Revision of the calcareous fen arachnofauna: habitat affinities of the fen-inhabiting spiders.

Calcareous fens are one of the most species-rich habitats of the temperate zone of the Northern Hemisphere. In spite of this species richness, however, calcareous fens are still rather poorly investigated. Consequently, the data of the fen-associated spider fauna are also largely lacking. The aim of the research was to study the spider fauna of the calcareous fens of Latvia and to draw conclusions about what kind of spider species and ecological groups typically inhabit calcareous fen habitats. Spiders were sampled in the summer months of 2010, 2011, and 2012 at nine different calcareous fens of the coastal lowland of Latvia. The spider collection was performed by pitfall traps and a sweep net. The examined material comprised 6631 adult spider individuals representing 21 families and 149 species. The main spider ecological groups that dominated in the studied calcareous fens were hygrophilous and photophilous species which largely reflect the main properties of our studied habitats, all of which were wet, open mire habitats. Nevertheless, the fen arachnofauna consisted also of spider groups which are less typical for moist, sun-exposed, and alkaline environments, like xerophilous, sciophilous, and sphagnophilous species, respectively. Finally, several spider species collected in this study have not been previously reported for the spider fauna of Latvia, and many more might still be undiscovered in these unique and poorly investigated habitats. Therefore, it is suggested that calcareous fens deserve special attention and they should definitely be investigated further.

Although both fens and bogs are mire habitats, there are several fundamental differences between them: (1) fens are mires that receive water and nutrients from groundwater and/or surface water, as well as from rainfall, while bogs depend solely on precipitation (McBride et al. 2011); (2) fens are mineral-rich type of mires which are usually characterized by basic or circumneutral conditions, while bogs are nutrient poor mires which have strongly acidic (pH < 5.0) soil conditions (Kellner 2003;Spitzer and Danks 2006;Horsák et al. 2011); (3) fens are dominated by brown mosses and sedges (e.g., Carex, Cladium, Schoenus), while bogs -by peat mosses (Sphagnum spp.) (Rydin and Jeglum 2006;Gałka et al. 2016); and (4) fens are rich in a floristic sense, while bogs have a low species diversity (Kellner 2003;McBride et al. 2011). Because of these differences between fens and bogs, and because of the fact that fens are much less studied than bogs, it is clear that more studies are needed in fen habitats.
Calcareous fens are one of those fen types which are especially worth studying because they belong to the most species-rich ecosystems of the temperate zone of the Northern Hemisphere (Joosten and Clarke 2002). In addition, there are some plant and animal species that occur almost exclusively in this habitat type. For example, Scorpidium cossonii, Schoenus ferrugineus, Carex davalliana, Ophrys insectifera, Saussurea esthonica and Juncus subnodulosus are plant species that can be found only within calcareous fens (Auniņš et al. 2013). Also, calcareous fens is a very important habitat for specially protected snail species, such as Vertigo genesii and V. geyeri (Cameron et al. 2003;Auniņš et al. 2013). Overall, calcareous fens are very rare in most of the countries in the European Union (Stanová et al. 2008), and they are considered priority habitats in Annex I of the EU Habitat Directive (EC 1992). Thus, because of the rarity of the calcareous fens and because of the presence of unique species within these habitats, it would be important and worthwhile to assess the quality of the calcareous fens, as well as to investigate their flora and fauna.
Spiders have been shown to be very good bioindicators (e.g., Marc et al. 1999;Pearce and Venier 2006), and thereby they are proposed as a group of organisms that are potentially useful tools for assessing the conservation value of rare and threatened habitats. Unfortunately, arachnids from calcareous fens are very poorly studied. In Latvia, spiders within calcareous fens have been investigated in some of our previous studies (Štokmane et al. 2013;Spuņģis 2014, 2016). These previous investigations had examined the influence of vegetation structure on spider diversity, while little attention was paid on the faunistic aspects of the fen spider communities. Thereby, the main purpose of the present study was to investigate the spider fauna of calcareous fens in greater detail and to analyse what kind of spider species and ecological groups are more typical for this habitat type.
A short overview of the used sampling methods is given in Table 1. As it can be seen, the spider capture methods as well as the time of sampling differed in each of the three study years. This was done in order to access more spider species and enlarge the species list, since it is well known that, firstly, each collecting method targets different spider species (e.g., Churchill and Arthur 1999) and, secondly, spiders exhibit seasonal variation in their occurrence (e.g., Marc et al. 1999). The sampling period of the present study, however, was limited to the summer months only (June/July/August). We decided that summer will be the optimal time for collecting spiders since it is the warmest season in Latvia (LEGMC 2014) when vegetation biomass is at its prime and food resources are plentiful, and thus we assumed that spider diversity as well as the number of species and individuals will be much greater at this period of time. Higher abundance and diversity of spiders during summer is also consistent with the findings of other researchers (e.g., Hatley and Macmahon 1980;Reddy and Venkataiah 1986).
A binocular microscope at 45× magnification was used to identify the spiders to species level. The unidentified adult species were recorded as morphospecies. Since juveniles could be identified only to family level, they were excluded from the study. The full species list of calcareous fen spiders as well as the number of collected individuals in each fen can be found in Appendix 1. The nomenclature of spider species followed the World Spider Catalog (2018).
