Terrestrial nematodes from the Maritime Antarctic

Abstract Background Soil nematodes are one of the most important terrestrial faunal groups in Antarctica, as they are a major component of soil micro-food webs. Despite their crucial role in soil processes, knowledge of their species diversity and distribution is still incomplete. Taxonomic studies of Antarctic nematodes are fragmented, which prevents assessment of the degree of endemicity and distribution of the species, as well as other aspects of biogeography. New information The present study is focused on the nematode fauna of one of the three Antarctic sub-regions, the Maritime Antarctic and summarises all findings published up to April 2023. A species list that includes 44 species, belonging to 21 genera, 16 families and eight orders is provided. A review of the literature on terrestrial nematodes inhabiting the Maritime Antarctic showed that the sites are unevenly studied. Three islands (Signy, King George and Livingston Islands) revealed highest species richness, probably due to the highest rates of research effort. Most species and four genera (Antarctenchus, Pararhyssocolpus, Amblydorylaimus and Enchodeloides) are endemic, proving that nematode fauna of the Maritime Antarctic is autochthonous and unique. Several groups of islands/sites have been revealed, based on their nematode fauna. The study showed that species with a limited distribution prevailed, while only two species (Plectusantarcticus and Coomansusgerlachei) have been found in more than 50% of the sites. Based on the literature data, details on species localities, microhabitat distribution, plant associations and availability of DNA sequences are provided.


Introduction
Soil nematodes are one of the most important groups of the terrestrial fauna in Antarctica (Maslen and Convey 2006) as they are abundant, taxonomically and functionally diverse and occupy a central position in the soil micro-food webs (Adams et al. 2014) and may have an impact on nutrient cycling and carbon dioxide emission, when soils thaw for a longer period of the year under climate change (van den Hoogen et al. 2019).In the challenging environmental conditions of the Antarctic, their distribution is limited to ice-free areas, where they have evolved throughout millions of years of climatic fluctuations in refugia (Ebach et al. 2008, Convey et al. 2020, Stevens and Mackintosh 2023).The glaciations, long-term isolation, harsh climate and the patchy distribution of ice-free areas (present today where at least partially ice-free throughout repeated glacial maxima (Newman et al. 2009)) are the main factors affecting the Antarctic nematode fauna origin/ genesis (i.e. the formation of fauna under the influence of multiple factors -historical, geographic and ecological) (Andrássy 1998, Convey andPeck 2019).In order to survive in the extreme environments, nematodes have developed exceptional cryptobiotic adaptations to manage freezing and desiccation stress (e.g.Pickup (1988), Wharton (1995), Treonis and Wall (2005), Kagoshima et al. (2019)).
Knowledge of the impact of climate change on nematode communities from extreme habitats and how they respond to these changes is insufficient (Freckman andVirginia 1997, Nielsen et al. 2011a).One of the main problems in predicting the effects of climate change in Antarctica is the limited knowledge on the diversity of terrestrial fauna, especially nematodes and the lack of comprehensive long-term studies (Gantait 2014).Data on species distribution and biogeography are not enough and the taxonomic information still remains confused or scarce (Andrássy 1998, Maslen and Convey 2006, Adams et al. 2014, Kagoshima et al. 2019).Nematodes possess high indicator potential for assessing various environmental changes in the soil environment because they are abundant, ubiquitous, utilise diverse trophic and live strategies and, thus, occupy key positions in soil micro-food webs (Neher 2001, Ferris et al. 2001, Neher 2010, Chauvin et al. 2020, Taylor et al. 2020, Ara Khanum et al. 2022, Du Preez et al. 2022).This highlights the need for research on the fauna of nematodes and their communities in extreme environments in view of the already occurring global change.
Antarctica represents three distinct climatic regions: the Sub-Antarctic, Maritime and Continental Antarctic (Holdgate 1977), with the Sub-Antarctic being the most favourable (mean air temperatures of most islands are low, but positive during the whole year), with the Continental Antarctic having the harshest conditions (the average monthly temperatures remain below freezing) (Convey 2017).The Continental Antarctic covers the territories of the continent, the Balleny Islands and the eastern side of the Antarctic Peninsula (Convey 2017).The Sub-Antarctic is the boundary zone that lies north of 56°S (Chown and Brooks 2019).The flora and fauna in this region are rather typical of temperate latitudes.In this paper, we focused on terrestrial nematodes from the Maritime Antarctic.This is a region with a strong influence of the Southern Ocean; it includes the western coast of the Antarctic Peninsula to ca. 72°S, the South Shetland, South Orkney and South Sandwich Islands and the isolated Bouvetøya and Peter I Øya (Convey 2006, Convey 2017).The Maritime Antarctic is characterised by more favourable conditions compared with the Continental Antarctic: mean air temperatures are positive for 1-4 months of the year (Convey 2017), the vegetation is predominantly cryptogamic (algae, mosses, liverworts, lichens); higher plants are represented by two species, Deschampsia antarctica Desv.(Poaceae) and Colobanthus quitensis Bartl.(Caryophyllaceae) ( Greene 1970, Longton 1979, Smith 1984).
According to Andrássy (1998), numerous studies have reported species as new records with no morphological description making it impossible to confirm identifications, especially when the collected material is no longer available for subsequent examination.Further, this has an impact on the potential to assess fauna endemicity, which is critical for examining Antarctic biogeography within a global context (Andrássy 1998).There are numerous cases of misclassification and underestimation of the diversity for most microfaunal groups th th Terrestrial nematodes from the Maritime Antarctic in Antarctica, likely due to poor taxonomic resolution caused by insufficient sampling and their difficult identification (Adams et al. 2006, Iakovenko et al. 2015, Carapelli et al. 2017, Short et al. 2022, Collins et al. 2023), as well as the low degree of the development and application of molecular taxonomy.
In recent years, molecular studies have become more important in these marginal habitats, as a powerful toolkit to complement the traditional taxonomy, species identification and descriptions and to assess biodiversity and biogeography (Courtright et al. 2000, Velasco-Castrillón et al. 2014b, Elshishka et al. 2015b, Elshishka et al. 2017, Czechowski et al. 2017, Velasco-Castrillón et al. 2018, Kagoshima et al. 2019).
The integrative approach (combining morphological and molecular data) is an effective way to understand the scale of endemism, evolution and distribution of the Antarctic nematode fauna.However, the main problem of not linking molecular data with morphology still remains for the vast majority of Antarctic nematode species.
The present paper aims to summarise all records of nematode species occurrence in the Maritime Antarctic between the years of 1904 and April 2023 as a basis for further studies and to present a snapshot of nematode species diversity in this part of the Antarctic.

