Human movements in the regions surrounding the Mediterranean Sea have caused a great impact in the composition of terrestrial fauna due to the introductions of several allochthonous species, intentionally or not. Reptiles are one of the groups where this anthropic impact is most evident, owing to the extensive intra-Mediterranean dispersals of recent chronologies. Chalcides ocellatus is a widespread skink with a natural distribution that covers almost the entire Mediterranean Basin. Two hypotheses have been proposed to explain its origin: natural dispersions and human translocations. Previous molecular data suggest the occurrence of a recent dispersal phenomenon across the Mediterranean Sea. In this study we present the first record of this species in the Iberian Peninsula, in Serra del Molar (South-east Spain). We combined molecular analyses and archaeological records to study the origin of this population. The molecular results indicate that the population is phylogenetically closely related to specimens from north-eastern Egypt and southern Red Sea. We suggest that the species arrived at the Iberian Peninsula most likely through human-mediated dispersal by using the trade routes. Between the Iron to Middle Ages, even now, the region surrounding Serra del Molar has been the destination of human groups and commercial goods of Egyptian origins, in which Chalcides ocellatus could have arrived as stowaways. The regional geomorphological evolution would have restricted its expansion out of Serra del Molar. These findings provide new data about the impact of human movements on faunal introductions and present new information relating to mechanisms of long-distance translocations.
The Mediterranean Basin has been and still is a fascinating biogeographical framework presenting constant faunal exchanges, which have affected many of the faunas present on every shore of the Mediterranean and which have been augmented by human intervention (Pooley and Queiroz, 2018). The distribution and inferred phylogeographic patterns of reptiles such as the circum-Mediterranean geckos Tarentola mauritanica and Hemidactylus turcicus have suggested the existence of possible human interventions in their intra-Mediterranean dispersals (Harris et al., 2004; Carranza and Arnold, 2006; Rato et al., 2011, 2016; Stöck et al., 2016). This phenomenon has also affected the Iberian Peninsula, which shares many species, both reptiles and amphibians, with North Africa (Pleguezuelos et al., 2008). Human translocations with North African origin are proposed for the Iberian populations of the chameleon Chamaeleo chamaeleon (Paulo et al., 2002) and the treefrog Hyla meridionalis (Recuero et al., 2007). Other dispersals, on the other hand, have their origins in the Iberian Peninsula, for example, the translocation of the lizard Podarcis vaucheri in Greece (Spilani et al., 2018). In the Western Mediterranean context, the human-mediated introductions of herpetofauna in the Balearic Islands stand out, where the successive arrivals of different human groups led to the extirpation of native species in most of the main islands (Alytes muletensis and Podarcis lilfordi), besides the introduction of new species from other Mediterranean regions (e.g. Bufotes balearicus, Emys orbicularis, Testudo hermanni, Podarcis sicula, and Hemorrhois hippocrepis), from the Neolithic-Bronze Ages to the present (Pinya and Carretero, 2011; Valenzuela et al., 2016; Silva-Rocha et al., 2018).
Another reptile species with a practically circum-Mediterranean distribution is the ocellated skink, Chalcides ocellatus, which can be considered a species complex, with several deep lineages across North Africa (Carranza et al., 2008). This skink is widely distributed in the southern, central and eastern regions of the Mediterranean basin (fig. 1), from the eastern Moroccan coast to Anatolia, as well as on various islands in the Aegean Sea, Euboea, Crete, Cyprus, Tunisian Tabarka, Malta, Sicily, Conigli, Lampedusa, Lampione, Linosa and Sardinia, with continental European populations in and into the Attica peninsula (Kornilios et al., 2010). The species’ distribution range also extends to the Near East, Mesopotamia, and the shores of the Red Sea, as far south as Somalia and Yemen, and in the Persian Sea region as far east as Pakistan (Anderson, 1999; Lavin and Papenfuss, 2012). There are currently introduced populations in Naples, Stromboli, Kasos and Sri Lanka (Caputo et al., 1997; Karunarathna et al., 2009; Lo Cascio and Grita, 2016; Kornilios and Thanou, 2016), as well as in Florida and Arizona (Krysko et al., 2011; Gunn et al., 2012). Three Algerian individuals were released in Marseille (France) and created a new population during the first decades of the 20th century, and an isolated individual was found in the railway goods yards of Cardiff (Wales) in 1944 (Siépi, 1913; Fitter, 1959; Kraus, 2009).