In order to understand what kind of spider species inhabit calcareous fens, we carried out a literature survey and prepared a short description on the habitat preferences for each of our collected spider species. The habitat affinities of the spider species were derived from many different literature sources, but mainly from Locket andMillidge (1951, 1953), Roberts (1996), Harvey et al. (2002a,b), Almquist (2005Almquist ( , 2006, Matveinen-Huju et al. (2006), Oxbrough et al. (2006), Nentwig et al. (2012)  and Arachnologische Gesellschaft (2018). Based on the literature analysis, all the collected spider species were sorted into a number of ecological groups. These groups were distinguished mainly by taking into account the spider requirements for moisture and light, since these two abiotic factors are among the most important determinants characterizing the habitats of spiders (Entling et al. 2007). When taking into account the moisture preferences, the species were classified as either being hygrophilous (water-loving) or xerophilous (drought-loving), but when taking into account the light preferences, the species were classified as either being photophilous (sun-loving) or sciophilous (shade-loving). Species with a wide ecological amplitude (= found in many different habitat types) were classified as being habitat generalists.
In addition, we have summarized the information whether our detected spider species have been found within other European mires. We have chosen to include in our summary those mire studies in which the full spider species list has been published. Thus, we included the following studies: Cera et al. (2010) (calcareous fens of Latvia), Šternbergs (1991) (Baži bog of Latvia), Kajak et al. (2000) (fens of Poland), Kupryjanowicz et al. (1998) (bogs of Poland), Vilbaste (1980) (fens and bogs of Estonia), Koponen (2002a,b) (bogs of Northern Europe, including Sweden, Finland and northern Norway), Rėlys and Dapkus (2002b) (bogs of Lithuania), and Rėlys et al. (2002) (bogs of Lithuania and Finland). This information is presented as presence data in Appendix 2.
All our collected spider specimens are stored in 70% ethanol, labelled, and deposited in the Department of Zoology and Animal Ecology, Faculty of Biology, University of Latvia, Riga.

Results
Overall, in the three study years a total number of 8,967 spider individuals (6631 adults and 2336 juveniles) were collected, representing 134 species and 15 morphospecies in 21 families. Most of the species (87 spp.) were collected only in a single year, while only five species were detected in all three study years (Dolomedes fimbriatus, Evarcha arcuata, Tibellus maritimus, Xysticus ulmi and Kaestneria pullata). Altogether eight spider species found during this investigation were registered as new species for the araneofauna of Latvia -Cheiracanthium punctorium (Eutichuridae), Gnaphosa lapponum (Gnaphosidae), G. nigerrima (Gnaphosidae), Bathyphantes parvulus (Linyphiidae), Centromerus semiater (Linyphiidae), Microlinyphia impigra (Linyphiidae), Pirata tenuitarsis (Lycosidae), and Leptorchestes berolinensis (Salticidae). The dominant spider species in each year and in each fen are given in Table 2. One of the most abundant and most frequently recorded species was Dolomedes fimbriatus, which occurred in the vast majority of the studied calcareous fens. Overall, however, there were rather large differences in spider species composition between fens, between study years, as well as between pitfall samples and the sweep-net samples.
By using relevant information from the literature (see the method section), we have prepared a short description of each of the collected spider species (Table 3). Also, all the collected spider species were sorted into ecological groups according to their habitat requirements. This classification was based mainly on spider requirements for moisture (hygrophilous/xerophilous species) and light (photophilous/sciophilous species). For some of the spider species we distinguished also sub-groups. In some cases, however, it was difficult to classify a spider species into a particular ecological group(-s), because the habitat preferences of some spiders are rather poorly defined (Eyre and Woodward 1996), and the information in the literature is sometimes contradictory (personal observation).  The spider ecological group composition in the studied calcareous fens and the number of spider species and individuals within each group is given in Figure 2. The most species-rich and the most abundant ecological group was hygrophilous species -more than a half of all spider species and individuals collected in the present study could be classified as hygrophilous (if including also hygrophilous-photophilous and hygrophiloussciophilouspecies). Photophilous species (including photophilous-hygrophilous and photophilous-xerophilous) was another large group in the studied fens -overall, 46 of our collected spider species (34% of all spiders) and 3088 individuals (48%) could be classified as photophilous species. The rest of the ecological groups, xerophilous, sciophilous, and habitat generalists, were represented by a rather low number of species and individuals.

Discussion
In the present study we investigated the spider fauna of the calcareous fens of the coastal lowland of Latvia. The full species list of the calcareous fen spiders is given in Appendix 1. The main purpose of the present study was to clarify the habitat preferences of the spider species collected during our investigations in the calcareous fens. The habitat preferences of each spider species are described in Table 3.
The arachnofauna of the studied calcareous fens consisted of a wide spectrum of different spider ecological groups. The vast majority of the spider species and individuals found in the fens were hygrophilous or photophilous or hygrophilous-photophilous. The dominance of these groups in the studied habitats is quite logical since all our studied fens were moist, sun-exposed habitats. Nevertheless, the fen arachnofauna consisted also of different other spider ecological groups, including even some groups which do not normally inhabit wet and alkaline environments, i.e., xerophilous and sphagnophilous species. The reason of the presence of such species within calcareous fens might be related with the fact that fens encompass a much broader range of microhabitat variation than other mire types. Fen surface often has a variable microrelief that consists of hummocks, hollows and pools, and since the tops of the hummocks are much drier than their lower part, they may serve as suitable habitat patches for the xerophilous species. Other researchers have also observed that drought-loving spider species can occasionally be found on raised, dry patches of vegetation within otherwise wet and marshy sites (Roberts 1996;Cattin et al. 2003). Similarly, the presence of sphagnophilous spider species within the studied mires might also be explained by the availability of hummocks. Usually these fen hummocks are dominated by acid-loving plant species (especially Sphagnum mosses) which are raised above the water level and thus protected from the influence of the alkaline groundwater (Rydin and Jeglum 2006). Consequently, the sphagnophilous spider species, which normally live in acid environments, especially bogs, and are related with Sphagnum mosses (e.g., Gnaphosa nigerrima, Pardosa sphagnicola, Pirata piscatorius), might also be supported in calcareous fens, since the Sphagnumdominated hummocks may serve as discrete habitat patches for them. These findings are supported by several other researchers who have also discovered that spiders can persist in very small microhabitats (Wing 1984;Foelix 2011;Cobbold and MacMahon 2012). Table 3. List of spider species collected in the calcareous fens of Latvia and description of their habitat preferences. The ecological group(s) of each species are also indicated (bold). For some of the species the ecological sub-group is given as well. Genera and species are sorted alphabetically within each family.

