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.
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
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
*Corresponding author; e-mail: email@example.com
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.,
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,
©Koninklijke Brill NV, Leiden, 2008. Also available online - www.brill.nl/amre
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-
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 (37◦5855N-
13◦0721E) 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
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
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.
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-
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
20◦C, 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
21 =76.6;P<0.001) and very signif-
icant for the percentage of prey biomass (χ2
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).
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
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
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
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
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Received: October 27, 2007. Accepted: March 26, 2008.