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 Author Sidorov, Dmitry 55C1264F-67C0-4C3F-8E6B-ECE1E821457B Federal Scientific Center of the East Asia Terrestrial Biodiversity, Russian Academy of Sciences, Vladivostok 690022, RF. biospeorossica@gmail.com Author Yalysheva, Elena 88AB67D2-1F97-4A2C-8121-362132FCF07C Municipal State Educational Institution “ Osinovskaya shkola ”, Osinovyy Mys 663457, RF. ona_vita@rambler.ru Author Sharyi-Ool, Mariana 9DE31FF0-0660-4980-B819-63305D609CAC Federal Scientific Center of the East Asia Terrestrial Biodiversity, Russian Academy of Sciences, Vladivostok 690022, RF. sharyiool@biosoil.ru text European Journal of Taxonomy 2021 2021-10-13 774 178 199 http://dx.doi.org/10.5852/ejt.2021.774.1541 journal article 4025 10.5852/ejt.2021.774.1541 bc28e8d7-56ea-4318-86bc-834a0fdb87c8 2118-9773 5567144 F8F78338-FF22-4E48-91FA-6EB1A9EC76EB Gammarus koshovi ( Bazikalova, 1946 ) Figs 2–5 Rivulogammarus koshovi Bazikalova, 1946: 677 , text-fig. (original description). Rivulogammarus koshovi – Barnard 1958: 72 . — Karaman 1991: 41 . Gammarus koshovi – Barnard & Barnard 1983: 467 . — Erbaeva et al. 1990: 56 . — Kozhova et al. 2000: 104 . — Safronov 2006: 210 . Diagnosis (both sexes) Compact, small-sized amphipod with well-developed eyes; coxal plates spacious (especially coxae 4 and 5); appendages comparatively short; antennae short and stout; peduncle articles 1–3 of antenna I comprise 33% of its total length, accessory flagellum 2-segmented with terminal article greatly reduced; antenna II with rare long setae on ventral face, males with calceoli; pereopods III and IV with clusters of setae on articles 4–6 posteriorly, carpi shortened; pereopod dactyli III–VII strong; uropod III endopodite somewhat shorter than exopodite, all margins with mix of setae of different length. BL = 8.0 mm ( ♀♀ ), 7.5 mm ( ♂♂ ). Material examined (new records) RUSSIA – Tuva Republic • 2 ♀♀ (BL = ca 8.0 mm); Tore-Khol Lake , isthmus ; 50°02′42.6″ N , 94°59′53.5″ E ; 6 m depth; 24 Jun. 2013 ; E.N. Yalysheva leg.; silted sand ; DAS 16-013 • 2 juv. ( BL = ca 5.0 mm); near Tore-Khol Lake , not far from recreation center ; 50°06′02.4″ N , 95°08′50.5″ E ; 7–8 m depth ; 14 Aug. 2011 , 27 Jun. 2015 ; E.N. Yalysheva leg.; silt ; DAS 16-013 • 1 ♂ (BL = ca 7.5 mm ); Sharlaa ; 50°01′32.6″ N , 95°03′28.4″ E ; 3 m depth ; 13 Aug. 2011 ; E.N. Yalysheva leg.; silted sand ; DAS 16-013 • 1 ♀ (BL = ca 8.0 mm); same collection data as for preceding; DAS 16-013 • 2 juv. (BL = ca 5.0 mm); same collection data as for preceding; DAS 16-013 • 1 ♀ (BL = ca 8.0 mm, damaged); site Yurty ; 50°04′44.6″ N , 95°08′38.5″ E ; 2 m depth ; 28 Jun. 2015 ; E.N. Yalysheva leg.; silted sand ; DAS 16-013 . Description Female SIZE. BL = 8.0 mm (oostegites developed, setose, bearing 13–15 small-sized eggs). GENERAL BODY MORPHOLOGY ( Figs 2A , 3A , 4G , 5A–B ). Body stout, clearly setose with minute setules. Head with inferior antennal sinus moderate, subrounded, eyes pigmented. Pleon armed with dorsal setae. Coxal plates I–IV broad, lateral margins strongly overlap neighboring plates, distal margins rounded, with 2 short setae each, coxae II–III tapering distally, coxa IV lobate; coxal plates V–VII broad (especially, plate V). Coxal gills II–VII stalked and saccular, coxal gill VII smallest. Oostegites II–V (brood plates) of gammarid shape, well developed, oblong, plates II–IV broad, plate V the smallest. Posterior margin of epimeral plates I and II convex, not produced, with 3 setules each, distoposterior corner of plate I small, slightly produced, ventral margin with cluster of setae distoanteriorly; distoposterior corner of plate II strongly produced, with 4 long setae distoanteriorly; plate III strongly produced, but less than plate II, only with 2 spines on ventral margin and 3 long setae distoanteriorly. Urosomites with mediodorsal elevations, armed with groups of 2 spines and 1–3 setae, spine formula I–III is: 2–2–2 // 2–2–2 // 2–0–2. Telson shorter than uropod III peduncle; its