The Asian clam, Corbicula fluminea, is an invasive alien species (IAS) originally from Asia that has spread worldwide causing major ecological and economic impacts in aquatic ecosystems. Here, we evaluated C. fluminea genetic (using COI mtDNA, CYTb mtDNA and 18S rDNA gene markers), morphometric and sperm morphology variation in Portuguese freshwater ecosystems. The COI marker revealed a single haplotype, which belongs to the Asian FW5 invasive lineage, suggesting a common origin for all the 13 Portuguese C. fluminea populations analysed. Morphometric analyses showed differences between the populations colonizing the North (with the exception of the Lima River) and the Centre/South ecosystems. The sperm morphology examination revealed the presence of biflagellate sperm, a distinctive character of the invasive androgenetic lineages. The low genetic variability of the Portuguese C. fluminea populations and the pattern of sperm morphology have been illuminating for understanding the demographic history of this invasive species. We hypothesize that these populations were derived from a unique introductory event of a Corbicula fluminea FW5 invasive androgenic lineage in the Tejo River, which subsequently dispersed to other Portuguese freshwater ecosystems. The C. fluminea asexual reproductive mode may have assisted these populations to become highly invasive despite the low genetic diversity.
Citation: Gomes C, Sousa R, Mendes T, Borges R, Vilares P, Vasconcelos V, et al. (2016) Low Genetic Diversity and High Invasion Success of Corbicula fluminea (Bivalvia, Corbiculidae) (Müller, 1774) in Portugal. PLoS ONE 11(7): e0158108. https://doi.org/10.1371/journal.pone.0158108
Editor: Donald James Colgan, Australian Museum, AUSTRALIA
Received: December 30, 2015; Accepted: June 12, 2016; Published: July 8, 2016
Copyright: © 2016 Gomes et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: All relevant data are within the paper. The sequences obtained in this study are available in Genbank database - Portugal COI (accession no. KJ909515), 18S rDNA (accession no. KT878642), CTYb (accession no. KT878643) and Morocco COI (accession no. KT006544).
Funding: Cidália Gomes was supported by a PhD grant from the Portuguese Foundation for Science and Technology (FCT) (SFRH/BD/71041/2010). This research was partially supported by the Strategic Funding UID/Multi/04423/2013 through national funds provided by FCT and European Regional Development Fund (ERDF) in the framework of the program PT2020, the Structured Program of R&D&I INNOVMAR - Innovation and Sustainability in the Management and Exploitation of Marine Resources (NORTE-01-0145-FEDER-000035; Research Line ECOSERVICES) by the Northern Regional Operational Program (NORTE2020) and ERDF, and the FCT project PTDC/AAG-GLO/6887/2014.
Competing interests: The authors have declared that no competing interests exist.
Biological invasions by bivalve mollusc species have become a worldwide problem due to their dispersal capacity and effects on biological diversity, and ecosystems functions and services [1–3]. The Asian clam Corbicula fluminea (Müller, 1774) is nowadays globally distributed and its invasive success is responsible for major ecological and economic impacts [4–6]. The successful invasive behaviour of C. fluminea may be related to their biological traits, namely: (i) rapid growth, (ii) early sexual maturation, (iii) short life span, (iv) high fecundity, (v) high filtration rates, (vi) broad dispersal capacities (which include natural vectors), (vii) ability to inhabit different substrate types, (viii) competitive success over native species and (ix) interactions with human activities [4,7,8]. Major ecological impacts are related with trophic and non-trophic (engineering) mechanisms [8,9]. As a filter feeder, this species is responsible for great changes in phytoplankton and zooplankton communities [8,9]. C. fluminea populations may reach great densities and biomass being also consumed by higher trophic levels [8,9]. Regarding non-trophic mechanisms major effects are related with engineering activities, which may be responsible for great changes in water clarity, bioturbation of sediments, nutrient cycling and substrate colonization mainly due to the massive presence of shells [6,10–12].
Even though the ecological and economic impacts caused by C. fluminea are substantially documented, their taxonomic status is still unclear. Initially, morphology-based taxonomy led to an excessive number of species within the genus Corbicula . This was mainly due to the observed high shell plasticity [14,15], which is attributed to different biotic (e.g. predation) and abiotic (e.g. water current, sediment) factors . However, additional studies relying mostly on genetic analyses—alloenzymes [17–19] and mitochondrial cytochrome c oxidase subunit I DNA sequences [20,21]–proposed the existence of fewer species [14,22,23].
Interestingly, the genus Corbicula presents different reproductive strategies, being able to reproduce sexually [24,25] and asexually [26–30]. Previous reports also indicate that a rare form of asexual reproduction known as androgenesis occurs within the genus Corbicula. This phenomenon occurs after the self-fertilization process—by an oocyte and a biflagellate sperm, which is a distinctive character of androgenetic lineages of the genus Corbicula found in both native and invasive populations [25,28,31–35]–where the maternal nuclear DNA is completely removed, while the retained male pronucleus develops into an “all-male” zygote nucleus, thus giving rise to a progeny of paternal clones [26,27,31,32]. In addition, “egg parasitism” also known as “mitochondrial DNA capture” may occur between the crossing of two different androgenic lineages of the genus Corbicula, with the sperm from one lineage being able to fertilize the egg of another lineage [34,36]. The maternal nuclear DNA of the second lineage is generally mostly extruded from the egg, while the paternal nuclear genome continues to develop but the maternal mitochondrial DNA from the second lineage is captured in this process, giving rise to offspring that possess cytoplasmic-nuclear disjunction [34,37–42]. However, occasionally during this process, part or the entire maternal nuclear DNA is not completely expelled from the egg, giving rise to “nuclear genome capture” whereby the offspring inherits a hybrid genome [35,36,39,40]. Such distinctive reproductive modes seem to benefit Corbicula species fitness and may contribute to the invasive success of the four genus Corbicula invasive lineages. Three of these, namely FW1 (form B), FW4 (form Rlc) and FW5 (forms A/R) have been reported in the native (Eastern Asia) and in the non-native range (Europe and North America). The fourth, FW17 (form C/S) has been detected outside the native range but not yet in Eastern Asia [35,36,40,42].
