Morphology and zoogeography of the burrower-like gammarid Gammarus koshovi (Bazikalova, 1946) (Crustacea, Amphipoda, Gammaridae) – An overlooked and poorly known species in the Siberian fauna

A comprehensive survey of several lakes in the Tuva Republic of Russia yielded, besides the ubiquitous Gammarus lacustris, the discovery of a small freshwater, lacustrine amphipod not previously recorded in this area. A comparative study of the Tore-Khol Lake gammarid population, probably conspecifi c with Gammarus koshovi (Bazikalova, 1946) originating from Khubsugul Lake, was conducted. The species, G. koshovi, is characterized by a specifi c habitus: a small-sized compact body, all limbs shortened, carpi of pereopods (PIII and PIV) reduced, coxal plates broad, and pereopod dactyli sturdy. It has been suggested that juveniles of the euryoecious G. lacustris or other large species could be confused with the relatively small G. koshovi. Consequently, we decided to present the distribution of gammarid species throughout south Siberia and Mongolia, referring to the sequences of works primarily by Soviet authors, which may be hard to access by international readers. We discuss affi nity with related groups, distribution, and ecology of G. koshovi to better understand their evolution. Additionally, the zoobenthic species diversity of widely represented groups in the ecosystems of Tore-Khol Lake is briefl y reviewed. An identifi cation key for the Siberian Gammarus with 10 species is provided.


Introduction
The territory at the southern end of Central Siberia is rich in various mountain pools (more than 430 lakes) lying within the Sayan Province of the Lena-Yenisei subregion of the Palearctic (Leontyev 1957). The fl uctuating geological history of the region (Misar 1997) and a signifi cant variety of natural landscapes (Sanders et al. 2021) created a wide variety of aquatic biotopes (rivers, springs, takyrs / playas) and ephemeral rivers, as well as a wide variety of invertebrate species that inhabit them. However, it should be noted that the amphipod fauna of the southern region of Central Siberia, and especially Tuva, is practically unknown (Dgebuadze et al. 2010;Kirova 2019). Currently, our study is a fi rst report for more than a century of amphipod research in Siberia.
The genus Gammarus J.C. Fabricius, 1775 is a large Holarctic / Sino-Indian epigean / subterranean group of amphipods that is typically found to be morphologically monotonous at high latitudes (Culver et al. 1995), with a number of distinctly divergent narrowly localized endemic species at the southern border of the range (Sidorov et al. 2018;Zheng et al. 2020). However, the historical reasons for this distribution are not entirely known. Despite the obvious inconsistencies in morphological distances between recent species and paraphyly within the group, the genus Gammarus reveals a certain unity and is known from at least the Oligocene-Eocene era (Hou et al. 2011). Earlier, in a series of publications, it was shown that the amazingly diverse endemic amphipod fauna of Baikal Lake, consisting of more than 354 species (Takhteev 2019), had radiated from a putative Gammarus-like ancestor in the Oligocene (28 Mya), coinciding with the formation of the lake (Ogarkov et al. 1997;Sherbakov et al. 1999;MacDonald et al. 2005;Hou & Sket 2016).
As part of the inventory of the genus Gammarus J.C. Fabricius, 1775 inhabiting Siberia, samples from three diff erent lakes of Tuva were collected. Two were small limnetic pools containing the ubiquitous Gammarus lacustris G.O. Sars, 1863, whereas in the dune subsaline lake Tore-Khol (also known as Döröö Nuur), a population of a poorly known species, Gammarus koshovi (Bazikalova, 1946) (sometimes misspelled as 'kozhovi', 'kozhowi,' or 'kozovi'), was apparently discovered. This species was described by A.Ya. Bazikalova in 1946 and originally placed in the now defunct genus Rivulogammarus S. Karaman, 1931. Type material of G. koshovi was either lost or not deposited, and the original description of the species by Bazikalova (1946) was so lacking in detail that its correct taxonomic status has remained problematic. It is noteworthy that G. koshovi was previously known only from the Mongolian Lake Khubsugul (Hövsgöl) that holds some endemic species of the purely Baikalian fl ocks (Kozhova et al. 1994;Martens & Segers 2009) and its discovery in Lake Tore-Khol not only signifi cantly expanded its range, but also raised additional questions. Lake Tore-Khol is located in the northeastern part of the Ubsunur Basin (1148 m a.s.l.), 20 km south of Erzin. The lake was formed as a result of damming one of the tributaries of the Tes-Khem River by a ridge of moving sands, the Eder-Elesin. The dimictic steppe lake has exclusively subterranean infl ow and an underground outflow, dominance of picoplanktic cyanobacteria, and strong calcite precipitation (Walther et al. 2020). It is oligotrophic, with a tendency towards being mesotrophic at the drainage area, with the following properties: total area 72 km 2 , approximately 7 m mean depth (max. 26-27 m, according to some sources 38.8 m), slightly subsaline (0.6-0.7 g/L), Mg/Ca-HCO 3 type, T 18.5°C (at max. depth T 6°C), conductivity 0.71 mS/cm, and pH 8.85-9.2. It has sandy sediments with a diff erent silting thickness overgrown with Chara sp. and Potamogeton pectinatus L., and a wide reed belt of Phragmites australis (Cav.) Steud. in places. Fishes include Oreoleuciscus sp. and Esox lucius Linnaeus, 1758 (artifi cially populated) (Zaika & Makarov 2000;Flößner et al. 2005;Paul 2012;Dulmaa 2013;Kalnaya et al. 2018). This is the only lake declared in 1982 as a natural monument of republican signifi cance with some areas later included in the protected zone of the 'Ubsunurskaya Kotlovina' Biosphere Reserve -a World Heritage Site of UNESCO since 2003 (Arakchaa & Laidyp 1994).
Considering the importance of fi nding this species in constructing the general zoogeography of the genus Gammarus, we also attempted to provide a brief description of the freshwater fauna of Tore-Khol Lake in highlighting the common species with Khubsugul.

