±0.1 – 0.071
Apidae 0.1 4.8 1.0 0.1±0.1 – 1.498
Thysanoptera 0.1 4.8 1.0 0.1±0.1 – 0.011
Ephemeroptera adults 0.1 9.5 1.0±0.00.1±0.1 – 0.121
XENOPUS 0.3 4.8 5.0 0.2±0.2 – 0.950
Eggs 0.3 4.8 5.0 0.2±0.2 – 0.950
OTHER Xenopus exuviae 33.3 – –
Vegetal remains 23.8
Und. fragments 61.9
Und. arthropods 38.1
which the diurnal water temperature went above
20C, considered to be the optimal temperature
stimulus for spawning (see Tinsley et al., 1996).
The most important prey category in terms of
mass is the nektonic one while zooplankton,
even if the most numerically abundant item,
represents only a small part of the total ingested
mass in the diet of X. laevis.
The comparison between the diet of adults
and juveniles showed a major tendency for the
former to prey on cladocerans and for the lat-
ter to practise cannibalism. The differences be-
tween the results for the two age classes are
highly significant for the percentage of preyed
items (χ2
21 =76.6;P<0.001) and very signif-
icant for the percentage of prey biomass (χ2
21 =
40.05;P<0.01). The t-tests show higher val-
ues for adults both for the number of prey in-
gested (t56 =2.30;P<0.05) and for the mean
mass of ingested prey (t56 =2.96;P<0.01).
Discussion
Although the point of first release for X. lae-
vis in Sicily is not known, it is evident that
this species has already achieved a remarkable
distribution. Currently, the present distribution
in Sicily is the largest known in Europe. The
French population reached an extent of about
100 km2in 2003 (Eggert and Foquet, 2006). X.
laevis was often considered strongly aquatic be-
cause of its distinctive characteristics and it is
Xenopus laevis in Italy 409
Tab le 2. Analysis of prey items of 38 juveniles of Xenopus laevis caught between 4 March and 21 April 2006 at a pond near
Poma Lake (Province of Palermo, Sicily). * mean of items of each taxon ingested from specimens eating a specific prey type.
Prey categories Stage % of % of prey Mean ±SE of Mean ±SE of Range % of total
X. laevis occurrence ingestion* ingested items prey biomass
eaten items in stomachs per stomach (1.768 g)
(N =728) (N =38) (N =38)
PLANKTON 55.4 44.7 23.7±12.110.6±5.7 1-201 1.442
Crustacea Calanoidea 3.0 13.2 4.4±1.90.6±0.3 1-10 0.030
Cyclopoidea 3.8 34.2 2.2±0.70.7±0.3 1-10 0.038
Cladocera 48.5 34.2 27.2±14.09.3±5.1 1-181 1.373
BENTHOS 22.4 73.7 5.8±1.24.3±1.01-25 5.001
Insecta Chironomidae larvae 20.6 68.4 5.8±1.14.3±1.0 1-20 4.637
pupae 0.7 10.5 1.3±0.30.1±0.1 1-2 0.258
Ceratopogonidae larvae 1.1 13.2 1.6±0.40.2±0.1 1-3 0.105
NEKTON 16.3 73.7 4.3±0.73.1±0.6 1-14 75.710
Insecta Ephemeroptera larvae 6.7 47.4 2.7±0.41.3±0.3 1-6 8.734
Zygoptera larvae 6.7 47.4 2.7±0.81.3±0.4 1-14 46.685
Anisoptera larvae 1.8 23.7 1.4±0.40.3±0.1 1-5 19.697
Leptoceridae larvae 1.0 7.9 2.3±0.70.2±0.1 1-3 0.593
Arachnida Acarina 0.1 2.6 1.0 0.0±0.0 – 0.001
TERRESTRIAL 1.2 15.8 1.5±0.30.2±0.1 1-3 9.496
Insecta Curculionidae 0.3 2.6 2.0 0.1±0.1 – 0.181
Staphylinidae 0.1 2.6 1.0 0.0±0.0 – 0.018
Formicidae 0.3 5.3 1.0±0.00.1±0.0 – 0.096
Thysanoptera 0.1 2.6 1.0 0.0±0.0 – 0.015
Ephemeroptera larvae 0.3 5.3 1.0±0.00.0±0.0 – 0.153
Crustacea Isopoda 0.1 2.6 1.0 0.1±0.0 – 9.034
XENOPUS 4.7 23.7 3.8±1.50.9±0.4 1-14 8.351
Eggs 4.4 23.7 3.6±1.40.8±0.4 1-13 7.743
Larvae 0.3 5.3 1.0±0.00.1±0.0 – 0.608
OTHER Xenopus exuviae 13.2 – –
Vegetal remains 50.0
Und. fragments 71.1
Und. arthropods 2.6 ––
considered quite unfit to terrestrial life (Elepfant
et al., 2000).
