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RESEARCH ARTICLE

Bifidochaetus, a new Arctic genus of freshwater Chaetonotida (Gastrotricha) from Spitsbergen revealed by an integrative taxonomic approach

Małgorzata Kolicka A E , Miroslawa Dabert B , Jacek Dabert C , Tobias Kånneby D and Jacek Kisielewski A
+ Author Affiliations
- Author Affiliations

A Department of Animal Taxonomy and Ecology, Institute of Environmental Biology, Faculty of Biology, Adam Mickiewicz University in Poznań, Umultowska 89, 61–614 Poznan, Poland.

B Molecular Biology Techniques Laboratory, Faculty of Biology, Adam Mickiewicz University in Poznań, Umultowska 89, 61–614 Poznań, Poland.

C Department of Animal Morphology, Institute of Environmental Biology, Faculty of Biology, Adam Mickiewicz University in Poznań, Umultowska 89, 61–614 Poznań, Poland.

D Department of Zoology, Swedish Museum of Natural History, PO Box 50007, SE–104 05 Stockholm, Sweden.

E Corresponding author. Email: kolicka@amu.edu.pl

Invertebrate Systematics 30(4) 398-419 https://doi.org/10.1071/IS16001
Submitted: 1 January 2016  Accepted: 14 April 2016   Published: 31 August 2016

Abstract

Gastrotricha is a cosmopolitan phylum of aquatic and semi-terrestrial invertebrates that comprises ~820 described species. To date, freshwater gastrotrichs have not been the subject of faunistic or taxonomic research in the polar regions. In this paper, we present the first species-level description of a freshwater gastrotrich from the Arctic (Svalbard Archipelago). Evidence from morphology, morphometry and molecular analyses reveals that the species represents a new genus in Chaetonotidae: Bifidochaetus arcticus, gen. et sp. nov. Taking into consideration many morphological similarities to Chaetonotus (Primochaetus) veronicae Kånneby, 2013 we propose to include C. (P.) veronicae in the newly established genus under the new combination Bifidochaetus veronicae (Kånneby, 2013), comb. nov. In the phylogenetic analysis based on nuclear 18S rRNA, 28S rRNA and mitochondrial cytochrome c oxidase subunit I sequence data, B. arcticus, gen. et sp. nov. is nested within the family Chaetonotidae, as the sister group to the genus Lepidochaetus Kisielewski, 1991. In this paper we also present new taxonomic characters useful for gastrotrich taxonomy: the pharynx-to-intestine length ratio (I) and the spine bifurcation ratio (B).


References

Amato, A. J., and Weiss, M. J. (1982). Developmental flexibility in the cuticular pattern of a cell-constant organism, Lepidodermella squammata (Gastrotricha). Transactions of the American Microscopical Society 101, 229–240.
Developmental flexibility in the cuticular pattern of a cell-constant organism, Lepidodermella squammata (Gastrotricha).Crossref | GoogleScholarGoogle Scholar |

Appeltans, W., Ahyong, S. T., Anderson, G., Angel, M. V., Artois, T., Bailly, N., Bamber, R., Barber, A., Bartsch, I., Berta, A., Błażewicz-Paszkowycz, M., Bock, P., Boxshall, G., Boyko, C. B., Brandão, S. N., Bray, R. A., Bruce, N. L., Cairns, S. D., Chan, T. Y., Cheng, L., Collins, A. G., Cribb, T., Curini-Galletti, M., Dahdouh-Guebas, F., Davie, P. J., Dawson, M. N., De Clerck, O., Decock, W., De Grave, S., de Voogd, N. J., Domning, D. P., Emig, C. C., Erséus, C., Eschmeyer, W., Fauchald, K., Fautin, D. G., Feist, S. W., Fransen, C. H., Furuya, H., Garcia-Alvarez, O., Gerken, S., Gibson, D., Gittenberger, A., Gofas, S., Gómez-Daglio, L., Gordon, D. P., Guiry, M. D., Hernandez, F., Hoeksema, B. W., Hopcroft, R. R., Jaume, D., Kirk, P., Koedam, N., Koenemann, S., Kolb, J. B., Kristensen, R. M., Kroh, A., Lambert, G., Lazarus, D. B., Lemaitre, R., Longshaw, M., Lowry, J., Macpherson, E., Madin, L. P., Mah, C., Mapstone, G., McLaughlin, P. A., Mees, J., Meland, K., Messing, C. G., Mills, C. E., Molodtsova, T. N., Mooi, R., Neuhaus, B., Ng, P. K., Nielsen, C., Norenburg, J., Opresko, D. M., Osawa, M., Paulay, G., Perrin, W., Pilger, J. F., Poore, G. C., Pugh, P., Read, G. B., Reimer, J. D., Rius, M., Rocha, R. M., Saiz-Salinas, J. I., Scarabino, V., Schierwater, B., Schmidt-Rhaesa, A., Schnabel, K. E., Schotte, M., Schuchert, P., Schwabe, E., Segers, H., Self-Sullivan, C., Shenkar, N., Siegel, V., Sterrer, W., Stöhr, S., Swalla, B., Tasker, M. L., Thuesen, E. V., Timm, T., Todaro, M. A., Turon, X., Tyler, S., Uetz, P., van der Land, J., Vanhoorne, B., van Ofwegen, L. P., van Soest, R. W., Vanaverbeke, J., Walker-Smith, G., Walter, T. C., Warren, A., Williams, G. C., Wilson, S. P., and Costello, M. J. (2012). The magnitude of global marine species diversity. Current Biology 22, 2189–2202.
The magnitude of global marine species diversity.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC38Xhs12gtb7K&md5=a94f52d1a66a8f4c6b252541a2273749CAS | 23159596PubMed |

