BACKGROUND
anguillicolidae is a family of ichthyoparasitic nematodes endemic to fresh-water eels of the genus anguilla around the pacific and indian oceans.
there are five morphospecies, recently divided into the genera anguillicola  and anguillicoloides
<cit> .
pre-adult and adult worms feed on blood from vessels within the wall of the swim bladder, and their basic life cycles, as far as known, are very similar, involving zooplanktonic and zoobenthic intermediate hosts
<cit> .
european populations of a. crassus have been described as being able to utilise several aquatic species  as paratenic hosts
<cit> .
in the last three decades, interest in this family of nematodes has been driven by a. crassus kuwahara, niimi and itagaki  <dig>  which has spread from its native host, the japanese eel anguilla japonica, to parasitise european eels anguilla anguilla, where it induces significant morbidity and mortality
<cit> .
data collected for three genes  for each of the five species of anguillicolidae.
#: successfully sequenced fragments, h: the number of unique sequences found, l: the length  of the fragments, gc : the gc-content .
in east asia, a. crassus parasitises the native japanese an.
japonica as well as introduced and cultured an.
anguilla and anguilla rostrata
<cit> .
in its native host this parasite is minimally pathogenic, with a small adult body mass and low infection intensity
<cit> .
in the 1980s the parasite was introduced from taiwan into europe as a result of the live eel trade
<cit> , and the parasite colonised wild european eels, an.
anguilla.
in this new host the parasite attains much higher intensities.
anguillicoloides crassus infections have since spread through wild and farmed populations of an.
anguilla in europe and north africa and are associated with cases of mass mortality when paired with environmental stressors such as high temperatures and low dissolved oxygen levels
<cit> .
a two-stage colonisation pattern has been described, consisting of rapid spread upon introduction into a water system followed by equilibration at ceiling levels
<cit> .
a. crassus was subsequently, and likely independently, introduced into populations of an.
rostrata in north america.
the introduction into north america is considered to have been from japan
<cit> .
a. crassus has also recently been reported from the island of réunion near madagascar, where it was found in three indigenous anguilla  species
<cit> .
being a “global invader” , it is considered an important pathogen of the economically relevant atlantic eel species an.
anguilla and an.
rostrata  <cit> .
a. novaezelandiae moravec and taraschewski  <dig> was first described from the short-fin eel anguilla australis from new zealand
<cit> .
however, an explant population of this species was recorded in lake bracciano in italy following stocking with an.
australis in  <dig>
<cit> .
after the introduction of a. crassus into lake bracciano in  <dig>  the a. novaezelandiae population appears to have disappeared
<cit> .
a. australiensis johnston and mawston  <dig> is recorded only from north-east australia where it parasitises the native long-fin eel anguilla reinhardtii  <cit> , and no evidence for pathological effects on the host have been found
<cit> .
a. papernai moravec and taraschewski  <dig> is endemic to south africa and madagascar where it parasitises the african long-fin eel anguilla mossambica.
a. papernai has been shown to be able to complete its life cycle in european intermediate and final hosts in laboratory infections
<cit> .
a. globiceps yamaguti  <dig> is known from certain prefectures of japan and provinces of china
<cit>  where it infects wild populations of the japanese eel an.
japonica.
a phylogenetically correct and robust taxonomy of nematoda is critical to understanding of biodiversity, biogeography and host-parasite coevolution
<cit> .
in the case of the anguillicolidae, it is important to understand the phylogenetic distribution of traits associated with colonisation of and pathogenicity in new hosts, such as the plasticity of life-cycle traits and the increased per-host intensity for invasive a. crassus.
dna sequence data are good characters for taxonomic inference  and for analysis of the deeper phylogenetic history of organisms.
however, loci with different rates of evolution are typically required to resolve these different types of questions.
the nuclear small subunit ribosomal rna gene  has been used extensively for analysis of nematode phylogenetics
<cit> , but has limited resolution at the congeneric level, including within anguillicolidae
<cit> .
in analyses of nematoda using nssu, a. crassus is placed within the entirely animal parasitic clade iii , consisting of the gnathostomatomorpha, ascaridomorpha, oxyuridomorpha, rhigonematomorpha and spiruromorpha.
de ley and blaxter
<cit>  placed anguillicolidae, along with other taxa in dracunculoidea, as incertae sedis within spirurina.
focussed analyses of spirurina using nssu have identified significant conflicts with both classical systematics and the revisions of de ley and blaxter
<cit> , in particular the non-monophyly of 'ascaridomorpha' and the placement of a. crassus together with the vertebrate-parasitic genus gnathostoma, and not allied to other dracunculoids.
the ichthyoparasitic cucullanid truttaedactinis truttae was identified as sister to other analysed spirurina
<cit> .
moravec in his synoptic revision
<cit>  removed the anguillicolidae from the superfamily dracunculoidea and erected the superfamily anguillicoloidea with the family anguillicolidae as its only member.
in addition, based on morphological characters, the sub-genera anguillicoloides and anguillicola were proposed to be promoted to the taxonomic rank of genera
<cit> .
the nuclear large subunit ribosomal rna gene , in particular the section spanning diversity loops d <dig> and d <dig>  is an attractive alternative to nssu, as there are both highly conserved parts and regions of more rapid evolution.
nlsu d1-d <dig> sequences have also been used for analyses of spirurina
<cit> , but the available data are much more sparse than for nssu.
a third candidate locus is the mitochondrial gene cytochrome oxidase i , which has been proposed as a universal dna taxonomy and dna barcoding target for metazoa
<cit> .
this mitochondrial locus evolves very rapidly compared to nssu, such that there is appreciable variation within species.
cox <dig> is beginning to be used widely in analyses of nematode population structure and phylogeography
<cit> , including anguillicola/anguillicoloides  <cit> , generating extensive datasets with, however, low taxonomic coverage.
the assumption underpinning dna taxonomy is the existence of a “barcoding gap”, resulting from the non-overlapping, discrete distribution of intra- and inter-specific variation of the dna fragment analysed
<cit> .
given this, unidentified specimens that differ by less than a threshold sequence divergence from reference voucher sequences  can be assigned to that species.
symmetrically, specimens yielding sequences that are more divergent than the specified threshold can be allocated to a different taxon.