Therefore the dispersion pattern of this species
was often described as strongly related to wa-
tercourses and ditches both in the native re-
gions (Measey, 2004) and in the Chilean popu-
lations (Lobos and Measey, 2002). Fouquet and
Measey (2006) pointed out that in the French
population dispersal by terrestrial movement
may be prevalent. The French population lives
in an oceanic climate region where abundant
rainfall may cause temporary flooding useful
as dispersal corridors for the frogs (Eggert and
Fouquet, 2006). Our data suggests that overland
migration may also be prevalent in Mediter-
ranean climate regions such as Sicily, where
the land surface is dry or arid for most of
the year and where irrigation channels are ab-
sent. Many ponds colonized by X. laevis are
in fact quite far from rivers. It is probable
that X. laevis could disperse over land during
the short periods of rainfall through a step-by-
step process influenced by the distribution of
ponds. In this “jumping island dispersal” model
(Pielou, 1979), ponds could act as “islands”
with suitable features for the colonization and
the development of reproductive populations.
These populations will be new dispersions for
the species (Pielou, 1979). The greatest distance
between two potentially linked knots within the
frogs overland movement seems to be at least
480 m. This is in fact the distance between the
410 F.P. Faraone et al.
most isolated pond containing X. laevis and the
closest pond to it. In the area colonized by X.
laevis, pond density attains a value of 13.7/km2.
There is a similar pond density in contiguous ar-
eas without X. laevis. Thus it is probable that the
spread of the species is still ongoing.
Despite X. laevis’s sex ratio generally being
unbiased both in natural populations (Tobias et
al., 1998; Du Preez et al., 2005) and in labo-
ratory experiments (Pickford et al., 2003; Levy
et al., 2004; Hayes et al., 2006), Lobos and
Measey (2002) found a female biased sex ratio
in allochthonous populations in Chile (M:F=
1:2.3). These authors considered the possibil-
ity of a sampling bias due to the catch method
(funnel-traps). Nevertheless our different catch
method also confirmed a sex ratio biased to-
wards females. There is much evidence of some
anthropogenic causes of imbalance in the sex-
ratio in X. laevis: water in which Bisphenol
A (Pickford et al., 2003; Levy et al., 2004),
Estradiol (Carr et al., 2003), industrial waste-
water (Bögi et al., 2003) and Atrazine is present
can cause the feminization of X. laevis larvae
(Hayes et al., 2006). The focal pond is located
in an area strongly affected by agricultural ac-
tivities, where it is probable that herbicides and
fertilizers have accumulated in the water. At the
same time it is known that, in amphibians, males
and females adopt different behavioural strate-
gies (Marsh, 2001). These differences in behav-
ioural strategies may result in different proba-
bilities of capturing specimens of one sex rather
than the other. It would be appropriate to evalu-
ate the effective sex ratio in the Sicilian popula-
tions through mark and recapture studies in or-
der to minimize sampling errors. Eventually it
may be necessary to investigate if this sex ratio
bias is due to to anthropogenic or behavioural
causes.
The diet analysis points out a considerable
difference between the mass contribution and
the numeric contribution of the prey types. For
example, small crustaceans are the most numer-
ous prey items ingested by both adults and ju-
veniles but contribute only 2.8% of the total in-
gested mass. Odonates and mayflies are fewer
in numbers but represent, along with some less
common nektonic groups, the most important
mass contribution both for adults (89.4%) and
juveniles (75.7%). It should be noted that the
quite high contribution in mass of terrestrial
prey (5.2%) is strongly influenced by a sin-
gle ingestion of a big isopod. The percentage
occurrence of prey in stomach contents shows
that nektonic and benthic prey are the most fre-
quently ingested, followed by planktonic prey
species. Planktonic prey is frequently ingested
but the quantity of ingested items is quite vari-
able, as suggested by the high standard error
(SE) values. This variability could be the re-
sult of incidental ingestion during feeding influ-
enced by the relative density of the concentra-
tions of small crustaceans present in the water.
