An overview of Estonian woodlice (Isopoda, Oniscidea)

Abstract An overview of the Estonian terrestrial isopod fauna is given, based on literature data and material collected from 1984 to 2021. The identified material consisted of 10915 specimens belonging to 14 species and collected from 172 localities throughout Estonia. In combination with previous data from the literature data, there are now reliable records of 16 species of woodlice from Estonia. Two species, viz. Platyarthrushoffmannseggii Brandt, 1833 and Hyloniscusriparius (C. Koch, 1838), are new for the fauna. The latter has probably colonised Estonia recently and range expansions have been reported elsewhere. The data on Philosciamuscorum (Scopoli, 1763) are dubious, and this species is currently excluded from the Estonian list.


Introduction
The knowledge on Estonian terrestrial isopods is scattered in various publications, without a modern overview of the fauna. Some publications are in Estonian and may be thus inaccessible to the wider audience.
Data on this group were first given by J. B. Fischer, who mentioned the presence of Oniscus asellus Linnaeus in Livonia (Fischer 1778: 167), an earlier administrative division, which covered the southern part of present-day Estonia and northern Latvia. The identity of the abovementioned species is unclear, as most of European species were yet to be described. At the beginning of 20 th century, W. Herold collected material in many places in Estonia and Latvia, published the results in several works (Herold 1927(Herold , 1928(Herold , 1930, and provided the first reliable overview of the fauna, which included 13 Estonian species. Later, Estonian entomologist J. Vilbaste published new records in three local faunistic studies (Vilbaste 1970;Vilbaste et al. 1985;Vilbaste and Vilbaste 1993) and K. Remm added one record (Remm 1988).
A lot of unidentified material from various research projects and fieldwork made over many years  has been stored in the entomological collection of Estonian University of Life Sciences (including the zoological collections of the former Institute of Zoology and Botany of the Estonian Academy of Sciences) and Tallinn University of Technology (TalTech) soil biology laboratory. Based on these materials and literature records, an account of the current knowledge is given below.

Material and methods
As complete as possible, a bibliography of historical records of terrestrial isopods in Estonia was compiled. New material was collected using: (1) pitfall traps, (2) Tullgren funnels, (3) sifting moss, leaf litter, and detritus with standard entomological sieves, (4) manual searching in suitable habitats, and (5) as bycatch of non-target species with window traps (attached to tree trunks; Sammet et al. 2016) and Malaise traps (for details of the Estonian Malaise trap project, see Tomasson et al. 2014). The material was collected from 172 localities throughout Estonia (Table 1; Figure  1). All studied material is preserved in 80% ethanol and deposited in the entomological collection of Estonian University of Life Sciences (IZBE) and soil biology laboratory of TalTech Tartu College (TTUSB), both in Tartu, Estonia. Various keys for European woodlice were used for identification (Palmen 1946;Frankenberger 1959;Vandel 1960Vandel , 1962Gruner 1966;Oliver and Meecham 1993). The distributions of Estonian species (Fig. 5) are presented in a 50 × 50 km UTM grid (compiled using Adobe Photoshop CS5 Extended). The images of the general habitus were combined using the LAS V.4.1.0 software from multiple gradually focused images of the specimens in alcohol taken by a Leica DFC 450 camera attached to Leica 205C stereomicroscope.

Results
Altogether 14142 specimens were collected. Of these, 10915 were identified to the species level. The following list contains all the known published records of Estonian woodlice, followed by numbers of studied specimens and collecting localities. Full details for each record from each locality are given in Suppl. material 1. An asterisk (*)   Table 1.

