Unsegmented annelids? Possible origins of four lophotrochozoan worm taxa

In traditional classification schemes, the Annelida consists of the Polychaeta and the Clitellata (the latter including the Oligochaeta and Hirudinida). However, recent analyses suggest that annelids are much more diverse than traditionally believed, and that polychaetes are paraphyletic. Specifically, some lesser-known taxa (previously regarded as separate phyla) appear to fall within the annelid radiation. Abundant molecular, developmental, and morphological data show that the Siboglinidae, which includes the formerly recognized Pogonophora and Vestimentifera, are derived annelids; recent data from the Elongation Factor-1α (EF-1α) gene also suggest that echiurids are of annelid ancestry. Further, the phylogenetic origins of two other lesser-known groups of marine worms, the Myzostomida and Sipuncula, have recently been called into question. Whereas some authors advocate annelid affinities, others argue that these taxa do not fall within the annelid radiation. With advances in our understanding of annelid phylogeny, our perceptions of body plan evolution within the Metazoa are changing. The evolution of segmentation probably is more plastic than traditionally believed. However, as our understanding of organismal evolution is being revised, we are also forced to reconsider the specific characters being examined. Should segmentation be considered a developmental process or an ontological endpoint?     

The role of character loss in phylogenetic reconstruction as exemplified for the Annelida

Annelid relationships are controversial, and molecular and morphological analyses provide incongruent estimates. Character loss is identified as a major confounding factor for phylogenetic analyses based on morphological data. A direct approach and an indirect approach for the identification of character loss are discussed. Character loss can frequently be found within annelids and examples of the loss of typical annelid characters, like chaetae, nuchal organs, coelomic cavities and other features, are given. A loss of segmentation is suggested for Sipuncula and Echiura; both are supported as annelid ingroups in molecular phylogenetic analyses. Moreover, character loss can be caused by some modes of heterochronic evolution (paedomorphosis) and, as shown for orbiniid and arenicolid polychaetes, paedomorphic taxa might be misplaced in phylogenies derived from morphology. Different approaches for dealing with character loss in cladistic analyses are discussed. Application of asymmetrical character state transformation costs or usage of a dynamic homology framework represents promising approaches. Identifying character loss prior to a phylogenetic analysis will help to refine morphological data matrices and improve phylogenetic analyses of annelid relationships.     

Trametes fumoso-avellanea (Aphyllophorales): a taxonomic study

The morphology, cultural features, sexuality, nuclear behavior and dolipore/parenthesome apparatus of Trametes fumoso-avellanea were studied in an attempt to settle its taxonomic position. Data provided by the micromorphology, biological (non-morphological) characters and the non-perforate parenthesomes sustain its redisposition in the genus Trichaptum Murr.     

Physiological Characterizations of Acanthamoeba Strains

ABSTRACT. Thirty-seven strains of Acanthamoeba are described using 69 physiological characters. The characterizations of the strains are based on quantitative estimates of growth responses which were standardized to eliminate any effects of differences in growth rates of the strains. Thirty-six distinct patterns are generated using numerical profiles, but it is possible to distinguish within this total diversity 20 sub-generic groups which, although not completely congruent with the morphological species, are consistent with other non-morphological characters. Evidence is presented that Acanthamoeba is a coherent, well-founded genus, but it is not desirable to recognize formal taxa at the sub-generic level.     

Analysis of characters and classification based on information theory in the species of the Greater Antilles of the fern familiy Gleicheniaceae

We define a set of characters for the study of the fern family Gleicheniaceae. These characters have no ordered states. We establish a classification, through a divisive strategy, on the basis of these characters applied to the species occurring in the Greater Antilles. We used a technique based on information theory. A computational program was implemented to obtain this classification. With the same technique a procedure is used in order to compare a set of classifications reached under a different set of characters. The results obtained under three classifications on the family of this study are shown.     