Currently, C. fluminea presents a widespread geographic distribution and has invaded ecosystems throughout Europe, North and South America, and more recently North Africa [43–47]. Records indicate that C. fluminea was first detected outside its native range (Eastern Asia) in 1924 in Vancouver Island, British Columbia [43,45]. By the 1970s it had spread throughout North and South America  and reached Europe at least as early as the 1980s . Britton and Morton  suggested that C. fluminea was firstly introduced in the North American continent as a food source for humans. However, the introductions into Europe and South America are believed to have occurred via ballast water . Consequently, C. fluminea dispersed within the continents by different dispersal vectors—commercial transportation and other human activities but also by natural vectors such as birds and mammals—which promoted their rapid spread [7,45,49,50]. In Portugal, C. fluminea was first detected in the Tejo River in 1980  and a few years later was reported in the Douro , Minho , Lima , Mondego  and Guadiana  Rivers. The history of introduction and further establishment of the species in the Tâmega, Tua, Sabor and Sado Rivers and Pateira de Fermentelos Lake are unknown.
The main objectives of this study were to assess the genetic variability and the phylogeography of C. fluminea Portuguese populations employing molecular, morphometric and morphological sperm analysis. The obtained results were compared with other available worldwide data of Corbicula spp. from native and invaded regions applying population genetics and phylogeographical inference methodologies.
Material and Methods
The study did not involve any kind of endangered or protected species. No specific scientific research permits were required for the sample collection of these highly invasive invertebrates.
Study area and sample collection
A total of 328 specimens of C. fluminea were randomly collected in 16 different sites belonging to 13 distinct ecosystems: Minho (four different sites, N = 100), Lima (two sites, N = 40), Tâmega (one site, N = 10), Tua (one site, N = 8), Sabor (one site, N = 10), Douro (one site, N = 10), Paiva (one site, N = 7), Mondego (one site, N = 30), Tejo (one site, N = 30), Sado (one site, N = 15), Mira (one site, N = 30), Guadiana (one site, N = 30) Rivers and Pateira de Fermentelos Lake (one site, N = 8), using a scoop net or by handpicking (Fig 1). Clams were immediately transported to the laboratory where all the soft body parts were isolated and individually stored at -80°C prior to DNA extraction.
Genomic DNA extraction, PCR gene amplification and sequencing
Total genomic DNA was extracted from 328 samples of C. fluminea foot tissue employing the salting-out method . The mitochondrial genes COI (N = 328) and CYTb (N = 110) and the nuclear gene 18S rDNA (N = 110), were amplified in a total volume of 40 μl per reaction containing: 1x PCR buffer, 2.5 mM MgCl2, 250 μM of each dNTP, 0.5 U of DNA Taq polymerase (Bioline, Luckenwalde, Germany), 10 pmol of a specific set of primers– LCO1490 and HC02198 for the COI , the HOLLAND18S1 and HOLLAND18S2 for the 18S rDNA  and CBF6 and CBR6 for the CYTb . The following PCR cycling conditions were used for the amplification of the mtDNA COI gene: 1 min at 94°C for initial denaturation, followed by 35 cycles of 1 min at 94°C, 30 s at 45°C, 1 min at 72°C and final extension of 10 min at 72°C . The 18S rDNA gene reactions were performed with the following PCR cycling parameters: 5 min at 95°C for initial denaturation, followed by 25 cycles of 4 s at 94°C, 2 min at 50°C, 1 min at 72°C and final extension of 8 min at 72°C . The CYTb amplifications were performed with the following PCR cycling conditions: 2 min at 94°C for initial denaturation, followed by 35 cycles of 3 min at 94°C, 45 s at 54°C, 2 min at 72°C and final extension of 5 min at 72°C . All PCR products were purified using Diffinity Rapid Tip (Diffinity Genomics, Inc, West Henrietta, NY) according to the manufacturer’s instructions. The final PCR amplifications were confirmed by electrophoresis in a 1.5% w/v agarose gel stained with ethidium bromide (Bio-Rad Laboratories Inc., California, USA) and followed by direct sequencing (Macrogen Amsterdam, Netherlands). The NCBI-BLAST program was employed for sequence identification and comparison [61,62].
Phylogenetic analysis—mtDNA COI, CYTb and 18S rDNA genes
A total of 93 Corbicula spp. sequences of the mtDNA COI gene were retrieved from the NCBI-GenBank [62,63] and Neocorbicula limosa was used as an outgroup for phylogenetic and phylogeographic analysis. The mtDNA, 18S rDNA and CYTb sequence alignments were performed employing the default parameters of ClustalW in MEGA 6 software [64,65]. DnaSP 5.10 was used for haplotype inference . Phylogenetic tree construction employed Bayesian Inference (BI) using MrBayes 3.1.2  and Maximum Likelihood (ML) using PhyML 3.0.1 . Both BI and ML employed the GTR + γ + I nucleotide evolutionary model based on the Akaike information criterion (with 95% confidence interval), using the jModelTest 2.1.1 [69,70]. The ML analysis used 1000 bootstrap replicates . The BI analysis was performed employing 5000000 generations, the trees were sampled every 1000th generation and a total of 25% of the generated trees were discarded. The tree convergence was evaluated in MrBayes by analysing the parameters set values of the Potential Scale Reduction factor (PSRF) and the Estimated sample size (ESS). In addition, further visual and numeric convergence was assessed using Tracer v1.6 software .
The shells of 275 C. fluminea specimens—from the Minho, Lima, Mondego, Sado, Mira, Tejo and Guadiana Rivers—were measured for length, height and width using a digital calliper (±0.2 mm). A Principal Component Analysis (PCA) was carried out using the three morphological measurements and the determination of the PCA components was performed using the correlation matrix in the “princomp” function of the R statistical software .