for metazoa, a general cox <dig> dna taxonomy threshold of 2% has been proposed
<cit> , but molecular operational taxonomic units
<cit>  can be defined at any cutoff, and exploration of the pattern of motu count and cutoff is warranted for previously understudied groups
<cit> .
this approach can simplify the identification of potentially morphologically cryptic taxa and encourage revisionary taxonomy
<cit> .
to investigate boundaries of species and genera in anguillicolidae, and assess placement of this family in the diversity of spirurina, we have generated data from nssu, nlsu and cox <dig> for a large sample of anguillicolidae encompassing all five nominal species.
we assessed the empirical support for barcoding gaps in the three genes by analysing the impact of increasing sequence divergence thresholds, and the congruence between motu derived from different loci and species assignments.
we analyse and compare the ability of the three genes to resolve the evolutionary relationships between the anguillicolidae and test whether the morphological taxonomy of this family is supported by molecular evidence.
additionally, we infer a phylogenetic tree for spirurina based on published and new nssu sequences, to further refine our understanding of the relationships of the anguillicolidae, and thus better inform hypotheses as to their origins.
RESULTS
multi-locus marker development from anguillicolidae
one hundred and fifty anguillicolid specimens were obtained representing all five morphologically defined species
<cit> .
primer sets were verified, and three genes  were amplified and sequenced .
we also amplified and sequenced these genes from an unidentified nematode larva  recovered from the serosa of the swim bladder of an.
mossambica from south africa.
not all specimens yielded amplicons for all genes.
for  <dig> specimens we obtained good sequences for all three genes and these form the nssu*, nlsu* and cox1* datasets analysed below.
for motu analyses we also considered an augmented cox <dig> dataset comprising all the sequences generated in this project and also additional published sequences from a. crassus and a. novaezelandiae  <cit>  for a total of  <dig> sequences.
sampling locations for the nematodes including label prefix, information on geographic position , host species and information about sampling.
* collectors are identified by their initials: ak: albert keim, bs: björn schäffner, eh: emanuel heitlinger, eg: ercüment genç, ht: horst taraschewski, jlc: josé lino costa, lp: lea perseke, ow: olaf weyl, pmr: pilar muñoz ruíz, kg: kerstin geiss and hs: hiroshi sato.
** month  and year.
*** a. papernai were obtained from an.
mossambica that were purchased from a commercial eel supplier.
according to the supplier, all eels originate from tributaries of the mangory river.
the exact localities of capture, holding conditions and periods prior to collection are however unknown.
species boundaries and motu in anguillicolidae
we used jmotu
<cit>  to infer the numbers of motu for each gene across a wide range of cutoffs .
there were five unique sequences in the nssu* dataset, and five motus between  <dig> –  <dig> % sequence divergence  and these motus were consistent with the morphological identifications of the specimens.
at  <dig> % sequence divergence cutoff three motus were defined .
at  <dig> % divergence, all the specimens were grouped into a single motu.
there were nine unique sequences in the nlsu* dataset, and for up to  <dig> % sequence divergence, independent motus were defined for a. australiensis, a. novaezelandiae and a. globiceps.
however, at this cutoff there were four motus for a. papernai and two for a. crassus.
an apparent barcoding gap  is evident at cutoffs between  <dig>  –  <dig> % sequence divergence, yielding  <dig> motus.
these motus do not correspond to the morphological species, as one groups all sequences from a. australiensis and a. novaezelandiae, while a. crassus sequences are divided into two motus.
within the cox1* dataset,  <dig> discrete sequences were found.
there is a clear barcoding gap yielding seven motus from  <dig>  –  <dig> % sequence divergence.
in this span motu composition resembles morphological species boundaries, except that a. papernai sequences are divided into  <dig> motus.
five motus are observed only for the narrow range of  <dig>  –  <dig> % sequence divergence, corresponding to a. crassusa.
globiceps, two motus identified as a. papernai and one motu combining a. australiensis and a. novaezelandiae.
the complete cox <dig> dataset had a total of  <dig> different sequences, but these had little impact on motu composition above a cut-off of  <dig> % sequence divergence.
however, motu richness was increased by one across the potential barcoding gap, due to sequences from specimens identified as a. novaezelandiae from new zealand forming a distinct motu from other a. novaezelandiae from tasmania.
<cit>  with motu composition at the critical cutoff intervals .
phylogenetic analyses of spirurina
we used these new sequences in conjunction with existing data to investigate the placement of anguillicolidae within the spirurina, and the interrelationships of anguillicolidae species.
the overall branching order of the phylogenetic tree of the spirurina, inferred using nssu sequences  was consistent with results of previous analyses
<cit> .
spirurina can be divided into three subclades.
spirurina "a", encompassing the cucullanid nematodes dichelyne mexicanus, cucullanus robustus and truttaedacnitis truttae , is the sister group to the other spirurina.
spirurina "b" comprises members of the gnathostomatidae, seuratidae and anguillicolidae, as well as the unidentified eel-parasitic larva snr <dig>  whereas the sister group, spirurina "c", consists of the remaining taxa, including oxyuridomorpha, spiruromorpha, ascaridomorpha, rhigonematomorpha and the remaining dracunculomorpha.
neither gnathostomatinae railliet  <dig>  nor seuratoidea chabaud campan-rouget et brigoo,  <dig> are monophyletic, as most recent common ancestors are shared between linstowinema sp.
and t. tiara , and between anguillicolidae and e. overstreeti .
phylogenetic relationships within the anguillicolidae
both nuclear  and mitochondrial  loci supported the monophyly of the anguillicolidae .
phylogenetic analyses of both nuclear loci, nssu  and nlsu , placed a. crassus as sister to the other four anguillicolids.
we note that the resolution of these analyses is likely limited by the low numbers of parsimony informative sites  present in either region .
analysis of cox <dig>  divided the anguillicolidae into two lineages with respect to morphospecies: a weakly supported clade formed by a. globiceps and a. papernai , and a robustly supported clade consisting of a. crassus, a. novaezelandiae and a. australiensis.
a close relationship of the oceania species a. novaezelandiae and a. australiensis was strongly supported .
in the cox <dig> analyses, the a. novaezelandiae population sampled in tasmania is more closely related to a. australiensis than to the a. novaezelandiae population from new zealand.
mirroring the motu analyses, there is significant divergence within a. papernai, with three strongly supported clades  as distinct from each other as are tasmanian a. novaezelandiae from a. australensis.
population diversity in anguillicola crassus
previous analyses of cox <dig> and microsatellite diversity in a. crassus  <cit>  have defined the likely origin of the european populations  and the effects of isolation-by-distance on the structure of these european populations.
analysis of our cox <dig> data in conjunction with these previous data reinforces patterns observed previously
<cit>  , with east asian populations harbouring the greatest diversity, european and north american populations being closely related to distinct east asian populations, and european populations showing some stratification by geographical location.
however, in our study additional unique haplotypes were found from turkish sampling locations that were grouped with haplotypes from taiwan and china.