Comments.
A locally abundant species, with no records from Estonian islands. It has been described as widespread in Estonia also in the past (Herold 1930). The findings are from different habitats: fresh to mesic forests, meadows, arable fields and gardens. Present also in Lithuania (Vilisics et al. 2012) and Latvia (Spuņģis 2008) but not Finland (Boxhall 2013).
Comments. Once reported as the most common species of Trichoniscidae (e.g. Herold 1927Herold , 1930, the species appears to have become less abundant. It is widespread in various habitats (bogs, different types of forests, meadows, and urban areas), but is more common in moist habitats and is often associated with decaying wood. The species is known to be mainly parthenogenetic (Gruner 1966;De Smedt et al 2016), and the collected material consisted only of female specimens. Thus, no male characters were available for study and it cannot be ruled out that some specimens were misidentified and other Trichoniscus species may also be present in Estonia as very rare (e.g. T. provisorius or T. pygmaeus). The taxonomic status of the described varieties T. elisabethae Herold, 1923 andT. elisabethae var. estoniensis Herold, 1927 is unclear, but we follow the Schmalfuss (2003) catalogue and treat them as T. pusillus. Present also in Latvia (Spuņģis 2008) and Finland (Palmén 1946;Vilisics and Terhivuo 2009).
Comments. The species is widespread and common, but has only recent records and is probably extending its range in the Europe. It has been often found in human settlements, but also seashore habitats and different types of forests, except the very dry ones. Present also in Latvia (Spuņģis 2008) and Finland (Vilisics and Terhivuo 2009). Fig. 5D Published source. Herold 1930: 479-480. Comments. Reported as rare, with only one finding locality in northern Estonia (Herold 1930). No recent records. Present also in Latvia (Spuņģis 2008) and Finland (Palmén 1946 Comments. A myrmecophilous species, found from a nest of Lasius niger (Linnaeus, 1758). There are no records from the northern Baltic region so far, but it has recently been found in Lithuania (Šatkauskienė 2017), and a population has also been found in Finland (Lehtinen 1961). However, due to the destruction of the only known locality, the species could be extinct there now (Vilisics and Terhivuo 2009).
Comments. One of the most common species in Estonia, in all kinds of habitats (both anthropogenic and natural, except bogs). It has also been described as widespread and common in Estonia in the past (Herold 1927(Herold , 1930. Present also in Leningrad region (European Russia, Kuznetsova and Gongalsky 2012), Latvia (Spuņģis 2008), and Finland (Palmén 1946).
Comments. The species was described as purely synanthropic in continental Estonia and free-living in western Estonian islands (Herold 1927(Herold , 1930. The studied material contains findings from and outside of human settlements (including different forests, grasslands, and seashore) both from western islands and continent. Present also in Novgorod region (European Russia, Kuznetsova and Gongalsky 2012), Latvia (Spuņģis 2008), and Finland (Palmén 1946).
Comments. This species is widespread and locally quite abundant, both in human settlements and in forests, under stones or in rotten logs. It has been described as widespread and mainly synanthropic in Estonia by W. Herold, with free-living populations in northern and western Estonian islands (Herold 1930). Present also in Latvia (Spuņģis 2008) and Finland (Palmén 1946).

Comments.
A rare synanthropic species with only one finding from Estonia (from suburban area in Tartu). Present also in Latvia (Spuņģis 2008) and Finland (Palmén 1946).
Comments. Common in western Estonia and islands, but rare elsewhere. Present in dry to mesic forests and different grasslands. Present also in Latvia (Spuņģis 2008) and Finland (Palmén 1946).

Discussion
There are reliable records of 16 species of terrestrial isopods from Estonia. One species has been dubiously claimed to occur in Estonia, and it is presently not included in the checklist. We failed to find any records or specimens to support the occurrence of Philoscia muscorum (Scopoli, 1763) in Estonia, although marked as "present" in Fauna Europaea database (Boxhall 2013). The species is, however, present in the neighbouring Latvia (Spuņģis 2008), and its occurrence in Estonia is not impossible. Two species, viz. Haplophthalmus mengii (Zaddach, 1844) and Porcellionides pruinosus (Brandt, 1833), have not been recently collected and are included here based on literature records only. The fauna is very similar to neighbouring Latvia and southern Finland, with which all species shared, except for Ligidium hypnorum and P. hoffmannseggii, but the apparent absence of the latter in Latvia can be possibly explained by its rarity and lack of studies in its specific habitat (ant nests). Comparing the recent records with older ones, it seems that the distribution and abundance of some species have remained approximately the same over the past century, whereas some other species appear to have become rarer or have expanded their ranges. Porcellio scaber was reported as a synanthropic species in continental Estonia (Herold 1930), but we found it also in the field there. The same applies to Oniscus asellus. The existing Baltic records of Hyloniscus riparius are from Lithuania and southern Latvia (Spuņģis 2008;Tuf et al. 2014). The first Estonian records are from 2015, and given that the species was found during the 2003-2007 studies only in south-western Latvia (Spuņģis 2008), its range may have shifted remarkably quickly (by more than 300 km northwards in only a decade). An expansion of the species range northwards and eastwards has also been detected in European Russia in recent decades (Gongalsky et al. 2013) and has recently reached also the Russian Far East (Gongalsky and Kuznetsova 2021). The first Finnish record of the species was from a greenhouse in 1946, but the first finding outside dates from 2007 (Vilisics and Terhivuo 2009). Several species are only found or are more common in western and northern Estonia, characterized by milder maritime climate and calcareous soil (Armadillidium opacum, A. pictum, A. pulchellum, A. zenckeri, Haplophthalmus mengii). Seven species are known from areas neighbouring Estonia and may have been not collected due to rarity or very local distribution: Porcellium conspersum (C.Koch, 1841), Philoscia muscorum (Scopoli, 1763), Haplophthalmus danicus Budde-Lund, 1879, Porcellio dilatatus Brandt, Porcellio laevis Latreille, 1804, and Armadillidium nasatum Budde-Lund 1885. The range of Trichoniscus provisorius Racovitza, 1908 reaches Poland (Jędryczkowski 1979), and there are other widespread Trichoniscus species, e.g. T. alemannicus or T. pygmaeus Sars 1898, in central Europe (the latter reaching southern Russia in the east; Kuznetsova and Gongalsky 2012), but due to lack of male specimens these species may remain as yet undetected in the Baltic countries. Introduced species can sometimes be found in greenhouses and may be expected to be found in the future too; several of the 25 species found in Finland (Vilisics and Terhivuo 2009) are found only indoors. It seems probable that the number of naturally occurring species might be closer to 19 as in neighbouring Latvia (Spuņģis 2008).