A sample of Corbicula fluminea (N = 10 from the Douro River) was collected to perform sperm morphology analyses. The sperm was obtained by collecting one drop of the specimens fresh gonadal tissue/fluid  in a glass slide and optical microscopy at 100x magnification was employed to observe the spermatozoa.
Mitochondrial DNA—COI gene
The obtained mtDNA COI sequences from all the 328 analysed individuals presented a unique haplotype which was phylogenetically compared with 93 other sequences from worldwide Corbicula spp. populations retrieved from GenBank [61–63]. Both BI and ML inferences implemented to reconstruct phylogenetic relationships between haplotypes displayed similar topologies (Fig 2).
Both Bayesian Inference posterior probabilities (BI) and Maximum Likelihood bootstrap values (ML) are indicated at the nodes.* Indicates the presence of monoflagellate sperm, ** indicates the presence of biflagellate sperm, α represents androgenetic lineages confirmed by cytological studies [25,32,73,74].
The phylogenetic analysis demonstrated the existence of two well supported clades: the estuarine and the freshwater (1.0/80 and 0.97/79 support values, respectively). The latter clade splits into five groups—I, II, III, IV and V—which includes Corbicula specimens from different geographical ranges within Asia, Europe, North and South America, Africa and Oceania (0.97/96 node support) and C. madagascariensis, which is an outgroup of the freshwater Corbicula spp. lineages. For phylogenetic analysis purposes the classification “Group I-V” was employed in this study and does not imply the existence of haplotype similarity.
The evaluation of Portuguese freshwater populations—Minho, Lima, Tâmega, Tua, Sabor, Douro, Paiva, Tejo, Sado, Mira, Guadiana Rivers and Pateira de Fermentelos Lake—revealed only one mtDNA COI haplotype (belonging to group IV with 0.90/85 node support), which is identical to previously reported haplotypes from Europe (form R), North America (form A), South America and the FW5 invasive lineage from Asia (Fig 2).
Group I (0.97/96 node support) encompasses two invasive lineages, the FW1 (form B) from Asia and North America and the FW4 (form Rlc) present in Asia and Europe, whereas group IV (0.90/85 node support) represents the invasive lineage FW5 (form A/R) from Asia, Europe and North America. Both of these groups include COI haplotypes from both the native and non-native range. Groups II and V (1.0/95 and 0.86/91 node support, respectively) are strictly confined to Eastern Asia. Group III (0.69/55 node support) is the only group that includes most of the genus Corbicula haplotypes from the non-Asian range, namely from: Europe, South America, Africa and Oceania (with the exception of Israel which is a Western Asian country), as well as the FW17 (C/S form), detected exclusively in non-native regions, namely Europe, Africa and South America.
CYTb and 18S rDNA genes
A subsample comprising a total of 110 specimens was used to evaluate the genetic variability of the mitochondrial CYTb and the nuclear 18S rDNA genes. The analysis revealed that only two haplotypes—one for the 18S rDNA (accession no. KT878642) and one for CTYb (accession no. KT878643)–were detected in Portuguese C. fluminea populations. This finding, suggests that Portuguese C. fluminea populations present a low genetic variability for these two markers. Further analyses employing these two markers were not carried out due to insufficient data available in the Genbank database.
The two PCA components explaining most variation (in total 99.4%) were jointly plotted to seek clusters related to the river/region location of each specimen (Fig 3). The PC1 and PC2 components suggest the existence of two clusters: north cluster (N), which includes C. fluminea specimens from the Minho River; and centre/south cluster (C/S) comprising specimens from Mondego, Tejo, Sado, Mira, Guadiana Rivers. However, the majority (31) of the specimens from the Lima River (N = 40), located in the North of Portugal, grouped within the C/S cluster. Therefore, morphologically they present more similarities with the C. fluminea populations from the centre and south.
The PCA showing the relationship of the PC1 and PC2 components of Corbicula fluminea populations from rivers of the North, Centre and South of Portugal, and the Lima River (North Portugal). Each circle represents one specimen corresponding to a specific river. The N cluster in the light red oval circle represents C. fluminea populations from the northern rivers and C/S in light green oval represents the centre and southern rivers cluster.
The sperm morphology analysis of the subset C. fluminea inhabiting the Douro River clearly revealed the presence of biflagellate spermatozoa (Fig 4), a distinctive character of androgenetic lineages of the genus Corbicula, which are associated with high invasive potential but low genetic variability [35,40–42,74].
Genetic and morphological analysis of the Portuguese Corbicula fluminea populations
The database sequence comparisons and the phylogenetic inference revealed the existence of a unique mtDNA COI haplotype in the studied Portuguese C. fluminea populations belonging to group IV. This haplotype is identical to the European haplotype I , the North American haplotype form A  and the Asian FW5 haplotype . In addition, the FW5 haplotype comprehends the majority of Corbicula spp. with biflagellate sperm which is indicative this lineage reproduces through androgenesis, a rare form of asexual reproduction [40,42].
A previous study also reported the existence of this mtDNA COI haplotype in C. fluminea populations of the Minho and Lima Rivers (38 out of 41 individuals analysed in six different sampling sites) and three other rare haplotypes (each represented by only one specimen) in the Minho River . However, in the present study, those rare haplotypes were not observed despite analysis of an eight times larger sample size (N = 328). The absence of the rare haplotype most probably resulted from the erosion of the genetic variability due to C. fluminea massive mortality events that occur recurrently in the Minho River [75,76] potentially leading to the loss of these rare alleles by genetic drift after a reduction in the population size . Even though the C. fluminea population recovered rapidly from these die-offs and attained its previous biomass and density , our data shows the lack of variation of the mtDNA COI in a considerable large number of individuals from the Minho River (N = 100).