<dig>
discussion
anguillicolidae species diversity analysed using dna taxonomy
primers that selectively amplify dna from certain taxa are crucial to surveys in which parasite dna has to be amplified over the background of host dna.
studies on anguillicolidae clearly fall into this category due to the haematophagous life-style of the pre-adult  stages, and thus host blood cells in their digestive system.
the primers used here
<cit>  were broadly successful in amplification of the desired nematode fragments for subsequent sequencing.
thus dna barcoding approaches are readily applicable to these nematodes.
although there were five distinct nssu sequences, corresponding to the five morphological species, these sequences differed by no more than  <dig> bases over the  <dig> bases sequenced.
this close similarity raises the issue of potential misidentification due to sequencing errors, and thus nssu may not be useful for species-level identification surveys of anguillicolids.
as expected the nlsu d2-d <dig> region displayed higher intra- and inter-specific variation among morphospecies than nssu, with  <dig> unique sequences.
this variation, along with the presence of indels, should also enable the design of species-specific amplification primers.
this nlsu fragment has been successfully amplified from partially degraded specimens of a. crassus encapsulated within host tissue
<cit> , and from morphologically indistinguishable life stages such as larvae.
nlsu could be used for pcr-based diagnostics of anguillicoloidosis in eels and can serve as a reliable tool in life-cycle studies.
the methodology of motu definition, as implemented in jmotu
<cit> , explores a defined range of sequence divergence thresholds upon which a plateau of motu richness, i.e.
potential barcoding gaps, can be identified.
for both the nuclear loci studied, no clear evidence of a barcoding gap was found.
the lack of interspecific diversity for the nssu locus limits the utility of this marker at congeneric levels.
multiple within-species nlsu sequence types were only observed in a. crassus  and a. papernai .
one a. crassus sequence type is found globally, while the second is confined to one japanese population, supporting the hypothesis of an asian origin of this species
<cit> .
the highest number of sequence types  was observed for the cox <dig> fragment, which has previously been used to investigate global population structure in a. crassus  <cit> .
we were able to obtain only a single cox <dig> fragment from a. globiceps that contained a correct open reading frame, despite multiple trials.
this phenomenon has been observed in other surveys of this species , and may be due to preferential amplification of nuclear copies of mitochondrial dna
<cit>  or the existence of rna editing of this mitochondrial transcript, as has been observed in other nematodes
<cit> .
sequencing of cox <dig> transcripts and the mitochondrial genome may be informative.
our a. crassus cox <dig> data, when added to those previously determined, reinforces the view that the invasion of west european and north american hosts has only happened a few times  and that the origins of the invading parasite differ.
north american a. crassus are robustly linked to japanese specimens, and european to taiwanese.
by sampling additional locations across china and japan, we identified many additional, unique cox <dig> haplotypes limited to east asia, affirming this as the area of highest diversity, and thus the likely origin of diversity of this species.
interestingly, turkish a. crassus may have distinct origins, as some haplotypes group with chinese and taiwanese sequences not previously observed in western europe.
the cox <dig> locus of the anguillicolidae exhibits a likely barcoding gap with over  <dig> % sequence divergence, exceeding by more than twofold the proposed threshold  for metazoa
<cit> .
although the species a. crassusa.
globiceps and a. australiensis were stably grouped into individual motus within this plateau, sequences of both a. novaezelandiae and a. papernai were found in multiple, distinct cox <dig> motus.
these distinct motus within a single nominal species are possible evidence of cryptic speciation.
the madagascar populations of a. papernai may be the source of introduction into south africa, since all sequence types from the five sampling sites in south africa occur in, or are closely related to, those from madagascar.
this inference is supported by the pattern observed for the four nlsu sequence types of a. papernai, which are all present in madagascar.
we note that the mitochondrial locus cox <dig> has uniparental inheritance and thus may not be an unbiased reporter of phylogeographic history, but propose these hypotheses as testable inferences from our data.
specimen snr <dig> was a single, unidentified larva from the swim bladder serosa of a south african a. mossambica, and the whole specimen was used for dna extraction.
its nssu sequence showed that it is a member of spirurina b. sampling site, host species and phylogenetic position in the spirurina suggest this specimen belongs to or is closely affiliated to paraquimperia africana
<cit> .
dna taxonomy and molecular phylogenetic systematics of the anguillicolidae
we found no support for the division of the anguillicolidae into the two genera anguillicola and anguillicoloides as proposed by moravec
<cit> , as anguillicola globiceps was recovered within the radiation of anguillicoloides species, suggesting that that the morphological criteria used to erect these putative genera  may not be phylogenetically informative.
as noted previously
<cit> , modifications of the structure of the oesophagus, a trait related to the mode of nutrition, may occur independently in the course of trophic adaptations.
our data support a single generic division for the species in anguillicolidae, which by priority should be called anguillicola, restoring anguillicoloides crassus to anguillicola crassus and ensuring continuity with historical literature on this important species.
based on the analysis of the dna sequences of three genes, a reevaluation of the taxonomy of the anguillicolidea may be necessary.
we identified eight discrete cox <dig> motus over a wide range of divergence, up to 5%, suggesting that there may be eight species-level taxa represented if a barcoding cutoff of 2% is accepted.
three motus are congruent with morphological species identifications: a. crassus, a. globiceps and a. australiensis.
however specimens unambiguously assigned to the morphological species a. novaezelandiae and a. papernai are found in multiple motus .