Materials and methods
The nematode species list has been composed, based on literature data and refers to the Maritime Antarctic.This list includes all species recovered in the Maritime Antarctic, as well as the islands and sites from where each species was reported, along with data on microhabitats and plant associations, accession numbers of published sequences in GenBank also included, if available.The type of microhabitat is reported as in the original paper, the scientific names of the plants being adapted according to the current systematics (Ochyra 1998).Geographical coordinates are presented additionally for each site if missing in the original paper.For the literature search, online bibliography search engine Google Scholar and the academic databases Scopus, Web of Science and CABI were used with search keywords "terrestrial nematode species*" and "Maritime Antarctic*".We focused on studies reporting nematode species (see Holovachov (2014a)) from the Maritime Antarctic and omitted those that provide data only at the generic or family level.
Several papers recording multiple unidentified taxa at generic level (Maslen andConvey 2006, Nielsen et al. 2011b) probably contain many undescribed nematode species from those regions suggesting that the nematode diversity there might be underestimated to a great extent.Overall, nematodes from 37 sites (34 islands and three localities on the Antarctic Peninsula) are included in the review.The taxonomic position of the Antarctic species was presented according to the current nematode nomenclature.Classification follows Andrássy (2005), Andrássy (2007) and Andrássy (2009); only for order Plectida classification follows Holovachov (2014b).The analyses are based on species presence/ absence data and Wizard > Matrix display function in PRIMER v.7.0 software (Clarke and Gorley 2015).The Matrix display wizard performs a sequence of sample and species resemblance calculations and clustering and seriation steps resulting in a shade plot which visualises the species presence/absence data and sites similarity.