The genetic analysis of the mtDNA CYTb and 18S rDNA also yielded only one sequence for each marker in the studied C. fluminea populations. The mtDNA CYTb haplotype has also been reported in Japanese populations . The 18S rDNA sequence corresponds to the same previously found in other European and North American populations namely, Spain , the United Kingdom  and the USA [80,81]. Therefore, the present study indicates a low genetic variability within the both mitochondrial (COI) and the nuclear markers (18S) in C. fluminea populations from the main Portuguese basins. Thus, this seems to be a general pattern in C. fluminea Portuguese populations. In addition, the sperm morphology revealed that at least one sample of the Portuguese C. fluminea lineage is biflagellate, a distinctive character of the asexual androgenetic lineages [28,31]. Given the lack of genetic variability detected in the studied populations with the employed genetic markers, we hypothesize that the Portuguese C. fluminea has derived from an androgenetic invasive asexual lineage with low mitochondrial genetic variability. In fact, some case-studies have reported successful invasions with low or no genetic variation in animals and plants regardless of their reproduction mode—in Africa diverse genotypes of the water flea Daphnia pulex have been replaced by a single non-native clone from the American continent , introduced populations of invasive Argentine ant Linepithema humile in California present a loss of genetic diversity which is associated with reduced intraspecific aggression and form interspecific dominant supercolonies , the Meditterranean bluespotted cornetfish Fistularia commersonii exhibits low genetic variability in comparison to the Indo-pacific native range  and the invasive water hyacinth Eichhornia crassipes from the Amazon basin presents one main clonal genotype in China [85,86]. In this case, C. fluminea asexual reproductive mode may assist these populations to become highly invasive despite their observed low genetic diversity [87,88]. However, further studies using other nuclear markers are required to confirm the low genetic variability at the nuclear level. Despite the low genetic variation in the mtDNA (COI and CYTb) and rDNA (18S) in C. fluminea populations found in the present study, we hypothesize that an asexual reproduction strategy might seem to increase their reproductive potential, thus contributing to their high invasive success [35,40–42,74]. Therefore, considering the invasive history of C. fluminea in Portugal, we hypothesize that the low genetic variability found in populations from main rivers all over the country, is a result of the introduction of an asexual lineage with a reduced genetic pool in Tagus River—where this species was first reported –that rapidly spread to other Portuguese freshwater ecosystems. This spread may have occurred through different dispersal mechanisms that may include human activities or natural dispersion by birds, mammals and fish as previously reported [4,7,49,89]. While the Portuguese C. fluminea populations exhibited low genetic variability, morphological differences have been detected in the present study (Fig 3). Two morphotypes were observed, one corresponding to C. fluminea populations from the northern rivers and, the other, corresponding to populations from centre/southern rivers. The exception is the population of the Lima River—located in the north of Portugal—that is morphologically more similar to Centre/South populations than to other northern populations. The observed morphological differences may be attributed to biotic or abiotic factors that influence shell morphology that may include avoidance of predation and parasitism, different current flow conditions, type of substratum, conductivity and calcium availability, among other factors [90–94]. The morphometric analysis of the studied C. fluminea populations may provide some ecologic insight, but further studies employing a hierarchal experimental design—composed of a robust C. fluminea sampling, evaluation of densities and the evaluation of both biotic and abiotic factors in these ecosystems—would be necessary to acquire a deeper ecological knowledge of the spatial variability of this species.
Global haplotype diversity and distribution of the genus Corbicula
Most of the groups resolved in the phylogenetic inference present polytomies (Fig 2), indicating that Corbicula spp. dispersal occurred in a short temporal scale . From a global perspective, we can observe that the native range presents higher Corbicula spp. haplotype diversity– 40 out of 47 haplotypes—in comparison to the invaded regions (Fig 5). However, the confinement or the absence from the species’ native range of some Corbicula spp. (groups II, V and III, respectively) still remains to be explained . Perhaps physiological and environmental constraints and/or Corbicula spp. habitats that are less subjected to human mediated activities may in fact be inhibiting the spread of these haplotypes that derive from well-established populations . Further studies should be performed to address these questions.
N indicates the number of haplotypes. Exclusively from the native-range two groups II and V (N = 10 haplotypes and N = 14 haplotypes, respectively). Group I is represented by 9 haplotypes and group IV by 7. The non-native range group I represents 4 haplotypes (North America, N = 1; South America, N = 2 and Europe N = 1). Group III englobes a total of 6 haplotypes; (Africa, N = 1; South America, N = 1; Europe N = 3 and Oceania, N = 1). Group IV presents a total of 3 haplotypes (North America N = 1, Europe N = 1 and Africa N = 1).
The dispersal trajectory of the Corbicula spp. invasive lineages
It is generally accepted that the Corbicula spp. invasion in Europe was exclusively by water ballast transport from America . However, recent genetic studies are not able to confirm whether the primary introduction of the invasive lineage FW5 (form A/R) in Europe was via North and/or South America . However, we cannot exclude the hypothesis that the Corbicula spp. may have also been introduced into the European continent directly from the Asian populations (Fig 6). The FW1 (form B) and the FW4 (form Rlc) invasive lineages (Figs 4 and 6) both clustered in group I. It has been proposed that both lineages may possess the same mitochondrial ancestor due to the detection of only one nucleotide difference in the mtDNA COI gene . Interestingly, the FW17 (form C/S) invasive lineage has not been detected in the eastern native range Corbicula sp. (Figs 4 and 5). In fact, a recent population genetic study  corroborates this result and hypothesizes an introduction of the FW17 invasive lineage from the African continent and subsequent spread to the South America and subsequently into Europe. Nevertheless, inferring introductions routes for the Corbicula species is indeed an arduous task, especially when considering the existent taxonomic controversy in this genus. Perhaps an integrative approach employing ecology, morphology and genetic techniques may provide further insights regarding the invasive dispersal trajectory of this IAS.