Agelenidae
Agelena labyrinthica (Clerck, 1757) It can be found in habitats such as sandy heathlands, banks of ditches (Almquist 2005), sunny forest edges (Nentwig et al. 2012), grasslands (Harvey et al. 2002b), bogs (Vilbaste 1980). This species can also occur in coastal sites -it has been found in coastal dune habitats in Latvia , as well as in salt marshes at the North Sea coast (Finch et al. 2007 It is usually found in damp, sheltered woodland clearings (Roberts 1996;Harvey et al. 2002b). It can also be found in humid forest edges, damp meadows, bogs (Vilbaste 1980;Almquist 2005;Nentwig et al. 2012 It is one of the most common and abundant species (Locket and Millidge 1953) which is rather ubiquitous (Cattin et al. 2003)it occurs wherever the habitat can provide supports for its large orb web (Harvey et al. 2002b). It can be found in a wide range of habitats, such as all types of woodland, grasslands, hedgerows, heathland, as well as roadside verges, quarries, gardens, buildings and different other places (Nyffeler and Benz 1987;Harvey et al. 2002b;Almquist 2005; Arachnologische Gesellschaft 2018). It, however, seems to prefer forest edges and gardens (Heimer and Nentwig 1991;Nentwig et al. 2012 It occurs in grasslands Benz 1987, 1989;Harvey et al. 2002b), especially in moist meadows (Almquist 2005;Nentwig et al. 2012). This species is found on vegetation which has sufficient height and strength to support its large orb web, such as tall grasses, heather and bushes such as gorse (Roberts 1996;Harvey et al. 2002b). A. quadratus can also be found in bogs (Vilbaste 1980;Almquist 2005) and fens (Vilbaste 1980;Kajak et al. 2000;Cera et al. 2010 Gesellschaft 2018). Most commonly, however, the species is found on trees and bushes in woodland, scrub and hedgerows, as well as on nearby low vegetation (Roberts 1996;Harvey et al. 2002b;Nentwig et al. 2012). Harvey et al. (2002b) wrote that the main tree that is inhabited by A. cucurbitina is oak, however Almquist (2005) mentions also pine, spruce and birch.
Overall, however, the species seems to prefer warm, dry and sunny places (Harvey et al. 2002b;Nentwig et al. 2012

Hygrophilous | Xerophilous; Photophilous | Sciophilous; Habitat generalist
Bathyphantes gracilis (Blackwall, 1841) A typical species for moist habitats (Koponen 2002b;Matveinen-Huju et al. 2006). It can be found in grasslands, heathlands, forests (Harvey et al. 2002a;Nentwig et al. 2012), as well as in fens (Vilbaste 1980;Kajak et al. 2000) and bogs (Koponen 2002a,b;Rėlys et al. 2002). Also, B. gracilis is an agrobiont -it is very common in open agricultural habitats, for example, meadows and fields (Bonte et al. 2002;Pommeresche 2004;Cristofoli et al. 2010). The species is a common aeronaut (Locket and Millidge 1953;Bonte et al. 2002;Harvey et al. 2002a It is a hygrophilous species (Aakra 2002;Matveinen-Huju et al. 2006) which seems to have an affinity for forests (Harvey et al. 2002a;Cristofoli et al. 2010). It mainly occurs in very damp and shadowed places, especially in bog forests (Nentwig et al. 2012). In Latvia it has been found on fens by Cera et al. (2010). It is predominantly a grassland spider that occurs in acid grasslands, chalk grasslands and meadows (Harvey et al. 2002a;Cristofoli et al. 2010). This species, however, can also be found on fens (Kajak et al. 2000), bogs (Koponen 2002a) and forests (Heimer and Nentwig 1991;Nentwig et al. 2012 It is indifferent as regards light intensity (Matveinen-Huju et al. 2006) and can be found in a variety of habitats -in grasslands, forest edges, coniferous and broad-leaved woodlands (Harvey et al. 2002a;Nentwig et al. 2012). It can also occur in fens and bogs (Vilbaste 1980;Arachnologische Gesellschaft 2018 It can be found in a wide variety of situations -in grasslands, hedgerows, gardens, humid forests, marshes and shadowed watersides (Harvey et al. 2002a;Nentwig et al. 2012). Overall, D. concolor seems to prefer forest habitats (Stańska et al. 2000;Pommeresche 2002Pommeresche , 2004Buchholz 2009;Gallé et al. 2011). Also, this species is quite common in habitats with a high level of human disturbance, such as vineyards (Harvey et al. 2002a;Isaia et al. 2007;Arachnologische Gesellschaft 2018 A species that is related with trees, particularly with conifers -it occurs in pine forests and in fir and spruce woodlands (Matveinen-Huju et al. 2006;Arachnologische Gesellschaft 2018). D. elevatus can be found mostly under pines, on the lower branches of pines, and also on heather, gorse and juniper (Locket and Millidge 1953;Harvey et al. 2002a;Nentwig et al. 2012). Can be also found in fens and bogs (Vilbaste 1980;Rėlys et al. 2002;Cera et al. 2010 It prefers humid conditions (Nentwig et al. 2012). This species is mainly associated with coastal habitats (Hänggi et al. 1995) -it occurs on the seashore and the shoreline of estuaries where it can be found amongst stones and seaweed (Locket and Millidge 1953;Harvey et al. 2002a). Irmler et al. (2002) have discovered E. arctica in the coastal salt marsh. Inland this species can also be found in saline areas (Harvey et al. 2002a;Duffey 2005 Both these spiders can be classified as pioneer species (Aakra 2002). E. atra is an universally