As would be predicted by size, the adults in-
gest a significantly greater mass of prey than ju-
veniles. But it is more difficult to explain the
significant difference between the prey types in-
gested by the two age classes. This difference
could possibly be explained by a difference in
prey preferences or by a difference in micro-
habitat use between the two age classes. Our
observations tend to support microhabitat dif-
ferences as an explanatin since, it is easier to
catch juvenile specimens of X. laevis by drag-
ging the dredge or the dip-net along the pond
shore, whereas it is easier to catch adults by
dragging in the middle and in the deepest parts
of the pond. This spatial segregation could re-
sult in the observed prey preferences merely re-
flecting the differential prey availability in these
different microhabitats.
A comparison of the percentages of ingested
prey of X. laevis (fig. 2) in the populations
in Sicily, Wales (Measey, 1998) and the two
populations in Chile (Lobos and Jaksic, 2005)
shows that zooplankton represents the numeri-
cally most abundant prey group only in Sicily
and in Wales. The high availability of chirono-
mid larvae in Wales and the frequent ingestion
of Physa sp. in Chile contribute significantly to
the trophic differences beween these two popu-
Xenopus laevis in Italy 411
Figure 2. Percentage of numeric prey occurrence in four feral populations of Xenopus laevis. See “Discussion” for details.
lations. These results should be considered with
caution because the data obtained by Measey
(1998) are the result of an annual sampling,
whereas the Chilean data (Lobos and Jaksic,
2005) and our data are the results of seasonal
samplings.
Although we observed the tendency towards
cannibalism of other X. laevis, we did not ob-
serve any evidence of predation on the eggs
or larval stages of autochthonous amphibians.
However, we also did not observe any spawn or
tadpoles in the pond during the sampling period.
Future studies should explain if the presence of
X. laevis could affect the syntopic populations
of autochthonous amphibians. During the sam-
pling period we did, in fact, record the presence
of larval and newly-metamorphosed Discoglos-
sus pictus,B. bufo and Hyla intermedia in some
neighbouring ponds, some with and others with-
out X. laevis.
Acknowledgements. The authors are gratefully indebted to
Giovanna Perricone for field support, Cosimo Marcenò for
botanical advice, Gentile F. Ficetola, G. John Measey and
two anonymous reviewers for comments on the manuscript,
John J. Borg and Fabrizio Lillo and L. Raw for the language
revision. This study was financially supported by “MIUR
Ex 60%”.
References
Bögi, C., Schwainger, J., Feeling, H., Mallow, U., Steineck,
C., Sinowats, F., Calbfus, W., Negele, R.D., Lutz, I.,
Kloas, W. (2003): Endocrine effects of environmental
pollution on Xenopus laevis and Rana temporaria.Env.
Res. 93: 195-201.
Carr, J.A., Gentles, A., Smith, E.E., Goleman, W.L.,
Urquidi, L.J., Thuett, K., Kendall, R.J., Jesy, J.P., Gross,
T.S., Solomo, K.L., Van Der Kraak, G. (2003): Re-
sponse of larval Xenopus laevis to Atrazine: assessment
of growth, metamorphosis and gonadal and laryngeal
morphology. Env. Tox. Chem. 22: 396-405.
Crayon, J.J. (2005): Species account: Xenopus laevis.In:
Amphibian Declines: The Conservation Status of United
States Species, Vol. 2, p. 522-525. Lannoo, M.J., Ed.,
Berkeley, Univerity of California Press.
Du Preez, L.H., Solomon, K.R., Carr, J.A., Giesy, J.P.,
Gross, T.S., Kendall, R.J., Smith, E.E., Van Der
Kraak, G.L., Weldon, C. (2005): Population structure
of the African Clawed Frog (Xenopus laevis) in maize-
growing areas with atrazine application versus non-
maize-growing areas in South Africa. Afri. J. Herpetol.
54: 61-68.
Eggert, C., Foquet, A. (2006): A preliminary biotelemetric
study of a feral invasive Xenopus laevis population in
France. Alytes 23: 144-149.
Elepfandt, A. (1996a): Sensory perception and the lateral
line system in the clawed frog, Xenopus. In: The Biology
of Xenopus, p. 97-116. Tinsley, R.C., Kobel, H.R., Eds,
Oxford, Oxford University Press.
Elepfandt, A. (1996b): Underwater acoustics and hearing in
the clawed frog, Xenopus. In: The Biology of Xenopus,
p. 177-191. Tinsley, R.C., Kobel, H.R., Eds, Oxford,
Oxford University Press.