C. fluminea populations from the main Rivers in Portugal revealed a low genetic variability with the employed genetic markers—COI, CYTb and 18S –despite the large number of individuals detected in the studied ecosystems. The mtDNA COI and the presence of biflagellate sperm indicate that Portuguese C. fluminea populations belong to the FW5 androgenetic invasive lineage. Thus, we suggest that a reduced genetic pool was probably recently introduced first in the Tagus River and afterwards spread quickly to other Portuguese freshwater ecosystems. At the moment is not possible to unambiguously infer neither the C. fluminea primary introductory route(s) within Portugal nor the main population source (North America and/or South America or directly from Asia).
Special thanks to Anoop Alex, João Machado, Cristiana Moreira and Daniela Almeida for their prompt support and knowledge sharing. We would also like to thank the editor and the two anonymous reviewers for the helpful recommendations on this manuscript.
Cidália Gomes was supported by a PhD grant from the Portuguese Foundation for Science and Technology (FCT) (SFRH/BD/71041/2010). This research was partially supported by the Strategic Funding UID/Multi/04423/2013 through national funds provided by FCT and European Regional Development Fund (ERDF) in the framework of the program PT2020, the Structured Program of R&D&I INNOVMAR—Innovation and Sustainability in the Management and Exploitation of Marine Resources (NORTE-01-0145-FEDER-000035; Research Line ECOSERVICES) by the Northern Regional Operational Program (NORTE2020) and ERDF, and the FCT project PTDC/AAG-GLO/6887/2014.
Conceived and designed the experiments: CG AA. Performed the experiments: CG. Analyzed the data: CG TM RB AA. Contributed reagents/materials/analysis tools: AA LG VV. Wrote the paper: CG AA. Revised the manuscript: AA RS LG VV. Performed sample collection: CG PV RS.
- 1. Higgins SN, Zanden MJV. What a difference a species makes: a meta–analysis of dreissenid mussel impacts on freshwater ecosystems. Ecol Monogr. 2010;80: 179–196.
- 2. Douda K, Vrtílek M, Slavík O, Reichard M. The role of host specificity in explaining the invasion success of the freshwater mussel Anodonta woodiana in Europe. Biol Invasions. 2012;14: 127–137.
- 3. Sousa R, Novais A, Costa R, Strayer DL. Invasive bivalves in fresh waters: impacts from individuals to ecosystems and possible control strategies. Hydrobiologia 2014;735: 233–251.
- 4. McMahon RF. Evolutionary and physiological adaptations of aquatic invasive animals: r selection versus resistance. Can J Fish Aquat Sci. 2002;59: 1235–1244.
- 5. Darrigran G. Potential Impact of Filter-feeding Invaders on Temperate Inland Freshwater Environments. Biol Invasions. 2002;4: 145–156.
- 6. Ilarri MI, Freitas F, Costa-Dias S, Antunes C, Guilhermino L, Sousa R. Associated macrozoobenthos with the invasive Asian clam Corbicula fluminea. J Sea Res. 2012;72: 113–120.
- 7. Karatayev AY, Padilla DK, Minchin D, Boltovskoy D, Burlakova LE. Changes in Global Economies and Trade: the Potential Spread of Exotic Freshwater Bivalves. Biol Invasions. 2007;9: 161–180.
- 8. Sousa R, Antunes C, Guilhermino L. Ecology of the invasive Asian clam Corbicula fluminea (Müller, 1774) in aquatic ecosystems: an overview. Ann Limnol—Int J Limnol. 2008;44: 85–94.
- 9. Novais A, Souza AT, Ilarri M, Pascoal C, Sousa R. From water to land: How an invasive clam may function as a resource pulse to terrestrial invertebrates. Sci Total Environ. 2015;538: 664–671. pmid:26327634
- 10. Sousa R, Gutiérrez JL, Aldridge DC. Non-indigenous invasive bivalves as ecosystem engineers. Biol Invasions. 2009;11: 2367–2385.
- 11. Ilarri MI, Souza AT, Modesto V, Guilhermino L, Sousa R. Differences in the macrozoobenthic fauna colonising empty bivalve shells before and after invasion by Corbicula fluminea. Mar Freshw Res. 2015;66: 549–558.
- 12. Novais A, Souza AT, Ilarri M, Pascoal C, Sousa R. Facilitation in the low intertidal: effects of an invasive species on the structure of an estuarine macrozoobenthic assemblage. Mar Ecol Prog Ser. 2015;522: 157–167.
- 13. Britton JC, Morton B. A dissection guide, field and laboratory manual for the introduced bivalve Corbicula fluminea. Malacol Rev Suppl. 1982;3: 1–8.
- 14. Kijviriya V, Upatham ES, Viyanant V, Woodruff DS. Genetic studies of Asiatic Clams, Corbicula, in Thailand—Allozymes of 21 Nominal Species are Identical. Am Malacol Bull. 1991;8: 97–106.
- 15. Tsoi SCM, Lee SC, Wu WL, Morton BS. Genetic variation in Corbicula fluminea (Bivalvia: Corbiculidae) from Hong Kong. Malacol Rev. 1991;24: 25–34.
- 16. Sousa R, Freire R, Rufino M, Méndez J, Gaspar M, Antunes C, et al. Genetic and shell morphological variability of the invasive bivalve Corbicula fluminea (Müller, 1774) in two Portuguese estuaries. Estuar Coast Shelf Sci. 2007;74: 166–174.
- 17. Hillis DM, Patton JC. Morphological and Electrophoretic Evidence for Two Species of Corbicula (Bivalvia: Corbiculidae) in North America. Am Midl Nat. 1982;108: 74–80.
- 18. Hatsumi M, Nakamura M, Hosokawa M, Nakao S. Phylogeny of three Corbicula species and isozyme polymorphism in the Corbicula japonica populations. Venus 1995;54: 185–193.
- 19. Lee J-S, Kim JB. Systematic study of the genus Corbicula (Bivalvia: Corbiculidae) in Korea. Korean. J Syst Zool. 1997;13: 233–246.
