Cell proliferation dynamics and morphological differentiation during regeneration in Dorvillea bermudensis (Polychaeta, Dorvilleidae)

Although some species of Annelida have an enormous capacity to regenerate, it is not yet known whether reestablishment of lost body parts is performed by stem cells, depends on preceding dedifferentiation of somatic cells, or is a combination of both. In order to clarify how, in the case of epimorphic regeneration, the blastemas are formed, we applied the thymidine analog 5'-bromo-2'-deoxyuridine (BrdU) in the dorvilleid polychaete Dorvillea bermudensis to identify cells in the S-phase of the cell cycle. Regeneration pulse-chase experiments were carried out to determine onset and dynamics of the proliferation process, and BrdU pulse-chase experiments were undertaken to follow cell fate. We found irregularly distributed S-phase cells throughout the body of adult specimens. Subsequent to amputation, these cells do not migrate from the amputee towards the wound site, where proliferation activity was documented no earlier than 16 h after fragmentation. In the initial phase, the proliferation rate at the anterior end exceeds the rate at the posterior end. Observance of identity could be demonstrated for the ectoderm and can be assumed for the two other germ layers. The anterior blastema transforms into the head, while the posterior forms the pygidium and persists as a proliferation zone; four or numerous segments are formed by intercalation between the former anterior or posterior blastema and the amputee.     

Nerve development, growth and differentiation during regeneration in Enchytraeus fragmentosus and Stylaria lacustris (Oligochaeta)

Enchytraeus fragmentosus (Enchytraeidae) and Stylaria lacustris (Naididae) are small terrestrial and limnetic oligochaetes that exclusively or seasonally reproduce by fragmentation and regeneration, respectively. We traced the neuronal development and differentiation during regeneration in order to gain information on the basic organization and evolution of the oligochaete nervous system. Subsequent to artificial amputation, the nervous systems have been stained with antibodies directed against acetylated alpha-tubulin. The staining was analyzed by indirect fluorescence in combination with confocal laser scanning microscopy. Both species show unique oligochaete neuronal regeneration patterns: (i) numerous fibers branch off from segmental nerves near the wound site and innervate the blastema; and (ii) the ventral cord is partly reestablished before the circumesophageal connectives develop. In the investigated 'Oligochaeta' the outgrowing fibers of the ventral nerve cord are soon bundled into at least two distinct connective pairs, which prolong into dorsal and ventral roots next to the mouth. Subsequent complete fusion of the doubled roots forms simple connectives. Thus, dorsal roots are not a unique feature for 'Polychaeta'. They occur as a transient structure in 'Oligochaeta' and might be part of the neuronal ground pattern of Annelida. The initially tetra or even pentaneuronal ventral nerve cord also differentiates into an unineuronal one by fusion.     

Morphology of the nervous system of Polychaeta (Annelida)

The article summarizes our up to date knowledge about the morphology of the annelid, especially the polychaete, central and peripheral nervous system. Since the cephalic nervous system was in the focus of controversial discussions for decades, the structure of its neuropile, associated ganglia and nerves is reviewed in detail. The enormous variation of the ventral nerve cord and peripheral nerves is presented as well as a theory how this might have evolved. A ground pattern of the polychaete nervous system is suggested, based on developmental and regeneration studies.     

Nephridial and gonoduct distribution patterns in Nerillidae (Annelida: Polychaeta) examined by tubulin staining and cLSM

The distribution and configuration of nephridia and gonoducts are described for seven species from seven genera of the interstitial polychaete family Nerillidae. The ciliated nephridia and gonoducts were identified by tubulin staining and examined with a confocal laser scanning microscope. The following species of the seven to nine-segmented nerillids were examined: Leptonerilla prospera, Nerilla antennata (nine segments); Nerillidium mediterraneum, Trochonerilla mobilis, Gen. sp. A (eight segments); and Aristonerilla brevis, Paranerilla limicola (seven segments). Two of the examined species are hermaphroditic (N. mediterraneum and Gen. sp. A). Segmented nephridia can be found from the first to the last segment, with a total of two to five pairs. One to three pairs of segmented spermioducts are present in all species. One pair of gonoducts is found in all species, except for P. limicola, where they are absent. Nephridia vary in length from half to almost twice the length of a segment and may be curled up in loops. In A. brevis and P. limicola the nephridia are discontinuously ciliated. The distribution and configuration of spermioducts and gonoducts are also variable, although to a lesser extent. The spermioduct distribution is generally consistent within genera and therefore of systematic significance. Nephridia and gonoducts are never found together in the same segments, and the results indicate that gonoducts and nephridia have developed from the same anlagen. The distribution patterns of nephridia and gonoducts are discussed with respect to segmentation, systematics, and development.     