these additional motus could represent morphologically cryptic, or previously unrecognised species.
in the case of a. novaezelandiae phylogenetic analyses of the cox <dig> locus suggest the paraphyly of this species, as the specimens from new zealand  are robustly placed as the sister group to the clade consisting of a. novaezelandiae specimens from tasmania  plus a. australiensis.
since nlsu d2-d <dig> sequences are not available for a. novaezelandiae from new zealand, additional representatives should be sampled to more thoroughly test the monophyly of this species.
a. papernai displays the greatest number of nlsu d2-d <dig> sequence types among the anguillicolidae and its cox <dig> locus displays a clear division into three clades and is represented by three motus.
whether these are distinct species level taxa, or merely distinct diverse populations of a widespread species will require analysis of additional loci.
a. papernai is found in well separated watersheds, with no linking waterways, and an absence of human-induced admixture through stocking.
the diversity within this species, together with the observation that it is able to complete its life cycle in european eel hosts
<cit> , supports the need for further research.
a similar branching pattern is observed for both nlsu and cox <dig> for all species analysed except a. crassus.
analyses of the two nuclear loci revealed a. crassus to be sister to the other sampled species within anguillicolidae.
however the mitochondrial cox <dig> gene placed it as the sister taxon to the clade comprising a. australiensis and a. novaezelandiae.
despite the low number of informative sites in the nlsu and nssu loci, we favour the hypothesis based on the nuclear loci, as no close outgroup sequences were available for cox <dig> and the sequences used  may have impacted on proper rooting of the cox <dig> tree.
reanalyses with cox <dig> data from seuratoidea or gnathostomatinae would address this issue.
these data suggest a scenario of asian origin for the anguillicolidae, since a. papernai is the only endemic species west of the  <dig> degree line of longitude.
speciation of a. crassus could have taken place in east asian waters, followed by the cladogeneses of the oceanian species and the clade comprising a. papernai and a. globiceps.
the biogeographic distribution of a. papernai observed today could thus be explained by a host-related dispersal of its ancestor.
comparing the anguillicolidae phylogenies with those of their hosts
<cit>  we can infer several host switches within the nematodes, even excluding recent host-range expansion by a. crassus and a. novaezelandiae .
the host phylogeny includes a deep split between an indo-pacific group, including an.
japonica, and an oceania/atlantic group, including an.
anguilla and an.
australis.
a. australiensis, parasitising an.
reinhardtii in the indo-pacific host group is sister to a. novaezelandiae parasitising an.
australis in the oceania host group.
if the relationship between anguillicola and its eel hosts was established in an ancestral indo-pacific host, a minimum of three host-capture events, two of them across the major split in the eel phylogeny, are required to explain the endemic distribution of these eel parasites.
the recent proven ability of a. crassus to exploit new, phylogenetically well-separated hosts may be a reflection of this evolutionary propensity for switching.
<cit>  and its swim bladder parasites, the family anguillicolidae  .
the known host parasite relationships are indicated by: black lines = traditional host-parasite relationship, displaying low abundances and low pathogenicity ; red lines = novel host-parasite relationship, displaying high abundances and pathogenicity; orange lines = novel host-parasite relationship, displaying low abundances and pathogenicity; dashed orange lines = novel host-parasite relationship where completion of the life cycle has not been demonstrated.
phylogenetic relationships within spirurina
spirurina includes parasites with a direct lifecyle as well as those that include biological vector and transport  hosts in complex, multi-species systems
<cit> .
although some applications of parsimony principles might suggest that the simpler, direct life cycle should be ancestral for the group, molecular phylogenetic analyses robustly place two clades of parasites  that have complex life histories as successive sister groups, at the base of the spirurina, to clades that include direct life cycle parasites.
this phylogenetic hypothesis favors intepretation of complex life histories as the ancestral state for the group.
we found that members of the ichthyoparasitic family cucullanidae  form a well-supported clade  that is sister to the remaining spirurina
<cit> .
anguillicolidae are placed in spirurina b along with certain members of the superfamily seuratoidea and all sampled gnathostomatoidea.
spirurina c includes the remaining taxa including the abundantly sampled ascaridomorpha and spiruromorpha.
previous analyses of taxa in spirurina c have suggested that many classical groups are non-monophyletic, in particular "ascaridomorpha" and "spiruromorpha"
<cit> , a result echoed here.
the position of the three gnathostoma species  outside of spirurina c has been observed previously
<cit>  and was recovered consistently in all analyses.
members of the other two genera of the gnathostomatinae, i.e.
t. tiara and e. overstreeti, are also displaced, and share direct ancestors with linstowinema sp.
and members of anguillicolidae, respectively.
this result indicates the paraphyly of both gnathostomatinae and seuratoidea.
the dracunculoidea is also rendered paraphyletic, as members are found in spirurina b and spirurina c. this phylogenetic hypothesis reveals an enormous definitive host diversity within the spirurina b, comprising fresh-water teleosts , chondrichthyes , mammals  and squamates , and an even wider diversity of vector and paratenic hosts.
denser taxon sampling in the spirurina, with special emphasis on the morphologically diverse spirurina b and c, is required to fully explore this fascinating group.
CONCLUSIONS
we have investigated the genetic diversity within, and genetic distinctiveness of the five described species of the family anguillicolidae.
we found no support for the erection of two genera within the family, and identified two species  where within-species divergences and phylogenetic tree topology suggests the presence of distinct cryptic taxa.
the role of host vicariance in the speciation of a. globiceps and a. papernai is an intriguing topic for future study, as is the evolutionary history of the oceanian species .
we revisited the phylogeny of the spirurina and identified a clade of vertebrate-parasitic taxa that includes anguillicoloidae and members of other families, which must therefore be suspected of being paraphyletic.
our analyses highlight the possibility that a. papernai might transfer to and be pathogenic in new, economically important, eel hosts, and the data generated here will, we hope, act as reference for future dna barcoding surveys of eel swim bladder parasites worldwide.