distributed species -it is one of the commonest spiders that often disperse aeronautically in large numbers (Locket and Millidge 1953;Harvey et al. 2002a). The second species -E. dentipalpis -occurs in a similarly wide range of habitats as E. atra, and is an equally common aeronaut (Locket and Millidge 1953;Harvey et al. 2002a). Both these linyphiids have also been described as ruderal species -they show a high frequency of occurrence in ruderal sites, fields and gardens, i.e., sites of agricultural disturbance (Bonte et al. 2002;Cole et al. 2003). In addition, both of these linyphiids have been found among the most abundant species in different European agroecosystems, and thus are also called agrobiont species (Thomas and Jepson 1997;Feber et al. 1998 It has been recorded in a wide variety of habitats (Harvey et al. 2002a;Nentwig et al. 2012; Arachnologische Gesellschaft 2018), but perhaps its main habitat is forest (Locket and Millidge 1953;Harvey et al. 2002a;Oxbrough et al. 2006;Nentwig et al. 2012).
The species can also occur in bogs (Vilbaste 1980 A strictly hygrophilous species -it is usually found near water (Hänggi et al. 1995;Nentwig et al. 2012). It occurs, for example, by the side of streams (Locket and Millidge 1953), in reed swamps (Duffey 2005) and in other flooded habitats (Harvey et al. 2002a;Cattin et al. 2003). Vilbaste (1980) has found this species in fens and bogs of Estonia. It can be found in wet habitats (Oxbrough et al. 2006). It occurs in damp situations in woodland, grasslands, and bogs (Harvey et al. 2002a It is common in wet habitats such as marshlands, reed-beds, seeps, drainage ditches, wet grasslands etc. (Harvey et al. 2002a;Oxbrough et al. 2006;Arachnologische Gesellschaft 2018). Can be also found in fens (Vilbaste 1980;Kajak et al. 2000;Cera et al. 2010) and bogs (Vilbaste 1980 It inhabits marshy habitats (Roberts 1996;Harvey et al. 2002a;Nentwig et al. 2012). It can be found in the littoral zone of inland surface waterbodies, reed-beds and mires (Arachnologische Gesellschaft 2018). Koponen (2000) has found this species on sandy shores (Koponen 2000 It has an affinity for moist open habitats (Heimer and Nentwig 1991;Nentwig et al. 2012). It can be found in heathland, dune, saltmarsh and other wet habitats, but is perhaps commonest in grasslands (Harvey et al. 2002a;Arachnologische Gesellschaft 2018). It has been found also in bogs (Vilbaste 1980) and fens (Vilbaste 1980;Kajak et al. 2000 It can be found on bushes and low vegetation and on tree trunks, logs and a variety of other structures in a range of habitats (Roberts 1996). This species, however, occurs mainly in woodland and other shady places (Harvey et al. 2002a;Oxbrough et al. 2006). It can be found also on bogs (Vilbaste 1980 It prefers humid conditions (Locket and Millidge 1953;Nentwig et al. 2012). It occurs in wet, marshy areas, especially in different kinds of mires, including fens (Vilbaste 1980;Kajak et al. 2000;Boyce 2004) and bogs (Vilbaste 1980;Šternbergs 1991;Kupryjanowicz et al. 1998;Pommeresche 2002;Rėlys and Dapkus 2002). The species is mainly found in tall moss (Sphagnum, Polytrichum), often under pine or other trees in the swampy places (Harvey et al. 2002a;Nentwig et al. 2012). N. sarcinatus can also be found in moist grasslands (Hänggi et al. 1995 (Heimer and Nentwig 1991;Nentwig et al. 2012). It seems to be a typical species in bogs (Vilbaste 1980;Kupryjanowicz et al. 1998;Rėlys and Dapkus 2002;Rėlys et al. 2002;Koponen 2003), and can be found also in fens (Vilbaste 1980;Kajak et al. 2000).

Sub-group: Alder forest species
Pardosa fulvipes (Collett, 1876) It is mainly associated with grasslands (Holm and Kronestedt 1970;Roberts 1996;Almquist 2005) and arable land (Holm and Kronestedt 1970;Huhta and Raatikainen 1974;Almquist 2005). This species can be found also on wetlands, including fens and bogs (Vilbaste 1980;Komposch 2000 It is a very common species in woods, especially on forest edges and in woodland clearings (Roberts 1996;Almquist 2005). It never seems to occur very far from woods (Locket and Millidge 1951) and can be found in the habitats edging forests (Aakra 2002;Biteniekyté and Rélys 2008). Koponen (2005) has recorded this species at the burned forest. P. lugubris occurs on mires as well -it has been found in fens (Kajak et al. 2000) and bogs (Vilbaste 1980;Kupryjanowicz et al. 1998;Rėlys et al. 2002 Both these species are often found together (Locket and Millidge 1951;Roberts 1996), however, P. prativaga is not so common as P. pullata which is one of the commonest species of the genus (Locket and Millidge 1951;Roberts 1996). Both species occur in a wide variety of open habitats, including grasslands, heathlands, woodland clearings, dunes, old quarries and roadside verges, as well as in wet places such as dyke edges, damp meadows, water borders and swampy areas (Locket and Millidge 1951;Harvey et al. 2002b;Almquist 2005;Nentwig et al. 2012;Arachnologische Gesellschaft 2018). Both of these species have been found in fens (Vilbaste 1980;Kajak et al. 2000) and bogs (Vilbaste 1980;Kupryjanowicz et al. 1998;Rėlys et al. 2002). P. pullata has been found in bogs also by Koponen (2002a,b) and Spuņģis (2008).