Balsamo, M., and Todaro, M. A. (1987). Aspidiophorus polystictos, a new marine species (Gastrotricha, Chaetonotida) and its life cycle. Bollettino di Zoologia 54, 147–153.
Aspidiophorus polystictos, a new marine species (Gastrotricha, Chaetonotida) and its life cycle.Crossref | GoogleScholarGoogle Scholar |

Balsamo, M., d’Hondt, J.-L., Kisielewski, J., and Pierboni, L. (2008). Global diversity of gastrotrichs (Gastrotricha) in freshwaters. Hydrobiologia 595, 85–91.
Global diversity of gastrotrichs (Gastrotricha) in freshwaters.Crossref | GoogleScholarGoogle Scholar |

Balsamo, M., Guidi, L., Ferraguti, M., Pierboni, L., and Kristensen, R. M. (2010). Diuronotus aspetos (Gastrotricha): new morphological data and description of the spermatozoon. Helgoland Marine Research 64, 27–34.
Diuronotus aspetos (Gastrotricha): new morphological data and description of the spermatozoon.Crossref | GoogleScholarGoogle Scholar |

Balsamo, M., Grilli, P., Guidi, L., and d’Hondt, J.-L. (2014). Gastrotricha: biology, ecology and systematics: families Dasydytidae, Dichaeturidae, Neogosseidae, Proichthydiidae. In ‘Identification Guides to the Plankton and Benthos of Inland Waters. 24’. (Ed. H. J. F. Dumont.) pp. 1–187. (Backhuys Publishers, Margraf Publishers GmbH: Weikeirsheim, Germany.)

Balsamo, M., d’Hondt, J.-L., Kisielewski, J., Todaro, M. A., Tongiorgi, P., Guidi, L., Grilli, P., and de Jong, Y. (2015). Fauna Europea: Gastrotricha. Biodiversity Data Journal 3, e5800.
Fauna Europea: Gastrotricha.Crossref | GoogleScholarGoogle Scholar | 26379467PubMed |

Coulson, S. J. (2007). The terrestrial and freshwater invertebrate fauna of the High Arctic archipelago of Svalbard. Zootaxa 1448, 41–58.

Coulson, S. J. (2013). The terrestrial invertebrate fauna of the Svalbard archipelago in a changing world: history of research and challenges. Canadian Entomologist 145, 131–146.
The terrestrial invertebrate fauna of the Svalbard archipelago in a changing world: history of research and challenges.Crossref | GoogleScholarGoogle Scholar |

Coulson, S. J., Hodkinson, I. D., Strathdee, A. T., Block, W., Webb, N. R., Bale, J. S., and Worland, M. R. (1995). Thermal environments of Arctic soil organisms during winter. Arctic and Alpine Research 27, 364–371.
Thermal environments of Arctic soil organisms during winter.Crossref | GoogleScholarGoogle Scholar |

