Water bugs (Heteroptera: Gerromorpha, Nepomorpha) of the Caucasus ecoregion

Abstract The Caucasian fauna of water bugs (Heteroptera: Gerromorpha, Nepomorpha) is reviewed based on data originating from literature survey, museum and private collections, and extensive field sampling. The diversity of Caucasus ecoregion is quite significant with 77 species and subspecies framed within 25 genera (17 in Nepomorpha, 8 in Gerromorpha), and 13 families (8 in Nepomorpha, 5 in Gerromorpha). Micronecta anatolica anatolica Lindberg, 1922, Sigara iranica Lindberg, 1964, S. samani tigranes Jansson, 1986, Velia kiritshenkoi Tamanini, 1958, and Gerris asper (Fieber, 1860) are reported for the first time from Georgia, M. anatolica anatolica, Notonecta maculata Fabricius, 1794, Mesovelia thermalis Horváth, 1915, and Gerris lacustris (Linnaeus, 1758) represent new records for Armenia, and Microvelia macani Brown, 1953 is new for Azerbaijan. A chorological and similarity analysis highlighted the connections with and between adjacent countries, with Caucasus ecoregion having the highest similarity of the water bug fauna with Turkey, whereas Iran and Russia (European territory) have the lowest. The largest number of shared species/subspecies was found between the Caucasus and Turkey (64 taxa), while the lowest was between Iran and Russia (40 taxa). Our results indicate that the Caucasus represents a composite of various faunal elements of different origin, ranging from Caucasian endemics to Afrotropico-Indo-Mediterranean and Holarctic.


Introduction
The water bugs (Hemiptera: Heteroptera: Gerromorpha, Nepomorpha) are worldwide spread, excepting polar regions, and occur in a remarkable broad range of aquatic ecosystems (Polhemus & Polhemus 2008;Schuh & Weirauch 2020). They comprise two relatively small, but very diverse groups of insects. The Gerromorpha, also known as semiaquatic bugs, are mostly living on the water surface of freshwater habitats, but some are adapted to moist terrestrial habitats or even marine environments (Andersen 1982;Spence & Andersen 1994). The Nepomorpha, or aquatic bugs, occur particularly in freshwater habitats, but some species are known to live in dry terrestrial or marine environments as well (Schuh & Slater 1995;Schuh & Weirauch 2020).
The world's gerromorphans are currently framed in eight, whereas nepomorphans in 13 families (Nieser 1999;Damgaard 2008b;Damgaard & Zettel 2014;Wang et al. 2021). The highest species richness is found in the Neotropical, Oriental, and Afrotropical regions, whereas in the Palaearctic the diversity is significantly lower. Overall, 17 families, 85 genera, and about 525 species and subspecies are known from the latter (Aukema & Riger 1995;Polhemus & Polhemus 2008;Kment et al. 2016). Although the Palaearctic water bug fauna is considered to be well documented in comparison with that of other regions, there are still large areas which lack adequate surveys (i.e., southeastern Europe, the Caucasus, and a large portion of the Asian territories) (see e.g., Aukema & Rieger 1995;Fent et al. 2011;Linnavuori et al. 2011;Ghahari et al. 2013;Kment et al. 2016;Csabai et al. 2017;Berchi et al. 2018aBerchi et al. , 2018bCianferoni 2019).
The Caucasus ecoregion is a biodiversity crossroads which completely or partially extends over the territories of Georgia, Azerbaijan, Armenia, Russia, Turkey, and Iran ( Figure 1). While being acknowledged as one of the richest biodiversity hotspots on Earth (Myers et al. 2000), and one of the Pleistocene refugia and post-glacial colonization source for northern Europe (Hewitt 2000), the Caucasus has supposedly received comparatively less scientific attention, mainly due to the political context of the last decades (Mumladze et al. 2020).
The most significant work on the water bugs of Caucasus is that of Kiritshenko (1918), which covers Armenia, Azerbaijan, Georgia, and areas of the south European Russia and northeastern Turkey. Additional studies for some smaller areas are listed by Aukema and Rieger (1995). More recently, Linnavuori & Hosseini (2000), Kanyukova et al. (2002), Prokin et al. (2008Prokin et al. ( , 2009, Neimorovets (2010), Shapovalov et al. (2012Shapovalov et al. ( , 2014Shapovalov et al. ( , 2018Shapovalov et al. ( , 2019 improved the knowledge in some areas, whereas the works of Kanyukova (2006), Fent et al. (2011), andGhahari et al. (2013) provide comprehensive lists and distributions by provinces/republics for the faunas of Russia, Turkey, and Iran respectively. Although, no comprehensive survey has been carried out on the Caucasian ecoregion as a whole.
The main aim of this paper is to improve the knowledge of the Caucasus ecoregion's water bug taxonomic diversity. Therefore, we provide an annotated catalogue of the gerromorphans and nepomorphans from this region based on a review of the published records, data from museum and private collections, and extensive sampling. A chorological analysis is performed in order to evaluate the Caucasus' composition of faunal elements. The faunal connections with and between adjacent countries are compared and discussed.