The large transcontinental distribution of C. ocellatus, together with the low molecular divergence and the hard polytomies observed among many of the populations assigned to the eastern subclades, have made it possible to postulate the human-mediated introduction in recent times as a possible mechanism for its expansion (Kornilios et al., 2010; Lavin and Papenfuss, 2012), and even link it, at least in the context of the Mediterranean Sea, to the trade during the Ancient Age (Kornilios et al., 2010).
In this study, we report for the first time the presence of a reproductive population of the ocellated skink, C. ocellatus, in Serra del Molar (fig. 2). The geographic location of the population is a small coastal mountainous area in the southeast of the Iberian Peninsula, localised between the Mediterranean Sea and marsh areas originated by the Vinalopó and Segura rivers, in the southern Valencian Country. These individuals were morphologically identified as C. ocellatus (body elongate and cylindrical, tail almost equal to body length, ear without lobules, tympanum exposed, multiple dorsal ocelli; Baha El Din, 2006). Given that this is a species without current herpetological or archaeo-paleontological records in the Iberian Peninsula, molecular analyses were performed to confirm its identification and to determine the biogeographical origin of this newly discovered population.
Material and methods
Individuals of C. ocellatus were identified for the first time in April 2017 in Serra del Molar (38.1439N, −0.6569W, WGS; fig. 1). This area is part of the Elx municipality, in the Baix Vinalopó comarca within the Valencian province of Alacant/Alicante, in the grid UTM 30SYH02. The area where the discovery took place is between 35 and 75 meters above sea level, presents a Mediterranean thermos-type and is biogeographically located in the Alicantino subsector of the Murciano-Almeriense sector (Rivas-Martínez, 1987). It is a predominantly sedimentary area, formed at its base by calcareous sandstones of coastal marine origin on which outcrop conglomerates of silt, marl and fluvio-lacustrine sands (Almela et al., 1978). The area presents the typical thermo-Mediterranean scrub communities (Brachypodium retusum, Macrochloa tenacissima, Rosmarinus officinalis, Chamaerops humilis) with concentrations of Pinus halepensis, and only the lower parts show evidence of abandoned plots for cultivation of olives (Olea europaea) and carobs (Ceratonia siliqua), although today most of them are covered by xerophilous and heliophilous scrubs. The archaeological remains of the Iberian settlements of La Escuera and El Oral, and the necropolis of El Molar are located in the surroundings (Grau and Moratalla, 2001). According to data from official herpetological agencies (AHE, 2018; BDB-GVA, 2018), the reptiles of this area had not been surveyed previously by any herpetologist or scientific group.
In collaboration with the Valencian environmental authorities, successive surveys were carried out in this locality to monitor their presence in the area, to obtain data on their spatial distribution and morphology, and to obtain tissue samples for genetic analyses. Twelve identified individuals were captured to obtain morphometric data and subsequently released (table 1). Each individual has been sexed by pressing gently around the cloacal region to find the hemipenises, if males. Importantly, this process was conducted avoiding the total eversion of hemipenises to avoid potential injuries for the animals.
Genetic sampling, DNA extraction and amplification
In order to perform molecular analyses, tissue samples were taken from five individuals with the following sample codes: CN12564 and CN12645 were collected during October 2017, and CN13391, CN13434 and CN13435 were collected in May 2018. To understand the geographic origin of these newly discovered Valencian individuals, sequences of other C. ocellatus specimens from distinct localities around the Mediterranean Basin and across the species’ distribution range, were retrieved from GenBank. Two specimens of Chalcides montanus have been included as outgroup (Carranza et al., 2008; Kornilios et al., 2010) (see supplementary table S1 and supplementary fig. S1).
