Delimiting species in the hoplonemertean genus Tetrastemma (phylum Nemertea): morphology is not concordant with phylogeny as evidenced from mtDNA sequences

Redescription of Emplectonema viride - a ubiquitous intertidal hoplonemertean found along the West Coast of North America

Emplectonema viride Stimpson, 1857, a barnacle predator, is one of the most common and conspicuous intertidal nemerteans found along the West Coast of North America from Alaska to California, but it is currently referred to by the wrong name. Briefly described without designation of type material or illustrations, the species was synonymized with the Atlantic look-alike, Emplectonema gracile (Johnston, 1837) by Coe. Here we present morphological and molecular evidence that E. viride is distinct from E. gracile. The two species exhibit differences in color of live specimens and egg size and are clearly differentiated with species delimitation analyses based on sequences of the partial regions of the 16S rRNA and cytochrome c oxidase subunit I genes. In order to improve nomenclatural stability, we re-describe E. viride based on specimens from the southern coast of Oregon and discuss which species should be the type species of the genus. Emplectonema viride was one of the two species originally included in the genus Emplectonema Stimpson, 1857, but subsequent synonymization of E. viride with E. gracile resulted in acceptance of the Atlantic species, E. gracile, as the type species of the genus. We resurrect E. viride Stimpson, 1857 and following Corrêa's designation, this should be the type species of the genus Emplectonema.

Carinina ochracea (Palaeonemertea: Tubulanidae) Reaches Its Southernmost Distribution: New Morphological and Molecular Data

New data for Carinina ochracea Sundberg et al., 2009 are provided for the Iberian Peninsula, establishing the southernmost limit of its known distribution. This species was previously known from only two localities: the type locality in Tjärnö (Sweden) and Pouldohan (Brittany, France). The material examined here was obtained during a faunal survey in the Villaviciosa Estuary (Asturias, northern Iberian Peninsula). The identity of the new specimen was confirmed both by DNA barcoding and anatomical examination. The molecular divergence of all available sequences of this species for four molecular markers, cytochrome c oxidase subunit I (COI), 16S, 18S and 28S rDNA, is discussed. For COI, four polymorphic sites were found, indicating: 1) a nuclear pseudogene; 2) heteroplasmy; or 3) gene duplication of a region of the mitochondrial genome. Two previously overlooked morphological characters were found: the presence of a colour ring and a postfixation staining band (pigmented band), which is histologically characterized. This species is the 12th palaeonemertean and the 75th nemertean reported from Iberian waters.

Integrative Taxonomy of the Micrura alaskensis Coe, 1901 Species Complex (Nemertea: Heteronemertea), with Descriptions of a New Genus Maculaura gen. nov. and Four New Species from the NE Pacific

Micrura alaskensis Coe, 1901 is a common intertidal heteronemertean known from eastern and northwest Pacific (Alaska to Ensenada, Mexico and Akkeshi, Japan, respectively). It is an emerging model system in developmental biology research. We present evidence from morphology of the adults, gametes, and sequences of cytochrome c oxidase subunit I and 16S rRNA genes that it is not one, but a complex of five, cryptic species. All five of these species co-occur at least in part of their geographic range (e.g. southern Oregon). Preliminary cross-hybridization experiments suggest that at least some of these species are reproductively isolated. The five species share characteristics of adult morphology (e.g. accessory buccal glands) and at least four are known to possess a unique larval morphotype--pilidium maculosum. We propose that these characters define a new genus, Maculaura gen. nov., which contains the following five species: Maculaura alaskensis comb. nov., Maculaura aquilonia sp. nov., Maculaura cerebrosa sp. nov., Maculaura oregonensis sp. nov., and Maculaura magna sp. nov. It is unclear which of the five species Coe originally encountered and described. We chose to retain the name "alaskensis" for the species that current researchers know as "Micrura alaskensis", although, presently, it is only known from Washington and Oregon, and has not been collected from Alaska. Maculaura aquilonia sp. nov. is the only member of the genus we have encountered in Alaska, and we show that it also occurs in the Sea of Okhotsk, Russia.

Thirty-Five Years of Nemertean (Nemertea) Research--Past, Present, and Future

Developments in nemertean research over the last 35+ years are reviewed from a systematist's perspective. Nemertean systematics and classification, until fairly recently, was not based on explicit phylogenetic hypotheses, but rather on subjective assessment of "important characters". The first cladistic analyses appeared in the 1980s and were criticized at the time by leading researchers in nemertean systematics for not taking into account convergent evolution in ribbon worm morphology. The first molecular study involving the phylum Nemertea appeared in 1992, followed by reports later in the 1990s and early 2000s. Molecular information is now commonplace in nemertean research, and has changed our understanding of evolutionary relationships within the phylum, as well as our view on species and intraspecific variation. Challenges in nemertean systematics and taxonomy are discussed, with special emphasis on future species descriptions, and how to deal with a number of species names that in all likelihood never will be encountered again. Suggestions for how to deal with these challenges are discussed.