 anguillicolidae is a family of ichthyoparasitic nematodes endemic to fresh-water eels of the genus anguilla around the pacific and indian oceans. there are five morphospecies, recently divided into the genera anguillicola and anguillicoloides. pre-adult and adult worms feed on blood from vessels within the wall of the swim bladder, and their basic life cycles, as far as known, are very similar, involving zooplanktonic and zoobenthic intermediate hosts. european populations of a. crassus have been described as being able to utilise several aquatic species as paratenic hosts. in the last three decades, interest in this family of nematodes has been driven by a. crassus kuwahara, niimi and itagaki which has spread from its native host, the japanese eel anguilla japonica, to parasitise european eels anguilla anguilla, where it induces significant morbidity and mortality. data collected for three genes for each of the five species of anguillicolidae. #: successfully sequenced fragments, h: the number of unique sequences found, l: the length of the fragments, gc: the gc-content. in east asia, a. crassus parasitises the native japanese an. japonica as well as introduced and cultured an. anguilla and anguilla rostrata. in its native host this parasite is minimally pathogenic, with a small adult body mass and low infection intensity. in the 1980s the parasite was introduced from taiwan into europe as a result of the live eel trade, and the parasite colonised wild european eels, an. anguilla. in this new host the parasite attains much higher intensities. anguillicoloides crassus infections have since spread through wild and farmed populations of an. anguilla in europe and north africa and are associated with cases of mass mortality when paired with environmental stressors such as high temperatures and low dissolved oxygen levels. a two-stage colonisation pattern has been described, consisting of rapid spread upon introduction into a water system followed by equilibration at ceiling levels. a. crassus was subsequently, and likely independently, introduced into populations of an. rostrata in north america. the introduction into north america is considered to have been from japan. a. crassus has also recently been reported from the island of réunion near madagascar, where it was found in three indigenous anguilla species. being a “global invader”, it is considered an important pathogen of the economically relevant atlantic eel species an. anguilla and an. rostrata. a. novaezelandiae moravec and taraschewski was first described from the short-fin eel anguilla australis from new zealand. however, an explant population of this species was recorded in lake bracciano in italy following stocking with an. australis in. after the introduction of a. crassus into lake bracciano in the a. novaezelandiae population appears to have disappeared. a. australiensis johnston and mawston is recorded only from north-east australia where it parasitises the native long-fin eel anguilla reinhardtii, and no evidence for pathological effects on the host have been found. a. papernai moravec and taraschewski is endemic to south africa and madagascar where it parasitises the african long-fin eel anguilla mossambica. a. papernai has been shown to be able to complete its life cycle in european intermediate and final hosts in laboratory infections. a. globiceps yamaguti is known from certain prefectures of japan and provinces of china where it infects wild populations of the japanese eel an. japonica. a phylogenetically correct and robust taxonomy of nematoda is critical to understanding of biodiversity, biogeography and host-parasite coevolution. in the case of the anguillicolidae, it is important to understand the phylogenetic distribution of traits associated with colonisation of and pathogenicity in new hosts, such as the plasticity of life-cycle traits and the increased per-host intensity for invasive a. crassus. dna sequence data are good characters for taxonomic inference and for analysis of the deeper phylogenetic history of organisms. however, loci with different rates of evolution are typically required to resolve these different types of questions. the nuclear small subunit ribosomal rna gene has been used extensively for analysis of nematode phylogenetics, but has limited resolution at the congeneric level, including within anguillicolidae. in analyses of nematoda using nssu, a. crassus is placed within the entirely animal parasitic clade iii, consisting of the gnathostomatomorpha, ascaridomorpha, oxyuridomorpha, rhigonematomorpha and spiruromorpha. de ley and blaxter placed anguillicolidae, along with other taxa in dracunculoidea, as incertae sedis within spirurina. focussed analyses of spirurina using nssu have identified significant conflicts with both classical systematics and the revisions of de ley and blaxter, in particular the non-monophyly of'ascaridomorpha' and the placement of a. crassus together with the vertebrate-parasitic genus gnathostoma, and not allied to other dracunculoids. the ichthyoparasitic cucullanid truttaedactinis truttae was identified as sister to other analysed spirurina. moravec in his synoptic revision removed the anguillicolidae from the superfamily dracunculoidea and erected the superfamily anguillicoloidea with the family anguillicolidae as its only member. in addition, based on morphological characters, the sub-genera anguillicoloides and anguillicola were proposed to be promoted to the taxonomic rank of genera. the nuclear large subunit ribosomal rna gene, in particular the section spanning diversity loops d and d is an attractive alternative to nssu, as there are both highly conserved parts and regions of more rapid evolution. nlsu d1-d sequences have also been used for analyses of spirurina, but the available data are much more sparse than for nssu. a third candidate locus is the mitochondrial gene cytochrome oxidase i, which has been proposed as a universal dna taxonomy and dna barcoding target for metazoa. this mitochondrial locus evolves very rapidly compared to nssu, such that there is appreciable variation within species. cox is beginning to be used widely in analyses of nematode population structure and phylogeography, including anguillicola/anguillicoloides, generating extensive datasets with, however, low taxonomic coverage. the assumption underpinning dna taxonomy is the existence of a “barcoding gap”, resulting from the non-overlapping, discrete distribution of intra- and inter-specific variation of the dna fragment analysed. given this, unidentified specimens that differ by less than a threshold sequence divergence from reference voucher sequences can be assigned to that species. symmetrically, specimens yielding sequences that are more divergent than the specified threshold can be allocated to a different taxon. for metazoa, a general cox dna taxonomy threshold of 2% has been proposed, but molecular operational taxonomic units can be defined at any cutoff, and exploration of the pattern of motu count and cutoff is warranted for previously understudied groups. this approach can simplify the identification of potentially morphologically cryptic taxa and encourage revisionary taxonomy. to investigate boundaries of species and genera in anguillicolidae, and assess placement of this family in the diversity of spirurina, we have generated data from nssu, nlsu and cox for a large sample of anguillicolidae encompassing all five nominal species. we assessed the empirical support for barcoding gaps in the three genes by analysing the impact of increasing sequence divergence thresholds, and the congruence between motu derived from different loci and species assignments. we analyse and compare the ability of the three genes to resolve the evolutionary relationships between the anguillicolidae and test whether the morphological taxonomy of this family is supported by molecular evidence. additionally, we infer a phylogenetic tree for spirurina based on published and new nssu sequences, to further refine our understanding of the relationships of the anguillicolidae, and thus better inform hypotheses as to their origins.  