Bythotrephes Cederströmii Schödler, 1877 Bythotrephes cederstroemii var. Cederströmii Bythotrephes cederstroemi Schödler, 1877 Bythotrephes cederströmii: Schödler 1863 B. cederströmii var. robustus B. cederströmii var. connectens B. longimanus var. cederströmii B. longimanus cederströmi B. cederströmii B. longimanus B. longimanus Redescription of Bythotrephes longimanus Leydig, 1860 and B. cederströmii Schödler, 1877 (Crustacea: Cladocera: Onychopoda), with notes on the morphology and systematics of the genus Bythotrephes Leydig, 1860 Korovchinsky, Nikolai M. Zootaxa 2015 3955 1 1 44 Schodler, 1877 Schodler 1877 [151,656,655,682] Branchiopoda Cercopagidae Bythotrephes Animalia Diplostraca 20 21 Arthropoda species cederstromii    Bythotrephes Cederströmii Schödler, 1877, pp. 233–234; Lieder 1988, pp. 125–127, Figs. 3, 4.   Bythotrephes cederstroemii var. Cederströmiis. str.: Lilljeborg 1901, pp. 619–621, Tab. 82, Fig. 11, Tab. 83, Fig. 1, 2.   Bythotrephes cederstroemi Schödler, 1877: Rylov 1935a, pp. 154–155, Fig. 233 (partim); Flössner 2000, pp. 377–379, Abb. 137 (partim). Non  Bythotrephes cederströmii: Schödler 1863, pp. 73–74; P.E. Müller 1868, pp. 203–214, Tab. 4, Fig. 29, Tab. 5, Fig. 1–18, Tab. 6, Fig. 7; Lilljeborg 1901, 621–623, Tab. 83, Fig. 3–5, Tab. 84, Fig. 1, 2(  B. cederströmii var. robustus), 623–625, Tab. 84, Fig. 3–6(  B. cederströmii var. connectens); Linko 1901, pp. 80 (  B. longimanus var. cederströmii); Chugunov 1922, pp. 2, 3; Ischreyt 1934b, pp. 181–202, Figs. 1–7; Spizharny 1929, pp. 17–23, Figs 1–8; Rylov 1935a, p. 154–155, Figs 231, 232 (partim); Vekhov 1987, pp. 28–29, Fig. 2(  B. longimanus cederströmi); Røen 1994, pp. 109–110, P. 44–47; Martin & Cash-Clark 1995, pp. 64–81, Figs. 1–20; Litvinchuk 2002, pp. 126–127, Figs. 32, 33, 2007, pp. 190–191.  Incertae sedis: Manuilova 1964, pp. 293–294 (  B. cederströmii); Mordukhai-Boltovskoi & Rivier 1987, pp. 149–152 (  B. longimanus); Rivier 1998, pp. 169–173, Fig. 215e, 216 (  B. longimanus).   Material examined. Typematerial.  Sweden: 1) A slide (MNB (N º1598) labelled “  Bythotrephes cedersrömiiSchödl., See Saxenin Wermland” with 1 ad.(  lectotype,designated by Lieder (1988))and 1 ad.(  paralectotype, the same designation) (for description of lectotypesee Lieder (1988)); 2) bottle (MNB (N º5537)) with a front label: “  Bythotrephes CederströmiSchödler, Sjöen Saxen, Wermland, Schweden, 18.7.1863, v. Cederström S., Syntypen”, other two labels are inside the bottle: “  Bythotrephes cederströmiiSchödl., Wermland, Saxensee, leg. 18/7 63Cedersr.”, “  Bythotrephes Cederströmii Schödl., 1877= Bythotr. CederstroemiSchödl, Syntypen, determ. Lieder, June 1976”. The bottle contains five small tubes: a) 2 ad., 1 juv (  paralectotypes), b) 4 ad., 4 juv.(  paralectotypes), c) 2 juv.(  paralectotypes), e) 9 ad., 5 juv.(  paralectotypes)(the fifth tube with a faded label “ Bythotr. Cederströmii,L 16”, possibly written by Schödler, contained specimens ( 4 ad.) of another species (presumably of “  B. brevimanus”- type); these specimens, which were not mentioned in Lieder (1988), were transferred by the author of the present paper into a separate bottle); 3) bottle (MNB (N º13752)) labelled with two labels: “  Bythotrephes cederströmiSchödler, See Nerika, Schweden, Holvuvistjärn, leg. v. Cederström, 19.7.1863” and “  Bythotrephes cederströmiiSchödl., Material as in See…, design. Lieder, June 86” and contained 6 ad.and 4 juv., 4) bottle (MNB (N º13753) with 12 small tubes, one of them illegibly labelled “N º. 14, Nerika, 19.7.1863, Sjön Holferssnon…” (possibly Holvuvistjärn) contained remains of females of  B.cederströmii– type.  