Coulson, S. J., Convey, P., Aakra, K., Aarvik, L., Avila-Jimenez, M. L., Babenko, A., Biersma, E. M., Bostrom, S., Brittain, J. E., Carlsson, A. M., Christoffersen, K., De Smet, W. H., Ekremj, T., Fjellberg, A., Fureder, L., Gustafssonm, D., Gwiazdowicz, D. J., Hansen, L. O., Holmstrup, M., Hulle, M., Kaczmarek, Ł., Kolicka, M., Kuklin, V., Lakka, H. K., Lebedeva, N., Makarova, O., Maraldo, K., Melekhina, E., Odegaard, F., Pilskog, H. E., Simon, J. C., Sohlenius, B., Solhoy, T., Soli, G., Sture, E., Tanasevitch, A., Taskaeva, A., Velle, G., Zawierucha, K., and Zmudczyńska-Skarbek, K. (2014). The terrestrial and freshwater invertebrate biodiversity of the archipelagoes of the Barents Sea; Svalbard, Franz Josef Land and Novaya Zemlya. Soil Biology & Biochemistry 68, 440–470.
The terrestrial and freshwater invertebrate biodiversity of the archipelagoes of the Barents Sea; Svalbard, Franz Josef Land and Novaya Zemlya.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC3sXhvFOlt7bI&md5=543f39713d8b29d0ab1833d485cf8c8eCAS |

Dabert, J., Ehrnsberger, R., and Dabert, M. (2008). Glaucalges tytonis sp. nov. (Analgoidea: Xolalgidae) from the barn owl Tyto alba (Strigiformes: Tytonidae): compiling morphology with DNA barcode data for taxa descriptions in mites (Acari). Zootaxa 1719, 41–52.

Dabert, M., Witalinski, W., Kaźmierski, A., Olszanowski, Z., and Dabert, J. (2010). Molecular phylogeny of acariform mites (Acari, Arachnida): strong conflict between phylogenetic signal and long branch attraction artifacts. Molecular Phylogenetics and Evolution 56, 222–241.
Molecular phylogeny of acariform mites (Acari, Arachnida): strong conflict between phylogenetic signal and long branch attraction artifacts.Crossref | GoogleScholarGoogle Scholar | 20060051PubMed |

De Smet, W. H. (1993). Report on rotifers from Barentsøya, Svalbard (78°30ʹN). Fauna Norvegica Serie A 14, 1–26.

De Smet, W. H., Van Rompu, E. A., and Beyens, L. (1987). Rotifera, Gastrotricha en Tardigrada uit Shetland, de Faroër en Spitsbergen (Rotifera, Gastrotricha et Tardigrada provenant du Shetland, les îles Féroé et du Spitsberg). Natuurwetenschappelijk Tijdschrift 69, 81–102.

De Smet, W. H., Van Rompu, E. A., and Beyens, L. (1988). Contribution to the rotifers and aquatic Tardigrada of Edgeøya (Svalbard). Fauna Norvegica Serie A 9, 19–30.

Felsenstein, J. (1985). Confidence limits on phylogenies: an approach using the bootstrap. Evolution 39, 783–791.
Confidence limits on phylogenies: an approach using the bootstrap.Crossref | GoogleScholarGoogle Scholar |

Fiers, F., and Kotwicki, L. (2013). The multiple faces of Nannopus palustris auct. reconsidered: a morphological approach (Copepoda: Harpacticoida: Nannopodidae). Zoologischer Anzeiger 253, 36–65.
The multiple faces of Nannopus palustris auct. reconsidered: a morphological approach (Copepoda: Harpacticoida: Nannopodidae).Crossref | GoogleScholarGoogle Scholar |

Fregni, E., Balsamo, M., and Tongiorgi, P. (1998). Interstitial gastrotrichs from lotic Italian fresh waters. Hydrobiologia 368, 175–187.
Interstitial gastrotrichs from lotic Italian fresh waters.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DyaK1cXmsFaju7k%3D&md5=b62b5d58518fe10a5f549a412eebc028CAS |

Goldman, N., and Yang, Z. (1994). A codon-based model of nucleotide substitution for protein-coding DNA sequences. Molecular Biology and Evolution 11, 725–736.
| 1:CAS:528:DyaK2cXmt1eit70%3D&md5=c6c79272c29b624b6037322c281d6881CAS | 7968486PubMed |