Elepfandt, A., Eistetter, I., Fleig, A., Gunther, E., Hainch,
M., Hepperle, S., Traub, B. (2000): Hearing threshold
and frequency discrimination in the purely acquatic frog
Xenopus laevis (Pipidae): measurement by means of
conditioning. J. Exp. Biol. 203: 3621-3629.
Ficetola, G.F., Coïc, C., Detaint, M., Berroneau, M., Lorv-
elec, O., Miaud, C. (2007): Pattern of distribution of the
American bullfrog Rana catesbeiana in Europe. Biol.
Invasions 9: 767-772.
Fouquet, A. (2001): Des clandestins aquatiques. Zamenis 6:
10-11.
412 F.P. Faraone et al.
Fouquet, A., Measey, J.G. (2006): Plotting the course of an
African clawed frog invasion in Western France. Animal
Biology 56: 95-102.
Garner, T.W.J., Perkins, M.W., Govindarajulu, P., Seglie,
D., Walker, S., Cunningham, A.A., Fisher, M.C. (2006):
The emerging amphibian pathogen Batrachochytrium
dendrobatidis globally infects introduced populations of
the North American bullfrog, Rana catesbeiana. Biol.
Lett. 2: 455-459.
Gurdon, J. (1996): Introductory comments: Xenopus as
a laboratory animal. In: The Biology of Xenopus,p.
3-6. Tinsley, R.C., Kobel, H.R., Eds, Oxford, Oxford
University Press.
Hayes, T.B., Haston, K., Tsui, M., Hoang, A., Haeffele, C.,
Vonk, A. (2002): Feminization of male frog in the wild.
Nature 419: 895-896.
Hayes, T.B., Stuart, A.A., Mendoza, M., Collins, A., Nor-
iega, N., Vonk, A., Johnston, G., Liu, R., Kpodzo, D.
(2006): Characterization of atrazine-induced gonadal
malformations in African clawed frogs (Xenopus lae-
vis) and comparisons with effects of an androgen an-
tagonist (Cyproterone Acetate) and exogenous estro-
gen (17β-Estradiol): support for the demasculiniza-
tion/feminization hypothesis. Env. Health Perspect. 114:
134-141.
Hecnar, S.J., M’Closkey, R.T. (1997): The effects of preda-
tory fish on amphibian species richness and distribution.
Biol. Cons. 79: 123-131.
Kats, L.B., Ferrer, R.P. (2003): Alien predators and amphib-
ians decline: review of two decades of science and the
transition to conservation. Div. Distrib. 9: 99-110.
Lanza, B. (1962): On the introduction of Rana ridibunda
Pallas and Rana catesbeiana Shaw in Italy. Copeia 3:
642-643.
Levy, G., Lutz, I., Kruger, A., Kloas, W. (2004): Bisphenol
A induces feminization in Xenopus laevis tadpoles. Env.
Res. 94: 102-111.
Lillo, F., Marrone, F., Sicilia, A., Castelli, G., Zava,
B. (2005): An invasive population of Xenopus laevis
(Daudin, 1802) in Italy. Herpetozoa 18: 63-64.
Lobos, G., Jaksic, F.M. (2005): The ongoing invasion of the
African clawed frog (Xenopus laevis) in Chile, causes of
concern. Biodiv. Conserv. 14: 429-439.
Lobos, G., Measey, G.J. (2002): Invasive population of
Xenopus laevis (Daudin) in Chile. Herpetol. J. 12: 163-
168.
Marsh, D.M. (2001): Fluctuations in amphibian popula-
tions: a meta-analysis. Biol. Conserv. 101: 327-335.
McCoid, M.J., Fritts, T.H. (1989): Growth and fat body
cycles in feral populations of the African clawed frog,
Xenopus laevis (Pipidae), in California with comments
on reproduction. South-west. Nat. 34: 499-505.
McCoid, M.J., Pregill, G.K., Sullivan, R.M. (1993): Possi-
bledeclineofXenopus populations in Southern Califor-
nia. Herpetol. Rev. 24: 29-30.
Measey, G.J. (1998): Diet of feral Xenopus laevis (Daudin)
in South Wales, U.K. J. Zool. 246: 287-298.