Other material.There are other three samples with  Bythotrephes cederströmiifrom the same set of typematerial collected by G. Cederström in Lake Saxen and kept in Museum of Evolution of Uppsala University ( MEUU). Specimens of the samples were investigated by Lilljeborg, not Schödler, the author of original description of the species under consideration, and for this reason formally they cannot be considered typespecimens ( ICZN72.4.1.1): 1) small bottle (№ 635a) labelled “  Bythotrephes cederstroemiiv. Schödler (=  B. borealisLilljeborg), Nerike, Sjön Saken, 18.7.1865[probably 1863], G.C. Cederström, Typ-ex”, 3ad., 1 juv., 2) sample Nº 2313 labelled “  Bythotrephes cederströmiiSchödler(  borealis), ♀, Sw., Nrk., Saxen, 18.7.1865[probably 1863], Cederström G.C., det. Lillj.”, 2 ad., 1 juv., 3) sample N º2666 labelled ”  B. cederströmii,Sw., Nrk., Saxen, 18.7.1863, G.C. Cederström”, 3 ad.   FIGURE 10.  Bythotrephes cederströmiiSchödler, parthenogenetic female (Schweden: Lake Saxen (Prov. Värmland) (lectotype), general lateral view (bend on spine is arrowed). Scale bar 1.0 mm. Specimens collected not in the typelocality but in three other Swedish water bodies and stored in MEUUare as follows: 1) subsample N º635c (specimens from sample № 2681) labelled “ B. c. robustus,Sw., Sm., Bolmen, 15.7.1885, F. Trybom”, 9 ad., 2 gam., 4 males; 2) sample N º2314 labelled “  Bythotrephes cederströmii,Sw., Sm., Bolmen, 15.7.1885, Trybom F.”, 2 dissected ad.; 3) sample N º2683 labelled “ B. c. robustus,Sw., Dsl., Stora Le, 17.8.1892, C. Aurivillius”, 3 ad., 1juv; 4) sample N º2686 labelled “  B. cederströmii s.str., Sw., Sm., Bolmen, 27.6.1893, F. Trybom”, 1 ad.; 5) sample N º2688 labelled “  B. ced. robustus,Sw., Vstm., Kopparberg, Ljusnaren, 23.7.1894, Trybom F.”, 4 ad.  The supposed hybrids  Bythotrephes cederströmiix “  B. crassicaudus“(the latter name is in inverted commas due to its questionable validity).   Sweden:Bottle (MNB, without a number) with a tube labelled “  B. cederströmiiSchödler, Karesuando, 31/7 1875, W. Lilljeborg ”, 22 gam., 5 males, 1 juv.   Russia: 1) bottle (MNB, without а number) with another tube labeled “Onega Lake, Aug 1890..?, H. Linder,  Bythotrephes CederströmiiSchödler”, 5 ad, 4 males, 1 juv; 2) bottle ( ZIN, N º73-915) labelled “Collection by A.K. Linko” with 17 tubes, one of them with a label “Lake Onega” (thus probably all samples originate from this lake and its vicinities), in total, 87 ad.and 27 juv.; 3) Lake Pozemskoje (Karelia) (65°9625’N; 34°1125’E), 0 7.08.2009, coll. A.V. Tiunov (AAK-M-1200, AAK-M-1201), 23 ad.; 4) Lake Kubenskoje (Vologodskaja region), 0 6.2011, coll. E.V. Labunicheva, 1 ad.( NMK); 5) River Shima (Vologodskaja region), 0 5.07.2012, coll. E.V. Labunicheva, 1 ad.( NMK); 6) Lake Kovzhozero (Vologodskaja region), 12.07.2011, coll. E.V. Labunicheva, 5 ad., 5 juv.( NMK); 7) Uglicheskoje Reservoir (River Volga, Central European Russia), summer 2005, coll. V.I. Lazareva, 10 ad., 1 male, 8 juv.( NMK); 8) Lake Taiylar (Viluisky district,Yakutia, Eastern Siberia), 0 8.2008, coll. I.G. Sobakina, 31 ad., 5 males, 3 juv.( NMK). Data on body and body parts measurements of specimens from three populations are presented in Table 3. The main part of the description is based on population from Lake Saxen ( Sweden).   Description. Female. General body appearance and segmentation.General body shape as in  B. longimanus( Fig.10, 11A). Postabdomen bears ventrally a pair of massive and long claws curved anteriorly. Caudal process is very long and thin, bearing similar but shorter claws situated distantly from the former ones and one from another and a conspicuous denticulated curve posteriorly. Body length of females (without caudal process) may reach 3 mmor slightly more (in the examined specimens it ranges from 1.94 to 3.04 mm), whereas the length of caudal process surpasses body length considerably.  