Genomic DNA was extracted from alcohol-preserved tissue samples using the SpeedTools Tissue DNA Extraction kit (Biotools, Madrid). A fragment of 303 bp of the mitochondrial gene Cytochrome b (cytb) was amplified by the Polymerase Chain Reaction (PCR). The following primers for amplification and sequencing were modified from Kocher et al. (1989): Cytb1 (5′-CCATCCAACATCTCAGCATGATGAAA-3′) and Cytb2 (5′-CCCTCAGAATGATATTTGTCCTCA-3′). We performed PCR in a volume of 25 μl with an initial denaturation step of 94°C for 5 min, followed by 35 cycles of denaturation at 94°C for 80 s, annealing at 50°C for 45 s, and extension at 72°C for 1 min; final extension step was set for 72°C for 5 min. Amplicons were visualized on a 1% agarose gel stained with SYBR Safe DNA gel stain (Invitrogen Corp., Carlsbad, CA, USA). Purification and bi-directional sequencing were carried out by Macrogen (Macrogen Inc.). Chromatographs were checked and the forward and reverse sequence contigs for each sample were assembled and edited using Geneious v.7.1.9 (Biomatter Ltd.). Sequences were aligned using MAFFT v.7.3 (Katoh and Standley, 2013) with default parameters. We translated the final alignment into amino acids and no stop codons were detected.
Phylogenetic analyses were performed under maximum likelihood (ML) and Bayesian inference (BI) frameworks. The ML analysis was conducted in RAxML v.8.1.2 as implemented in raxmlGUI v.1.5 (Silvestro and Michalak, 2012). The analysis was performed with the GTR+G model of sequence evolution and 100 random addition replicates. Nodal support was assessed with 1000 bootstrap replicates. The BI analysis was conducted with BEAST v.1.8.4 (Drummond et al., 2012). We used jModelTest v.2.1.7 (Guindon and Gascuel, 2003; Darriba et al., 2012) to select the best model of nucleotide substitution under the Bayesian information criterion (BIC). We carried out the BEAST analysis with the following priors (otherwise by default): TrN+G model; Coalescent: Constant size tree model; random starting tree; alpha prior uniform (0–10); uncorrelated relaxed clock (uniform distribution; 0–1). Three individual runs of 2 × 106 generations were carried out, with sampling at intervals of 2 × 103 generations. Convergence, posterior trace plots, effective sample sizes (>200), and burn-in were evaluated with Tracer v.1.6 (Rambaut et al., 2014). The tree runs were combined in LogCombiner discarding the first 10% of the trees as burn-in and the ultrametric tree was generated with TreeAnnotator (both available in the BEAST package). Phylogenetic trees were visualized with FigTree v.1.4.3 (Rambaut and Drummond, 2010).
Between April 2017 and May 2018, 35 individuals of C. ocellatus were identified, from which morphometric measurements of 12 distinct individuals could be obtained, all were adults (snout-vent length, SVL > 55 mm, Çiçek et al., 2013) (table 1). The different attested stages of ontogenetic development have allowed verifying the existence of a breeding population, with young and adult individuals, besides the presence of gravid females (fig. 2c). The population is distributed at least in a spatial range of 5.2 km2.
The preliminary taxonomic assignment of Serra del Molar individuals into C. ocellatus has been confirmed, following the criteria of Baha El Din (2006) and Carranza et al. (2008): primitive corporal form within the Chalcides genus, elongate and cylindrical; black and white dorsal ocelli; atrial overture markedly visible, with exposed tympanum; biometric tail/body ratio (tail length divided by distance from snout tip to cloaca) very close to value 1 (average: 1.05, range: 1.14–0.96; table 1); pentadactyl front limbs with phalangeal formula 188.8.131.52.3; and pentadactyl hind limbs with phalangeal formula 184.108.40.206.3. The animals present the typical body-form and colouration pattern of the subspecies C.ocellatus ocellatus (fig. 2).
The dataset of the cytb gene used in the phylogenetic analyses included 153 sequences of C. ocellatus and had a total length of 303 bp: five newly discovered individuals from Spain, 146 sequences from across the Mediterranean Sea and adjacent regions, and two specimens of C. montanus (see supplementary table S1 and fig. S1). The ML and BI phylogenetic trees present a structure of three clades within C. ocellatus (fig. 3, supplementary fig. S1) separated into the eastern, central and western coastal areas of the Mediterranean Sea. Within these clades, a geographic grouping of specimens is apparent, although with an unsupported topology. In both the ML and BI analyses, Serra del Molar individuals are nested within a clade with specimens from the eastern Mediterranean region.