multi-locus marker development from anguillicolidae one hundred and fifty anguillicolid specimens were obtained representing all five morphologically defined species. primer sets were verified, and three genes were amplified and sequenced. we also amplified and sequenced these genes from an unidentified nematode larva recovered from the serosa of the swim bladder of an. mossambica from south africa. not all specimens yielded amplicons for all genes. for specimens we obtained good sequences for all three genes and these form the nssu*, nlsu* and cox1* datasets analysed below. for motu analyses we also considered an augmented cox dataset comprising all the sequences generated in this project and also additional published sequences from a. crassus and a. novaezelandiae for a total of sequences. sampling locations for the nematodes including label prefix, information on geographic position, host species and information about sampling. * collectors are identified by their initials: ak: albert keim, bs: björn schäffner, eh: emanuel heitlinger, eg: ercüment genç, ht: horst taraschewski, jlc: josé lino costa, lp: lea perseke, ow: olaf weyl, pmr: pilar muñoz ruíz, kg: kerstin geiss and hs: hiroshi sato. ** month and year. *** a. papernai were obtained from an. mossambica that were purchased from a commercial eel supplier. according to the supplier, all eels originate from tributaries of the mangory river. the exact localities of capture, holding conditions and periods prior to collection are however unknown. species boundaries and motu in anguillicolidae we used jmotu to infer the numbers of motu for each gene across a wide range of cutoffs. there were five unique sequences in the nssu* dataset, and five motus between – % sequence divergence and these motus were consistent with the morphological identifications of the specimens. at % sequence divergence cutoff three motus were defined. at % divergence, all the specimens were grouped into a single motu. there were nine unique sequences in the nlsu* dataset, and for up to % sequence divergence, independent motus were defined for a. australiensis, a. novaezelandiae and a. globiceps. however, at this cutoff there were four motus for a. papernai and two for a. crassus. an apparent barcoding gap is evident at cutoffs between – % sequence divergence, yielding motus. these motus do not correspond to the morphological species, as one groups all sequences from a. australiensis and a. novaezelandiae, while a. crassus sequences are divided into two motus. within the cox1* dataset, discrete sequences were found. there is a clear barcoding gap yielding seven motus from – % sequence divergence. in this span motu composition resembles morphological species boundaries, except that a. papernai sequences are divided into motus. five motus are observed only for the narrow range of – % sequence divergence, corresponding to a. crassusa. globiceps, two motus identified as a. papernai and one motu combining a. australiensis and a. novaezelandiae. the complete cox dataset had a total of different sequences, but these had little impact on motu composition above a cut-off of % sequence divergence. however, motu richness was increased by one across the potential barcoding gap, due to sequences from specimens identified as a. novaezelandiae from new zealand forming a distinct motu from other a. novaezelandiae from tasmania. with motu composition at the critical cutoff intervals. phylogenetic analyses of spirurina we used these new sequences in conjunction with existing data to investigate the placement of anguillicolidae within the spirurina, and the interrelationships of anguillicolidae species. the overall branching order of the phylogenetic tree of the spirurina, inferred using nssu sequences was consistent with results of previous analyses. spirurina can be divided into three subclades. spirurina "a", encompassing the cucullanid nematodes dichelyne mexicanus, cucullanus robustus and truttaedacnitis truttae, is the sister group to the other spirurina. spirurina "b" comprises members of the gnathostomatidae, seuratidae and anguillicolidae, as well as the unidentified eel-parasitic larva snr whereas the sister group, spirurina "c", consists of the remaining taxa, including oxyuridomorpha, spiruromorpha, ascaridomorpha, rhigonematomorpha and the remaining dracunculomorpha. neither gnathostomatinae railliet nor seuratoidea chabaud campan-rouget et brigoo, are monophyletic, as most recent common ancestors are shared between linstowinema sp. and t. tiara, and between anguillicolidae and e. overstreeti. phylogenetic relationships within the anguillicolidae both nuclear and mitochondrial loci supported the monophyly of the anguillicolidae. phylogenetic analyses of both nuclear loci, nssu and nlsu, placed a. crassus as sister to the other four anguillicolids. we note that the resolution of these analyses is likely limited by the low numbers of parsimony informative sites present in either region. analysis of cox divided the anguillicolidae into two lineages with respect to morphospecies: a weakly supported clade formed by a. globiceps and a. papernai, and a robustly supported clade consisting of a. crassus, a. novaezelandiae and a. australiensis. a close relationship of the oceania species a. novaezelandiae and a. australiensis was strongly supported. in the cox analyses, the a. novaezelandiae population sampled in tasmania is more closely related to a. australiensis than to the a. novaezelandiae population from new zealand. mirroring the motu analyses, there is significant divergence within a. papernai, with three strongly supported clades as distinct from each other as are tasmanian a. novaezelandiae from a. australensis. population diversity in anguillicola crassus previous analyses of cox and microsatellite diversity in a. crassus have defined the likely origin of the european populations and the effects of isolation-by-distance on the structure of these european populations. analysis of our cox data in conjunction with these previous data reinforces patterns observed previously, with east asian populations harbouring the greatest diversity, european and north american populations being closely related to distinct east asian populations, and european populations showing some stratification by geographical location. however, in our study additional unique haplotypes were found from turkish sampling locations that were grouped with haplotypes from taiwan and china. discussion anguillicolidae species diversity analysed using dna taxonomy primers that selectively amplify dna from certain taxa are crucial to surveys in which parasite dna has to be amplified over the background of host dna. studies on anguillicolidae clearly fall into this category due to the haematophagous life-style of the pre-adult stages, and thus host blood cells in their digestive system. the primers used here were broadly successful in amplification of the desired nematode fragments for subsequent sequencing. thus dna barcoding approaches are readily applicable to these nematodes. although there were five distinct nssu sequences, corresponding to the five morphological species, these sequences differed by no more than bases over the bases sequenced. this close similarity raises the issue of potential misidentification due to sequencing errors, and thus nssu may not be useful for species-level identification surveys of anguillicolids. as expected the nlsu d2-d region