Head.Comparatively very large (35.6‒48.1 % of body length) and divided into two parts—rounded anterior part mostly filled by large compound eye and posterior part, bearing dorsally a large saddle-shaped neck organ, swimming antennae and mouth parts.  Antennulesand swimming antennaeare as to those of  B. longimanus.  Mouth partsas well as thoracic limbscould not be studied in detail because only the museum’s specimens, not accessible to dissection, were at author’s disposal. But generally it seems that they are similar to those ones of  B. longimanus.   TABLE 3.Data on body and body parts measurements of  Bythotrephes cederströmiifrom three localities (specimens from type localities: l—lectotype, p—paralectotypes; R—range, M—mean).    N BL, mm HL:BL, % T1L:BL, % E3L:E1L, % CPL:BL, % PBD:CPL, % PCL:BL, % CPCL:BL, % ICD:BL. % ICTh:ICD, %  Lake Saxen (Värmland, Sweden)  1-l 2.75 47.4 75.4 76.6 410.5 62.4 35.1 28.1 – 15.8 84.2 – 77.2 5.8 – 4.5  2-p 2.30 45.2 95.1 93.2 - - 52.8 33.3 – 20.8 119.4 – 100.0 5.3 – 4.9  3-p 2.11 - - - 545.5 61.1 - - - -  4-p 2.38 43.0 82.5 90.0 - - 51.0 30.9 – 21.5 115.4 – 96.9 5.2 – 4.9  5-p 2.50 37.1 88.1 85.1 - - 47.2 32.7 – 20.8 89.3 – 90.6 6.4 – 5.5  6-p 2.93 39.9 - - - - 35.0 32.2 – 19.7 69.9 – 67.8 5.4 – 5.6  7-p 2.32 44.1 92.4 90.7 496.6 60.0 41.4 33.1 – 22.8 88.3 – 91.0 6.2 – 5.7  8-p 3.04 39.5 73.7 85.4 - - 32.1 25.8 – 17.9 82.1 – 76.3 5.7 – 5.5  9-p 2.99 41.2 73.8 85.1 - - 37.4 24.1 – 14.4 89.8 – 79.1 4.8 – 4.7  10-p 2.75 41.9 67.4 94.6 - - 46.5 27.9 102.3 4.0  11-p 2.56 35.6 85.6 89.1 - - 35.0 27.5 – 19.4 91.3 – 88.8 5.8 – 5.0  12-p 2.10 38.9 84.0 74.4 412.2 50.7 34.4 22.9 91.6 5.8  13 2,16 48.1 104.6 89.2 525.0 64.4 50.9 37.0 – 25.0 117.6 – 106.5 5.5 – 5.6  14 2.62 44.3 82.4 83.3 385.5 60.8 35.9 26.0 – 18.3 74.0 – 71.0 6.8 – 5.9  Lake Holfuvistjärn (Närke, Sweden)  15-p 2.27 39.6 75.4 73.0 - - 47.2 29.6 – 19.7 162.0 – 130.3 4.0 – 3.8  R M 2.10–3.04 2.52  35.6–48.1 41.8 67.4–104.6 83.1 73.0 –94.6 85.4 385.5– 545.5 462.5 50.7–64.4 59.9 32.1 – 52.8 41.6 24.1 – 37.0; 14.4 – 25.0 29.4 – 19.7 69.9 – 162.0; 67.8 – 130.5 98.4 – 89.6 4.0 – 6.8; 3.8 – 5.9 5.5 –5.1  Lake Bolmen (Småland, Sweden)  1 2.04 38.2 74.5 85.1 523.5 71.2 16.7 13.7 – 6.9 146.1 – 118.6 2.7 – 3.3  2 1.94 38.1 62.9 77.3 432.0 56.8 13.4 10.3 127.8 3.2   FIGURE 11.  Bythotrephes cederströmiiSchödler, parthenogenetic female (Schweden: Lake Saxen (Prov. Värmland) (paralectotypes): A–H, J, K, L; Lake Holvuvistjarn (Prov. Närke) (paralectotypes): M,N; Lake Bolmen (Prov. Småland): G, I. A, general lateral view. B, F, G, caudal process. C, I, caudal curve. D, denticles of caudal curve. E, proximal parts of tl I–tl III, outer view. H, claws of caudal process, dorsal view. J, tl I. K, N, apical setae of first segment of endopodite of tl I. L, M, apical setae of second segment of endopodite of tl I.   FIGURE 12. The supposed hybrids  Bythotrephes cederströmiix “  B. crassicaudus”; parthenogenetic females (Schweden, Karesuando (Prov. Norrbotten): A, F; Russia, Lake Onega: C, I; Lake Pozemskoje (Karelia): E, J; Uglecheskoje Reservoir (Central European Russia): B, G, H; Lake Taiylar (Yakutia, Eastern Siberia): D). A–E, proximal part of caudal process. F–J, curve of caudal process.  