Górska, B., Grzelak, K., Kotwicki, L., Hasemann, C., Schew, I., Soltwed, T., and Włodarska-Kowalczuk, M. (2014). Bathymetric variations in vertical distribuction patterns of meiofauna in the surface sediments of the deep Arctic ocean. Deep-sea Research. Part I, Oceanographic Research Papers 91, 36–49.
Bathymetric variations in vertical distribuction patterns of meiofauna in the surface sediments of the deep Arctic ocean.Crossref | GoogleScholarGoogle Scholar |

Grzelak, K., and Kotwicki, L. (2012). Meiofaunal distribution in Horsund fjord, Spitsbergen. Polar Biology 35, 269–280.
Meiofaunal distribution in Horsund fjord, Spitsbergen.Crossref | GoogleScholarGoogle Scholar |

Guerne, J. (1888). Campagnes scientifiques du Yacht monégasque l’Hirondelle, treoisième année 1887. [Scientific surveys of the Monaco Yacht Hirondelle, third year 1887]. Paris Gauthier-Villars 10, 72–78.

Hagen, J. O., Kohler, J., Melvold, K., and Winther, J. G. (2003). Glaciers in Svalbard: mass balance, runoff and freshwater flux. Polar Research 22, 145–159.
Glaciers in Svalbard: mass balance, runoff and freshwater flux.Crossref | GoogleScholarGoogle Scholar |

Hochberg, R. (1998). Postembryonic growth and morphological variability in Turbanella mustela (Gastrotricha, Macrodasyida). Journal of Morphology 237, 117–126.
Postembryonic growth and morphological variability in Turbanella mustela (Gastrotricha, Macrodasyida).Crossref | GoogleScholarGoogle Scholar |

Hochberg, R., and Litvaitis, M. K. (2000). Phylogeny of Gastrotricha: a morphology-based framework of Gastrotricha relationships. The Biological Bulletin 198, 299–305.
Phylogeny of Gastrotricha: a morphology-based framework of Gastrotricha relationships.Crossref | GoogleScholarGoogle Scholar | 1:STN:280:DC%2BD3c3ks1Khsw%3D%3D&md5=c05e4d3df6563850134e75bed179ca38CAS | 10786949PubMed |

Hummon, W. D., Balsamo, M., and Todaro, M. A. (1992). Italian marine Gastrotricha: I. Six new and one redescribed species of Chaetonotida. Bollettino di Zoologia 59, 499–516.
Italian marine Gastrotricha: I. Six new and one redescribed species of Chaetonotida.Crossref | GoogleScholarGoogle Scholar |

Jankowska, E., Włodarska-Kowalczuk, M., Kotwicki, L., Balazy, P., and Kukliński, K. (2014). Seasonality in the vegetation cover and its effects on sediment characteristics and meiofauna in the Baltic seagrass meadows. Estuarine, Coastal and Shelf Science 139, 159–170.
Seasonality in the vegetation cover and its effects on sediment characteristics and meiofauna in the Baltic seagrass meadows.Crossref | GoogleScholarGoogle Scholar |

Kånneby, T. (2013). New species and records of freshwater Chaetonotus (Gastrotricha: Chaetonotidae) from Sweden. Zootaxa 3701, 551–588.
New species and records of freshwater Chaetonotus (Gastrotricha: Chaetonotidae) from Sweden.Crossref | GoogleScholarGoogle Scholar | 26191602PubMed |

Kånneby, T. (2015). A redescription of Chaetonotus (Primochaetus) veronicae Kånneby, 2013 (Gastrotricha: Chaetonotidae). Zootaxa 4027, 442–446.
A redescription of Chaetonotus (Primochaetus) veronicae Kånneby, 2013 (Gastrotricha: Chaetonotidae).Crossref | GoogleScholarGoogle Scholar | 26624190PubMed |

Kånneby, T., Todaro, M. A., and Jondelius, U. (2012). A phylogenetic approach to species delimitation in freshwater Gastrotricha from Sweden. Hydrobiologia 683, 185–202.
A phylogenetic approach to species delimitation in freshwater Gastrotricha from Sweden.Crossref | GoogleScholarGoogle Scholar |

Kånneby, T., Todaro, M. A., and Jondelius, U. (2013). Phylogeny of Chaetonotidae and other Paucitubulatina (Gastrotricha: Chaetonotida) and colonization of aquatic ecosystems. Zoologica Scripta 42, 88–105.
Phylogeny of Chaetonotidae and other Paucitubulatina (Gastrotricha: Chaetonotida) and colonization of aquatic ecosystems.Crossref | GoogleScholarGoogle Scholar |

Kieneke, A., and Schmidt-Rhaesa, A. (2015). Gastrotricha. In ‘Handbook of Zoology. Vol. 3 Gastrotricha and Gnathifera’. (Ed. A. Schmidt-Rhaesa.) pp. 1–134. (De Gruyter: Berlin, Boston.)