Four of the five Spanish specimens (CN12645, CN13391, CN13434 and CN13435) cluster together with two Egyptian individuals with high support (bootstrap and posterior probability values, 96 and 1, respectively). These Egyptian specimens (co50-FJ980237 and co51-FJ980238; Kornilios et al., 2010) were collected from Ras El Barr, in the Damietta Branch of the eastern Nile Delta. The sequences of these six specimens (Serra del Molar-Spain and Ras El Barr-Egypt) are almost identical, apart from one single mutation in position 159 where the Spanish specimens have an A, whereas the Egyptian specimens have a G. The fifth Spanish individual (CN12564) has a different phylogenetic position, clustering with samples from Egypt (including four Egyptian specimens that were collected from Ras El Barr, co48-FJ980235, co49-FJ980236, co52-FJ980239 and co53-FJ980240), Somalia, Libya, Yemen, Turkey, Syria, Greece, and Cyprus, although with no support. This specimen’s sequence is different from the other Spanish individuals in seven positions (in sites: 60, G vs. A; 96, A vs. C; 105, T vs. C; 129, C vs. G; 144, T vs. C; 204, G vs. A; 285, C vs. T, respectively).
Two hypotheses have been proposed to explain the origin of the Mediterranean populations of C. ocellatus: natural dispersal (maritime, continental or through temporary terrestrial bridges) and human translocations (Lavin and Papenfuss, 2012). The molecular results rule out any scenario of natural colonization of C. ocellatus in Serra del Molar: the phylogenetic tree assigned Serra del Molar individuals within subclade A2, distributed across the eastern Mediterranean basin (Kornilios et al., 2010) and exclude their arrival by “rafting” or other natural dispersal by sea from the Tunisian, Algerian or Moroccan coasts, which are the nearest natural populations of ocellated skinks (Martín et al., 2017; Beddek et al., 2018). Additionally, the populations of C. ocellatus from these Maghrebian countries belong to other phylogenetic clades (Carranza et al., 2008; Kornilios et al., 2010; fig. 3, supplementary fig. S1). Moreover, the extremely low genetic divergence found in the molecular data and the phylogenetic position of the Serra del Molar individuals allow to rule out their spread by land through a continental bridge within a very old paleogeographic scenario (for example, during the Messinian Salinity Crisis). Therefore, the genetic assignation in the eastern Mediterranean subclade implies that the only reasonable and plausible scenario is the human-mediated translocation.
The presence of a genetic admixture among the ocellated skinks from Serra del Molar (supplementary table S1 and supplementary fig. S1), which has been identified in other human-mediated lizard introductions (Kolbe et al., 2007; Santos et al., 2019), suggests the concurrence of at least two translocation events with distinct sources that originated the new population, or a single introduction with individuals from multiples origins, or that the admixture was already present at the original population or locality, as happens in Ras El Barr.
The dispersal mechanism linked to human activities has been one of the explanations given to the wide distribution of C. ocellatus (Schneider, 1981; Anderson, 1999) and is consistent with the phylogenetic studies (Carranza et al., 2008, Kornilios et al., 2010; Lavin and Papenfuss, 2012). In fact, there is a historical record of its introduction into Naples in a shipment of orange trees from Sicily during the 18th century AD (Caputo et al., 1997). Other authors propose different translocation methods, like the use of sand ballasts and their subsequent abandonment in port areas of the Persian Sea (Anderson, 1999). The widespread distribution of the subclade A2 in Kornilios et al. (2010) has been associated with maritime exportation routes of the Silphium plant during the Ancient Age (7th-2nd centuries BC), a trade originating in Libyan-Hellenic Cyrenaica (Amigues, 2004), as one of the ways by which C. ocellatus expanded through the Eastern Mediterranean (Kornilios et al., 2010).
The regional archaeological record of Serra del Molar has many peculiarities that allow us to postulate the possibility of an introduction due to intra-Mediterranean maritime trade. The surrounding region presents an important archaeological record linked to trade with the Eastern Mediterranean and Phoenician colonization during the second quarter of the 1st millennium BC, associated with the Phoenician colony of La Fonteta (Guardamar del Segura), which is located 2.5 kilometres from Serra del Molar (González, 2010a, b; Doménech, 2010) (fig. 4b).
