Nematodes from terrestrial and freshwater habitats in the Arctic

Abstract We present an updated list of terrestrial and freshwater nematodes from all regions of the Arctic, for which records of properly identified nematode species are available: Svalbard, Jan Mayen, Iceland, Greenland, Nunavut, Northwest territories, Alaska, Lena River estuary, Taymyr and Severnaya Zemlya and Novaya Zemlya. The list includes 391 species belonging to 146 genera, 54 families and 10 orders of the phylum Nematoda.


Introduction
Nematodes are one of the most numerous and abundant multicellular organisms on the planet in general and in the Arctic in particular. There are over 70 research papers published which include data on the fauna and distribution of nematodes in the region. Few faunistic overviews of the Arctic nematodes exist in the literature. The "Catalogue of free-living soil and fresh-water nematodes of Arctic and Subarctic" (Kuzmin and Gagarin 1990) includes a large number of subarctic nematodes and is normally inaccessible. Subsequent publication by Gagarin (2001b) focuses only on the fresh-water nematodes from the Russian Arctic and Subarctic, while Coulson and Refseth (2004) give a list of terrestrial and freshwater nematodes from Svalbard and Jan Mayen. The present paper aims to provide a comprehensive overview of all findings of nematodes in the entire Arctic region between the years of 1883 and 2013, with all species names adjusted according to the most recent classification and nomenclature.

Materials and methods
The species list has been compiled based on literature data and refers to the area of the Arctic (Fig. 1) as defined by the Circumpolar Arctic Vegetation Map (CAVM Team 2003) and Meltofte et al. (2013). The list below includes all species found in the Arctic, and specifies the regions where each species was found, as well as all relevant references. Current taxonomic status and systematic position of all species was updated according to the most recent views on nematode systematics. Section on nomenclature includes those synonymous names, and sometimes mistyped names, which were used in the original publications on the Arctic nematodes; it does not include any other existing synonyms or combinations. Asterisks indicate those new species names, which were originally proposed based on populations described from the Arctic.
Taxa identified in the literature to the genus level only were not considered in the review for the following reasons: 1) It is impossible to make sure how many actual species were considered by the author -just one, or if s/he had treated multiple species together in the same unit. 2) Taxa identified only to the genus level have limited usefulness when Known localities in the Arctic for which detailed information about nematode fauna (identified to species level) is available in the literature.
analyzing distribution, endemism and other aspects of biogeography. Nematode systematics is continuously improving. Menzel (1920) described Dorylaimus sp. from Spitzbergen (Svalbard). The context of the genus Dorylaimus had changed considerably since 1920 and now includes members of at least families Dorylaimidae, Qudsianematidae, Aporcelaimidae and Nordiidae, making it practically impossible to use Menzel's record in modern faunistic studies. Even genus-level identifications done more recently face similar problems. Species that are now considered to belong to the same genus may in future, when more data becomes available, be split into different, sometimes distantly related genera. It would thus be impossible to know in the future what author of the paper meant when he identified his material.

Discussion
Despite of more than 130 years of research, our knowledge on the diversity and distribution of Arctic nematodes is still very incomplete and limited to only few relatively small areas of the region. The first studies of Arctic nematodes date back to 1883, when the first nematode was described from Spitzbergen (Aurivillius 1883). The taxonomic position of the species Aphelenchus nivalis Aurivillius, 1883 remains uncertain and its type material can not be located for re-study. After Aurivillius, the nematode fauna of Svalbard archipelago was studied by von Linstow (1900), Menzel (1920, Boström (1987a), Boström (1987b), Boström (1989), Zell (1993 and to a lesser extent by Brzeski (1962), Siddiqi (1979), Janiec (1997), Klekowski and Opalinski (1986), Klekowski and Opalinski (1990), , Klekowski (1992), Holovachov et al. (2003) adding up to the list of 101 species. Most of the sampling on Svalbard was concentrated around the west part of the Spitsbergen island and a large part of the archipelago remains unstudied. From other territories belonging to the Kingdom of Norway, 43 species of nematodes were found on the small island of Jan Mayen (Allgén 1953, Steiner 1916a, while no information is available for the northernmost part of the Finnmark. Most of the nematological research in the Iceland was focused in southern and western part of the island (De Coninck 1940, De Coninck 1943, De Coninck 1944, which is considered as Subarctic in the current publication. Sampling sites from the northern part of the country were also outside the boundary considered in the present paper (Kreis 1963, Starmühlner 1969. A small part of the Arctic of the Iceland has not been studied to any considerable extent -only six species from the genus Plectus are listed from the areas near Húsavík and Tjörnes by Zell (1993). Terrestrial nematode fauna of Greenland, on the other hand, received considerable attention from Ditlevsen (1927), Allgén (1954), Zell (1993, Brzeski (1999), who altogether reported 43 species of nematodes collected in several distant localities along the eastern (Little Pendulum Island and Angmagssalik) and western (Disko island, Kangerlussuaq, Kapisillit, Sukkertoppen and Upernavik) sides of the island.
Fauna of the Russian part of the Arctic lists the largest number of nematode species, including many typical freshwater nematodes, but known from only three distinct geographic regions: Novaya Zemlya archipelago together with Vaygach island, Taymyr peninsula together with Severnaya Zemlya, and the Lena River estuary. So far, only 37 species of freshwater nematodes are know from the Lena River estuary (Gagarin 1995, Gagarin 1999b, Gagarin 2000, Gagarin 2001b). The list of nematodes from the Novaya Zemlya archipelago and Vaygach island includes 81 nematode species, both terrestrial (Steiner 1916b) and freshwater (Gagarin 1997c, Gagarin 1997b, Gagarin 1997a, Gagarin 1999a, Gagarin 2000, Gagarin 2001c, Gagarin 2001b. Arctic areas of the Taymyr peninsula and Severnaya Zemlya are the most intensively studied part of the Arctic with localities including the Taymyr Lake, Enisei River, Putorana Plateau, Cape Chelyuskin -229 species of freshwater and terrestrial nematodes were reported from different sampling sites of this region over the past 40 years (Elshishka et al. 2012, Elshishka et al. 2013, Gagarin 1990, Gagarin 1991a, Gagarin 1991b, Gagarin 1993, Gagarin 1996, Gagarin 1999a, Gagarin 2001b, Gagarin 2001a, Gagarin 2008, Gagarin 2009, Gagarin and Kuzmin 1972, Gagarin and Kuzmin 1983, Kuzmin 1972, Kuzmin 1976, Kuzmin 1986, Kuzmin and Gagarin 1990. In summary, 391 species, belonging to 146 genera, 54 families, and 10 orders of the phylum Nematoda are known from the entire Arctic (including species of dubious taxonomic status, but exclusing single nomen nudum). Even taking into consideration relatively low variety of habitats in this region, comparing to other areas of the world, as well as the rather harsh environmental conditions, this list is considered to be very incomplete and preliminary.
"Systematics of Swedish free-living nematodes of the orders Desmodorida and Araeolaimida".