Kieneke, A., and Zekely, J. (2007). Desmodasys abyssalis sp. nov. – first record of a deep-sea gastrotrich from hydrothermal vents. Marine Biodiversity Records 1, 1–7.

Kisielewski, J. (1981). ‘Gastrotricha from Raised and Transitional Peat Bogs in Poland. Monografie Fauny Polski, 11.’ (Polska Akademia Nauk: Kraków, Poland.)

Kisielewski, J. (1991). Inland-water Gastrotricha from Brazil. Annales Zoologici 43, 1–168.

Kisielewski, J. (1997). ‘Brzuchorzeski (Gastrotricha). Fauna Słodkowodna Polski, Zeszyt 31.’ (Wydawnictwo Uniwersytetu Łódzkiego: Łódź, Poland.)

Kolicka, M. (2014). Gastrotricha from Copenhagen Palm House – contribution to the knowledge of Lepidodermella intermedia Kånneby, Todaro & Jondelius, 2012 (Chaetonotida, Gastrotricha). Zoosystema 36, 713–722.
Gastrotricha from Copenhagen Palm House – contribution to the knowledge of Lepidodermella intermedia Kånneby, Todaro & Jondelius, 2012 (Chaetonotida, Gastrotricha).Crossref | GoogleScholarGoogle Scholar |

Kolicka, M. (2016). Gastrotrichs in bromeliads – newly recorded Chaetonotus (Hystricochaetonotus) furcatus Kisielewski, 1991 (Chaetonotida) from the Łódź Palm House. Zoosystema 38, 141–155.
Gastrotrichs in bromeliads – newly recorded Chaetonotus (Hystricochaetonotus) furcatus Kisielewski, 1991 (Chaetonotida) from the Łódź Palm House.Crossref | GoogleScholarGoogle Scholar |

Kolicka, M., Kisielewski, J., Nesteruk, T., and Zawierucha, K. (2013). Gastrotricha from the Poznań Palm House – one new subgenus and three new species of freshwater Chaetonotida (Gastrotricha). Zootaxa 3717, 231–279.
Gastrotricha from the Poznań Palm House – one new subgenus and three new species of freshwater Chaetonotida (Gastrotricha).Crossref | GoogleScholarGoogle Scholar | 26176105PubMed |

Kolicka, M., Jankowska, E., and Kotwicki, L. (2015). Baltic Sea Gastrotricha – one new species and one new record of Chaetonotida from Poland. Zootaxa 4027, 487–508.
Baltic Sea Gastrotricha – one new species and one new record of Chaetonotida from Poland.Crossref | GoogleScholarGoogle Scholar | 26624193PubMed |

Kotwicki, L., De Troch, M., Urban-Malinga, B., Gheskiere, T., and Węsławski, J. M. (2005). Horizontal and vertical distribution of meiofauna on sandy beaches of the North Sea (The Netherlands, Belgium, France). Helgoland Marine Research 59, 255–264.
Horizontal and vertical distribution of meiofauna on sandy beaches of the North Sea (The Netherlands, Belgium, France).Crossref | GoogleScholarGoogle Scholar |

Leasi, F., and Todaro, M. A. (2008). The muscular system of Musellifer delamarei (Reanud-Mornant, 1968) and other chaetonotidans with implication for the phylogeny and systematisation of the Paucitubulatina (Gastrotricha). Biological Journal of the Linnean Society. Linnean Society of London 94, 379–398.
The muscular system of Musellifer delamarei (Reanud-Mornant, 1968) and other chaetonotidans with implication for the phylogeny and systematisation of the Paucitubulatina (Gastrotricha).Crossref | GoogleScholarGoogle Scholar |

Leasi, F., and Todaro, M. A. (2009). Meiofaunal cryptic species revealed by confocal microscopy: the case of Xenotrichula intermedia (Gastrotricha). Marine Biology 156, 1335–1346.
Meiofaunal cryptic species revealed by confocal microscopy: the case of Xenotrichula intermedia (Gastrotricha).Crossref | GoogleScholarGoogle Scholar |

Mock, H. (1979). Chaetonotidea (Gastrotricha) from the North Sea Island of Sylt. Mikrofauna Meeresbodens 78, 1–107.