Analysis Results
To date, 44 species of terrestrial nematodes, belonging to 21 genera, 16 families and eight orders have been recorded in the Maritime Antarctic (Table 1, Fig. 1).Nematodes have been reported from 34 islands and three sites on the Antarctic Peninsula (Fig. 2).Several groups of islands/sites have been revealed, based on their nematode fauna.Those groups form a gradient from north (the group of Livingston, King George and Signy Islands) to south (the group of Adelaide, Charcot, Alexander, Leonie and Alamode Islands).
The order Dorylaimida is the best represented order in this Antarctic Region with five families, six genera and 13 species.The order Mononchida is represented by only one family (one genus and species).
The families Aphelenchoididae, Cephalobidae, Monhysteridae, Plectidae, Qudsianematidae, Peloderidae and Rhabditidae have a cosmopolitan distribution and, in the Maritime Antarctic, they are represented by one to two genera and two to ten species.The family Plectidae is the most diverse (10 species).Seven families (Amphidelidae, Anguinidae, Aporcelaimidae, Mononchidae, Nordiidae, Pararhyssocolpidae and Psilenchidae) are represented by only one species each.
Almost all species and four genera (Antarctenchus, Pararhyssocolpus, Amblydorylaimus and Enchodeloides) are endemic.Four species generally known as cosmopolitan are reported in some ecological studies in the Maritime Antarctic: Eumonhystera vulgaris (de Man 1880) Andrássy (1981), E. filiformis (Bastian 1865) Andrássy (1981), Geomonhystera villosa (Bütschli 1873) Andrássy (1981) and Plectus armatus Bütschli 1873.Of these, a description and illustrations were provided only for E. vulgaris (Tsalolikhin 1989).In most of the literature sources, there are data on the microhabitats in which nematode species occurred.The nematodes have been recorded from various microhabitats: bare soil, microbial mats, moss, lichens and algae and soil around the two species of higher plants occurring in the Maritime Antarctic (Fig. 6).
DNA data have been generated for 11 species, but sequences for only three of them (Amblydorylaimus isokaryon (Loof 1975), P. paradoxus and E. signyensis) are supported by full morphological descriptions as per the modern taxonomic standards (Elshishka et al. 2015b, Elshishka et al. 2017, Kagoshima et al. 2019).
The review of the literature related to terrestrial nematodes from the Maritime Antarctic showed that the different parts are unevenly studied and three islands, Livingston (31 species), King George (28 species) and Signy (25 species) exhibited the richest nematode fauna (Fig. 7).Signy Island is the best studied Antarctic island with 12 new species Terrestrial nematodes from the Maritime Antarctic described.This is due to the intensive studies on the nematode fauna in the 1970s and 1980s undertaken by the British Antarctic Survey (Spaull 1973a, Spaull 1973b, Spaull 1973c, Loof 1975, Maslen 1979a, Maslen 1979b, Caldwell 1981, Maslen 1981, Pickup 1988, Pickup 1990 etc.).