width 88% of length, completely cleft, lobes apically with 2–3 notched spines accompanied by 3 long setae, laterally 1 spine or seta. ANTENNAE ( Figs 2A , 3B–C ). Antenna I 40% of body length, peduncle articles in relation 1:0.6:0.35, flagellum of 17 articles, which are approximately twice as long as wide; accessory flagellum of 2 articles (1 long + 1 reduced); peduncle articles with short setae on ventral face and apically, articles of main flagellum with short setae, each flagellar article bearing minute aesthetasc, shorter than setae. Antenna II 67% of antenna I length, peduncle articles (4–5) slightly longer than flagellum; peduncle articles 4–5 bearing rare long setae on ventral face and short setae in 10 clusters along their entire lengths; flagellum modestly equipped with short setae, no calceoli. MOUTH PARTS (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. GNATHOPODS ( 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 defining 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 defining angle; posterior margin twice as long as palm with ca 5 sets of setae; dactylus similar to that of gnathopod I. PEREOPODS ( 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 Fig. 2. Gammarus koshovi ( Bazikalova, 1946 ) , DAS 16-013. A–C . ♀, 8.0 mm. A . Habitus, lateral view. B . Gnathopod I. C . Gnathopod II. — D–G . ♂, 7.5 mm. D . Antenna II. E . Antenna II, calceolus (× 2800). F . Gnathopod I, setation omitted. G . Gnathopod II, setation omitted. Fig. 3. Gammarus koshovi ( Bazikalova, 1946 ) , DAS 16-013, ♀, 8.0 mm. A . Head. B . Antenna I. C .Antenna II. D . Mandible, right. E . Mandible, lacinia mobilis (× 600). F . Mandible, left (part.). G . Lower lip. H . Upper lip. I . Maxilla I, right. J . Maxilla I, outer plate (× 600). K . Palp of maxilla I, left. L . Maxilla II. M . Maxilliped. Fig. 4. Gammarus koshovi ( Bazikalova, 1946 ) , DAS 16-013. A–E . ♀, 8.0 mm. A . Pereopod III. B . Pereopod IV. C . Pereopod V. D . Pereopod VI. E . Pereopod VII. — F–G . ♂, 7.5 mm. F . Uropod III. G . Telson. Black arrowheads indicate reduced carpi. 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. PLEOPODS AND UROPODS ( 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. Fig. 5. Gammarus koshovi ( Bazikalova, 1946 ) , DAS 16-013, ♀, 8.0 mm. A . Epimeral plates I–III. B . Urosoma, dorsal view. C–E . Pleopods I–III (part.). F . Uropod I. G . Uropod II. H . Uropod III. I . Telson. Male Single, apparently subadult male, with habitus smaller (BL = 7.5 mm ) and more slender; flagellum 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 defining 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 superficially; 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 differences, 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 riffle (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 different. 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, 2018 in Sidorov et al. 2018: 445 ). Furthermore, there is no close similarity with species of the highly diversified 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. Accompanying fauna Mollusca, Ostracoda (subfossil, represented by separate valves), Coleoptera , larvae of Chironomidae and Ephemeroptera. Table 1. Comparison of character differences of Gammarus koshovi ( Bazikalova, 1946 ) from two lacustrine populations.

Feature	Tore-Khol Lake	Locality Khubsugul Lake
Antenna I covers	40% of body length	25% of body length
Antenna I, primary flagellum with	19 ♂, 16–17 ♀♀ articles	11–12 ♂♂, 8–12 ♀♀ articles
Antenna II, flagellum with	10 ♂, 7–8 ♀♀ articles	6–7 ♂♂, 4–6 ♀♀ articles
Pereopods V–VII, along posterior margins of basipodites	from 5 to 9 setae	from 14 to 16 setae
Epimeral plates I–III	I (set of setae disto-anteriorly), II (setae disto-anteriorly), III (2 spines)	I (a row of setae along ventral margin), II (setae cover 2/3 along ventral margin), III (spines absent)
Telson, lobes apically	with 2–3 spines each	with 4 spines each

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.