- 20. Renard E, Bachmann V, Cariou ML, Moreteau JC. Morphological and molecular differentiation of invasive freshwater species of the genus Corbicula (Bivalvia,Corbiculidea) suggest the presence of three taxa in French rivers. Mol Ecol. 2000;9: 2009–2016. pmid:11123613
- 21. Siripattrawan S, Park J-K, Foighil DÓ. Two lineages of the introduced Asian freshwater clam Corbicula occur in North America. J Molluscan Stud. 2000;66: 423–429.
- 22. Morton B. Corbicula in Asia—an updated synthesis. Am Malacol Bull. 1986;2: 113–124.
- 23. Araujo R, Moreno D, Ramos MA. The Asiatic clam Corbicula fluminea (Müller, 1774) (Bivalvia: Corbiculidae) in Europe. Am Malacol Bull. 1993;10: 39–49.
- 24. Okamoto A, Arimoto B. Chromosomes of Corbicula japonica, C. sandai and C. (Corbiculina) leana (Bivalvia: Corbiculidae). Venus 1986;45: 194–202.
- 25. Glaubrecht M, Rintelen TV, Korniushin AV. Toward a systematic revision of brooding freshwater Corbiculidae in southeast Asia (Bivalvia, Veneroida): on shell morphology, anatomy and molecular phylogenetics of endemic taxa from islands in Indonesia. Malacologia 2003;45: 1–40.
- 26. Komaru A, Konishi K, Nakayama I, Kobayashi T, Sakai H, Kawamura K. Hermaphroditic Freshwater Clams in the Genus Corbicula Produce Non-Reductional Spermatozoa With Somatic DNA Content. Biol Bull. 1997;193: 320–323.
- 27. Komaru A, Konishi K. Non-reductional spermatozoa in three shell color types of the freshwater clam Corbicula fluminea in Taiwan. Zoolog Sci. 1999;16: 105–108.
- 28. Byrne M, Phelps H, Church T, Adair V, Selvakumaraswamy P, Potts J. Reproduction and development of the freshwater clam Corbicula australis in southeast Australia. Hydrobiologia. 2000;418: 185–197.
- 29. Qiu AD, Shi AJ, Komaru A. Yellow and brown shell color morphs of Corbicula fluminea (Bivalvia: Corbiculidae) from Sichuan Province, China, are triploids and tetraploids. J Shellfish Res. 2001;20: 323–328.
- 30. Korniushin AV. A revision of some Asian and African freshwater clams assigned to Corbicula fluminalis (Muller, 1774) (Mollusca: Bivalvia: Corbiculidae), with a review of anatomical characters and reproductive features based on museum collections. Hydrobiologia 2004;529: 251–270.
- 31. Konishi K, Kouichi K, Hirofumi F, Akira K. Spermatogenesis of the freshwater clam Corbicula aff. fluminea Müller (Bivalvia: Corbiculidae). J Shellfish Res. 1998;17: 185–189.
- 32. Ishibashi R, Ookubo K, Aoki M, Utaki M, Komaru A, Kawamura K. Androgenetic Reproduction in a Freshwater Diploid Clam Corbicula fluminea (Bivalvia: Corbiculidae). Zoolog Sci. 2003;20: 727–732. pmid:12832823
- 33. Park J-K, Kim W. Two Corbicula (Corbiculidae: Bivalvia) mitochondrial lineages are widely distributed in Asian freshwater environment. Mol Phylogenet Evol. 2003;29: 529–539. pmid:14615191
- 34. Lee T, Siripattrawan S, Ituarte CF, Foighil DO. Invasion of the clonal clams: Corbicula lineages in the New World. Am Malacol Bull. 2005;20: 113–122.
- 35. Pigneur L-M, Hedtke SM, Etoundi E, Van Doninck K. Androgenesis: a review through the study of the selfish shellfish Corbicula spp. Heredity 2012;108: 581–591. pmid:22473310
- 36. Hedtke SM, Glaubrecht M, Hillis DM. Rare gene capture in predominantly androgenetic species. Proc Natl Acad Sci U S A. 2011;108: 9520–9524. pmid:21606355
- 37. Komaru A, Kumamoto A, Ishibashi R. Possible elevation of ploidy levels by accidental formation of female pronucleus in androgenetic clam Corbicula leana. Dev Biol. 2001;18: 87.
- 38. Park J-K, Lee J-S, Kim W. A single mitochondrial lineage is shared by morphologically and allozymatically distinct freshwater Corbicula clones. Mol Cells. 2002;14: 318–322. pmid:12442908
- 39. Komaru A, Kumamoto A, Kato T, Ishibashi R, Obata M, Nemoto T. A Hypothesis of Ploidy Elevation by Formation of a Female Pronucleus in the Androgenetic Clam Corbicula fluminea in the Tone River Estuary, Japan. Zoolog Sci. 2006;23: 529–532. pmid:16849840
- 40. Hedtke SM, Stanger-Hall K, Baker RJ, Hillis DM. All-male asexuality: origin and maintenance of androgenesis in the Asian clam Corbicula. Evol Int J Org Evol. 2008;62: 1119–1136.
- 41. Hedtke SM, Hillis DM. The Potential Role of Androgenesis in Cytoplasmic–Nuclear Phylogenetic Discordance. Syst Biol. 2010; syq070.
- 42. Pigneur L-M, Marescaux J, Roland K, Etoundi E, Descy J-P, Doninck KV. Phylogeny and androgenesis in the invasive Corbicula clams (Bivalvia, Corbiculidae) in Western Europe. BMC Evol Biol. 2011;11: 147. pmid:21619630
- 43. Counts CL. Corbicula fluminea (Bivalvia: Corbiculidea) in. British Columbia. Nautilus 1981;95: 12–13.
- 44. Mouthon J. Sur la présence en France et au Portugal de Corbicula (Bivalvia, Corbiculidae) originaire d’Asie. Basteria. 1981;45: 109–116.