Discussion
Our knowledge of the nematode species diversity in the Maritime Antarctic is still insufficient and fragmented.The different study efforts at the various sites do not allow gaining a clear picture of trends in the diversity and distribution of nematode species in the target Antarctic Region.Yet, the analysis provided on the basis of species presence/ absence data revealed several groups of sites with similar nematode fauna forming a latitudinal gradient (Fig. 1).The high level of endemism at both the species and genus level is a characteristic feature of the nematode fauna of the region as was mentioned above.This high degree of endemism can be explained by the long-term isolation and the harsh conditions of the region (Convey et al. 2008, Nielsen et al. 2011a).It has been suggested that the Antarctic terrestrial fauna might have survived glaciation in ice-free areas and some species might be remnants of the fauna of the Gondwana super-continent (Andrássy 1998, Maslen and Convey 2006, Chown and Convey 2016, Convey et al. 2020).
The physical isolation and harsh environment of Antarctic terrestrial ecosystems is the major reason for the difficult colonisation by non-native biota (Convey and Peck 2019).In recent decades, human visits and activities in the Antarctic have provided ways (e.g.cargo, vehicles, scientific equipment, fresh food, clothing, people) to overcome these barriers (Lee and Chown 2009, Hughes et al. 2010, Chwedorzewska et al. 2013, Adams et al. 2014).So far, the probability that introduced invertebrates will become established and spread is considered to be quite low; most of them are not able to complete the life cycle and establish a stable population outside the station (Chwedorzewska et al. 2013).Although these organisms cannot survive outside at present, they are potential colonisers, which could be established in the future following the climate warming (Convey and Peck 2019).
Тhe four cosmopolitan nematode taxa (E.vulgaris, E. filiformis, G. villosa and P. armatus) also reported from the Maritime Antarctic are considered to be of non-native origin by Andrássy (1998).Due to the absence or scarcity of data on the morphology of these species, at present, their origin cannot be confirmed.Future studies using an integrated taxonomic approach (i.e.simultaneous molecular and morphological characterisation) of materials obtained from pristine areas may help clarify their status.The gap in knowledge of nematode diversity, both in terms of taxonomy and distribution, is essential when assessing the introduction of non-native species.Nematode species richness in the Maritime Antarctic, which is underestimated (Nielsen et al. 2011b) may be compromised with increasing human impact in Antarctica.
The risk to Antarctic biodiversity is not limited to the transfer of alien species originating from other regions of Earth, but also concerns the transfer of native or endemic species from one part of Antarctica to another where they are not part of the indigenous biota (Convey 2008, Hughes et al. 2019, Hughes et al. 2020).This risk is greater because such species are likely to adapt well to the new location, unlike most non-native species that have been transferred to Antarctica from elsewhere (Convey 2015).The transfer of species across natural biogeographic boundaries can affect endemism in these areas.Antarctica is one of the few regions on the Planet where such boundaries still exist (Convey 2008).The nematode faunas of the Maritime and the Continental Antarctic are characterised by their uniqueness, as no overlap at the species level of the two local faunas exists (Andrássy 1998, Maslen and Convey 2006, Convey et al. 2020).This is indicative of an ancient geographical divide between these areas (Andrássy and Gibson 2007) and led Chown and Convey (2006) to define the Gressitt Line, which is located across the base of the Antarctic Peninsula.
So far, there is no evidence for the transfer and establishment of nematode species from the Continental to the Maritime Antarctic.Some nematological reports have included data on the presence of species that are emblematic of the Continental Antarctic (Plectus murrayi Yeates 1970 and P. frigophilus Kirjanova 1958) in the Maritime part, without morphological data (see Velasco-Castrillón et al. (2014a)).In our study, these records are not included as they are most likely due to misidentification.
Regarding the biotope/microhabitat distribution of the species, the incomplete and insufficient data do not allow a definite conclusion, taking into account also the lack of research in the more inaccessible areas of the Antarctic Peninsula and the islands.Most likely the micro biotope distribution pattern is similar to that shown in the study of the nematode fauna of Cape Chelyuskin in the Arctic (Chernov et al. 1979), where species show very low biotopic associations and most of them inhabit all possible microhabitats (i.e. the majority of species are polytopic); this is also a characteristic feature of other groups of organisms in the polar regions (Chernov et al. 1979).
The major life strategy of organisms living in extreme environments is the development of tolerance and plasticity and not lack of competition and specialisation, which is typical of other biomes (Convey 1996, Chernov et al. 2011).
Comparing the two parts of the Antarctic shows that the nematode studies in the Maritime Antarctic are less represented, whereas investigations in the Continental Antarctic have been more intensive.However, the latter are primarily related to ecology (Adams et al. 2014, Velasco-Castrillón et al. 2014a, Velasco-Castrillón et al. 2018) and have identified to date 34 species of soil nematodes (Velasco-Castrillón et al. 2014a).The smaller number of species in the Continental Antarctic is associated with the harsher and more unfavourable environmental conditions.This zone includes ecosystems with the simplest terrestrial fauna on the Planet, where even nematodes are absent (Convey andMcInnes 2005, Convey 2017).
The two opposite polar regions of the Earth are unevenly studied with respect to soil nematodes (Peneva et al. 2009, Holovachov 2014a).Despite the fewer taxonomic studies of terrestrial nematodes in the Arctic, 391 species have been recorded there (Holovachov 2014a).Key geographical and ecological features of both regions, such as geological history, climate, landscape, dispersal barriers and vegetation are responsible for the lower nematode diversity in the Antarctic than in the Arctic (Nielsen and Wall 2013).
Studies that include molecular data for the nematodes in the Maritime Antarctic are too rare to provide valuable information regarding nematode diversity, phylogenetics and endemism (Elshishka et al. 2015b, Elshishka et al. 2017, Kagoshima et al. 2019).The taxonomic position of only three Antarctic dorylaimid species, A. isokaryon, P. paradoxus and E. signyensis, was reconsidered on the basis of morphological and molecular characteristics of 18S rDNA (SSU rDNA) and the D2-D3 expansion fragments of 28S rDNA (LSU rDNA) (Elshishka et al. 2015b, Elshishka et al. 2017) and two new endemic genera were proposed (Pararhyssocolpus and Enchodeloides).
To advance the understanding of phylogeny and phylogeography of Antarctic nematodes, studies are required of other genes with higher evolutionary rates than 18S rDNA, such as 28S rDNA, the internal transcribed spacer (ITS in the ribosomal RNA locus) or the mitochondrial cytochrome c oxidase subunit I (COI).These genes should be included in future taxonomic analyses of Antarctic nematodes (Kagoshima et 2019).
The application of integrated taxonomy and DNA barcoding will substantially assist in nematode diversity studies, phylogenetics and especially the recognition of cryptic species.Further, comprehensive molecular studies will provide valuable information on the patterns of species distribution and for gaining additional knowledge on evolutionary processes and biogeography of Antarctic nematodes.
The scant studies of polar regions, in particular of the Maritime Antarctic, demand more intensive sampling and research, especially in the territories that have so far remained unexplored, in order to give a clearer and more adequate view of species diversity and trends in their microhabitat and geographical distribution.Therefore, further efforts aiming at targeted and systematic integrative studies are needed.
. antarcticus, Pararhyssocolpus paradoxus (Loof 1975), Eudorylaimus spaulli Loof 1975, E. coniceps Loof 1975, Enchodeloides signyensis ( Loof 1975)) with C. gerlachei and P. antarcticus being the most widespread (reported from more than half of the sites) (Figs 3,4, 5).There are no particular trends in the distribution of most common species (occurring in more than 22% of the sites, 1/4 of the species) related to longitude or latitude, only P. paradoxus and Mesodorylaimus imperator Loof 1975 have not been reported from the most southern sites, whereas G. villosa -from the most northern localities.