Nicholas, K. B., and Nicholas, H. B. (1997). GeneDoc: a tool for editing and annotating multiple sequence alignments. Pittsburgh Supercomputing Center’s National Resource for Biomedical Supercomputing. Available at http://www.nrbsc.org/downloads/ [Accessed 10 January 2014]

Norwegian Meteorological Institute (2014). Available at http://retro.met.no/observasjoner/svalbard/index.html [Accessed 12 July 2014]

Nylander, J. A. A. (2004). ‘MrModeltest v2. Program distributed by the author.’ (Evolutionary Biology Centre, Uppsala University: Uppsala, Sweden.)

Page, R. D. M. (1996). TREEVIEW: an application to display phylogenetic trees on personal computers. Computer Applications in the Biosciences 12, 357–358.
| 1:STN:280:DyaK2s%2FlvVSgtg%3D%3D&md5=0b115f1c34d59a6279f7f1ccf2b946f9CAS |

Pfannkuche, O., and Thiel, H. (1987). Meiobenthic stocks and benthic activity on the NE-Svalbard Shelf and in the Nansen Basin. Polar Biology 7, 253–266.
Meiobenthic stocks and benthic activity on the NE-Svalbard Shelf and in the Nansen Basin.Crossref | GoogleScholarGoogle Scholar |

Pilskog, H. E., Solhøy, T., Gwiazdowicz, D. J., Grytnes, J.-A., and Coulson, S. J. (2014). Invertebrate communities inhabiting nests of migrating passerine, wild fowl and sea birds breeding in the High Arctic, Svalbard. Polar Biology 37, 981–998.
Invertebrate communities inhabiting nests of migrating passerine, wild fowl and sea birds breeding in the High Arctic, Svalbard.Crossref | GoogleScholarGoogle Scholar |

Posada, D. (2008). jModelTest: phylogenetic model averaging. Molecular Biology and Evolution 25, 1253–1256.
jModelTest: phylogenetic model averaging.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1cXotlKgsb4%3D&md5=53928922981f8b5b3119f47393588dfaCAS | 18397919PubMed |

Rao, G. C., and Clausen, C. (1970). Planodasys marginalis gen. et. sp. nov. and Planodasyidae fam. nov. (Gastrotricha Macrodasyoidea). Sarsia 42, 73–82.

Rambaut, A. (2014). FigTree v. 1.4.2. Available at http://tree.bio.ed.ac.uk/software/figtree/ [Accessed 10 January 2014]

Rambaut, A., and Drummond, A. J. (2007). Tracer v1.4. Available at http://beast.bio.ed.ac.uk/Tracer [Accessed 12 July 2014]

Remane, A. (1936). Gastrotricha. In ‘Klassen Ordnungen des Tierreichs, Band 4, Abteilung II, Buch I, Teil 2, Liefrungen 1–2’. (Ed. H. G. Bronns.) pp. 1–385. (Akademie Verlagsgesellschaft: Berlin, Germany.)

Ronquist, F., Teslenko, M., van der Mark, P., Ayres, D. L., Darling, A., Höhna, S., Larget, B., Liu, L., Suchard, M. A., and Huelsenbeck, J. P. (2012). MrBayes 3.2: efficient Bayesian phylogenetic inference and model choice across a large model space. Systematic Biology 61, 539–542.
MrBayes 3.2: efficient Bayesian phylogenetic inference and model choice across a large model space.Crossref | GoogleScholarGoogle Scholar | 22357727PubMed |

Scourfield, D. J. (1897). Contribution to the non-marine fauna of Spitsbergen. Part I. Preliminary notes, and reports on the Rhizopoda, Tardigrada, Entomostraca, etc. Catalogue of the Birds in the British Museum 20, 784–792.