Historical review
In 1851, J.F. Brandt reported on Gammarus in the Nachiki hot spring basin on E Kamchatka, which he believed was indistinguishable from Gammarus pulex (De Geer, 1778) (Brandt 1851). Gammarus ermanni Milne-Edwards, 1840 was mentioned among others reported in Kamchatka's thermal spring by H. Milne-Edwards in 1840, and was subsequently assigned to the genus Crangonyx Spence Bate, 1859 (Bate 1862;Karaman 1991). In a historical retrospective for Siberia, the indication of the binomial name Gammarus pulex is not unexpected. For example, a wide geographical distribution of this species was reported by R.K. Maak (1886) according to the results of the expedition of 1853-1855 to Angara, Lena, Vilyui, in the Ural region, Pyasina (Taimyr), in the vicinity of Olenek, beyond the Arctic Circle (see also Gerstfeldt 1858). Probably the binomen 'Gammarus pulex' was used as a collective name for any species of Gammarus, Eulimnogammarus Bazikalova, 1945, or Pallasea Spence Bate, 1862, and even Synurella Wrześniowski, 1877 that are usually common in these areas (see Stebbing 1906;Martynov 1930;Lepneva 1933). Later, in Mongolia, B.I. Dybowsky (1901) found G. pulex off the northern shore of Lake Khuvsgul, later referred to as Gammarus lacustris G.O. Sars, 1863(Karaman 1991. In addition to this species, two more are known from Lake Khubsugul: Gammarus koshovi Bazikalova, 1946(Erbaeva et al. 1990Kozhova et al. 1994) and Gammarus hanhi Safronov, 2006(Kozhova et al. 2000Safronov 2006).
Studies of the hydrobiology of the lakes of the Ubsunur Basin do not cover an extended period. On the fauna of gammarids inhabiting the freshwater lakes of the Ubsunur depression, in particular Tore-Khol Lake, the information is either absent (Zaika 1993;Zaika & Makarov 2000) or short (Dolgin & Yalysheva 2008;Yalysheva 2010). Paul (2012) provides data on the euryhaline G. lacustris that stands as a typical component of the bottom fauna in salt lakes. In general, it should be noted that G. lacustris (partially identifi ed as 'Gammarus sp.', 'Gammaridae', or 'gammarids') is a permanent component of the various aquatic biotopes of Tuva and Mongolia, often dominating the lotic and lentic communities (Dulmaa 1979;Gundriser et al. 1986 Other species of the genus Gammarus in Siberia, not mentioned above, that are known include: Gammarus korbuensis Martynov, 1930;Gammarus teletzkensis Martynov, 1930; Gammarus angulatus (= Gammarus ocellatus angulatus Martynov, 1930) from the mountain river Korbu and Teletskoye Lake; Gammarus angustatus Martynov, 1930 from the stream in the Inya River basin, Ob' River near Novosibirsk (Martynov 1930); Gammarus pellucidus Gurjanova, 1930 from the Ladeysky riffl e at the Yenisey (sandbank) (Gurjanova 1930); Gammarus barnaulensis Schellenberg, 1937 (locus typicus is unknown), which A. Schellenberg indicated from Barnaul (Tomsk) in lakes, Burgusutai (W Siberia), which has led to confusion (Schellenberg 1937 Gurjanova (1930) from the Yenisey near Nyasha and a channel of the Yenisei near Khudonogovo (Gurjanova 1930).