Measey, G.J. (2004): Species account: Xenopus laevis
(Daudin 1802). In: Atlas and Red Data Book of the
Frogs of South Africa, Lesotho and Swaziland, p. 266-
267. Minter, L.R., Burger, M., Harrison, J.A., Bishop,
P.J., Braack, H., Eds, Smithsonian Institution Press.
Measey, G.J., Tisley, R.C. (1998): Feral Xenopus laevis in
south Wales. Herpetological Journal 8(11): 23-27.
Pearl, C.A., Adams, M.J., Bury, R.B., McReary, B. (2004):
Asymmetrical effects of introduced bullfrog (Rana
catesbeiana) on native ranid frogs in Oregon. Copeia 1:
11-20.
Phillips, B.L., Brown, G.P., Shine, R. (2004): Assessing the
potential for an evolutionary response to rapid environ-
mental change: invasive toads and an Australian snake.
Evol. Ecol. Res. 6: 799-811.
Pickford, D.B., Hetheridge, M.J., Caunter, J.E., Tilghman,
H., Hutchinson, T.H. (2003): Assessing chronic toxicity
of bisphenol A to larvae of the African clawed frog
(Xenopus laevis) in a flow-through exposure system.
Chemosphere 53: 223-235.
Pielou, E.C. (1979): Biogeography. New York, John Wiley
and Sons.
Solé, M., Beckmann, O., Pelz, B., Kwet, A., Engels, W.
(2005): Stomach-flushing for diet analysis in anurans: an
improved protocol evaluated in a case study in Araucaria
forests, southern Brazil. Studies on Neotropical Fauna
Env. 40: 23-28.
Stöck, M., Sicilia, A., Belfiore, N.M., Buckley, D., Lo
Brutto, S., Lo Valvo, M., Arculeo, M. (2008): Post-
Messinian evolutionary relationship across the Sicilian
channel: Mitochondrial and nuclear markers link a new
green toad from Sicily to African relatives. BMC Evol.
Biol. 8(in press).
Tinsley, R.C., Loumont, C., Kobel, H.R. (1996): Geograph-
ical distribution and ecology. In: The Biology of Xeno-
pus, p. 35-39. Tinsley, R.C., Kobel, H.R., Eds, Oxford,
Oxford University Press.
Tinsley, R.C., McCoid, M.J. (1996): Feral Population of
Xenopus outside Africa. In: The Biology of Xenopus,p.
81-94. Tinsley, R.C., Kobel, H.R., Eds, Oxford, Oxford
University Press.
Tobias, M.L., Viswanathan, S.S., Kelley, D.B. (1998): Rap-
ping, a female receptive call, initiates male-female duets
in the South African clawed frog. Proc. Nat. Acad. Sci.
USA 95: 1870-1875.
Toshiaki, H. (2004): Diet composition of introduced bull-
frog, Rana catesbeiana, in the Mizorogaike Pond of Ky-
oto, Japan. Ecol. Res. 19: 375-380.
Videler, J.J., Jorna, J.T. (1985): Functions of the sliding
pelvis in Xenopus laevis. Copeia 1985: 251-254.
Weldon, C., du Preez, L.H., Hyatt, A.D., Muller, R., Speare,
R. (2004): Origin of the amphibian Chytrid Fungus.
Emerg. Infect. Dis. 12: 2100-2105.
Received: October 27, 2007. Accepted: March 26, 2008.
... Correlative SDM approaches revealed the Mediterranean region of Portugal as climatically highly suitable for X. laevis (Measey et al. 2012;Ihlow et al. 2016). However, while the populations in Sicily and France are spreading fast (Faraone et al. 2008;Louppe et al. 2017), the expansion of the Portuguese populations was comparatively slow -approximately 30 years after the introduction event, the species was still confined to a 30 km 2 region (Sousa et al. 2018). ...
... Although X. laevis can live, disperse through and successfully reproduce in flowing waters (Lobos and Jaksic 2005;Courant et al. 2017;Moreira et al. 2017), these seem not to be ideal for the species. Lotic habitats have been mostly identified as pathways for dispersal, while breeding is commonly referred to take place in lentic water bodies, like pools or ponds (Fouquet and Measey 2006;Faraone et al. 2008;Measey 2016). In Portugal and probably due to the poor habitat quality of the two streams and/or presence of predatory fish, the number of metamorphs is much lower in lotic than in lentic environments; metamorph size is also smaller, whereby reaching the reproductive size takes longer (Moreira et al. 2017). ...