DNA barcoding supports identification of Malacobdella species (Nemertea: Hoplonemertea)

BACKGROUND

Nemerteans of the genus Malacobdella live inside of the mantle cavity of marine bivalves. The genus currently contains only six species, five of which are host-specific and usually found in a single host species, while the sixth species, M. grossa, has a wide host range and has been found in 27 different bivalve species to date. The main challenge of Malacobdella species identification resides in the similarity of the external morphology between species (terminal sucker, gut undulations number, anus position and gonad colouration), and thus, the illustrations provided in the original descriptions do not allow reliable identification. In this article, we analyse the relationships amongthree species of Malacobdella:M.arrokeana,M.japonica andM.grossa,adding new data for the M.grossa and reporting the first for M. japonica, analysing 658 base pairs of the mitochondrial cytochrome c oxidase subunit I gene(COI).Based on these analyses, we present and discuss the potential of DNA barcoding for Malacobdellaspecies identification.

RESULTS

Sixty-four DNA barcoding fragments of the mitochondrial COI gene from three different Malacobdella species (M. arrokeana, M. japonica and M. grossa) are analysed (24 of them newly sequenced for this study, along with four outgroup specimens) and used to delineate species. Divergences, measured as uncorrected differences, between the three species were M.arrokeana-M. grossa11.73%,M.arrokeana-M.japonica 10.62%and M.grossa-M. japonica 10.97%. The mean intraspecific divergence within the ingroup species showed a patent gap with respect to the interspecific ones: 0.18% for M.arrokeana,0.13% for M.grossa and0.02% for M.japonica (rangesfrom 0 to 0.91%).

CONCLUSIONS

We conclude that there is a clear correspondence between the molecular data and distinguishing morphological characters. Our results thus indicate that some morphological characters are useful for species identification and support the potential of DNA barcoding for species identification in a taxonomic group with subtle morphological external differences.

The necessity of DNA taxonomy to reveal cryptic diversity and spatial distribution of meiofauna, with a focus on Nemertea

Meiofauna represent one of the most abundant and diverse communities in marine benthic ecosystems. However, an accurate assessment of diversity at the level of species has been and remains challenging for these microscopic organisms. Therefore, for many taxa, especially the soft body forms such as nemerteans, which often lack clear diagnostic morphological traits, DNA taxonomy is an effective means to assess species diversity. Morphological taxonomy of Nemertea is well documented as complicated by scarcity of unambiguous character states and compromised by diagnoses of a majority of species (and higher clades) being inadequate or based on ambiguous characters and character states. Therefore, recent studies have advocated for the primacy of molecular tools to solve the taxonomy of this group. DNA taxonomy uncovers possible hidden cryptic species, provides a coherent means to systematize taxa in definite clades, and also reveals possible biogeographic patterns. Here, we analyze diversity of nemertean species by considering the barcode region of the mitochondrial gene Cytochrome Oxidase subunit I (COI) and different species delineation approaches in order to infer evolutionarily significant units. In the aim to uncover actual diversity of meiofaunal nemerteans across different sites in Central America, COI sequences were obtained for specimens assigned here to the genera Cephalothrix, Ototyphlonemertes, and Tetrastemma-like worms, each commonly encountered in our sampling. Additional genetic, taxonomic, and geographic data of other specimens belonging to these genera were added from GenBank. Results are consistent across different DNA taxonomy approaches, and revealed (i) the presence of several hidden cryptic species and (ii) numerous potential misidentifications due to traditional taxonomy. (iii) We additionally test a possible biogeographic pattern of taxonomic units revealed by this study, and, except for a few cases, the putative species seem not to be widely distributed, in contrast to what traditional taxonomy would suggest for the recognized morphotypes.

New insights into the phylogeny, systematics and DNA barcoding of Nemertea

Although some clades of ribbon worms (phylum Nemertea) are consistently recovered with high support in molecular phylogenies, the placement and inter-relationships of some taxa have proven problematic. Herein, we performed molecular phylogenetic analyses aimed at resolving these recalcitrant splits, using six loci (nuclear 18S rRNA, 28S rRNA, histones H3 and H4, and mitochondrial 16S rRNA and COI) for 133 terminals, with particular emphasis on the problematic families Hubrechtidae and Plectonemertidae. Three different datasets were used for phylogenetic analyses and both maximum likelihood and maximum parsimony methodologies were applied. All but one of the resulting tree topologies agree on the paraphyly of the class Palaeonemertea, whereas Heteronemertea, Hoplonemertea, Polystilifera, Monostilifera and Hubrechtidae are always recovered as reciprocally monophyletic. Hubrechtidae is sister group to Heteronemertea (the Pilidiophora hypothesis) only when length variable regions of 18S rRNA and 28S rRNA are excluded. Moreover, the terrestrial and freshwater family Plectonemertidae is recovered with high support and the implications of this finding are further discussed. Finally, we evaluate the utility of DNA barcoding for specimen identification within Nemertea using an extended dataset containing 394 COI sequences. Results suggest that DNA barcoding may work for Nemertea, insofar as a distinct barcoding gap (the gap between the maximum intraspecific variation and the minimum interspecific divergence) may exist, but its recognition is regularly hampered by low accuracy in species level identifications.