Giant pelagic larvae of Phyllodocidae (Polychaeta,Annelida)

The microscopic anatomy of giant pelagic larvae of Phyllodocidae was studied using routine histological, SEM, and TEM techniques. The larvae consist of two distinct regions: a large spherical trochophore measuring up to 2 mm in diameter and a posterior, long (up to 10 mm length), narrow rudiment of the adult body with up to 120 segments. The larvae have an unusual mixture of larval and adult features, including a very complex, well-developed brain and ganglia in the ventral nerve cord, and only a single pair of protonephridia located in the hyposphere of the trochophore. A muscular pharynx is not developed. The intestinal wall, especially in the trochophore region, consists of endodermal cells containing considerable nutritive material in the form of yolk-like globular inclusions. The digestive tract of all larvae was empty. The position of the frontal sensory organ and the prototroch, the structure of the parapodia and setae, and the three pairs of tentacular cirri dictate inclusion of the larvae in the family Phyllodocidae. The relatively enormous size and unusual pattern of development of the adult body may be adaptations for a long pelagic life and rapid settlement of the species, which inhabits slopes of islands and underwater mounts located far apart. J. Morphol. 238:93–107, 1998. © 1998 Wiley-Liss, Inc.     

Structure and anterior regeneration of musculature and nervous system in Cirratulus cf. cirratus (Cirratulidae,Annelida)

Annelids provide suitable models for studying regeneration. By now, comprehensive information is restricted to only a few taxa. For many other annelids, comparative data are scarce or even missing. Here, we describe the regeneration of a member of the Cirratulus cirratus species complex. Using phalloidin‐labeling and antibody‐stainings combined with subsequent confocal laser scanning microscopy, we provide data about the organization of body wall musculature and nervous system of intact specimens, as well as about anteriorly regenerating specimens. Our analyses show that C. cf. cirratus exhibits a prominent longitudinal muscle layer forming a dorsal muscle plate, two ventral muscle strands and a ventral‐median muscle fiber. The circular musculature forms closed rings which are interrupted in the area of parapodia. The nervous system of C. cf. cirratus shows a typical rope‐ladder like arrangement and the circumesophageal connectives exhibit two separate roots leading to the brain. During regeneration, the nervous system redevelops remarkably earlier than the musculature, first constituting a tripartite loop‐like structure which later become the circumesophageal connectives. Regeneration of longitudinal musculature starts with diffuse ingrowth and subsequent structuring into the blastema. In contrast, circular musculature develops independently inside the blastema. Our findings constitute the first analysis of regeneration for a member of the Cirratuliformia on a structural level. Summarizing the regeneration process in C. cf. cirratus, five main phases can be subdivided: 1) wound closure, 2) blastema formation, 3) blastema differentiation, 4) resegmentation, and 5) growth, respectively elongation. Additionally, the described tripartite loop‐like structure of the regenerating nervous system has not been reported for any other annelid taxon. In contrast, the regeneration of circular and longitudinal musculature originating from different groups of cells seems to be a general pattern in annelid regeneration. J. Morphol. 275:1418–1430, 2014. © 2014 Wiley Periodicals, Inc.     

CLSM analysis of the phalloidin-stained muscle system in Nerilla antennata, Nerillidium sp. and Trochonerilla mobilis (Polychaeta; Nerillidae)

The entire muscle system of Nerilla antennata, Nerillidium sp. and Trochonerilla mobilis was three-dimensionally reconstructed from whole mounts. In juvenile and adult specimens the F-actin musculature subset was stained with FITC-conjugated phalloidin and visualized with a confocal laser scanning microscope (cLSM). The muscle system shows the following major organization: 1) circular muscles are totally absent in the body wall; 2) the longitudinal muscles are confined in two ventral and two dorsal thick bundles; 3) additional longitudinal muscles are located in the ventro- and dorsomedian axis; 4) three segmental pairs of ventral oblique muscles elongate into the periphery: the main dorsoventral muscles that run along the body side posterior and dorsally and the anterior and posterior oblique parapodial muscles, which contribute to the ventral chaetal sacs; 5) one segmental pair of dorsal oblique parapodial muscles, contributing to the dorsal chaetal sacs; 6) five to seven small dorsoventral muscles per segment; and 7) complex head and pharyngeal musculature. These results support the belief that absence of circular muscles in the polychaete body wall is much more widely distributed than is currently presumed.     