Materials and methods
The study is based on data originating from literature survey, museum and private collections, and  G. M. Berchi et al. our own extensive field sampling. Most of the newly obtained material was collected in the period 2013-2020, and is preserved in GMB's private collection (see below), unless otherwise stated in the material examined; the collectors, which were not repeated in the material examined, are Gavril Marius Berchi, Denis Copilaș-Ciocianu, and Levan Mumladze. Specimens were collected from 116 localities in Armenia, Azerbaijan, and Georgia (Table I) Abbreviations of the wing morphs: ap = apterous, br = brachypterous, ma = macropterous, mi = micropterous.
The geographical coordinates of the newly collected material are given in the World Geodetic System (WGS84) (see Table I). The Caucasus ecoregion is defined as proposed by Olson et al. (2001). The map ( Figure 1) was created with ArcGIS 10 (ESRI 2011). When referring to the published records from Iran, Russia, and Turkey, we only considered those administrative units (provinces, republics) completely or partially overlapping the ecoregion.
The material examined is arranged alphabetically by country and locality; each location of the newly collected material has a code which corresponds to the specific additional information (habitat, geographical coordinates, elevation, and the date of sampling) presented in Table I. Information about the material in HNHM, MSHC, and NMPC is presented as almost as written on the labels and is not included in Table I.
In order to analyse community similarity among neighboring countries we used the Sørensen index (Magurran 2004).
Remarks: First records from Armenia and Georgia.

Water bugs of Caucasus
Remarks: First record from Georgia.
Remarks: A. kolenatii was described by Kiritshenko (1925) based on a female specimen collected from the former Elisabethpol (today's Ganja, in Azerbaijan). The male genitalia of A. kolenatii differs distinctly from A. aestivalis (see Kanyukova 1974;Linnavuori 1994Linnavuori , 2006. Currently, it is known only in macropterous form in various hydrographic basins flowing into the Black and Caspian Seas and the Persian Gulf. In the light of the recent discoveries of the cryptic Aphelocheirus species from the Iberian Peninsula and Morocco (Carbonell et al. 2011;Millán et al. 2016;Roca-Cusachs et al. 2020), the pattern of sympatric occurrence of A. aestivalis and A. kolenatii differs considerably, but further investigation is needed. However, the lack of representative material of Aphelocheirus from Caucasus, Turkey and the Near East greatly limits the possibilities to elucidate this problem.

Remarks:
Hungerford (1934) described N. g. poissoni based on "a large transverse band across the hemelytra", and a larger size comparing to the average specimens of the nominotypical subspecies N. glauca glauca. Later, Lindberg (1964) upgraded its rank to species based on differences of the male genitalia, but he figured the characters of N. maculata instead of those of N. g. poissoni. Kanyukova (1973a) downgraded its rank back to subspecies arguing that the species is in all structural characters (including male and female genitalia) identical with N. g. glauca, differing from it only by colour pattern of hemelytron and suspected allopatric distribution. Still, the taxonomic validity of this taxon is questionable, since the differentiation from N. g. glauca is based only on chromatic features. Both subspecies are mentioned in most of the Caucasian countries, but according to our data N. g. poissoni predominantly occurs at high elevations (see Material examined sections and Table I). This darker coloration of the hemelytra of N. g. poissoni could fit in the thermal melanism hypothesis, which predicts that "exposure to lower temperatures during development will result in darker pigmentation if the adult coloration is phenotypically plastic" (cf. Sibilia et al. 2018). Melanotic forms of N. g. glauca have also been reported (Kanyukova 1973a;Fent et al. 2011), and likewise documented for other species of Notonectidae i.e., N. meridionalis (Fent et al. 2011;Berchi et al. 2012;Berchi 2013). On the other hand, Fent et al. (2011) reported syntopic occurrence of N. g. glauca and N. g. poissoni in Tödürge Lake in Sivas province, Turkey. Therefore, an integrative approach based on molecular, cytological, morphometric and morphologic data is necessary in order to shed light on the validity of this taxon.
Caucasian distribution: Armenia and Iran (East Azerbaijan) (Ghahari et al. 2013;this paper). Also reported outside of the Caucasian area in Turkey (Fent et al. 2011).
Remarks: First record from Armenia.
Remarks: First record from Armenia.

Hebrus (Hebrus) liliimacula Horváth, 1929
Caucasian distribution: The exact collecting site is unknown, but expected to be located in Armenia or Azerbaijan (Kanyukova 1997(Kanyukova , 2006Kment et al. 2016). Also reported by Sakenin et al. (2020) from Ardabil Province in Iran, but this record needs verification.
Remarks: First record from Azerbaijan.