Tamura, K., Stecher, G., Peterson, D., Filipski, A., and Kumar, S. (2013). MEGA6: molecular evolutionary genetics analysis version 6.0. Molecular Biology and Evolution 30, 2725–2729.
MEGA6: molecular evolutionary genetics analysis version 6.0.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC3sXhvVKhurzP&md5=03ff7cc7d33ff928f46a243ed7d97fb2CAS | 24132122PubMed |

Telford, M. J., Lockyer, A. E., Cartwright-Finch, C., and Littlewood, D. T. J. (2003). Combined large and small subunit ribosomal RNA phylogenies support a basal position of the acoelomorph flatworms. Proceedings. Biological Sciences 270, 1077–1083.
Combined large and small subunit ribosomal RNA phylogenies support a basal position of the acoelomorph flatworms.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD3sXnsFGqsbg%3D&md5=64e96565e3270c3629d3391f61de7141CAS |

Thompson, J. D., Gibson, T. J., Plewniak, F., Jeanmougin, F., and Higgins, D. G. (1997). The ClustalX windows interface: flexible strategies for multiple sequence alignment aided by quality analysis tools. Nucleic Acids Research 25, 4876–4882.
The ClustalX windows interface: flexible strategies for multiple sequence alignment aided by quality analysis tools.Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DyaK1cXntFyntQ%3D%3D&md5=8581e86bc906dce14b2b746bc9a7ce44CAS |

Todaro, M. A. (2015). Gastrotricha. World Register of Marine Species. Available at http://www.marinespecies.org/aphia.php?p=taxdetails&id=2078 [Verified 1 September 2015].

Todaro, M. A., Balsamo, M., and Kristensen, R. (2005). A new genus of marine chaetonotids (Gastrotricha), with a description of two new species from Greenland and Denmark. Journal of the Marine Biological Association of the United Kingdom 85, 1391–1400.
A new genus of marine chaetonotids (Gastrotricha), with a description of two new species from Greenland and Denmark.Crossref | GoogleScholarGoogle Scholar |

Todaro, M. A., Leasi, F., Bizzarri, N., and Tongiorgi, P. (2006). Meiofauna densities and gastrotrich community composition in a Mediterranean sea cave. Marine Biology 149, 1079–1091.
Meiofauna densities and gastrotrich community composition in a Mediterranean sea cave.Crossref | GoogleScholarGoogle Scholar |

Todaro, M. A., Dal Zotto, M., Maiorova, A. S., and Adrianov, A. V. (2009). A new species of Aspidiophorus (Gastrotricha, Chaetonotida) from the Russian Far East with a key to marine species of the genus. Marine Biology Research 5, 297–303.
A new species of Aspidiophorus (Gastrotricha, Chaetonotida) from the Russian Far East with a key to marine species of the genus.Crossref | GoogleScholarGoogle Scholar |

Urban-Malinga, B., Wiktor, J., Jabłońska, A., and Moens, T. (2005). Intertidial meiofauna of high-latitude glacial Arctic fjord (Kongsfjorden, Svalbard) with emphasis on the structure of free-living Nematoda communities. Polar Biology 28, 940–950.
Intertidial meiofauna of high-latitude glacial Arctic fjord (Kongsfjorden, Svalbard) with emphasis on the structure of free-living Nematoda communities.Crossref | GoogleScholarGoogle Scholar |

Węsławski, J. M., Zajączkowski, M., Wiktor, J., and Szymelfening, M. (1997). Intertidal zone of Svalbard. 3. Littoral of subartic, oceanic island: Bjornoya. Polar Biology 18, 45–52.

Włodarska-Kowalczuk, M., Szymfeling, M., and Kotwicki, L. (1999). Macro- and meiobenthic fauna of Yaldiabucta glacial bay (Isfjorden, Spitsbergen). Polish Polar Research 4, 367–386.

Zawierucha, K., Coulson, S. J., Michalczyk, Ł., and Kaczmarek, Ł. (2013). Current knowledge of the Tardigrada of Svalbard with the first records of water bears from Nordaustlandet (High Arctic). Polar Research 32, 20886.
Current knowledge of the Tardigrada of Svalbard with the first records of water bears from Nordaustlandet (High Arctic).Crossref | GoogleScholarGoogle Scholar |

Zawierucha, K., Kolicka, M., Takeuchi, N., and Kaczmarek, Ł. (2015). Which animals can live in cryoconite holes of glaciers? A faunal review. Journal of Zoology 295, 159–169.
Which animals can live in cryoconite holes of glaciers? A faunal review.Crossref | GoogleScholarGoogle Scholar |

Zwickl, D. J. (2006). ‘Genetic algorithm approaches for the phylogenetic analysis of large biological sequence data sets under the maximum likelihood criterion.’ PhD Thesis. (The University of Texas, Austin, TX.)