Hygrophilous | Xerophilous;
Photophilous | Sciophilous; Habitat generalist Pardosa proxima (C. L. Koch, 1847) It can be found in a variety of sparsely vegetated habitats but usually in moist and marshy places (Locket and Millidge 1951;Harvey et al. 2002b). This species is most likely to be found at coastal sites including earthy cliffs, saltmarsh, dune slacks and in streamside habitats (Harvey et al. 2002b). It often occurs also in grasslands and fields, in damp situations (Roberts 1996;Nentwig et al. 2012).
In addition, P. proxima seems to be associated with habitats of anthropogenic disturbances, for example, gardens and arable land (Bonte et al. 2002;Harvey et al. 2002b;Arachnologische Gesellschaft 2018 It occurs mainly in forests (Bonte et al. 2002;Harvey et al. 2002b;Nentwig et al. 2012;Barsoum et al. 2014). It prefers broadleaved deciduous woodland, but can also occur in coniferous woodland (Arachnologische Gesellschaft 2018). The species might also be found in other habitats, for example, in anthropogenic herb stands, hedgerows, vineyards etc. (Arachnologische Gesellschaft 2018).

Lycosidae
Pirata piscatorius (Clerck, 1757) It is always found in very damp areas (Harvey et al. 2002b;Glime and Lissner 2013), most often near standing or slowly flowing water (Harvey et al. 2002b;Nentwig et al. 2012). Peus (1928) has classified P. piscatorius as a tyrphobiontic species. Indeed, this species is more typical for bog habitats (Koponen 2002a,b;Bruun, Toft 2004;Oliger 2004), however it can also be found in other wetlands as well, including fens (Vilbaste 1980;Kajak et al. 2000;Glime and Lissner 2013). This species shows a clear preferences for Sphagnum mosses -in bogs it is usually confined to the Sphagnum area of the habitat (Bruun, Toft 2004), and, in addition, the species can also be found in Sphagnum birch woods (Arachnologische Gesellschaft 2018).
This species inhabits open marshes, fens, bogs, reed belts, humid grasslands (Roberts 1996;Harvey et al. 2002b;Buchholz and Schröder 2013). Vilbaste (1980) has found P. latitans in Estonian fens, while Kajak et al. (2000) have found it among the dominant spider species in natural fens of Poland. P. latitans is less associated with Sphagnum bogs than other species of the Pirata genus, though it can be found on Sphagnum (Harvey et al. 2002b).

Habitat generalist
Xerolycosa nemoralis (Westring, 1861) It seems to prefer dry places -it can be found in heathlands, stony chalk grasslands, forest edges and woodland clearings (Locket and Millidge 1951;Roberts 1996;Harvey et al. 2002b;Nentwig et al. 2012). The species also occurs in forests (Cristofoli et al. 2010;Arachnologische Gesellschaft 2018) and bogs (Vilbaste 1980). In addition, X. nemoralis favors open, dry and warm areas, which are human-influenced, e.g., sparsely vegetated ground at postindustrial sites (Harvey et al. 2002b), dried peat bogs (Koponen 1979), burned sites (Harvey et al. 2002b;Moretti et al. 2002;Koponen 2005 It is associated mainly with forests -it can be found in or near woods (Harvey et al. 2002b), in woodland clearings (Roberts 1996), in moist forest meadows (Almquist 2006). The species can, however, also occur in heather (Locket and Millidge 1951;Almquist 2006) and in other habitats (Arachnologische Gesellschaft 2018). A widespread and common species (Locket and Millidge 1951;Nentwig et al. 2012). Although it is suggested to be a grassland spider by some authors (Harvey et al. 2002b;Cristofoli et al. 2010), it seems to be indifferent as regards light intensity (Matveinen-Huju et al. 2006), and can be found also in forests (Rėlys and Dapkus 2002;Biteniekyté and Rélys 2008), as well as in a wide variety of other habitats (Locket and Millidge 1951;Roberts 1996;Harvey et al. 2002b;Biteniekyté and Rélys 2008;Arachnologische Gesellschaft 2018). Thus, Z. spinimana could be classified as an ubiquitous species (Roberts 1996;Koponen 2002b;Cattin et al. 2003). Z. spinimana can also be found in fens (Vilbaste 1980;Kajak et al. 2000) and bogs (Vilbaste 1980;Kupryjanowicz et al. 1998;Koponen 2002b;Rėlys and Dapkus 2002;Rėlys et al. 2002 (Roberts 1996;Almquist 2005;Aakra and Berggren 2007;Nentwig et al. 2012). The species can also be found in fens and bogs (Vilbaste 1980;Almquist 2005;Aakra and Berggren 2007 It seems to prefer dry habitats (Heimer and Nentwig 1991;Nentwig et al. 2012), especially dry grasslands (Cera 2013;Arachnologische Gesellschaft 2018). It can also be found in forests (Almquist 2006;Arachnologische Gesellschaft 2018), wet heathland (Roberts 1996) and mires (Vilbaste 1980;Harvey et al. 2002b;Koponen 2002a It occurs in both humid and dry, but sunny habitats (Nentwig et al. 2012). The main habitat types of T. maritimus seems to be seashores, coastal sand dunes and marshes with Carex and Cladium mariscus (Roberts 1996;Bonte et al. 2002;Gajdoš and Toft 2002;Duffey 2005;Almquist 2006). The species can also be found in fens (Vilbaste 1980;Kajak et al. 2000;Cera et al. 2010) and bogs (Vilbaste 1980). T. maritimus is usually found close to the