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.

Pseudocnidae of ribbon worms (Nemertea): ultrastructure, maturation, and functional morphology.

The fine structure of mature pseudocnidae of 32 species of nemerteans, representatives of 20 genera, six families, and two classes was investigated with scanning and transmission electron microscopy. Pseudocnidae are composed of four layers (cortex, medulla, precore layer, and core) in most species investigated, but the degree of development and position of each layer can vary between different species. The secretion products comprising immature pseudocnidae segregate into separate layers: a thin envelope, which subsequently separates into the cortex and medulla and an extensive internal layer. We distinguish two pseudocnida types: type I is characterized by a two-layered core and type II by a three-layered core. Type I pseudocnidae are present in archinemertean species, Carinoma mutabilis, and in all pilidiophoran species, except Heteronemertea sp. 5DS; type II pseudocnidae occur in all studied species of Tubulanidae and the basal Heteronemertea sp. 5DS. Based on the structure of the discharged pseudocnidae observed in eleven species of palaeonemerteans and in eight species of pilidiophorans, we distinguish three different mechanisms (1-3) of core extrusion/discharge with the following characteristics and distribution: (1) the outer core layer is everted simultaneously with the tube-like layer and occurs in type I pseudocnidae of most species; (2) the extruded core is formed by both eversion of the outer core layer and medullar layer, and occurs in type I pseudocnidae of Micrura cf. bella; (3) the eversion of the outer core layer begins together with the core rod and core rod lamina and occurs in type II pseudocnidae. Morpho-functional comparison with other extrusomes (cnidae, sagittocysts, rhabdtites, and paracnids) confirm that pseudocnidae are homologous structures that are unique to nemerteans.

Pseudocnidae of archinemerteans (Nemertea, Palaeonemertea) and their implications for nemertean systematics.

The structure of pseudocnidae of 16 species of Palaeonemertea clade Archinemertea (= Cephalotrichida s.l.) was investigated with confocal laser, scanning, and transmission electron microscopy (TEM). All species of the genus Cephalothrix possess two kinds of pseudocnidae, large and small. Only one type of pseudocnida is present in Balionemertes and Cephalotrichella. TEM revealed variation in the ultrastructure of large and small pseudocnidae of four species of Cephalothrix. Pseudocnidae of Balionemertes, Cephalotrichella, and Cephalothrix differ in substructure: in Balionemertes and Cephalotrichella the medulla is located in the basal half of the pseudocnidae with а precore layer situated in the apical half, whereas in Cephalothrix spp. and other palaeonemerteans the medulla surrounds a precore layer. Our results confirm the division of archinemerteans into Cephalotrichidae (with genus Cephalothrix) and Cephalotrichellidae (with genera Cephalotrichella and Balionemertes). The synapomorphy of Cephalotrichidae is pseudocnida dimorphism and the synapomorphies of Cephalotrichellidae are the position of the pseudocnidae on epithelial ridges and the distinct organization of pseudocnida layers, specifically the relative position of the medulla and precore layers. The pseudocnida lateral process, one or more of which is present in most species observed, is a probable synapomorphy of the clade Archinemertea. This is the first application of pseudocnida features to distinguish super-generic nemertean taxa and the results suggest that pseudocnidae provide a useful source of characters for nemertean systematics.

High Rates of Self-Fertilization in a Marine Ribbon Worm (Nemertea).

Organisms capable of self-fertilization ("selfing") typically exhibit two evolutionary syndromes: uniting high inbreeding depression with low levels of selfing, or low inbreeding depression with high levels of selfing. We examined the effect of inbreeding on fecundity and time to first reproduction in an apparently self-compatible, simultaneously hermaphroditic marine nemertean worm Prosorhochmus americanus. Adult and juvenile worms were raised in isolation or in pairs. Isolated worms produced significantly more offspring than paired worms (in the adult experiment), and did not exhibit inbreeding avoidance (in the juvenile experiment). The selfing rate of six natural populations was evaluated using 17 species-specific, microsatellite markers, and was consistent with preferential selfing (mean: 0.843, SD: 0.027). Our results showed that P. americanus exhibited an interesting suite of life-history traits, uniting high colonization potential through self-fertilization and high fecundity, with no dispersive larval stage, and with moderate levels of gene flow. We believe that P. americanus is an ideal model system for studies of mating system evolution, inbreeding, and sex allocation.