Syllidae (Annelida, Polychaeta) from the Caribbean coast of Venezuela

Venezuela possesses a great variety of coastal environments allowing for a high diversity of marine species. However, systematic studies on marine invertebrates are scarce, especially on polychaetes. The family Syllidae is poorly known, and only 14 genera and 42 species have been reported from this country. A total of 13 genera and 26 species the Syllidae were identified from benthic samples collected on different substrata of the northeastern coast of Venezuela. Of these, seven genera and 16 species constitute new records for Venezuela: Odontosyllis guillermoi, Syllides floridanus, Salvatoria clavata, Salvatoria limbata, Sphaerosyllis longicauda, Parapionosyllis longicirrata, Trypanosyllis parvidentata, Trypanosyllis vittigera, Opisthosyllis sp., Syllis amica, Syllis armillaris, Syllis gracilis, Syllis pseudoarmillaris, Syllis vittata, Parasphaerosyllis indica and Myrianida convoluta.     

Comparative analysis of the nervous systems in presumptive progenetic dinophilid and dorvilleid polychaetes (Annelida) by immunohistochemistry and cLSM

Entire nervous systems of the dinophilids Dinophilus (two species) and Trilobodrilus (three species) and the dorvilleids Parapodrilus psammophilus and Ophryotrocha gracilis (larva) were stained with antisera directed against serotonin, Phe‐Met‐Arg‐Phe‐NH₂ (FMRFamide) and acetylated α‐tubulin and analysed by confocal laser scanning microscopy (cLSM). Adult dinophilids and the dorvilleid larva exhibit the same structure of the ventral nerve cord, with two main nerves spaced far apart, one median and two paramedian nerves. A serotonergic plexus is situated between the paramedian nerve pair, above the ventral locomotory ciliary band. These similarities between adults and larva corroborate the presumed progenetic origin of dinophilids. However, since larval nervous systems of other polychaete taxa also seem to be organized in this way, this result cannot support the view that dinophilids originate from dorvilleids. In P.psammophilus the main nerve cords are widely separated only in the last segment, indicating that this pattern may be correlated with the absence of parapodia. The unpaired median nerve of dinophilids, P. psammophilus and many other polychaetes, is considered to be part of the basic annelid body plan. The ground pattern of the ventral paired dinophilid ganglia is represented by three (anterior, main, posterior) commissures, conserved in most of the ganglia in the Dinophilus species and mostly reduced to a main commissure in the Trilobodrilus species. The dinophilid species and P. psammophilus possess six pairs of ganglia indicating six trunk segments – in contrast to former views. The two rings behind the prostomium in both the dinophilid and the dorvilleid species contain one pair of ganglia only, corroborating the presumed homology of this peristomial region in the two taxa. The Dinophilus nervous system with 12 longitudinal nerves and three perpendicular nerve rings per segment resembles orthogonal nervous structures characteristic of platyhelminths.     

A new genus of Syllidae (Polychaeta) from Western Australia

During a study carried out on the subfamily Exogoninae (Syllidae) from Australia, several specimens of a new genus and species were found in samples of dead coral substrate from Western Australia. They have long palps, fused except for a terminal notch, long median and two short lateral antennae, a single pair of short tentacular cirri, and short dorsal cirri, somewhat longer than the parapodial lobes. These characters resemble those of the genus Exogone Örsted, 1845. However, all these appendages are articulated. The chaetae are very similar to those of several species of Syllis Lamarck, 1818, having coarse spines on the margin of compound chaetal blades and truncated dorsal simple chaetae. Furthermore, the pharynx begins in chaetiger 3, posterior to the peristomium, as in many species of the genus Syllis; this condition does not occur in any described species of Exogone. The new genus is provisionally proposed to belong to the subfamily Syllinae, although it has some characters typical of the Exogoninae. Examination under the SEM shows another peculiar feature, the nuchal organs are distinctly laterally located. Within the Syllinae, only Paratyposyllis Hartmann-Schröder, 1962 has a single pair of tentacular cirri, but in that genus, the palps are only basally fused.     