sea, however, it can also be detected further inland (Roberts 1996;Duffey 2005 It occurs in situations similar to those of T. maritimus (Locket and Millidge 1951), however T. oblongus is commoner inland and in damper habitats (Roberts 1996). T. oblongus can be found in a variety of dry and damp sunny habitats, including seashores, coastal dunes and grasslands of most types (Hänggi et al. 1995;Harvey et al. 2002b;Almquist 2006;Cera et al. 2010;Nentwig et al. 2012;Cera 2013). The species can also occur on fens (Vilbaste 1980;Cera et al. 2010) and bogs (Vilbaste 1980 It can be found in grasslands, dune heaths, quarries, gardens, and in a variety of similar situations, in both dry and wet conditions (Roberts 1996;Koponen 2000;Harvey et al. 2002b;Almquist 2006;Batáry et al. 2008). Koponen (2005) has recorded it at the burned forest. This species has been found also on bogs (Vilbaste 1980;Rėlys et al. 2002 It occurs in wet, swampy areas (Cattin et al. 2003;Oxbrough et al. 2006;Nentwig et al. 2012). The main habitat of this species is Sphagnum bogs and pools, however it can also inhabit moist meadows, alluvial forests, water margins of ditches, ponds, streams and other habitats (Harvey et al. 2002b;Almquist 2005;Nentwig et al. 2012). The literature suggests that D. fimbriatus is being found only in those swamps or streams which do not dry up, because this species needs permanent pools of water (Locket and Millidge 1951;Roberts 1996). D. fimbriatus can be found in fens and bogs of Europe (Vilbaste 1980;Kajak et al. 2000;Koponen 2002b;Rėlys et al. 2002 A species that is very rarely found (Nentwig et al. 2012), and is thought to be in decline throughout Europe (Collins and Wells 1987). This species is associated with damp places such as mires, wet meadows, ponds, banks of rivers, lakes and ditches (Andrušaitis 1998;Holec 2000;Almquist 2005). The main habitats of D. plantarius, however, seems to be fens (Collins and Wells 1987;Helsdingen 1993;Roberts 1996;Andrušaitis 1998) and the littoral zone of inland surface waterbodies (Holec 2000;Arachnologische Gesellschaft 2018). D. plantarius strongly depends on the presence of water -a permanent, whole year round water surface is obligatory for this species (Helsdingen 1993 It is common almost everywhere (Locket and Millidge 1951;Cattin et al. 2003), but seems to prefer open habitats (Nentwig et al. 2012). It can be found in grasslands, heathlands, open woods, woodland clearings, gardens and other places (Locket and Millidge 1951;Roberts 1996;Harvey et al. 2002b;Almquist 2005;Arachnologische Gesellschaft 2018). The species has been found also in fens and bogs (Vilbaste 1980 It is the commonest species of the genus (Roberts 1996) which can be found in various habitats (Harvey et al. 2002b;Arachnologische Gesellschaft 2018), including forests, meadows and bogs (Vilbaste 1980;Roberts 1996;Rėlys et al. 2002;Almquist 2006). In Latvia this species has previously been found in Baži bog (Šternbergs 1991;Spuņģis 2008 (Cattin et al. 2003;Nentwig et al. 2012). It occurs mainly on heathland in damp areas (Roberts 1996;Harvey et al. 2002b), although it can also be found on dry heathland (Harvey et al. 2002b;Nentwig et al. 2012). E. arcuata can occur also in meadows (Nyffeler and Benz 1988;Almquist 2006), fens (Vilbaste 1980;Cera et al. 2010) and bogs (Vilbaste 1980;Šternbergs 1991;Rėlys et al. 2002;Spuņģis 2008 It seems to prefer sunny conditions (Harvey et al. 2002b;Arachnologische Gesellschaft 2018). Overall, however, it can be found in a variety of situations -meadows, woods, forest edges, glades, raised bogs, shingle beaches etc. (Roberts 1996;Harvey et al. 2002b;Almquist 2006;Nentwig et al. 2012). It can also be found in disturbed habitats such as wastelands and quarries (Harvey et al. 2002b It is associated with wet habitats -it can be found in swamps with Cladium mariscus and Carex elata, on shores of lakes among Iris pseudacorus (Almquist 2006), on cattail in still water (Nentwig et al. 2012), in fens (Vilbaste 1980;Cera et al. 2010) and bogs (Vilbaste 1980). According to Holec (2000) M. radiata is a specialist of the eulittoral zone. Overall, in the continental Europe this species is widespread in wet habitats generally, while in the United Kingdom it seems to be confined to fens (Harvey et al. 2002b It does not appear to be restricted to any particular habitat type apart from the need for dry, warm and sparsely vegetated places (Heimer and Nentwig 1991;Harvey et al. 2002b;Nentwig et al. 2012;Arachnologische Gesellschaft 2018). The species occurs among short vegetation (grass, heather) and amongst stones (Locket and Millidge 1951;Roberts 1996;Harvey et al. 2002b). This species can also be found in human-influenced sites such as sand or chalk quarries and post-industrial sites (Harvey et al. 2002b It seems to have an affinity for swampy areas (Roberts 1996;Harvey et al. 2002b). It has been found in fens, bogs, Carex-swamps, damp meadows and moors (Vilbaste 1980;Kajak et al. 2000;Almquist 2006;Nentwig et al. 2012 It occurs in dry and warm