... As noted by other authors on less hilly landscapes (cf. Faraone et al. 2008), small water bodies can be used as stepping stones during the rainy season to reach suitable habitat. However, we cannot fully exclude other non-accounted factors, such as the intentional release by amphibian keepers. ...
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... The African clawed frog Xenopus laevis tadpoles sampled during the present study are indigenous to southern Africa, although this species has also been introduced and become an alien invasive in various regions across the globe, mainly as a direct result of its use in human pregnancy tests (McCoid and Fritts, 1980;Measey, 1998;Measey and Tinsley, 1998;Lobos and Measey, 2002;Lillo et al., 2008). In warmer climates, this amphibian species grows all ...
... year round and the diet of the adults mainly consists of zooplankton and slow-moving macroinvertebrates (McCoid and Fritts, 1980;Measey, 1998;Lillo et al., 2008), while tadpoles are generally reported as feeding on epilithon and detritus (Altig et al., 2007). The reported dietary preferences of both the tadpoles and adult frogs therefore make X. laevis a potential competitor with insectivorous feeding fish such as M. capensis (Carassou et al., 2017) in those instances where they inhabit the same aquatic system. ...
Thesis
South Africa has more than 200 Temporarily Open/Closed Estuaries (TOCEs) along its approximately 3000 km coastline. In addition, there are approximately 200 micro-estuaries and micro-outlets on the subcontinent, many of which share some of the characteristics with TOCEs but differ in that they are supplied by localised coastal stream catchments with a more limited supply of freshwater and less connectivity to the marine environment. Between 2015 and 2016, this study endeavoured to investigate the physico-chemical properties, microalgal dynamics as well as the seasonal species abundance and composition of aquatic invertebrates and fishes in four micro-estuaries and four micro-outlets along the warm-temperate section of the Eastern Cape coast. The micro-outlets were dominated by oligohaline or freshwater conditions, while the micro-estuaries were mostly mesohaline. Microalgal biomass in these systems was influenced by temperature, nutrient conditions and light availability. Chironomidae and Ephemeroptera dominated benthic macro-invertebrate communities in the micro-outlets, while Amphipoda and Mysida were most dominant in micro-estuaries. Copepoda and Amphipoda dominated the zooplankton assemblages in both system types. Nekton communities in micro-outlets were dominated by juvenile Myxus capensis and tadpoles (mostly Xenopus laevis), while Oreochromis mossambicus, Gilchristella aestuaria, M. capensis and Mugil cephalus were the four most common species in the micro-estuaries. Seasonal variations in physico-chemical parameters and community composition of both the nekton and invertebrates were recorded during this study. Micro-estuaries and micro-outlets exhibited relatively lower diversity and abundances of biota than TOCEs. Furthermore, the micro-estuaries have a low estuarine function when compared to TOCEs, while the micro-outlets have very limited estuarine characteristics and were dominated by freshwater biota. The micro-systems are smaller in size and contain lower proportions of estuary-associated taxa when compared to larger estuarine systems such as TOCEs and permanently open estuaries. The results of this study confirm that there are distinctions between the biota of the two micro-system types, thus making biotic assemblages, together with physico-chemical parameters, a viable tool for differentiating between micro-estuaries and micro-outlets. This study represents the first investigation into the ecological functioning of micro-estuaries and micro-outlets. As a whole, micro-systems serve as nursery areas for freshwater-associated invertebrate taxa. Due to their limited periods of stability, micro-estuaries serve as intermediate nursery areas for 0+ juveniles of estuary-associated marine fish which, after less than a year, then migrate to larger estuarine systems located nearby. The small catchment size and short period of stability in these systems heightens their level of sensitivity to water flow alterations through dams, direct water abstraction, afforestation and watercourse diversion, while their lower buffering capacity makes them more susceptible to pollution and nutrient inputs from intensive agricultural activities and urban expansion.
... For X. laevis, occurrence points were restricted to the native range of the species as defined by Furman et al. (2015), which restricts the species to southern Africa. Additional occurrences were added from the literature for D. melanostictus (Crottini et al., 2014;Moore et al., 2015) and X. laevis (Arao and Kitano, 2006;Faraone et al., 2008;Kobayashi and Hasegawa, 2005). Occurrences were reduced to a binary of presence/putative-absence for georeferenced points placed on a global grid with intervals of 20 km of latitude and longitude using a projected equal area coordinate system (World Behrmann). ...