- 45. McMahon R.F. The occurrence and spread of the introduced Asiatic freshwater clam, Corbicula fluminea (Müller) in North America: 1924–1982. Nautilus 1982;96: 134–141.
- 46. Ituarte CF. Corbicula and Neocorbicula (Bivalvia: Corbiculidae) in the Paranà, Uruguay, and Río de La Plata basins. Nautilus 1994;107: 129–135.
- 47. Clavero M, Araujo R, Calzada J, Delibes M, Fernández N, Gutiérrez-Expósito C, et al. The first invasive bivalve in African fresh waters: invasion portrait and management options. Aquat Conserv Mar Freshw Ecosyst. 2012;22: 277–280.
- 48. Britton JC, Morton B. Corbicula in North America: the evidence reviewed and evaluated. Britton JC (ed) Proceedings of the First International Corbicula Symposium. Fort Worth, TX: Texas Christian University Research Foundation Publication; 1979. pp. 249–287.
- 49. McMahon RF. Invasive characteristics of the freshwater bivalve Corbicula fluminea. Claudi R, Leach JH (eds), Nonindigenous Freshwater Organisms. Vectors, Biology, and Impacts. Lewis Publishers; 1999. pp. 315–343.
- 50. Belz CE, Darrigran G, Netto OSM, Boeger WA, Ribeiro PJ. Analysis of Four Dispersion Vectors in Inland Waters: The Case of the Invading Bivalves in South America. J Shellfish Res. 2012;31: 777–784.
- 51. Nagel K-O. Ein weiterer Fundort von Corbicula fluminalis (Müller 1774) (Mollusca: Bivalvia) in Portugal. Mitteilungen Dtsch Malakozool Ges. 1989;17: 44–45.
- 52. Sousa R, Antunes C, Guilhermino L. Factors influencing the occurrence and distribution of Corbicula fluminea (Müller, 1774) in the River Lima estuary. Ann Limnol—Int J Limnol. 2006;42: 165–171.
- 53. Ferreira V, Graça MAS, Feio MJ, Mieiro C. Water quality in the Mondego river basin: Pollution and habitat heterogeneity. Limnetica. 2004;23: 295–306.
- 54. Chícharo L, Chícharo MA, Amaral A, Condinho S, Dias S, Morais P. Valorização dos recursos pesqueiros do estuário do Guadiana. Relatório final do projecto ODIANA-VALPEG, Universidade do Algarve. 2000.
- 55. Dill WA. Inland Fisheries of Europe. Food & Agriculture Org.; 1993.
- 56. Teles M, Pacheco M, Santos MA. Endocrine and metabolic responses of Anguilla anguilla L. caged in a freshwater-wetland (Pateira de Fermentelos—Portugal). Sci Total Environ. 2007;372: 562–570. pmid:17150242
- 57. Miller SA, Dykes DD, Polesky HF. A simple salting out procedure for extracting DNA from human nucleated cells. Nucleic Acids Res. 1988;16: 1215. pmid:3344216
- 58. Folmer O, Black M, Hoeh W, Lutz R, Vrijenhoek R. DNA primers for amplification of mitochondrial cytochrome c oxidase subunit I from diverse metazoan invertebrates. Mol Mar Biol Biotechnol. 1994;3: 294–299. pmid:7881515
- 59. Adamkewicz SL, Harasewych MG, Blake J, Saudek D, Bult CJ. A molecular phylogeny of the bivalve mollusks. Mol Biol Evol. 1997;14: 619–629. pmid:9190063
- 60. Yamada M, Ishibashi R, Kawamura K, Komaru A. Interrelationships of the freshwater clams Corbicula leana Prime, 1864 and C. fluminea (Müller, 1774) distributed in Japan inferred from shell type and 11 mitochondrial DNA Cyt b region. Nippon Suisan Gakkaishi. 2010;76: 926–938 Nippon Suisan Gakkaishi 76: 926–938 (In Japanese with English abstract).
- 61. Altschul SF, Madden TL, Schäffer AA, Zhang J, Zhang Z, Miller W, et al. Gapped BLAST and PSI-BLAST: a new generation of protein database search programs. Nucleic Acids Res. 1997;25: 3389–3402. pmid:9254694
- 62. Sayers EW, Barrett T, Benson DA, Bryant SH, Canese K, Chetvernin V, et al. Database resources of the National Center for Biotechnology Information. Nucleic Acids Res. 2009;37: D5–D15. pmid:18940862
- 63. Benson DA, Karsch-Mizrachi I, Lipman DJ, Ostell J, Wheeler DL. GenBank. Nucleic Acids Res. 2005;33: D34–38. pmid:15608212
- 64. Larkin MA, Blackshields G, Brown NP, Chenna R, McGettigan PA, McWilliam H, et al. Clustal W and Clustal X version 2.0. Bioinformatics. 2007;23: 2947–2948. pmid:17846036
- 65. Tamura K, Stecher G, Peterson D, Filipski A, Kumar S. MEGA6: Molecular Evolutionary Genetics Analysis Version 6.0. Mol Biol Evol. 2013;30: 2725–2729. pmid:24132122
- 66. Librado P, Rozas J. DnaSP v5: a software for comprehensive analysis of DNA polymorphism data. Bioinformatics. 2009;25: 1451–1452. pmid:19346325
- 67. Ronquist F, Huelsenbeck JP. MrBayes 3: Bayesian phylogenetic inference under mixed models. Bioinformatics. 2003;19: 1572–1574. pmid:12912839
- 68. Guindon S, Dufayard J-F, Lefort V, Anisimova M, Hordijk W, Gascuel O. New Algorithms and Methods to Estimate Maximum-Likelihood Phylogenies: Assessing the Performance of PhyML 3.0. Syst Biol. 2010;59: 307–321. pmid:20525638
- 69. Guindon S, Gascuel O. A simple, fast, and accurate algorithm to estimate large phylogenies by maximum likelihood. Syst Biol. 2003;52: 696–704. pmid:14530136
- 70. Darriba D, Taboada GL, Doallo R, Posada D. jModelTest 2: more models, new heuristics and parallel computing. Nat Methods. 2012;9: 772.