Evolution of interstitial Polychaeta (Annelida)

An update of the systematics is given for the eight most important interstitial polychaete families: Diurodrilidae, Nerillidae, Protodrilidae, Protodriloididae, Saccocirridae, Parergodrilidae, Polygordidae and Psammodrilidae. Additional information and new observations are presented for the Diurodrilidae, Nerillidae and Psammodrilidae. Three new supplementary evolutionary hypotheses for these families are here suggested: (I) basal position of Diurodrilidae in Polychaeta, (2) evolution of Nerillidae in mud, and (3) evolution from meio- to macrofaunal forms of Psammodrilidae.     

Global diversity of polychaetes (Polychaeta; Annelida) in freshwater

A literature review of Polychaeta (Annelida) including Aphanoneura (the oligochaete-like Aeolosomatidae and Potamodrilidae), living in freshwater yielded 168 species, 70 genera and 24 families representing all of the major polychaete clades, but less than 2% of all species. The best-represented families were, in order, Nereididae, Aeolosomatidae, Sabellidae, Spionidae and Histriobdellidae. Fourteen families were represented by a single species and genus. Regions supporting the highest diversity of freshwater polychaetes were in order, Palaearctic, Neotropical, Oriental, Nearctic, Australasian, and Afrotropical. More than half of all species and genera inhabitat lakes and rivers, followed by lagoons/estuaries, which have a high proportion of euryhaline species, and inland seas. Less common, atypical polychaete habitats include subterranean waters, the hyporheic zone of rivers and plant container habitats (phytotelmata). At least three distinct ecological/historical processes appear to account for the colonisation of continental waters: invasion of a clade prior to the break-up of Gondwana, as in Aphanoneura, Namanereis, Stratiodrilus, and Caobangia; relatively recent stranding of individual species (relicts); and the temporary visitation of euryhaline species. Guest editors: E. V. Balian, C. Lévêque, H. Segers & K. Martens Freshwater Animal Diversity Assessment     

Fine structure of the spermatozoon of <Emphasis Type="Italic">Diopatra neapolitana</Emphasis> (Polychaeta,Onuphidae)

The identification of Diopatra species lacks of clear diagnostic features of taxonomic importance and the knowledge of their reproductive characters is scant. The spermatozoa of Diopatra neapolitana were ultrastructurally investigated by electron microscopy in order to correlate the mode of reproduction with sperm cells morphology. The mature male gamete has a depressed subspherical nucleus, a cone-like acrosome, and a long flagellum. The acrosome is conical in shape and radially symmetrical, with a base diameter twice the height. Within the acrosome vesicle, the basal region includes a very electron-dense thickened ring composed of paracrystalline substances. The subacrosomal space is filled with a poorly electron-dense material, with straight filaments axially arranged to form a perforatorium. The nucleus contains the complete axial canal, holding the hind perforatorium region. The middle piece consists of five mitochondria with well-distinct membranes and tubulo-vesicular cristae. Two centrioles are located perpendicularly to each other. The proximal one lies in the central fossa and the distal one, slightly eccentric to the sperm axis, anchors to the plasma membrane by nine satellite rays of the pericentriolar complex. The axoneme has a 9+2 arrangement of microtubules. In general, the spermatozoon of D. neapolitana conforms exteriorly to the typical ect-aquasperm; the acrosome complex ultrastructure, however, shows noticeable modifications from the basic form. This finding agrees with the previously observed reproductive pattern (broadcast spawning—free-swimming larvae) of D. neapolitana belonging to Santa Gilla population, and may be helpful to solve the taxonomic problems of the D. neapolitana complex as well.     

The formation of the nervous system during larval development in Triops cancriformis (Bosc) (crustacea,Branchiopoda): An immunohistochemical survey

We provide data of the development of thenervous system during the first five larval stages of Triops cancriformis. We use immunohistochemical labeling (against acetylated α‐tubulin, serotonin, histamine, and FMRFamide), confocal laser scanning microscopy analysis, and 3D‐reconstruction. The development of the nervous system corresponds with the general anamorphic development in T. cancriformis. In larval stage I (L I), all brain parts (proto‐, deuto‐, and tritocerebrum), the circumoral connectives, and the mandibular neuromere are already present. Also, the frontal filaments and the developing nauplius eye are already present. However, until stage L III, the nauplius eye only consists of three cups. Throughout larval development, the protocerebral network differentiates into distinct subdivisions. In the postnaupliar region, additional neuromeres and their commissures emerge in an anteroposterior gradient. The larval nervous system in L V consists of a differentiated protocerebrum including a central body, a nauplius eye comprising four cups, a circumoral nerve ring, mandibular‐ and postnaupliar neuromeres up to the seventh thoracic segment, each featuring an anterior and a posterior commissure, and two parallel connectives. The presence of a protocerebral bridge is questionable. The distribution of neurotransmitters in L I is restricted to the naupliar nervous system. Over the course of the five stages of development, neurotransmitter distribution also follows an anteroposterior gradient. Each neuromere is equipped with two ganglia innervating the locomotional appendages and possesses a specific neurotransmitter distribution pattern. We suggest a correlation between neurotransmitter expression and locomotion. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Homology of prostomial and pharyngeal structures in eunicida (Annelida) based on innervation and morphological similarities