displayed higher intra- and inter-specific variation among morphospecies than nssu, with unique sequences. this variation, along with the presence of indels, should also enable the design of species-specific amplification primers. this nlsu fragment has been successfully amplified from partially degraded specimens of a. crassus encapsulated within host tissue, and from morphologically indistinguishable life stages such as larvae. nlsu could be used for pcr-based diagnostics of anguillicoloidosis in eels and can serve as a reliable tool in life-cycle studies. the methodology of motu definition, as implemented in jmotu, explores a defined range of sequence divergence thresholds upon which a plateau of motu richness, i.e. potential barcoding gaps, can be identified. for both the nuclear loci studied, no clear evidence of a barcoding gap was found. the lack of interspecific diversity for the nssu locus limits the utility of this marker at congeneric levels. multiple within-species nlsu sequence types were only observed in a. crassus and a. papernai. one a. crassus sequence type is found globally, while the second is confined to one japanese population, supporting the hypothesis of an asian origin of this species. the highest number of sequence types was observed for the cox fragment, which has previously been used to investigate global population structure in a. crassus. we were able to obtain only a single cox fragment from a. globiceps that contained a correct open reading frame, despite multiple trials. this phenomenon has been observed in other surveys of this species, and may be due to preferential amplification of nuclear copies of mitochondrial dna or the existence of rna editing of this mitochondrial transcript, as has been observed in other nematodes. sequencing of cox transcripts and the mitochondrial genome may be informative. our a. crassus cox data, when added to those previously determined, reinforces the view that the invasion of west european and north american hosts has only happened a few times and that the origins of the invading parasite differ. north american a. crassus are robustly linked to japanese specimens, and european to taiwanese. by sampling additional locations across china and japan, we identified many additional, unique cox haplotypes limited to east asia, affirming this as the area of highest diversity, and thus the likely origin of diversity of this species. interestingly, turkish a. crassus may have distinct origins, as some haplotypes group with chinese and taiwanese sequences not previously observed in western europe. the cox locus of the anguillicolidae exhibits a likely barcoding gap with over % sequence divergence, exceeding by more than twofold the proposed threshold for metazoa. although the species a. crassusa. globiceps and a. australiensis were stably grouped into individual motus within this plateau, sequences of both a. novaezelandiae and a. papernai were found in multiple, distinct cox motus. these distinct motus within a single nominal species are possible evidence of cryptic speciation. the madagascar populations of a. papernai may be the source of introduction into south africa, since all sequence types from the five sampling sites in south africa occur in, or are closely related to, those from madagascar. this inference is supported by the pattern observed for the four nlsu sequence types of a. papernai, which are all present in madagascar. we note that the mitochondrial locus cox has uniparental inheritance and thus may not be an unbiased reporter of phylogeographic history, but propose these hypotheses as testable inferences from our data. specimen snr was a single, unidentified larva from the swim bladder serosa of a south african a. mossambica, and the whole specimen was used for dna extraction. its nssu sequence showed that it is a member of spirurina b. sampling site, host species and phylogenetic position in the spirurina suggest this specimen belongs to or is closely affiliated to paraquimperia africana. dna taxonomy and molecular phylogenetic systematics of the anguillicolidae we found no support for the division of the anguillicolidae into the two genera anguillicola and anguillicoloides as proposed by moravec, as anguillicola globiceps was recovered within the radiation of anguillicoloides species, suggesting that that the morphological criteria used to erect these putative genera may not be phylogenetically informative. as noted previously, modifications of the structure of the oesophagus, a trait related to the mode of nutrition, may occur independently in the course of trophic adaptations. our data support a single generic division for the species in anguillicolidae, which by priority should be called anguillicola, restoring anguillicoloides crassus to anguillicola crassus and ensuring continuity with historical literature on this important species. based on the analysis of the dna sequences of three genes, a reevaluation of the taxonomy of the anguillicolidea may be necessary. we identified eight discrete cox motus over a wide range of divergence, up to 5%, suggesting that there may be eight species-level taxa represented if a barcoding cutoff of 2% is accepted. three motus are congruent with morphological species identifications: a. crassus, a. globiceps and a. australiensis. however specimens unambiguously assigned to the morphological species a. novaezelandiae and a. papernai are found in multiple motus. these additional motus could represent morphologically cryptic, or previously unrecognised species. in the case of a. novaezelandiae phylogenetic analyses of the cox locus suggest the paraphyly of this species, as the specimens from new zealand are robustly placed as the sister group to the clade consisting of a. novaezelandiae specimens from tasmania plus a. australiensis. since nlsu d2-d sequences are not available for a. novaezelandiae from new zealand, additional representatives should be sampled to more thoroughly test the monophyly of this species. a. papernai displays the greatest number of nlsu d2-d sequence types among the anguillicolidae and its cox locus displays a clear division into three clades and is represented by three motus. whether these are distinct species level taxa, or merely distinct diverse populations of a widespread species will require analysis of additional loci. a. papernai is found in well separated watersheds, with no linking waterways, and an absence of human-induced admixture through stocking. the diversity within this species, together with the observation that it is able to complete its life cycle in european eel hosts, supports the need for further research. a similar branching pattern is observed for both nlsu and cox for all species analysed except a. crassus. analyses of the two nuclear loci revealed a. crassus to be sister to the other sampled species within anguillicolidae. however the mitochondrial cox gene placed it as the sister taxon to the clade comprising a. australiensis and a. novaezelandiae. despite the low number of informative sites in the nlsu and nssu loci, we favour the hypothesis based on the nuclear loci, as no close outgroup sequences were available for cox and the sequences used may have impacted on proper rooting of the cox tree. reanalyses with cox data from seuratoidea or gnathostomatinae would address this issue. these data suggest a scenario of asian origin for the anguillicolidae, since a. papernai is the only endemic species west of the degree line of longitude. speciation of a. crassus could have taken place in east asian waters, followed by the cladogeneses of the oceanian species and the clade comprising a. papernai and a. globiceps. the biogeographic distribution of a. papernai observed today could thus be explained by a host-related dispersal of its