Sampling
Specimens of gammarids were collected with a Petersen bottom grab sampler and a common hand net in three lakes of the territory of the Tuva Republic of Russia (see Fig. 1): Tore-Khol Lake, a small freshwater lake near Shara-Nur Lake and 'Dashtyg' alpine lake (see below). Samples were fi xed and stored in a ca 80% ethanol solution. Fig. 1. Distribution map of Gammarus J.C. Fabricius, 1775 in Siberia. G. koshovi (Bazikalova, 1946) in Tore-Khol and Khubsugul Lakes (empty circles); balcanicus group (yellow squares); pulex group (fi lled circles) with G. lacustris G.O. Sars, 1863 (purple circles) coinciding with the entire Siberian part of the group's range. A contour line overview (top right) shows principal study area. Each point may represent 1-3 closely adjacent localities (for further explanation, see text).

Morphology
Specimens were dissected using a dissecting microscope Lomo MBS-9 and mounted on microscope slides in polyvinyl lactophenol (PVL) and stained with methylene blue (Sigma-Aldrich Company, Inc.); dissected appendages were then covered with a coverslip and edged by clear nail polish. Prior to dissection, body length (BL) was recorded by holding the specimen straight and measuring the distance along the dorsal side of the body from the base of the fi rst antennae to the base of the telson. All pertinent morphological structures were drawn using a Carl Zeiss NU-2 compound microscope equipped with a drawing device as modifi ed by Gorodkov (1961). The nomenclature for setal patterns on article 3 of the mandibular palp follows the standard described by Karaman (1970) and Stock (1974). A geographical map ( Fig. 1), with the location of the sampling sites, was constructed with the open source software Generic Mapping Tools, GMT ver. 4.5.14. The description given here is based on the type series which is deposited in the private collection of D.A. Sidorov (prefi x DAS).

Material examined
. Antenna I 40% of body length, peduncle articles in relation 1:0.6:0.35, fl agellum of 17 articles, which are approximately twice as long as wide; accessory fl agellum of 2 articles (1 long + 1 reduced); peduncle articles with short setae on ventral face and apically, articles of main fl agellum with short setae, each fl agellar article bearing minute aesthetasc, shorter than setae. Antenna II 67% of antenna I length, peduncle articles (4-5) slightly longer than fl agellum; peduncle articles 4-5 bearing rare long setae on ventral face and short setae in 10 clusters along their entire lengths; fl agellum modestly equipped with short setae, no calceoli.