Eunicidan bristle worm families are commonly identified by the shape of their prostomia and pharyngeal structures. However, current hypotheses of homology among these structures are conflicting, making it difficult to assess morphological evolution, reconstruct phylogeny, and produce a stable classification. To generate more consistent hypotheses of homology among eunicidan anterior structures, the author examined the anterior morphology and the nervous system stained with anti‐α‐tubulin and serotonin antibodies in representative species of Eunicidae, Onuphidae, Oenonidae, Dorvilleidae and Lumbrineridae. The shape of the brain varied conspicuously among families; however, it has mostly the same commissures (usually two of the dorsal and five of the ventral roots of the circumoesophageal connective). The stomatogastric system is also conservative in composition, having two main pairs of stomatogastric nerves which vary in their relative position among the different families. Innervation similarities combined with correspondence and topological morphological similarities made it possible to present explicit hypotheses of primary homology of features, such as buccal lips, pharyngeal fold, and dorsolateral fold anterior extension. Buccal lips are present in all families; however, ventral pads on the prostomium of the Dorvillea line of Dorvilleidae are anterior prolongations of the pharyngeal fold and not buccal lips. All examined taxa, except dorvilleid species, have conspicuous dorsolateral fold anterior extension. In Eunicidae, this anterior extension is a transverse band, while in other families it is a pair of folds. Observed similarities also gave insights on the homology of maxillary elements of Dorvilleidae, providing background knowledge for future studies. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Ophryotrocha longidentata sp.n. and Dorvillea erucaeformis (Malmgren) (Polychaeta, Dorvilleidae) from the West Coast of Scandinavia

Ophryotrocha longidentata sp.n. is described from the Skagerak, West Coast of Sweden. The species lives on muddy bottoms at a depth of about 100 m. It is distinguished by having biarticulated palps and by the structure of the jaw apparatus. Dorvillea erucaeformis (Malmgren) from the West Coast of Norway is redescribed and re-established. This species was earlier considered identical with D. rubrovittata (Grube).     

Developmental architecture of the nervous system in Themiste lageniformis (Sipuncula): New evidence from confocal laser scanning microscopy and gene expression

Sipuncula is a clade of unsegmented marine worms that are currently placed among the basal radiation of conspicuously segmented Annelida. Their new location provides a unique opportunity to reinvestigate the evolution and development of segmented body plans. Neural segmentation is clearly evident during ganglionic ventral nerve cord (VNC) formation across Sedentaria and Errantia, which includes the majority of annelids. However, recent studies show that some annelid taxa outside of Sedentaria and Errantia have a medullary cord, without ganglia, as adults. Importantly, neural development in these taxa is understudied and interpretation can vary widely. For example, reports in sipunculans range from no evidence of segmentation to vestigial segmentation as inferred from a few pairs of serially repeated neuronal cell bodies along the VNC. We investigated patterns of pan-neuronal, neuronal subtype, and axonal markers using immunohistochemistry and whole mount in situ hybridization (WMISH) during neural development in an indirect-developing sipunculan, Themiste lageniformis. Confocal imaging revealed two clusters of 5HT⁺ neurons, two pairs of FMRF⁺ neurons, and Tubulin⁺ peripheral neurites that appear to be serially positioned along the VNC, similar to other sipunculans, to other annelids, and to spiralian taxa outside of Annelida. WMISH of a synaptotagmin1 ortholog in T. lageniformis (Tl-syt1) showed expression throughout the centralized nervous system (CNS), including the VNC where it appears to correlate with mature 5HT⁺ and FMRF⁺ neurons. An ortholog of elav1 (Tl-elav1) showed expression in differentiated neurons of the CNS with continuous expression in the VNC, supporting evidence of a medullary cord, and refuting evidence of ontogenetic segmentation during formation of the nervous system. Thus, we conclude that sipunculans do not exhibit any signs of morphological segmentation during development.     