A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale

Wiwaxia corrugata and the indisputable polychaetes of the Middle Cambrian Burgess Shale, particularly Canadia spinosa, have figured prominently in recent hypotheses about the early evolution of polychaete annelids. Based on similarities between the sclerites of Wiwaxia and the notochaetae of Canadia with the broad notochaetae (paleae) of Recent chrysopetalid polychaetes, these two fossil taxa have been variously treated as closely related to the most highly derived stem forms of the polychaete (and annelid) crown group or as members of a specific, Recent subgroup within Polychaeta, the order Phyllodocida. Chrysopetalidae is a member of Phyllodocida, which is part of the major polychaete clade Aciculata; the latter two taxa are distinguished by four and six well defined autapomorphic characters, respectively. The best preserved or otherwise appropriate fossils of Wiwaxia corrugata, Canadia spinosa and the other polychaetes of the Burgess Shale have been studied in detail in order to determine whether they possess any characters that could support the homology of wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae. Most of these fossil taxa have significant autapomorphies but the specific characters of the Aciculata and Phyllodocida are entirely absent. It is demonstrated that constraining cladograms in such a manner that wiwaxiid sclerites, canadiid notochaetae and chrysopetalid paleae are homologous leads to results that are markedly unparsimonious. Furthermore, Canadia and the other polychaetes of the Burgess Shale cannot be referred to any extant subgroup within the Polychaeta and cannot be used to polarize character evolution within the annelid crown group. Apart from its dubious sclerites, Wiwaxia has no characters that could indicate any close relationship with Polychaeta or Annelida.     

An Early Cambrian stem polychaete with pygidial cirri

The oldest annelid fossils are polychaetes from the Cambrian Period. They are representatives of the annelid stem group and thus vital in any discussion of how we polarize the evolution of the crown group. Here, we describe a fossil polychaete from the Early Cambrian Sirius Passet fauna, Pygocirrus butyricampum gen. et sp. nov., with structures identified as pygidial cirri, which are recorded for the first time from Cambrian annelids. The body is slender and has biramous parapodia with chaetae organized in laterally oriented bundles. The presence of pygidial cirri is one of the characters that hitherto has defined the annelid crown group, which diversified during the Cambrian-Ordovician transition. The newly described fossil shows that this character had already developed within the total group by the Early Cambrian.     

Enalikter is not an annelid: homology,autapomorphies and the interpretation of problematic fossils

A megacheiran arthropod, Enalikter aphson, was recently described by Siveter et al. (2014) from the mid‐Silurian (late Wenlock) of Herefordshire. Previously, megacheirans had only been recognized from the Cambrian. Struck et al. (2015) considered the body plan of Enalikter to be incompatible with this affinity, arguing that many of the arthropod features were either not present or misinterpreted. Instead, they compared Enalikter to polychaete annelids, identifying characters from numerous polychaete lineages which they considered to be present in Enalikter. A reply to this critique by Siveter et al. (2015) reaffirmed arthropod affinities for Enalikter by presenting additional evidence for key arthropod features, such as arthropodized appendages. Here, we augment Siveter et al. by critically addressing the putative annelid characters of Enalikter presented by Struck et al. and additionally explore the morphological and phylogenetic implications of their hypothesis. We conclude that similarities between Enalikter and polychaetes are superficial and that character combinations proposed by Struck et al. are not present in any annelid, living or extinct. This taxon highlights the importance of using a phylogenetic framework for interpreting fossils that present unusual morphologies, such that proposed shared characters are hypotheses of homology rather than merely phenotypic similarities. Crucially, we argue that autapomorphic characters of subgroups of large taxa (like families or classes within phyla) should not be used to diagnose problematic fossils.     

Mouthparts of the Burgess Shale fossils Odontogriphus and Wiwaxia: implications for the ancestral molluscan radula

The Middle Cambrian lophotrochozoans Odontogriphus omalus and Wiwaxia corrugata have been interpreted as stem-group members of either the Mollusca, the Annelida, or a group containing Mollusca + Annelida. The case for each classification rests on the organisms' unusual mouthparts, whose two to three tooth-rows resemble both the molluscan radula and the jaws of certain annelid worms. Despite their potential significance, these mouthparts have not previously been described in detail. This study examined the feeding apparatuses of over 300 specimens from the 505-million-year-old Burgess Shale, many of which were studied for the first time. Rather than denticulate plates, each tooth row comprises a single axial tooth that is flanked on each side by eight to 16 separate shoehorn-shaped teeth. Tooth rows sat on a grooved basal tongue, and two large lobes flanked the apparatus. New observations-the shape, distribution and articulation of the individual teeth, and the mouthparts' mode of growth-are incompatible with an annelid interpretation, instead supporting a classification in Mollusca. The ancestral molluscan radula is best reconstructed as unipartite with a symmetrical medial tooth, and Odontogriphus and Wiwaxia as grazing deposit-feeders.     