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... Capture-mark-recapture trials in the native region showed that X. laevis can move 2.36 km Euclidean distance on land, but mostly move between 200 and 400 m per dispersal event . Further, it is assumed that X. laevis uses small waterbodies during the rainy season to reach more distant locations (Faraone et al., 2008;Measey, 2016). ...
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... Currently, they are known from multiple countries on four continents (Measey et al. 2012). Invasions are relatively well studied in Italy (Sicily: Giacalone et al. 2008), Chile (Lobos and Measey 2002;Lobos and Jaksic 2005;Lobos et al. 2013), Portugal (Rebelo et al. 2010;Moreira et al. 2017), UK (Measey and Tinsley 1998;Measey 1998Measey , 2001 and USA (McCoid andFritts 1980a, b, 1989). Adults are principally predators of aquatic macroinvertebrates (Measey 1998;Amaral and Rebelo 2012;Courant et al. 2017cCourant et al. , 2018a, but there is evidence that they adversely affect native amphibian populations in their introduced areas (Lillo et al. 2011;Courant et al. 2018). ...
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This review provides the first assessment of animal species that are native to South Africa and invasive elsewhere in the world. While around a twelfth of all naturalised plants in the world are native to South Africa, there are very few examples of South African native marine, terrestrial, or freshwater animals becoming invasive elsewhere. We provide a narrative of each of the 34 cases that we could find. Three of these species, the Common Waxbill, Estrilda astrild, the Mozambique Tilapia, Oreochromis mossambicus and the African Clawed Frog, Xenopus laevis, were widely traded, and introduced on several continents with invasive populations becoming the subject of substantial research. Most other species are poorly documented in the literature such that it is often not known whether South African populations are the source of invasions. These species demonstrate the same trend in pathways of animals entering South Africa, moving from deliberate to accidental through time. The role of mavericks, individuals whose deliberate actions wilfully facilitate invasions, is highlighted. While South Africa has acted as an important bridgehead for the invasions of forestry pests, crayfish, fish and amphibians on the continent, it is clearly not a major donor of animal invasions, but rather a recipient. This could be due to South African ecosystems being fundamentally more invasible, or South African fauna showing reduced invasiveness, though it is likely that substantial differences in historical pathways also played a crucial role.
... The African clawed frog, Xenopus laevis (Daudin, 1802), is one of the world's most widely distributed amphibians with populations originating from the Cape region in South Africa having become established on four continents (North America, South America, Asia and Europe) due to both accidental escape and voluntary release of laboratory animals [43][44][45][46][47][48][49][50][51]. While the establishment of introduced populations was most successful in areas with a Mediterranean climate, which closely resembles the environmental conditions of the Western Cape region, the persistence of several populations in cooler environments for decades suggests a capacity for long-term adaptation [52]. ...
Thesis
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Because of the current global biodiversity decline, understanding the consequences of each threat on biodiversity is crucial for conservation biology. Invasive species are among the main threats at the global scale, and can locally imply harmful damages on ecosystems. Studying the phenomena driving the effects and potential for expansion of these species appears as a crucial element to assess their long terms impacts. In this study, we focused our efforts on an invasive population of the African clawed frog, Xenopus laevis, in France, to bring insight about the interactions of this population with its environment and to study the changes in resource allocation to the life history traits, related to reproduction, survival and dispersal probabilities, during the range expansion of the population. We studied the diet in the French invasive population and in other invasive and native populations, and found that this species can expand by predating a narrow, as well as a broad, range of prey categories. We also detected an impact of X. laevis on the native amphibian community in France. In the second section of the thesis, we reported a decrease in reproductive investment, and an increased dispersal allocation of resources at the range edge. We finally studied population dynamics and detected a lower survival probability and density at the range core. All these results combined suggest that the potential for long term impacts is important in France for X. laevis as well as in other areas where the species has been, or will be, introduced.