A transcriptomic approach to ribbon worm systematics (nemertea): resolving the pilidiophora problem.

Resolving the deep relationships of ancient animal lineages has proven difficult using standard Sanger-sequencing approaches with a handful of markers. We thus reassess the relatively well-studied phylogeny of the phylum Nemertea (ribbon worms)-for which the targeted gene approaches had resolved many clades but had left key phylogenetic gaps-by using a phylogenomic approach using Illumina-based de novo assembled transcriptomes and automatic orthology prediction methods. The analysis of a concatenated data set of 2,779 genes (411,138 amino acids) with about 78% gene occupancy and a reduced version with 95% gene occupancy, under evolutionary models accounting or not for site-specific amino acid replacement patterns results in a well-supported phylogeny that recovers all major accepted nemertean clades with the monophyly of Heteronemertea, Hoplonemertea, Monostilifera, being well supported. Significantly, all the ambiguous patterns inferred from Sanger-based approaches were resolved, namely the monophyly of Palaeonemertea and Pilidiophora. By testing for possible conflict in the analyzed supermatrix, we observed that concatenation was the best solution, and the results of the analyses should settle prior debates on nemertean phylogeny. The study highlights the importance, feasibility, and completeness of Illumina-based phylogenomic data matrices.

Morphology of the proboscis of Hubrechtella Juliae (Nemertea, Pilidiophora): implications for pilidiophoran monophyly.

The proboscis of Hubrechtella juliae was examined using transmission electron microscopy, scanning electron microscopy, and confocal laser scanning microscopy to reveal more features of basal pilidiophoran nemerteans for morphological and phylogenetic analysis. The proboscis glandular epithelium consists of sensory cells and four types of gland cells (granular, bacillary, mucoid, and pseudocnidae-containing cells) that are not associated with any glandular systems; rod-shaped pseudocnidae are 15-25 µm in length; the central cilium of the sensory cells is enclosed by two rings of microvilli. The nervous plexus lies in the basal part of glandular epithelium and includes 26-33 (11-12 in juvenile) irregularly anastomosing nerve trunks. The proboscis musculature includes four layers: endothelial circular, inner diagonal, longitudinal, and outer diagonal; inner and outer diagonal muscles consist of noncrossing fibers; in juvenile specimen, the proboscis longitudinal musculature is divided into 7-8 bands. The endothelium consists of apically situated support cells with rudimentary cilia and subapical myocytes. Unique features of Hubrechtella's proboscis include: acentric filaments of the pseudocnidae; absence of tonofilament-containing support cells; two rings of microvilli around the central cilium of sensory cells; the occurrence of subendothelial diagonal muscles and the lack of an outer diagonal musculature (both states were known only in Baseodiscus species). The significance of these characters for nemertean taxonomy and phylogeny is discussed. The proboscis musculature in H. juliae and most heteronemerteans is bilaterally arranged, which can be considered a possible synapomorphy of Hubrechtellidae + Heteronemertea (= Pilidiophora).

Disentangling ribbon worm relationships: multi-locus analysis supports traditional classification of the phylum Nemertea.

The phylogenetic relationships of selected members of the phylum Nemertea are explored by means of six markers amplified from the genomic DNA of freshly collected specimens (the nuclear 18S rRNA and 28S rRNA genes, histones H3 and H4, and the mitochondrial genes 16S rRNA and cytochrome c oxidase subunit I). These include all previous markers and regions used in earlier phylogenetic analyses of nemerteans, therefore acting as a scaffold to which one could pinpoint any previously published study. Our results, based on analyses of static and dynamic homology concepts under probabilistic and parsimony frameworks, agree in the non-monophyly of Palaeonemertea and in the monophyly of Heteronemerta and Hoplonemertea. The position of Hubrechtella and the Pilidiophora hypothesis are, however, sensitive to analytical method, as is the monophyly of the non-hubrechtiid palaeonemerteans. Our results are, however, consistent with the main division of Hoplonemertea into Polystilifera and Monostilifera, the last named being divided into Cratenemertea and Distromatonemertea, as well as into the main division of Heteronemertea into Baseodiscus and the remaining species. The study also continues to highlight the deficient taxonomy at the family and generic level within Nemertea and sheds light on the areas of the tree that require further refinement. © The Willi Hennig Society 2011.