FIne structure of the spermatozoon of an onychophoran, Peripatopsis

The spermatozoon of the Onychophoran Peripatopsis moseleyi is described. This cell is characterized by the general filiform shape, the absence of an acrosome, the presence of mitochondrial derivatives, of an annulus and of nine accessory tubules and a manchette of microtubules around the axoneme. All of these characters are typical of highly evolved sperm models, like those of insects and mammals, and suggest a long evolutionary history. Only the position of the mitochondria, inserted between nucelus and axoneme, is reminiscent of annelid features.     

Annelid phylogeny and the status of Sipuncula and Echiura

Background  

Annelida comprises an ancient and ecologically important animal phylum with over 16,500 described species and members are the dominant macrofauna of the deep sea. Traditionally, two major groups are distinguished: Clitellata (including earthworms, leeches) and "Polychaeta" (mostly marine worms). Recent analyses of molecular data suggest that Annelida may include other taxa once considered separate phyla (i.e., Echiura, and Sipuncula) and that Clitellata are derived annelids, thus rendering "Polychaeta" paraphyletic; however, this contradicts classification schemes of annelids developed from recent analyses of morphological characters. Given that deep-level evolutionary relationships of Annelida are poorly understood, we have analyzed comprehensive datasets based on nuclear and mitochondrial genes, and have applied rigorous testing of alternative hypotheses so that we can move towards the robust reconstruction of annelid history needed to interpret animal body plan evolution.     

Short term dynamics of the dominant annelids in a polyhaline temperate estuary

Weekly sampling over a two year period from a muddy sand bottom in the polyhaline York River, Virginia, U.S.A., clearly identified the pattern of recruitment and survival of the dominant annelid species. Three intermingled recruitment strategies and two survival patterns were observed, ranging from the classic opportunistic life style of mass recruitment over short time periods followed by mass mortality to prolonged recruitment with lower mortality. Qualitatively the annelid assemblage was very similar from year to year with most of the changes being quantitative. Oligochaetes, Tubificoides spp., were the most stable and characteristic members of the annelid assemblage.Contribution No. 1118 of the Virginia Institute of Marine Science     

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.     

Evolution of habitat preference in Clitellata (Annelida)

Clitellata (earthworms, leeches, and allies) is a clade of segmented annelid worms that comprise more than 5000 species found worldwide in many aquatic and terrestrial habitats. According to current views, the first clitellates were either aquatic (marine or freshwater) or terrestrial. To address this question further, we assessed the phylogenetic relationships among clitellates using parsimony, maximum likelihood and Bayesian analyses of 175 annelid 18S ribosomal DNA sequences. We then defined two ecological characters (Habitat and Aquatic‐environment preferences) and mapped those characters on the trees from the three analyses, using parsimony character‐state reconstruction (i.e. Fitch optimization). We accommodated phylogenetic uncertainty in the character mapping by reconstructing character evolution on all the trees resulting from parsimony and maximum likelihood bootstrap analyses and, in the Bayesian inference, on the trees sampled using the Markov chain Monte Carlo algorithm. Our analyses revealed that an ‘aquatic’ ancestral state for clitellates is a robust result. By using alterations of coding characters and constrained analyses, we also demonstrated that the hypothesis for a terrestrial origin of clitellates is not supported. Our analyses also suggest that the most recent ancestor of clitellates originated from a freshwater environment. However, we stress the importance of adding sequences of some rare marine taxa to more rigorously assess the freshwater origin of Clitellata. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 447–464.     

Information theory and character selection in the numerical taxonomy of some male Diaspididae (Hemiptera: Coccoidea)

Abstract. Data on twenty-six species of male Diaspididae, two of them dimorphic, are analysed in relation to the information content and other quantitative attributes of the characters used to construct analysis. It is shown that simple information theory and the use of other numerical criteria enable one to select a relatively small number of characters which effectively reproduce the salient features of referednce classification based on the entire set of available characters. The conclusion are discussed in relation to views on characters weighting and the stability of numeriocal classificatins.     