Thoracic limbs.Limbs of first pair (tl I) are especially long and strong (62.9–104.6 % of body length) ( Fig. 10, 11A, 11J). Externally, their protopodite bears small outgrowth ( Fig. 11E). The first segment of endopodite is long and bears 6–8 anterior lateral setae ( Fig. 2A, 2J). Distally, this segment bears rather long anterior seta of type“j” (see Fig. 4J) and longer posterior finely setulated seta of type“k” (see Fig. 4K) ( Fig. 11K, 11N). Second segment of endopodite is conspicuously shorter and bears only two apical setae: comparatively long anterior seta of type“j” and longer posterior one ( Fig. 11L, 11M). The terminal, third segment of endopodite is also long but somewhat shorter than first proximal segment (73.0–94.6 % of the latter) and bears apically four long roughly spinulated setae, two of them terminally and two subterminally. The limbs of second and following pairs have not been studied closely but judging from the observation of the supposed hybrid specimens (author’s unpubl. data) (see further), the structure and armament of  B. cederströmiilimbs seem quite similar to those of  B. longimanus(in particular, the protopodites of tl II–tl III bear similar external outgrowths ( Fig. 11E)). The same may be said about the abdomen(metasome).  Postabdomenbears large stout claws with apical ends curved forward (32.1–52.8 % of body length; 13.4 –16.7 % in specimens from Lake Bolmen) that flare outwards from the midline ( Figs. 10, 11A, 11F–11H).  Caudal processis directly connected with postabdomen and then proceeds as a very long spine-like structure variable in its length (385.5–545.5 % of body length), thus surpassing body length in 3.9–5.5 times ( Fig. 10, 11A, 11B, 11F). It is thin (4.0–6.8 % and 3.8–5.9 % of distances between pairs of claws, respectively) and in its proximal half, it bears one or two pairs of claws similar to those on postabdomen but smaller (anterior pair 24.1–37.0 % and posterior pair 14.4–25.0 % of body length; in specimens from Lake Bolmen they are smaller, 6.9–13.7 % of body length) ( Fig. 10, 11A, 11B, 11F, 11G). All pairs of claws sit quite distantly one from another (distance between three pairs: 69.9–162.0 and 67.8–130.5 % of body length, respectively; in specimens from Lake Bolmen, 118.6– 146.1 % of body length), the posterior pair of claws is often turned upwards ( Fig. 11F, 11G). In its middle part or usually more distally (in 50.7–64.4 % of length of caudal process), caudal process bears a prominent curve ( Fig. 10, 11B, 11C) covered by numerous small curved denticles directed forward, which sit on its dorsal and ventral sides ( Fig. 11C, 11D). In specimens from Lake Saxen, this curve is moderately developed ( Fig. 11D, 11C), while in those from Lake Bolmen it can be more strongly pronounced and is situated closer to apical end of caudal process ( Fig. 11I, Table 3).  Malesand gamogenethic femalesare not known, but again, judging from such specimens of supposed hybrids studied in detail (authors’ unpubl. data), these individuals do not differ in their specific structures from those in  B. longimanus.  Juvenile females.Two studied juvenile females differed from adults in slightly thicker caudal process and position of caudal curve situated closer to postabdomen (in 35.9–37.4 % of length of caudal process).   Remarks.Specimens from Lake Holfuvistjärn have comparatively shorter tl I and their distal segment, apical setae of first and second segment of endopodite are stouter with more rough armament. Specimens from Lakes Bolmen and Store Le possess especially prominent curves on extremely long caudal processes. Those from the former locality have unusually small claws of postabdomen and caudal process. It was found that the lectotypeand paralectotypeon a slide ( Fig. 10) were in a quite good condition, while the condition of specimens in liquid samples was not so satisfactory. Their head and eye were most readily deformed ( Fig. 11A). For this reason, measurements of such individuals could not be provided as precisely as usual. In total, 17 adultparthenogenetic females and 12 juvenilefemales were recorded typespecimens ( lectotypeand paralectotypes).  