Article
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Predators can play an important role in regulating prey abundance and diversity, determining food web structure and function, and contributing to important ecosystem services, including the regulation of agricultural pests and disease vectors. Thus, the ability to predict predator impact on prey is an important goal in ecology. Often predators of the same species are assumed to be functionally equivalent, despite considerable individual variation in predator traits known to be important for shaping predator-prey interactions, like body size. This assumption may greatly oversimplify our understanding of within species functional diversity and undermine our ability to predict predator effects on prey. Here we examine the degree to which predator-prey interactions are functionally homogenous across a natural range of predator body size. Specifically, we quantify the size-dependence of the functional response of African clawed frogs ( Xenopus laevis ) preying on mosquito larvae ( Culex pipiens ). Three size classes of predators, small (15-30mm snout-vent length), medium (50-60mm) and large (105-120mm), were presented with five densities of prey to determine functional response type and to estimate search efficiency and handling time parameters generated from the models. The results of mesocosm experiments show that functional response of X. laevis changed with size: small predators exhibited a Type II response, while medium and large predators exhibited Type III responses. Both functional response and behavioural data showed an inversely proportional relationship between predator attack rate and predator size. Small and medium predators had highest and lowest handling time respectively. That the functional response changed with the size of predator suggests that predators with overlapping cohorts may have a dynamic impact on prey populations. Therefore, predicting the functional response of a single size-matched predator in an experiment may be a misrepresentation of the predator’s potential impact on a prey population.
Preprint
Full-text available
Predators can play an important role in regulating prey abundance and diversity, determining food web structure and function, and contributing to important ecosystem services, including the regulation of agricultural pests and disease vectors. Thus, the ability to predict predator impact on prey is an important goal in ecology. Often predators of the same species are assumed to be functionally equivalent, despite considerable individual variation in predator traits known to be important for shaping predator-prey interactions, like body size. This assumption may greatly oversimplify our understanding of within species functional diversity and undermine our ability to predict predator effects on prey. Here we examine the degree to which predator-prey interactions are functionally homogenous across a natural range of predator body size. Specifically, we quantify the size-dependence of the functional response of African clawed frogs ( Xenopus laevis ) preying on mosquito larvae ( Culex pipiens ). Three size classes of predators, small (15-30mm snout-vent length), medium (50-60mm) and large (105-120mm), were presented with five densities of prey to determine functional response type and to estimate search efficiency and handling time parameters generated from the models. The results of mesocosm experiments show that functional response of X. laevis changed with size: small predators exhibited a Type II response, while medium and large predators exhibited Type III responses. Both functional response and behavioural data showed an inversely proportional relationship between predator attack rate and predator size. Small and medium predators had highest and lowest handling time respectively. That the functional response changed with the size of predator suggests that predators with overlapping cohorts may have a dynamic impact on prey populations. Therefore, predicting the functional response of a single size-matched predator in an experiment may be a misrepresentation of the predator’s potential impact on a prey population.
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Full-text available
Predators can play an important role in regulating prey abundance and diversity, determining food web structure and function, and contributing to important ecosystem services, including the regulation of agricultural pests and disease vectors. Thus, the ability to predict predator impact on prey is an important goal in ecology. Often, predators of the same species are assumed to be functionally equivalent, despite considerable individual variation in predator traits known to be important for shaping predator–prey interactions, like body size. This assumption may greatly oversimplify our understanding of within-species functional diversity and undermine our ability to predict predator effects on prey. Here, we examine the degree to which predator–prey interactions are functionally homogenous across a natural range of predator body sizes. Specifically, we quantify the size-dependence of the functional response of African clawed frogs (Xenopus laevis) preying on mosquito larvae (Culex pipiens). Three size classes of predators, small (15–30 mm snout-vent length), medium (50–60 mm) and large (105–120 mm), were presented with five densities of prey to determine functional response type and to estimate search efficiency and handling time parameters generated from the models. The results of mesocosm experiments showed that type of functional response of X. laevis changed with size: small predators exhibited a Type II response, while medium and large predators exhibited Type III responses. Functional response data showed an inversely proportional relationship between predator attack rate and predator size. Small and medium predators had highest and lowest handling time, respectively. The change in functional response with the size of predator suggests that predators with overlapping cohorts may have a dynamic impact on prey populations. Therefore, predicting the functional response of a single size-matched predator in an experiment may misrepresent the predator’s potential impact on a prey population.
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Despite its prominence as the • ·standard laboratory amphibian", the ecology of the African clawed frog , Xenopus laevis, has been neglected. Feral populations have been documented in several countries with Mediterranean climates, but established populations are also known from the UK. Long term studies of individually-marked X laevis in South Wales reveal large demographic fluctuations and the ability to migrate overland. Maximum longevity recorded from recapture of marked individuals was 14 years. Diet analysis demonstrates a major reliance on b~nthic invertebrates and zooplankton components of the pond fauna. Skeletochronological studies of growth rings in bone showed that lines of arrested growth are formed annually and allow calculation of age. Data on population age structure indicate successful recruitment is infrequent, with dominant cohorts originating in perhaps only four summers during the past 20 years.
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