localities, amongst low vegetation, on sandy to rocky ground (Nentwig et al. 2012). This species has mainly been found in sand dunes on the coast and among similar vegetation in fens (Locket and Millidge 1951;Roberts 1996;Harvey et al. 2002b;Duffey 2005). Gajdoš and Toft (2002) recorded this species in dune habitats on the Danish North Sea coast, while Perttula (1984) found it on the coastal sand dunes in Finland. Cera et al. (2010) have also detected this species in a couple of coastal habitats in Latvia. S. venator can also be found in birch woods, bogs, fens, on cultivated land, on walls of buildings and on other artificial habitats (Roberts 1996;Almquist 2006;Nentwig et al. 2012;Arachnologische Gesellschaft 2018 It occurs mainly in dry, warm, sunny habitats with bare surfaces (Harvey et al. 2002b;Nentwig et al. 2012). It has been found in dune heaths, grasslands, sandy or stony banks, quarries and old railway embankments (Harvey et al. 2002b;Duffey 2005;Almquist 2006). It has been found also in fens and bogs (Vilbaste 1980 It might be found in quite different habitats (Cattin et al. 2003;Arachnologische Gesellschaft 2018). According to Roberts (1996) and Harvey et al. (2002b) it prefers damp sheltered woodlands and woodland clearings, but according to Nentwig et al. (2012) the species prefers sunny and warm conditions. M. virescens has been found also on fens (Vilbaste 1980;Cera et al. 2010) and bogs (Vilbaste 1980;Biteniekyté, Rélys 2008 It seems to occur mostly near water, e.g., at the edges of ponds, rivers and streams (Harvey et al. 2002b;Nentwig et al. 2012). This species has been found, for example, in damp meadows (Almquist 2005;Nentwig et al. 2012), bogs (Vilbaste 1980;Rėlys et al. 2002), fens (Vilbaste 1980;Kajak et al. 2000), and swamp forests (Stańska et al. 2000 It is most commonly found on trees and shrubs (Roberts 1996;Harvey et al. 2002b;Nentwig et al. 2012). Interestingly, that Glime and Lissner (2013) suggest that this species is largely confined to branches of trees growing on Sphagnum bogs and fens, and is only rarely found on the same tree species growing outside bogs and fens. In other literature, however, it is stated that T. nigrita can be found on trees in damp woodland (Nentwig et al. 2012), on trees that grow on shores, as well as on fruit trees (Almquist 2005). Although the species is most often found near water (Nentwig et al. 2012), it can also be found in drier situations (Harvey et al. 2002b It lives in wet swampy places such as fens and bogs (Locket and Millidge 1953;Vilbaste 1980;Roberts 1996;Almquist 2005;Nentwig et al. 2012). This species has also been found in several bogs of Latvia (Šternbergs 1991;Spuņģis 2008). Also, C. sticta has been recorded from damp heaths, on shingle and amongst marram on dunes (Harvey et al. 2002a (Harvey et al. 2002a;Nentwig et al. 2012), but still it needs the presence of shrubs, bushes, trees or the vicinity of woods (Almquist 2005;Isaia et al. 2007). This species is typical of open habitats containing low broad-leaved vegetation, for example, road verges, domestic gardens and woodland glades (Harvey et al. 2002a). E. ovata might also be found in different kinds of forests, dry grasslands, woodland fringes, vineyards near woods and elsewhere (Isaia et al. 2007;Arachnologische Gesellschaft 2018). The species has also been found in fens (Kajak et al. 2000) and bogs (Vilbaste 1980 It occurs in a wide variety of habitats -in grasslands, mires, woodland clearings, forests etc. (Harvey et al. 2002a;Rėlys and Dapkus 2002;Arachnologische Gesellschaft 2018). Overall, however, it seems to prefer damp situations (Harvey et al. 2002a;Oxbrough et al. 2006;Nentwig et al. 2012). The species is usually found on shrubs and bushes (Heimer and Nentwig 1991;Almquist 2005;Nentwig et al. 2012 It is reported to be found in damp or boggy places (Locket and Millidge 1953;Roberts 1996;Oxbrough et al. 2006), however, in central Europe it is mainly found in dry habitats, for example, in chalk grasslands, heathlands, coastal dunes (Bonte et al. 2003(Bonte et al. , 2004Arachnologische Gesellschaft 2018). The species can also occur in different types of forests -coniferous, deciduous, as well as in mixed forests (Nentwig et al. 2012;Arachnologische Gesellschaft 2018). Koponen (2005) has recoraded E. flavomaculata at the burned forest in Finland. In Europe, this species can also be found on fens (Kajak et al. 2000) and bogs (Vilbaste 1980;Kupryjanowicz et al. 1998;Koponen 2002b;Rėlys et al. 2002;Spuņģis 2008 It seems to be able to live under variable conditions (Harvey et al. 2002a;Nentwig et al. 2012;Arachnologische Gesellschaft 2018). Most records of this species, however, have been from open habitats, especially meadows (Nyffeler and Benz 1988;Matveinen-Huju et al. 2006). Heimer and Nentwig (1991) suggest that this species occurs mainly in roadsides. It lives in permanent contact with water, and can be found in marshes (Almquist 2005), in very damp meadows (Nentwig et al. 2012) and in fens (Vilbaste 1980;Kajak et al. 2000).