M
(typical gammarid, Fig. 3D-K). Mandibular palp with article 2 the longest, with 9 stiff setae, article 3 bearing 3 A-setae, 3 B-setae, 16 D-setae and 4 E-setae. Maxilla I asymmetric, palps broad, apically with ca 5 or 6 strong spines accompanied by thin setae; outer lobe with 12 subequal pectinate spines with ca 6 denticles each. Maxilla II inner plate with oblique row of 30 plumose setae on inner margin. Maxilliped basal endite (= inner plate) with 3 simple strong cuspidate spines on distal margin (+1 spine located subdistally); rest without peculiarities. G ( Fig. 2B-C). Gnathopod I basis stout with long simple setae on anterior and posterior margins; carpus (article 5) triangular, 0.7 × as long as propodus; propodus ovate, palm oblique, straight with cutting margin developed (no spines on medial face) and armed with 4 distally notched spines at defi ning angle; posterior margin as long as palm, bearing 4 sets of moderate setae; dactylus with 1 seta on outer face. Gnathopod II larger than gnathopod I; basis stout with long simple setae on anterior and posterior margins; carpus (article 5) 0.73 × as long as propodus; propodus narrow, subrectangular, palm subtransverse, slightly concave, with cutting margin developed (spines on medial face lacking) and armed with 2 distally notched spines at defi ning angle; posterior margin twice as long as palm with ca 5 sets of setae; dactylus similar to that of gnathopod I. P (Fig. 4A-E). Pereopods III-IV subsimilar, but pereopod III densely covered with sets of moderate (as long as article widths) setae along posterior margins of articles 4-6; carpi (articles 5) reduced, about 0.6 × as long as corresponding propodus, each with 1 set of very long setae apically. Pereopods V-VII strong, subsimilar, pereopod VI as long as pereopod VII; basipodites shortened, besides basipodite VII   with convex posterior margins, tapering distally, with 3 small spines and a tuft of setae along anterior and over 9 small setae along posterior margin; articles 4-5 with 4 or 5 pairs of paired spines along margins; dactylus ca 35% of propodus (article 6) length, with a short nail. P (Fig. 5C-H). Pleopods ordinary, rami subequal in length, segmented with 12-14 articles and fringed with plumose setae; peduncle with groups of thin setae, each retinacula two-hooked, accompanied by 1-2 slender simple stiff setae. Uropods I-II peduncles approximately reaching the end of uropod III peduncle, uropod I rami slightly beyond end of uropod III; peduncles with ca 4 or 6 spines along edges, uropod I with 1 basofacial spine, with 1 or 2 single spines along rami, with 3 apical and 2 subapical spines. Uropod III peduncle with 1 facial spine and 4 stiff setae and ca 8 weak spine-setae on apical margin; endopodite (= inner ramus) comprises 90% of exopodite (= outer ramus) length, with cluster of 1 spine and very long setae apically; terminal article of exopodite long, with tuft of short setae apically; both rami with moderately dense marginal brushes with mix of relatively long simple and plumose setae.

Male
Single, apparently subadult male, with habitus smaller (BL = 7.5 mm) and more slender; fl agellum of antenna II with 10 articles, each with calceoli of gammarid type (type 1) (Lincoln & Hurley 1981); gnathopods subsimilar to those of female, but propodi heavily armed at defi ning angle, each palm bearing 1 mid-palmar spine; uropod III weakly fringed with setae; in all other characters similar to female.

Variation
Not observed.

Taxonomic remarks
The enigmatic G. koshovi (Bazikalova, 1946) was collected in Lake Khubsugul of northeastern Mongolia (Fig. 1) and was reported in several previous publications (see Erbaeva et al. 1990;Safronov 2006;Dulmaa 2009), which has now necessitated a comparative study of this species with that in the remote Tore-Khol population of the Uvs Nuur Basin. Gammarus koshovi from Khubsugul was described rather superfi cially; a holotype was not deposited, which complicates detailed comparison. Furthermore, actual sampling of the Khubsugulian gammarids was not possible for correct comparisons. Although the original G. koshovi vs that in the Tore-Khol population exhibit diff erences, their pattern and variability are not clear (Table 1). Nevertheless, we consider these two forms to be closely related and belonging to the same lineage and united by the following: common fossorial morphotype (compact body, abbreviated antennae, spacious coxal plates, and strong, short pereopods with robust dactyli), presence of calceoli in antenna II of males, armament and gnathopod shape in both sexes, armament of urosomal segments, furnishing of pereopods, uropods, and telson. Bazikalova (1946) ignored the morphology of the carpi of pereopods III and IV. Additionally, her indication that antenna I comprised 25% of the entire body length with the number of articles 11-12 (♂♂) and 8-12 (♀♀) for 7-8 mm long specimens is doubtful. It should be noted that G. koshovi is related to Gammarus sp. in Gurjanova (1930) described from Yenisey's riffl e (downstream of Krasnoyarsk) based on several juvenile specimens with well-developed marsupial plates. Gurjanova (1930) compared the Yenisey form with juveniles of G. pulex (= G. lacustris G.O. Sars, 1863) from Karelia and the Polar Urals (see also Kessler 1868) and found them sharply diff erent. Among the Central Asian members of Gammarus (Martynov 1935), the Tuvan-Mongolian G. koshovi, because of its small size, gravitates to a poorly distinguishable species from the springs of Turkestan, but the forms described by Martynov, despite their small size, still possess elongated antennae and not shortened pereopods, and are clearly distinguishable by their reduced eyes (see also Gammarus parvioculatus Sidorov, Hou & Sket, 2018in Sidorov et al. 2018. Furthermore, there is no close similarity with species of the highly diversifi ed Altay and Tian-Shan group of Gammarus (Zhao et al. 2017;Zheng et al. 2020). This indicates that the territory of Siberia / Mongolia, possibly the basin of the middle Yenisey, is inhabited by a small burrower-like form of Gammarus, the origin and phylogenetic relationships of which are unresolved.