The five sequenced individuals from Serra del Molar belong to the same subclade A2 in Kornilios et al. (2010) and appear to be mainly linked with the Egyptian specimens sampled from Ras El Barr, in the Nile Delta. During the ancient past, in this same area of Lower Egypt were seaports that traded with other regions across the Mediterranean Sea, such as Naukratis, Thonis-Heracleion, Tamiat, Pelusium and Tell el-Ghaba (Stanley et al., 2008; Pfeiffer, 2010; Lupo and Kohen, 2010), all of them were active harbours during the same period as La Fonteta colony (8th–6th centuries BC). In this Phoenician settlement and in nearby contemporary sites, a large amount of Egyptian objects has been discovered together with other manufactures from eastern workshops (Padró, 1975; Doménech, 2010; Escolano, 2012; López and Velázquez, 2012; González, 2014; Martínez and Vilaplana, 2014).
A possible route for the ocellated skink translocation is its arrival as an unintentional cargo passenger along with merchandises from Egypt (or another nearby eastern Mediterranean region) to the Phoenician colony of La Fonteta and its later establishment in the periphery of one of its associated Iberian settlements located in Serra del Molar, which was part of its most direct area of influence. The skinks could have arrived inside imported plants or soils, as is proposed by Kornilios et al. (2010) as way for human-mediated dispersals of C. ocellatus in the eastern Mediterranean regions, in which the skinks were transported as involuntary stowaways. For example, another agricultural product that could be the transport means for the translocation were eastern varieties of vine strains (Vitis vinifera), during the introduction of viticulture in Iberia by the Phoenicians themselves (Buxó, 2008; Iriarte et al., 2016). The molecular data of the conifer Tetraclinis articulata from the nearby Sierra de Cartagena (Murcia), which suggest the introduction or local genetic substitution through translocations from Tunisia by the Phoenicians/Carthaginians (Sánchez-Gómez et al., 2013), show the human-mediated mobility of plant species across the Mediterranean Sea during this same chronology.
In the context of ancient Mediterranean interactions, similar mechanisms of translocation have been proposed for the colonization of Vipera aspis hugyi on the island of Montecristo (Masseti and Zuffi, 2011) and the introduction of Eryx jaculus in the Licata region, Sicily (Insacco et al., 2015). The presence of ocellated skinks in this Iberian region, archaeologically linked to the Eastern Mediterranean, reinforces the hypothesis of Kornilios et al. (2010) about the ancient trade as the main phenomenon for the dispersal of C. ocellatus, and reinforces Egypt as the possible main point of the human-mediated translocations.
However, the links between the Iberian south-east with the Eastern Mediterranean, and especially with Egypt, are not limited to the phenomena of Iron Age trade and Phoenician colonization, because the intra-Mediterranean contacts continued during the Roman dominion. More recent historical relations with Egypt occurred during early Middle Age, when Islamic Egyptian troops colonized the surrounding region of Tudmîr in 743 AD (Gutiérrez, 1996; Vallvé, 1999). The Middle Age manuscripts also relay the existence of direct trade between Tudmîr and the harbours of the Fatimite Caliphate and Ayyubid Sultanate, particularly the Egyptian port of Alexandria (Azuar, 2016).
One of the most important contributions that the Muslim period had in the southern Valencian area was the introduction of the “oasis crop”. Its greatest exponent is the “Hort de Palmeres d’Elx” (Palm Grove of Elche), a large monoculture concentration of the date palm (Phoenix dactylifera) originated in the 10th century AD that continues at present (Azuar, 1998). During the last decades, palm trees have additionally been used as ornament in private gardens and public parks. This requirement for new trees, which could not be serviced only by local production, was supplied by importing plants from Argentina and Egypt (Berbegal, 2017). Due to the growing demand, the invasive red palm weevil (Rhynchophorus ferrugineus) was introduced in Spain mainland through the entry of Egyptian palms without phytosanitary control in 1995 (Ferry and Gómez, 2002). The importation of Egyptian palm trees or another anthropogenic factors (including the current pet trade) may also explain a more recent origin for the translocation and colonization of Serra del Molar. In addition, C. ocellatus have demonstrated a high capacity to colonize new areas due to passive dispersals, recently documented in the islands of Stromboli (Italy) and Kasos (Greece) or even in America mainland (Florida and Arizona), where the ocellated skink was absent in thorough herpetological surveys a few years ago (Krysko et al., 2011; Gunn et al., 2012; Lo Cascio and Grita, 2016; Kornilios and Thanou, 2016).