- 71. Rambaut A, Suchard MA, Xie D, Drummond AJ. Tracer V1.6. 2014. Available from http://beast.bio.ed.ac.uk/Tracer
- 72. Dray S, Dufour A-B, others. The ade4 package: implementing the duality diagram for ecologists. J Stat Softw. 2007;22: 1–20.
- 73. Komaru A, Kawagishi T, Konishi K. Cytological evidence of spontaneous androgenesis in the freshwater clam Corbicula leana Prime. Dev Genes Evol. 1998;208: 46–50. pmid:9518524
- 74. Pigneur L-M, Etoundi E, Aldridge DC, Marescaux J, Yasuda N, Van Doninck K. Genetic uniformity and long-distance clonal dispersal in the invasive androgenetic Corbicula clams. Mol Ecol. 2014;23: 5102–5116. pmid:25208249
- 75. Ilarri MI, Antunes C, Guilhermino L, Sousa R. Massive mortality of the Asian clam Corbicula fluminea in a highly invaded area. Biol Invasions. 2011;13: 277–280.
- 76. Sousa R, Varandas S, Cortes R, Teixeira A, Lopes-Lima M, Machado J, et al. Massive die-offs of freshwater bivalves as resource pulses. Ann Limnol—Int J Limnol. 2012;48: 105–112.
- 77. Nei M. Molecular Evolutionary Genetics. Columbia Universty press; 1987.
- 78. Espiñeira M, González-Lavín N, Vieites JM, Santaclara FJ. Development of a method for the genetic identification of commercial bivalve species based on mitochondrial 18S rRNA sequences. J Agric Food Chem. 2009;57: 495–502. pmid:19128038
- 79. Taylor JD, Williams ST, Glover EA, Dyal P. A molecular phylogeny of heterodont bivalves (Mollusca: Bivalvia: Heterodonta): new analyses of 18S and 28S rRNA genes. Zool Scr. 2007;36: 587–606.
- 80. Giribet G, Wheeler W. On bivalve phylogeny: a high-level analysis of the Bivalvia (Mollusca) based on combined morphology and DNA sequence data. Invertebr Biol. 2002;121: 271–324.
- 81. Frischer ME, Hansen AS, Wyllie JA, Wimbush J, Murray J, Nierzwicki-Bauer SA. Specific amplification of the 18S rRNA gene as a method to detect zebra mussel (Dreissena polymorpha) larvae in plankton samples. Hydrobiologia 2002;487: 33–44.
- 82. Mergeay J, Verschuren D, De Meester L. Invasion of an asexual American water flea clone throughout Africa and rapid displacement of a native sibling species. Proc Biol Sci. 2006;273: 2839–2844. pmid:17015310
- 83. Tsutsui ND, Suarez AV, Holway DA, Case TJ. Reduced genetic variation and the success of an invasive species. Proc Natl Acad Sci. 2000;97: 5948–5953. pmid:10811892
- 84. Golani D, Azzurro E, Corsini-Foka M, Falautano M, Andaloro F, Bernardi G. Genetic bottlenecks and successful biological invasions: the case of a recent Lessepsian migrant. Biol Lett. 2007;3: 541–545. pmid:17686751
- 85. Ren M-X, Zhang Q-G, Zhang D-Y. Random amplified polymorphic DNA markers reveal low genetic variation and a single dominant genotype in Eichhornia crassipes populations throughout China. Weed Res. 2005;45: 236–244.
- 86. Liu J, Dong M, Miao SL, Li ZY, Song MH, Wang RQ. Invasive alien plants in China: role of clonality and geographical origin. Biol Invasions. 2006;8: 1461–1470.
- 87. Roman J, Darling JA. Paradox lost: genetic diversity and the success of aquatic invasions. Trends Ecol Evol. 2007;22: 454–464. pmid:17673331
- 88. Vrijenhoek RC. Animal Clones and Diversity. BioScience. 1998;48: 617–628.
- 89. Prezant RS, Chalermwat K. Flotation of the Bivalve Corbicula fluminea as a Means of Dispersal. Science. 1984;225: 1491–1493. pmid:17770078
- 90. Stanley SM. Adaptative morphology of the shell in bivalves and gastropods. Trueman E R & Clarke M R (eds), The MolluscaThe Mollusca. New York: Academic Press; 1983.
- 91. Gardner JPA, Skibinski DOF. Biological and physical factors influencing genotype dependent mortality in hybrid mussel populations. Mar Ecol Prog. 1991;71: 235–243.
- 92. Willis GL, Skibinski DOF. Variation in strength of attachment to the substrate explains differential mortality in hybrid mussel (Mytilus galloprovincialis and M. edulis) populations. Mar Biol. 1992;112: 403–408.
- 93. Norberg J, Tedengren M. Attack behaviour and predatory success of Asterias rubens L. related to differences in size and morphology of the prey mussel Mytilus edulis L. J Exp Mar Biol Ecol. 1995;186: 207–220.
- 94. Baker AM, Bartlett C, Bunn SE, Goudkamp K, Sheldon F, Hughes JM. Cryptic species and morphological plasticity in long-lived bivalves (Unionoida: Hyriidae) from inland Australia. Mol Ecol. 2003;12: 2707–2717. pmid:12969474
- 95. Crespo D, Dolbeth M, Leston S, Sousa R, Pardal MÂ. Distribution of Corbicula fluminea (Müller, 1774) in the invaded range: a geographic approach with notes on species traits variability. Biol Invasions. 2015;17: 2087–2101.
- 96. Kinzelbach R. Die Körbchenmuscheln Corbicula fluminalis, Corbicula fluminea und Corbicula fluviatilis in Europa (Bivalvia: Corbiculidae). Mainz Naturwissenschaftliches Arch. 1991;29: 215–218.