On the associations between Haplosyllis (Polychaeta, Syllidae) and gorgonians (Cnidaria, Octocorallaria), with the description of a new species

The present paper includes a morphological, ecological and biological updating of the three gorgonian associated species of Haplosyllis (Polychaeta, Syllidae) known to date: H. chamaeleon (symbiont with Paramuricea clavata in the Mediterranean), H. anthogorgicola (symbiont with Anthogorgia bocki in the Japanese seas) and H. villogorgicola , a new species living symbiotically with Villogorgia bebrycoides which is only known from Tenerife (Canary Islands, Eastern Central Atlantic). The new species is described on the basis of ecological, morphological, morphometric and statistical analysis of relevant characteristics. Each host colony harboured about 15 pale-yellowish worms, whose cryptic colouration mimicked that of the host. They occurred either on the host branches or partly hidden inside cavities formed by the fusion of two branches. The new species is characterized by the presence of simple chaetae with clearly bidentate tips all along the body, the presence of gland pore aggregates distributed in two lateral rows and two ventral patches on each palp and the absence of ciliary tufts on the pharyngeal papillae. H. villogorgicola sp. nov. is closely related to H. chamaeleon . Thus, it is compared with two populations of this species collected in the north-west and south-west Mediterranean. Stolons of H. chamaeleon are re-described as tetracerous and a peculiar posterior end regeneration process occurring in adult worms during the stolon formation is described. H. anthogorgicola is also re-described, with particular emphasis on its appendage and chaetal arrangements. The main features of the three associations are discussed in light of the current knowledge on symbiotic polychaetes, particularly cnidarian-associated syllids.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 455–477.     

Novel aspects of the nervous system of Bonellia viridis (Echiura) revealed by the combination of immunohistochemistry, confocal laser-scanning microscopy and three-dimensional reconstruction

The Echiura have been placed in close phylogenetic affinity to the Annelida on the basis of numerous homologous characters including the mode of development, the nearly identical formation of a trochophore larva, as well as the development and ultrastructure of chaetae and spermatozoa. Furthermore, phylogenetic analysis of elongation factor-1 gene sequences supports placement of the Echiura within the Annelida. Nevertheless, the Echiura are generally excluded from the Annelida due to their lack of segmentation. However, it must be considered that this lack could represent a secondary condition and that Echiura are derived from formerly segmented ancestors. In the present study, the combination of methods applied reveals several novel aspects of the central nervous system in developmental stages of Bonellia viridis. The most important of these is the metameric organization of the ventral nerve cord. Antibodies against different neurotransmitters label discrete repetitive units of perikarya in the ventral nerve cord. This organisation is additionally supported by the distribution of peripheral nerves as shown by labelling of neurotubules. These nerves are clearly paired and are evenly distributed, corresponding to the serial units of serotoninergic neurons. Different methods of computer-aided three-dimensional reconstruction display the precise spatial distribution of perikarya and peripheral nerves allowing the repetitive units to be discerned on the basis of relative size, position and number of labelled cells. The repetitive units in the nervous system of B. viridis correspond to segmental ganglia of various Annelida and are interpreted as an indication that Echiura are derived from formerly segmented ancestors, thus supporting the systematic inclusion of the Echiura within the Annelida.     

Phylogenetic relationships of Serpulidae (Annelida: Polychaeta) based on 18S rDNA sequence data, and implications for opercular evolution

Phylogenetic relationships of (19) serpulid taxa (including Spirorbinae) were reconstructed based on 18S rRNA gene sequence data. Maximum likelihood, Bayesian inference, and maximum parsimony methods were used in phylogenetic reconstruction. Regardless of the method used, monophyly of Serpulidae is confirmed and four monophyletic, well-supported major clades are recovered: the Spirorbinae and three groups hitherto referred to as the Protula-, Serpula-, and Pomatoceros-group. Contrary to the taxonomic literature and the hypothesis of opercular evolution, the Protula-clade contains non-operculate (Protula, Salmacina) and operculate taxa both with pinnulate and non-pinnulate peduncle (Filograna vs. Vermiliopsis), and most likely is the sister group to Spirorbinae. Operculate Serpulinae and poorly or non-operculate Filograninae are paraphyletic. It is likely that lack of opercula in some serpulid genera is not a plesiomorphic character state, but reflects a special adaptation.