Figure 1 .
Figure 1.Terrestrial nematodes from the Maritime Antarctic -visual representation of the data matrix (shade plot): in the columns are the 37 sites and in the rows -44 species.White and black spaces denote absence or presence of a particular species at a given site; sites and species are arranged according to the groups derived by the clustering analyses.Significant clusters were identified with SIMPROF test and visualised in red dashed lines and a range of coloured dots.Each colour represents a group of sites/islands with similar nematode fauna.

Figure 2 .
Figure 2. Distribution of terrestrial nematodes in the Maritime Antarctic.In green are presented the sites with records of terrestrial nematodes.

Figure 3 .
Figure 3. Described species and their occurrences presented as percentages.

Figure 4 .
Figure 4. Distribution of P. antarcticus, C. gerlachei and Pararhyssocolpus paradoxus in the Maritime Antarctic.In red are presented the sites with records of these species, in green are presented the sites with records of the other Antarctic terrestrial nematodes.

Figure 5 .
Figure 5. Distribution of Eudorylaimus spaulli, E. coniceps and Enchodeloides signyensis in the Maritime Antarctic.In red are presented the sites with records of these species, in green are presented the sites with records of the other Antarctic terrestrial nematodes.

Figure 7 .
Figure 7. Bar chart visualising the described species (left axis) and literature sources (right axis) per each island/site.