Theridion varians
Hahn, 1833 It is found in a variety of different habitats, for example, in forests, grasslands, hedgerows, woodland fringes, mires and in other places (Arachnologische Gesellschaft 2018). This species can be found mainly on trees and shrubs, and also on other structures, for example, buildings and walls (Locket and Millidge 1953;Roberts 1996;Harvey et al. 2002a;Nentwig et al. 2012 It is usually found in damp, marshy areas (Locket and Millidge 1951;Roberts 1996). It has been found in marshes, in damp alder forests and near the sea (Locket and Millidge 1951;Vilbaste 1980;Harvey et al. 2002b;Almquist 2006). It can, however, also be found in heathlands, grasslands and other drier habitats (Harvey et al. 2002b by the quality of adjacent habitats (e.g., Uetz et al. 1999;Cobbold and MacMahon 2012). In addition, the quantity of nearby habitats also seems to be important: Gallé et al. (2011), for example, discovered that the number of forest specialist species increases in open habitats with increasing proportion of surrounding forests. Thereby, we must emphasise that different spatial scales should always be taken into account in the arachnological investigations, since not only local but also landscape variables could affect the spider fauna of the studied habitat. Spiders in the present study were sampled by two different techniques, pitfall trapping and sweep netting. Both these methods are among the most popular techniques in spider surveys while pitfall traps have been used extensively for studies on surfacedwelling spiders (e.g., Rėlys et al. 2002;Koponen 2003;Seyfulina 2005;Fetykó 2008), the sweep-net is perhaps the most widely used piece of equipment for sampling spiders from vegetation (e.g., Turnbull 1960;Seyfulina 2005;Fetykó 2008;Horváth et al. 2009). It has been shown that pitfall trapping and sweep netting target different species (Samu and Sarospataki 1995). This was also true in our study: overall, quite different spider species (and even families) were collected with each of these methods (see Appendix 1). We need to emphasize, however, that it is quite hard to compare the obtained data, since using various methods in different sites may influence the results. Other studies have shown that the efficiency of pitfall trapping and sweep netting varies greatly with the structure of the surrounding vegetation (Henderson 2003;Sutherland 2006;Samways et al. 2010). Greenslade (1964), for example, has found that taller vegetation in the vicinity of the pitfall traps hinders invertebrate movement. The sweep netting possess some problems as well -although this method can be used on most vegetation, it is ineffective in some vegetation types, e.g. tall reeds, very short vegetation or flattened vegetation (Sutherland 2006;Henderson 2003). Also, sweep netting is relatively ineffective on sparsely vegetated ground (Sutherland 2006). Thus, we must conclude that it is very difficult to compare catches between different calcareous fens, since our studied fens differed quite greatly from each other in terms of the plant species composition and vegetation height (Štokmane et al. 2013;Spuņģis 2014, 2016). Furthermore, pitfall trapping and sweep netting tend to collect spider species that exhibit different foraging strategies. For example, pitfall traps collect mostly surface-living spiders with an active hunter lifestyle, e.g., many lycosids (Topping and Sunderland 1992;Mallis and Hurd 2005), however, some ground web builders such as those from the family Linyphiidae, can also be adequately sampled (Coyle 1981;Standen 2000). Pitfall traps will not efficiently sample spiders which inhabit the upper vegetation layers (Standen 2000). Sweep netting, on the contrary, is used to catch spiders which occur on the top of the vegetation (Southwood and Henderson 2000). This method is appropriate in low vegetation (Sutherland 2006) and it only collects those spiders that do not fall off on the approach of the collector (Henderson 2003). The sweep net captures primarily aerial web builders (e.g., Araneidae), however aerial hunters could also be collected (Coyle 1981). Overall, it can be concluded that pitfall trapping and sweep netting are methods that successfully complement each other.
In the present study we have also sampled several new spider species for the fauna of Latvia. Recording new species could mainly be explained by insufficient studies in calcareous fens, therefore we suggest that these habitats should be investigated further. In the future studies it would be worthwhile to use a combination of different other spider collection methods (e.g., hand collecting, beating, sieving, suction sampling, etc.) so that a greater variety of microhabitats is accessed. Also, it would be desirable to extend the sampling season throughout the spring, summer and autumn as well as to include both day and night collection, since it is known that spiders exhibit different seasonal and diel activity patterns (Coddington et al. 1996;Marc et al. 1999). Ideally, if the researchers could follow a standardized and optimized sampling protocol when collecting spiders (such as already-existing methodology prepared by Cardoso (2009)), because this could allow future studies in cooperation between different research teams.
The main conclusion from the present study is that calcareous fens are very diverse habitats not only structurally and floristically but also from the arachnofaunistic point of view. Our study showed that calcareous fens contain a very wide range of different spider species, including not only those that need wet and open habitats but also those that prefer other types of microhabitats (e.g., dry, shady, acid, salty, etc.). Besides, we found that along with the terrestrial spider ecological groups, calcareous fens can harbour also semi-aquatic and even aquatic spider species. Overall, however, calcareous fens are poorly investigated habitats, and therefore many spider species might still be undiscovered in this unique mire habitat type. Thereby, in order to get a more accurate picture of the spider fauna of the calcareous fens, these habitats should definitely be investigated further.