Regardless of the possibility of an introduction during Phoenician times, Islamic period or more recently, the evolution of the Serra del Molar’s environment could explain the survival of an allochthonous population. Although currently it is connected to the continent (fig. 4c), in the recent past Serra del Molar was an isolated island in front of the deltas of the Vinalopó and Segura rivers (fig. 4a). Recently, the sediments by both rivers settled and filled the area, encircling Serra del Molar to the west and north sides to form a lagoon environment of marshes, swamps and flood plains, which remained with brackish marsh conditions until the 18th century AD (Blázquez, 2001; Grau and Moratalla, 2001; Blázquez and Usera, 2010; Tent-Manclús, 2012) (fig. 4b).
The preservation of quasi insular conditions until relatively recent times (fig. 4c) could have enabled the survival of C. ocellatus, in case of an ancient introduction. To date, we were only able to locate individuals of ocellated skink in the north-east quadrant of Serra del Molar, which comparatively has suffered less anthropic impact, although this range may be much larger due to the fossorial cryptic nature of this skink. Most of the area preserve the typical autochthonous scrubs with Aleppo pines and only the lowlands show evidences of old plots, though nowadays are abandoned. Today, the biggest anthropic impact is the massive urbanization, such as estates and roads, in the southern half of Serra del Molar. New faunal surveys will help to clarify and more accurately assess the distribution of C. ocellatus, since it is a generalist species, cryptic and adaptable to the presence of humans and agriculture (Schneider, 1981; Schleich et al., 1996).
An alien population of C. ocellatus in the Iberian Peninsula raises a new management problem: the possibility of an ancient introduction in Serra del Molar, in addition to the semi-insular character of this area, could be arguments for its conservation. On the other hand, the possible competition with the native skink Chalcides bedriagai and the current connectivity with the surrounding regions, may raise arguments to control its population and even propose its eradication. Following the guidelines of the recent review about the status of allochthonous herpetofauna in Spain and management proposals (Santos et al., 2015), we believe it would be necessary to first carry out field surveys, with monitoring of individuals and experimental studies to assess the C. ocellatus interactions with the native biota, specifically the skink C. bedriagai, as well as to perform ecological niche models that might hint on the future distribution trends and possible expansion of this species. The results of these proposed studies will help to evaluate the criteria in future management for C. ocellatus in Serra del Molar.
Finally, new efforts are needed in the herpetofaunal studies in archaeological contexts of the Iberian Mediterranean regions. The currently known data show that the first records of some Maghrebian species in the Iberian Peninsula are dated in recent chronology, during the Holocene (Bisbal-Chinesta and Blain, 2018). Paleo-Archaeoherpetology can help us to identify their arrival and the influences of the human factor on them.
We are very grateful to the Editor Sebastian Steinfartz, associated editor and four anonymous reviewers for their constructive comments on earlier draft of this manuscript. The authors thank the staffs of the Wildlife Recovery Center La Granja del Saler of València city and Santa Faç of Alacant for all their assistance as well as to the herpetologists I. Lacomba and V. Sancho. F. Prados is thanked for his constructive comments about Phoenician colonization in southern Valencian Country. To Conselleria de Medi Ambient of Generalitat Valenciana for the granting of faunistic prospecting permits (166/17FAU17_017, 000/18FAU18_002). Finally, to our companions of the Associació Herpetològica Timon (AHT) during the surveys: J. Burgos, E. Rosillo, M. Real, P. Luna, Á. Mondejar and E. Berdún. This paper is part of project CGL2016-80000-P of the Spanish Ministry of Economy and Competitiveness and SGR2017-859 of the Generalitat de Catalunya. J.F. Bisbal-Chinesta is supported by a FI Predoctoral Fellowship 2016FI_B00286 with the financial sponsorship of the Agència de Gestió d’Ajuts Universitaris i de Recerca and the Departament d’Empresa i Coneixement of the Generalitat de Catalunya. S. Carranza and K. Tamar are supported by project CGL2015-70390-P of the Spanish Ministry of Economy and Competitiveness (cofunded by FEDER).
Supplementary material is available online at: https://doi.org/10.6084/m9.figshare.9703460
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Associate Editor: Matthias Stöck.