Distribution and ecology
Previously recorded from the area of Pupok Island in Khubsugul Lake (Fig. 1), a burrower-like gammarid form that dwells on silty sand at a depth of 15-16 m (Bazikalova 1946). Details of its biology in Khubsugul are lacking (Dulmaa 2009), but for Bazikalova to rely on the ʻburrowing morphotypeʼ implies its fossorial behavior. We have no direct observations of the burrowing behavior of the species, except that the samples were collected by bottom grabs in the Tore-Khol on muddy bald patches devoid of vegetation.

Remarks
Considering the outstanding morphogenetic polymorphism (Sket et al. 2019) and the very wide distribution of Holarctic / Sino-Indian G. lacustris in the water bodies of Siberia (Tuva) and Mongolia (Fig. 1), it is highly probable that juveniles of this species can be misidentifi ed as small species, such as G. koshovi. However, G. lacustris was absent in our samples from Tore-Khol, which we associated, fi rst of all, with partial acidifi cation of waters in the Russian part owing to the increased anthropogenic pressure because of intensive grazing, fi shing, and other economic activities. It was previously reported that G. lacustris   (Bazikalova, 1946) from two lacustrine populations.
critically avoids water environments with pH < 5 (Moiseenko & Yakovlev 1990). However, the exact reasons for this observation remain to be solved.

Key to the Siberian species of Gammarus (based on adults of both sexes)
Gammarus barnaulensis Schellenberg, 1937 is not included in the key as the original description does not provide appropriate morphological features.  Gurjanova (1930) * Conventionally proposed group to which we additionally assign Gammarus sp. in Gurjanova (1930).