Discussion
From a Palaearctic perspective, the Caucasus ecoregion comprises 8 of 11 nepomorphan, and 5 of 6 gerromorphan families. The nepomorphan Belostomatidae, Gelastocoridae, Helotrephidae and gerromorphan marine Hermatobatidae are not reported from the Caucasian fauna, and except the first family, the occurrence of all the others can be excluded due to ecological and geographical reasons. The largest two families of water bugs in the Palaearctic region, Corixidae and Gerridae with 14 and 20 genera, and more than 120 and 100 species and subspecies, respectively (Aukema & Rieger 1995;Aukema et al. 2013), have also the highest richness in the Caucasus, both in terms of genera (Corixidae: 8; Gerridae: 3) and species and subspecies (Corixidae: 29; Gerridae: 12).
The chorological analysis (Figure 2) (9). Eleven chorotypes are represented by only one taxon each. The majority of taxa are widely spread throughout parts of Asia, Europe, northern Africa or even the entire Palaearctic. One taxon (i.e., Anisops sardeus sardeus) has an Afrotropico-Indo-Mediterranean distribution, whereas another one (i.e., Limnoporus rufoscutellatus) is Holarctic.
Territories of the neighboring countries (Turkey: ~785,000 km 2 , Iran: ~1,745,000 km 2 , Russia (European part): ~3,969,000 km 2 ) are significantly larger than the Caucasus ecoregion (~580,000 km 2 ). Therefore, we analyzed the total and shared number of species/subspecies, and compared the similarity of the water bug taxa with and between the ecoregion and these countries.
Our results indicate an overall number of 77 species and subspecies in the Caucasus ecoregion. Eighty three taxa are listed from Turkey by Fent et al. (2011); however, Velia rhadamantha rhadamantha Hoberlandt, 1941 is endemic to Crete (see Csabai et al. 2017;Berchi et al. 2018b), Hebrus pusillus pusillus has been recently excluded by Kment et al. (2016), whereas two more species have been recorded afterwards: Hydrometra gracilenta (Banbal & Fent 2016;Çerçi & Koçak 2016), and V. mancinii mancinii (Berchi et al. 2018b). Yet, the presence of other seven species is considered doubtful. From Iran, 82 species have been reported by Ghahari et al. (2013), but the occurrence of some is questionable (see e.g., Kment et al. 2016), whereas some certainly does not occur in this country i.e., Velia caprai caprai Tamanini, 1947, Gerris gibbifer Schummel, 1832, which for e.g., in eastern Europe are restricted to north of the Mediterranean peninsulas and Caucasus (see e.g., Fent et al. 2011;Küçükbasmacı & Kıyak 2015;Berchi et al. 2018aBerchi et al. , 2018b, G. lateralis Schummel, 1832, which is also a Nordic species, and H. pusillus pusillus, which has been recently excluded by Kment et al. (2016). Two other taxa, Ranatra parvipes vicina Signoret, 1880 and Sigara marginata (Fieber, 1851) have been listed by Andersen (1995e) and omitted by Ghahari et al. (2013), whereas the occurrence of H. pusillus arabicus Linnavuori, 1971 has been recently confirmed by Kment et al. (2016). According to Aukema & Rieger (1995), Kanyukova et al. (2002), Kanyukova (2006), and Shapovalov et al. (2018), 83 species are known from the European territory of Russia. Table III shows the Sørensen's similarity indices with and between the ecoregion and the neighboring countries. The Caucasus ecoregion and Turkey had the highest similarity index of 0.800, followed by Caucasus ecoregion and Russia, and Turkey and Iran, which had 0.763 and 0.695 respectively. Russia and Iran had the least similarity index of 0.488. The largest number of shared species/subspecies is between the Caucasus and Turkey (64 taxa) and Caucasus and Russia (61 taxa), whereas the lowest was between Iran and Russia (40 taxa) ( Figure 3).

Conclusion
The significant number of taxa concentrated on a relatively small area acknowledge the expected richness, whereas the relatively similar number of shared species with the adjacent countries emphasize that the Caucasus ecoregion represents a water bug fauna crossroad. This is also highlighted by the variety of chorotypes which represents a composite of various faunal elements of diverse biogeographic origin. The overall number of species reported in the Caucasus ecoregion is large, but definitely incomplete, and notwithstanding this reason, it is likely that several additional taxa will be discovered. Some possible taxonomic issues were identified

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G. M. Berchi et al. and are awaiting to be solved. We hope that our work will encourage further studies in the area, in order to create a solid base for other research.  Figure 3. Diagram of the total and shared number of taxa in and between the Caucasus ecoregion and neighboring countries. The total number is presented under the country/region name, and numbers within overlapping areas represent the shared number of taxa.