A revision of the classification of the phylum Sipuncula

Stephen, A. C.

doi:10.1080/00222936408651485

Cited by 22 publications

(11 citation statements)

References 5 publications

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“…However, this ranking did not gain much acceptance until Hyman (1959) proposed the spelling Sipunculida as she 'obliterated' the biologically meaningless construct Gephyrea. The present name, Sipuncula, and the use of 'sipunculan' for the vernacular name (not sipunculid) was proposed by Stephen (1964) and restated by Stephen & Edmonds (1972). Prior to this latter work there had been only two 20th century informal proposals regarding the arrangement of genera into unnamed family-like sets (Pickford, 1947;Å kesson, 1958).…”

Section: Evolutionary Origin Of the Sipunculamentioning

confidence: 96%

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Reconstructing the phylogeny of the Sipuncula

2005

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Sipunculans are marine spiralian worms with possible close affinities to the Mollusca or Annelida. Currently 147 species, 17 genera, 6 families, 4 orders and 2 classes are recognized. In this paper we review sipunculan morphology, anatomy, paleontological data and historical affiliations. We have conducted cladistic analyses for two data sets to elucidate the phylogenetic relationships among sipunculan species. We first analyzed the relationships among the 45 species of Phascolosomatidea with representatives of the Sipunculidea as outgroups, using 35 morphological characters. The resulting consensus tree has low resolution and branch support is low for most branches. The second analysis was based on DNA sequence data from two nuclear ribosomal genes (18S rRNA and 28S rRNA) and one nuclear protein-coding gene, histone H3. Outgroups were chosen among representative spiralians. In a third analysis, the molecular data were combined with the morphological data. Data were analyzed using parsimony as the optimality criterion and branch support evaluated with jackknifing and Bremer support values. Branch support for outgroup relationships is low but the monophyly of the Sipuncula is well supported. Within Sipuncula, the monophyly of the two major groups, Phascolosomatidea and Sipunculidea is not confirmed. Of the currently recognized families, only Themistidae appears monophyletic. The Aspidosiphonidae, Phascolosomatidae and Golfingiidae would be monophyletic with some adjustments in their definition. The Sipunculidae is clearly polyphyletic, with Sipunculus nudus as the sister group to the remaining Sipuncula, Siphonosoma cumanense the sister group to a clade containing Siphonosoma vastum and the Phascolosomatidea, and Phascolopsis gouldi grouping within the Golfingiiformes, as suggested previously by some authors. Of the genera with multiple representatives, only Phascolosoma and Themiste are monophyletic as currently defined. We are aiming to expand our current dataset with more species in our molecular database and more detailed morphological studies.

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Section: Evolutionary Origin Of the Sipunculamentioning

confidence: 96%

“…3). Phascolopsis gouldi has had a confusing nomenclatural history, but prior to Stephen (1964) who moved it into its own monotypic genus and Cutler & Gibbs (1985) who shifted the genus into a different family, it was associated with species that are currently members of the genus Golfingia.…”

Section: Combined Analysis Of Molecular and Morphological Datamentioning

confidence: 98%

Reconstructing the phylogeny of the Sipuncula

2005

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“…This was later adopted by Sedgwick (1898), who eliminated the Gephyrea. Yet, based on his elaborate developmental studies of Urechis caupo Fisher andMacGinitie, 1928, Newby (1940) established the Echiuroidea as a separate phylum in terms of traditional classification, which was later renamed Echiura by Stephen (1965) to conform to nomenclature standards.…”

Section: Introductionmentioning

confidence: 98%

Metameric organisation of the nervous system in developmental stages of Urechis caupo (Echiura) and its phylogenetic implications

Hessling

2002

Zoomorphology

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The phylogenetic position of Echiura is still in continuous debate. The commonly accepted view regards Echiura as a distinct taxon, often classified as phylum, which forms the sister group of the Articulata. The alternative view considers Echiura to be a subtaxon of Annelida, which is supported by numerous shared characters. The correct systematic position of Echiura is inevitably linked to the presence or absence of true segmentation. The apparent lack of segmentation in Echiura is considered to be either primary, thereby supporting their exclusion from Annelida, or alternatively to be the result of reduction. The latter would clearly substantiate their classification as a subtaxon of Annelida. Immunohistochemical methods and confocal laser-scanning microscopy clearly demonstrate a metameric organisation of the nervous system in different larval stages of Urechis caupo, which corresponds to the segmental arrangement of ganglia in "typical" Annelida. This segmental pattern is reflected in the serially repetitive distribution of neurons containing the neurotransmitter serotonin (5-hydroxytryptamine) and also in the corresponding distribution of strictly paired peripheral nerves. Precisely two pairs of peripheral nerves are associated with each of the repetitive units. This metameric pattern also corresponds to the transient annulation of the trunk, which is found in late larval stages. Other characters of the nervous system including the paired origin of the ventral nerve cord, the anterior-posterior development gradient and the presence of a distinct suboesophageal ganglion are also found accordingly in typical Annelida. These results are interpreted as an indication that Echiura are derived from formerly segmented ancestors, and thus support their systematic inclusion within Annelida.

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“…On the basis of developmental studies, Newby (1940) established the Echiuroidea as a separate phylum, which was later renamed Echiura by Stephen (1965).…”

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confidence: 99%

Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis

Hessling¹,

Westheide²

2002

Journal of Morphology

154

105

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Despite several unquestionably homologous characters with Annelida, the Echiura have generally been considered to form a distinct taxon due to apparent lack of segmentation: neither in the body cavity nor in any other structures of the adult animals have clear signs of a metameric organization been observed. However, it must be considered that this lack of segmentation could represent a secondary condition and that Echiura are derived from segmented ancestors. An immunohistochemical analysis visualized with confocal laser-scanning microscopy (cLSM) shows the development of serially repeated units in the nervous system of Bonellia viridis. This organization corresponds to the metameric ganglia typical of Annelida. Antibodies against the neurotransmitters serotonin (5-hydroxytryptamine) and FMRFamide label distinct subsets of neurons. Their perikarya are arranged in discrete repetitive units in the ventral nerve cord of different larval stages. Labeling of neurotubuli using antibodies against different tubulin isoforms provides additional support for the metameric character of the nervous system. Contrary to previous descriptions, the peripheral nerves extending from the ventral nerve cord into the body wall musculature are paired and are evenly distributed; their arrangement corresponds to that of the serotoninergic perikarya. Morphological and neurochemical differentiation of the nervous system clearly proceeds from anterior to posterior, indicating the occurrence of a posterior growth zone. The serial ganglionic organization of the nervous system of Articulata is generally regarded as having evolved in conjunction with repetitive muscular units, in particular with the formation of typical annelid segments. Therefore, these results are interpreted as an indication that Echiura are derived from segmented ancestors and thus support the systematic inclusion of the Echiura within the Annelida.

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A revision of the classification of the phylum Sipuncula

Cited by 22 publications

References 5 publications

Reconstructing the phylogeny of the Sipuncula

Reconstructing the phylogeny of the Sipuncula

Metameric organisation of the nervous system in developmental stages of Urechis caupo (Echiura) and its phylogenetic implications

Are Echiura derived from a segmented ancestor? Immunohistochemical analysis of the nervous system in developmental stages of Bonellia viridis

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