Discussion
During a detailed and comparative study of the lacustrine amphipod fauna in Inner Asia, a remote population of the poorly known Gammarus koshovi (Bazikalova, 1946), which was previously known only from the fl owing Lake Khubsugul, was discovered. The history of lake formation in Inner Asia and the evolutionary transformations that they underwent are closely related to global processes throughout the Mesozoic-Cenozoic epoch (Gladkochub & Donskaya 2009;Shuvalov 2013). Khubsugul Lake is the largest and deepest freshwater lake in Mongolia and the second largest (after Lake Baikal) in Inner Asia. It is associated with Baikal by a common origin and confi nement to the Baikal Rift Zone, a common history of development, and modern ʻarcticʼ runoff through the Egiin and Selenga Rivers (AOW -Arctic Ocean Watershed in Maasri & Gelhaus 2012). The discovery of a small burrower-like form of the genus Gammarus raises a number questions, viz., why is G. koshovi absent in Baikal Lake and in other pools of its catchment area, and does it claim the role of a supposed ancestor for Palearctic forms? The burrowing lifestyle and special fossorial morphology is a fairly common phenomenon among marine amphipods, such as Haustoriidae Stebbing, 1906(Bousfi eld 1970, as well as freshwater amphipods like the Baikal Micruropus Stebbing, 1899, Crypturopus Sowinsky, 1915(Bazikalova 1962Takhteev 2000), and the Caspian Pontogammarus Sowinsky, 1904, and Niphargoides G.O. Sars, 1894(Copilaş-Ciocianu & Sidorov 2021. However, it is rather unique for lacustrine amphipods of the genus Gammarus. Amongst 264 described species of the genus Gammarus, no lacustrine burrowing forms are known (Horton et al. 2021).
A number of endemic species in the Mongolian Khubsugul are prescribed a common genetic relatedness with the Baikalian groups. However, this has not been confi rmed for gammarids, indicating their independent origin in these lakes (Kozhova et al. 2000;Goulden et al. 2006). Khubsugul has a less diverse set of biotopes and is much younger than Baikal, with the beginning of sedimentation expected to be no older than the Pliocene (Zorin et al. 1989). The ancient Mongolian multi-lake area in the Neogene is characterized by a developed lake-river system and is fed by numerous large rivers originating in the neighboring mountain structures of Khangai (Tes, Dzavkhan), Altay (Khovd, Bulgan), Khingan (Khalkhin), and Khentei (Kerulen) (Shuvalov & Deviatkin 2013). Prozorova & Zasypkina (2010) analyzed the distribution of Odhneripisidium Kuiper, 1962 and had the opinion that a shallow-water molluscan fauna formed in the fl owing areas of the basin of a giant paleolake that existed in the Neogene up to the Late Pliocene (Florensov 1968). Herein, our original data for the Tore-Khol concerns only amphipods (2 spp.). However, it is known that the fauna of the lake is quite distinctive, although it mainly contains the typical inhabitants of the lentic environments of Inner Asia. Among others, the following elements are known: sponges (Spongillidae) (Wiens et al. 2009), taxonomically rich zooplankton (Flößner et al. 2005;Kirova et al. 2020), Oligochaeta, and amphibious Insecta (Trichoptera, Ephemeroptera, Chironomidae), mostly represented by taxa indicating a Palearctic distribution (Zaika & Makarov 2000;Paul 2012). In numerous springs surrounding the lake, the following are common -Odonata, Plecoptera, Turbellaria, water boatman bugs (Corixidae), and water beetles (Dytiscidae) (Zaika 2011). Therefore, a couple of species are common to both lakes Tore-Khol and Khubsugul -Anisus (G.) terekholicus Prozorova & Starobogatov, 1997, also known from Nogoon and Durgun Lakes, and the aforementioned G. koshovi (Bazikalova, 1946) (see Kozhova et al. 1994;Prozorova & Starobogatov 1997;Sitnikova et al. 2011). The presence of the widespread amphipod G. lacustris in these lakes, as well the taxonomic status of the Khubsugulian G. hanhi Safronov, 2006, requires further confi rmation.
An interesting observation by Belostotsky (1958) is that it is likely that from the Jurassic (Mesozoic) to the Pleistocene (Cenozoic) era, the upper part of the Yenisey basin was discharged into the Ubsunur system through the Samagaltay passage (located between Tannu-Ola Ridge and the western spurs of Sangilen Upland) (Fig. 1). This point of view is supported by a number of authors (Izzatullaev & Starobogatov 1985;Tshernyshev 2010;Lukashov & Smoktunovich 2018). However, based on some elusive similarity of the Yenisey Gammarus sp. in Gurjanova (1930) and G. koshovi, along with the unclear taxonomic status of the former, such an observation does not seem obvious. However, if the Yenisey acts as a ʻfaunistic donorʼ for the Ubsunur Basin, it is an interesting observation that the basin, as the largest endorheic basin (CAIW -Central Asian Inland Watershed in Maasri & Gelhaus 2012), accumulates peripheral biodiversity.
It is well known that amphipods are amazingly plastic in terms of adaptations to various environmental conditions (they occupy all types of biotopes from oceanic trenches to the interstitial areas of sandy beaches and cave pools), show response to habitat drying (Gilbert et al. 2018), and have an eff ective ability to survive (Thorp & Rogers 2011) and disperse (Rachalewski et al. 2013). The unlimited dispersal ability of epigean gammarids within drainage areas (Altermatt et al. 2014) in contrast to increased diversifi cation rates due to isolation through mountain barriers (Hou et al. 2014), in combination with multiple rearrangements of the hydrographic networks of Inner Asia during the historical period (Zherikhin 2003;Zabelin & Zaika 2021) could have positively aff ected the expansion of a species range. Therefore, we assume that upon careful research, G. koshovi will also be found in other lakes of the region.
Finally, we have concluded that an excessive presence of ʻgray spotsʼ in gammarid taxonomy signifi cantly violates the accuracy of biogeographic information at a global scale. According to rough estimates, more than 80% of the described species of the genus Gammarus inhabiting the crucial region of Central Asia, Siberia, and the Far East need to be re-examined, and the number of undescribed species cannot be properly estimated.