ancestor. comparing the anguillicolidae phylogenies with those of their hosts we can infer several host switches within the nematodes, even excluding recent host-range expansion by a. crassus and a. novaezelandiae. the host phylogeny includes a deep split between an indo-pacific group, including an. japonica, and an oceania/atlantic group, including an. anguilla and an. australis. a. australiensis, parasitising an. reinhardtii in the indo-pacific host group is sister to a. novaezelandiae parasitising an. australis in the oceania host group. if the relationship between anguillicola and its eel hosts was established in an ancestral indo-pacific host, a minimum of three host-capture events, two of them across the major split in the eel phylogeny, are required to explain the endemic distribution of these eel parasites. the recent proven ability of a. crassus to exploit new, phylogenetically well-separated hosts may be a reflection of this evolutionary propensity for switching. and its swim bladder parasites, the family anguillicolidae. the known host parasite relationships are indicated by: black lines = traditional host-parasite relationship, displaying low abundances and low pathogenicity; red lines = novel host-parasite relationship, displaying high abundances and pathogenicity; orange lines = novel host-parasite relationship, displaying low abundances and pathogenicity; dashed orange lines = novel host-parasite relationship where completion of the life cycle has not been demonstrated. phylogenetic relationships within spirurina spirurina includes parasites with a direct lifecyle as well as those that include biological vector and transport hosts in complex, multi-species systems. although some applications of parsimony principles might suggest that the simpler, direct life cycle should be ancestral for the group, molecular phylogenetic analyses robustly place two clades of parasites that have complex life histories as successive sister groups, at the base of the spirurina, to clades that include direct life cycle parasites. this phylogenetic hypothesis favors intepretation of complex life histories as the ancestral state for the group. we found that members of the ichthyoparasitic family cucullanidae form a well-supported clade that is sister to the remaining spirurina. anguillicolidae are placed in spirurina b along with certain members of the superfamily seuratoidea and all sampled gnathostomatoidea. spirurina c includes the remaining taxa including the abundantly sampled ascaridomorpha and spiruromorpha. previous analyses of taxa in spirurina c have suggested that many classical groups are non-monophyletic, in particular "ascaridomorpha" and "spiruromorpha", a result echoed here. the position of the three gnathostoma species outside of spirurina c has been observed previously and was recovered consistently in all analyses. members of the other two genera of the gnathostomatinae, i.e. t. tiara and e. overstreeti, are also displaced, and share direct ancestors with linstowinema sp. and members of anguillicolidae, respectively. this result indicates the paraphyly of both gnathostomatinae and seuratoidea. the dracunculoidea is also rendered paraphyletic, as members are found in spirurina b and spirurina c. this phylogenetic hypothesis reveals an enormous definitive host diversity within the spirurina b, comprising fresh-water teleosts, chondrichthyes, mammals and squamates, and an even wider diversity of vector and paratenic hosts. denser taxon sampling in the spirurina, with special emphasis on the morphologically diverse spirurina b and c, is required to fully explore this fascinating group.  we have investigated the genetic diversity within, and genetic distinctiveness of the five described species of the family anguillicolidae. we found no support for the erection of two genera within the family, and identified two species where within-species divergences and phylogenetic tree topology suggests the presence of distinct cryptic taxa. the role of host vicariance in the speciation of a. globiceps and a. papernai is an intriguing topic for future study, as is the evolutionary history of the oceanian species. we revisited the phylogeny of the spirurina and identified a clade of vertebrate-parasitic taxa that includes anguillicoloidae and members of other families, which must therefore be suspected of being paraphyletic. our analyses highlight the possibility that a. papernai might transfer to and be pathogenic in new, economically important, eel hosts, and the data generated here will, we hope, act as reference for future dna barcoding surveys of eel swim bladder parasites worldwide. 
the propensity for host switching, and thus the potential for invasive behaviour, is found in a. crassus, a. novaezelandiae and a. papernai, and thus may be common to the group. anguillicolidae yamaguti, is a family of parasitic nematode infecting fresh-water eels of the genus anguilla, comprising five species in the genera anguillicola and anguillicoloides. genetic diversity within the african species anguillicoloides papernai was suggestive of cryptic taxa, as was the finding of distinct lineages of anguillicoloides novaezelandiae in new zealand and tasmania. the described species may hide cryptic taxa, identified here by dna taxonomy, and this dna barcoding approach may assist in tracking species invasions. anguillicoloides crassus is of particular importance, as it has recently spread from its endemic range in the eastern pacific to europe and north america, where it poses a significant threat to new, naïve hosts such as the economic important eel species anguilla anguilla and anguilla rostrata.
 anguillicolidae yamaguti, is a family of parasitic nematode infecting fresh-water eels of the genus anguilla, comprising five species in the genera anguillicola and anguillicoloides. anguillicoloides crassus is of particular importance, as it has recently spread from its endemic range in the eastern pacific to europe and north america, where it poses a significant threat to new, naïve hosts such as the economic important eel species anguilla anguilla and anguilla rostrata. the anguillicolidae are therefore all potentially invasive taxa, but the relationships of the described species remain unclear. anguillicolidae is part of spirurina, a diverse clade made up of only animal parasites, but placement of the family within spirurina is based on limited data.  we generated an extensive dna sequence dataset from three loci for the five species of anguillicolidae and used this to investigate specific and generic boundaries within the family, and the relationship of anguillicolidae to other spirurine nematodes. neither nuclear nor mitochondrial sequences supported monophyly of anguillicoloides. genetic diversity within the african species anguillicoloides papernai was suggestive of cryptic taxa, as was the finding of distinct lineages of anguillicoloides novaezelandiae in new zealand and tasmania. phylogenetic analysis of the spirurina grouped the anguillicolidae together with members of the gnathostomatidae and seuratidae.  the anguillicolidae is part of a complex radiation of parasitic nematodes of vertebrates with wide host diversity, most closely related to other marine vertebrate parasites that also have complex life cycles. molecular analyses do not support the recent division of anguillicolidae into two genera. the described species may hide cryptic taxa, identified here by dna taxonomy, and this dna barcoding approach may assist in tracking species invasions. the propensity for host switching, and thus the potential for invasive behaviour, is found in a. crassus, a. novaezelandiae and a. papernai, and thus may be common to the group. anguillicolaanguillicoloidesinvasivehost switchcryptic speciesdna-taxonomybarcoding 