Lilljeborg (1901: p. 619)recorded rather considerable variability of “  B. cederströmii s. str.” from some Swedish lakes. But mode of his measurements is not known and only single individuals were measured. His limited data on individuals from typelocality, Saxen See, do not correspond well to data of present investigation (see Table 3).  Intra- and interpopulation variability.The variability of some structures of typespecimens from Lake Saxen seems considerable ( Table 3). This mostly concerns length of tl I and their distal segment, length of caudal process and size of claws.  Differential diagnosis.  B. cederströmiidiffers from  B. longimanusin the presence of shorter tl I (62.9–104.6 % of body length) and their distal segment, the apical anterior setae of first and second segments of endopodite of tl I are long and roughly armed. Caudal process of the former species is much longer (385.5–545.5 % of body length), thinner, and possesses denticulated curve. The postabdominal and caudal claws of its adults are more numerous (two or three pairs), situated distantly one from another, usually conspicuously larger (14.4–52.8 % of body length), stouter, flared outwards and their distal ends are curved forward.   FIGURE 13. Comparative proportions of body parts of  Bythotrephes cederströmii s. str.(Southern Sweden) and hybrids (Lake Pozemskoje, Karelia, Russia). A, length of postabdominal claws. B, interclaw distance.   FIGURE 14. Schemes of morphological structure of tl II–tl III (A), tl IV (B), and abdomen (C) (lateral view). (abd = three abdominal segments, cp = caudal process, end = endopodite, end/prot = internal part of protopodite fused with endopodite, pgnat = pseudognathobase, p/abd = postabdomen, tr = trunk).  Notes on the supposed interspecific hybrids.In many samples from water bodies of different type, there appeared specimens much similar to  B. cederströmiibecause of presence of more or less expressed curve on caudal process ( Fig. 12F–12J) and large postabdominal and caudal claws with apical end curved forward ( Fig. 12A–12E). For a long time, such specimens were identified either as true  B. cederströmii(or its varieties) or sometimes considered as a variety or subspecies of  B. longimanus(see e.g., Lilljeborg 1901; Spizharny 1922; Ischreyt 1934b; Vekhov 1987; Yurista 1992; Martin & Cach-Clark 1995).  Litvinchuk (2002, 2007), who made morphometric and biochemical analyses of  Bythotrephes,suggested consideration of different forms close at least in some respects to  B. cederströmii—either true  B. cederströmiior hybrids of the species with two other species (  B. cederströmiix  B. brevimanusand  B. cederströmiix “  B. crassicaudus”). However, present investigation has revealed that the typespecimens of  B. cederströmii s.str.differ conspicuously in some morphological parameters from all other  B. cederströmii–like forms, including those studied by Litvinchuk, in the presence, first of all, of an especially long and thin caudal process with the curve shifted closer to its end, and widely separated and usually very large postabdominal and caudal claws. Some  B. cederströmii–like forms are reminiscent of the nominal species ( Figs. 12C, 12D, 12E), but nevertheless differ from it in some parameters ( Figs. 13A, 13B) (p <0.05; p <0.01).