Statistical parsimony networks and species assemblages in Cephalotrichid nemerteans (nemertea)

BACKGROUND

It has been suggested that statistical parsimony network analysis could be used to get an indication of species represented in a set of nucleotide data, and the approach has been used to discuss species boundaries in some taxa.

METHODOLOGY/PRINCIPAL FINDINGS

Based on 635 base pairs of the mitochondrial protein-coding gene cytochrome c oxidase I (COI), we analyzed 152 nemertean specimens using statistical parsimony network analysis with the connection probability set to 95%. The analysis revealed 15 distinct networks together with seven singletons. Statistical parsimony yielded three networks supporting the species status of Cephalothrix rufifrons, C. major and C. spiralis as they currently have been delineated by morphological characters and geographical location. Many other networks contained haplotypes from nearby geographical locations. Cladistic structure by maximum likelihood analysis overall supported the network analysis, but indicated a false positive result where subnetworks should have been connected into one network/species. This probably is caused by undersampling of the intraspecific haplotype diversity.

CONCLUSIONS/SIGNIFICANCE

Statistical parsimony network analysis provides a rapid and useful tool for detecting possible undescribed/cryptic species among cephalotrichid nemerteans based on COI gene. It should be combined with phylogenetic analysis to get indications of false positive results, i.e., subnetworks that would have been connected with more extensive haplotype sampling.

DNA barcoding should accompany taxonomy - the case of Cerebratulus spp (Nemertea)

Many issues in DNA barcoding need to be solved before it can reach its goal to become a general database for species identification. While species delimitations are more or less well established in several taxa, there are still many groups where this is not the case. Without the proper taxonomic background/knowledge and corroboration with other kinds of data, the DNA barcoding approach may fail to identify species accurately. The classification and taxonomy of phylum Nemertea (nemerteans, ribbon worms) are traditionally based on morphology, but are not corroborated by an increasing amount of genetic data when it comes to classification either into species or into higher taxa. The taxonomy of the phylum needs to be improved before the full potential of DNA barcoding can be utilized to make sure that valid Linnean names accompany the barcode sequences. We illustrate the problematic situation in the phylum Nemertea by a case study from the genus Cerebratulus.

Open-ocean barriers to dispersal: a test case with the Antarctic Polar Front and the ribbon worm Parborlasia corrugatus (Nemertea: Lineidae)

Open-ocean environments provide few obvious barriers to the dispersal of marine organisms. Major currents and/or environmental gradients potentially impede gene flow. One system hypothesized to form an open-ocean dispersal barrier is the Antarctic Polar Front, an area characterized by marked temperature change, deep water, and the high-flow Antarctic Circumpolar current. Despite these potential isolating factors, several invertebrate species occur in both regions, including the broadcast-spawning nemertean worm Parborlasia corrugatus. To empirically test for the presence of an open-ocean dispersal barrier, we sampled P. corrugatus and other nemerteans from southern South America, Antarctica, and the sub-Antarctic islands. Diversity was assessed by analyzing mitochondrial 16S rRNA and cytochrome c oxidase subunit I sequence data with Bayesian inference and tcs haplotype network analysis. Appropriate neutrality tests were also employed. Although our results indicate a single well-mixed lineage in Antarctica and the sub-Antarctic, no evidence for recent gene flow was detected between this population and South American P. corrugatus. Thus, even though P. corrugatus can disperse over large geographical distances, physical oceanographic barriers (i.e. Antarctic Polar Front and Antarctic Circumpolar Current) between continents have likely restricted dispersal over evolutionary time. Genetic distances and haplotype network analysis between South American and Antarctic/sub-Antarctic P. corrugatus suggest that these two populations are possibly two cryptic species.

Genus Tetrastemma Ehrenberg, 1831 (Phylum Nemertea)--a natural group? Phylogenetic relationships inferred from partial 18S rRNA sequences

We investigated the monophyletic status of the hoplonemertean taxon Tetrastemma by reconstructing the phylogeny for 22 specimens assigned to this genus, together with another 25 specimens from closely related hoplonemertean genera. The phylogeny was based on partial 18S rRNA sequences using Bayesian and maximum likelihood analyses. The included Tetrastemma-species formed a well-supported clade, although the within-taxon relationships were unsettled. We conclude that the name Tetrastemma refers to a monophyletic taxon, but that it cannot be defined by morphological synapomorphies, and our results do not imply that all the over 100 species assigned to this genus belong to it. The results furthermore indicate that the genera Amphiporus and Emplectonema are non-monophyletic.