The phylogenetic position of the Clitellata and the Echiura - on the problematic assessment of absent characters*

Absent characters (negative characters) are difficult to assess and their correct interpretation as symplesiomorphies, synapomorphies or convergencies (homoplasies) is one of the greatest challenges in phylogenetic systematics. Different phylogenetic assessments often result in contradictory phylogenetic hypotheses, in which the direction of evolutionary changes is diametrically opposed. Especially in deciding between primary (plesiomorphic) and secondary (apomorphic) absence, false conclusions may be reached if only the outgroup comparison and the principle of parsimony are employed without attempting any biological evaluation or interpretation of characters. For example, in the higher‐level systematization of the Annelida and related taxa different assessments of absent characters have led to conflicting hypotheses about the phylogenetic relationships and the ground pattern of the annelid stem species. Varying phylogenetic interpretations regarding the absence of the chemosensory nuchal organs in the clitellates and their presence in polychaetes initiated a controversy that produced two alternative phylogenetic hypotheses: (1) the Clitellata are highly derived Annelida related to a subtaxon within the, in this case, paraphyletic ‘Polychaeta’ or (2) the Clitellata are comparatively primitive Annelida representing the sister group of a monophyletic taxon Polychaeta. In the former, the absence of nuchal organs in the Clitellata is regarded as a secondary character, in the latter as primary. As most Clitellata are either limnetic or terrestrial, we must ask which characters are plesiomorphies, taken from their marine stem species without changes. In addition to a thorough investigation and evaluation of clitellate characters, a promising approach to these questions is to look for such characters in limnetic and terrestrial annelids clearly not belonging to the Clitellata. A similar problem applies to the evaluation of the position of the Echiura, which lack both segmentation and nuchal organs. Evidence is presented that in both taxa these absent characters represent derived, apomorphic character states. The consequences for their phylogenetic position and the questionable monophyly of the Polychaeta are discussed. The conclusion drawn from morphological character assessments is in accordance with recently published hypotheses based on molecular data.     

A preliminary introduction to mathematical methods for taxonomy

In this paper, the various mathematical methods applied to taxonomy are introduced to readers. Some approaches to the classification induced by statistics, graph theory, information theory, fuzzy mathematics are discussed. An example of classification (6 OTU’s with 8 characters) is given for convenience of discussion. The original data matrix of this example is obtained from 6 species in the family of Campanulaceae.     

Immunohistochemical investigations of Myzostoma cirriferum and Mesomyzostoma cf. katoi (Myzostomida,Annelida) with implications for the evolution of the myzostomid body plan

Although part of the annelid radiation, Myzostomida exhibit a highly specialized body plan that lacks many typical annelid characters. Their annelid ancestry is evident from their trochophora-like larvae, adult myoanatomy and parts of the nervous system, whereas segmentation is considered at best to be incomplete in myzostomids. We analyzed the morphology of two myzostomid species, the ectocommensal Myzostoma cirriferum and the endoparasitic Mesomyzostoma cf. katoi using a broad set of fluorescent markers to reveal the degree of segmentation in myzostomids. We used immunocytochemical and classical fluorescent staining methods combined with confocal laser-scanning microscopy to visualize tissues labeled with antibodies directed against classical invertebrate neurotransmitters (serotonin, dopamine, FMRFamide), synapsin, which labels nerve cell terminals, and the marker phalloidin–rhodamine which binds F-actin in muscle. Our data provide a broad body of additional evidence for the segmented origin of Myzostomida. It becomes apparent that the adult nervous system of M. cirriferum exhibits signs of pseudoradial symmetry with repetitive patterns of putative FMRFamide, serotonin and dopamine-like immunoreactivity. An analysis of the staining patterns in juvenile M. cirriferum yielded evidence for positional changes, as well as additions and reductions of neuronal structures during development. Interestingly, the neuroanatomy and myoanatomy of Mesomyzostoma cf. katoi indicate further reductions of neuronal and myoanatomical patterns in this species. Notably this taxon shows a presumably secondarily evolved cylindrical and strictly bilateral morphology, which is supposed to have evolved from a flat, disk-shaped Myzostoma-like ancestor with an underlying pseudoradial symmetry.