Bythotrephes longimanus Leydig 1860
Creators: name: Korovchinsky, Nikolai M.
Publisher: name: Zenodo
Year: 2015
Subjects: Biodiversity
Taxonomy
Animalia
Arthropoda
Branchiopoda
Diplostraca
Cercopagidae
Bythotrephes
Bythotrephes longimanus

description: Bythotrephes longimanus Leydig, 1860   
(Figs. 2–9)  
 Bythotrephes longimanus: Leydig 1860, pp. 244‒246, Taf. 10, Fig. 73‒75; Weismann 1876, pp. 12‒14, Fig. 1; Ischreyt 1934a, pp. 261, 277, Abb. 16c.; 1935, p. 199; 1936, p. 361; 1937, p. 55; 1939, p. 110, 118, 119, 128, Abb. 1, 3, 4, 5; Margaritora 1985, pp. 338‒341, Fig. 134, 135; Litvinchuk 2002, p. 128, Fig. 38; 2007, p. 192.   
 B. longimanus longimanus: Flössner 1972, pp. 405–406, Abb. 190a; 2000, p. 375, Abb. 136d.   
 B. longimanus brigantinus Ischreyt, 1930, pp. 266, 302, 320‒321, Fig. 6 b, 9c, 12, 13a–c, 14‒16.   
 Bythotrephes lariano (B. longimanus lariano) Agnesotti, 1935, pp. 157‒172, Fig. 1, 2.   
 B. longimanus v. carnica Ischreyt, 1939, p. 110, 119, 125‒126, 128, Abb. 6.   
 B. Cederströmii Schödler: Beck 1883, pp. 780–782, pl. 12, figs 1–8.   
 Non Bythotrephes longimanus Leydig: Lilljeborg 1861, p. 268, Taf. 8, Fig. 23‒29; Sars 1861 [1993], p. 156, Plates 106‒109; Sars 1890, p. 51; Lilljeborg 1901, pp. 604‒617, Tab. 80, fig. 10‒19; Tab. 81, fig. 1‒2; Tab. 82, fig. 1‒10; Ekman, 1904, pp. 28–29; Rylov 1935a, pp. 152–155, figs 230–233; Behning 1941, pp. 352‒356, Fig. 146; Manuilova 1964, pp. 291‒293, Fig. 160; Flössner 1972, pp. 403‒408, Abb. 189, 190b (partim); 2000, pp. 371‒376, Abb. 136, a‒c, e‒g; Mordukhai- Boltovskoi & Rivier 1987, pp. 149‒152, Fig. 44, 93; Vekhov 1987, pp. 28‒29, Fig. 1, 2; Røen 1995, pp. 322‒324, Fig. 146; Rivier 1998, pp. 169‒173, Fig. 215, 216.   
 Material examined. Germany: Boden See (type locality), leg. H.-B. Stich, having three main areas: 1) Zeller See, 0 4.05.2009, 4 ad.; 2) Fischbach-Uttwil, 0 7.09.2009, numerous ad. and juv.; 3) Bregenzer Bucht, 0 7.09.2009, 10 ad., 1 juv.; 4) “ Bythotrephes longimanus Leydig, Bodensee, 1892, Lampert G.”, 2 females of first generation, 5 ad., 1 juv., 1 male, (MNB, № 9596). Switzerland: 1) Lake Geneva (Lac Leman, Genfer See): a) “ Bythotrephes longimanus, Lac Leman, Forel, 1879 ”, 11 ad., 2 juv. (deformed), ZIN (№ 1638), b) 2 ad., 3 juv., “ B. longimanus s.str., Swit., Lac Leman, 0 7.1887, coll. P.T. Cleve”, MEUU (№ 2671), c) “ B. longimanus, Genfer See, 24.10.1899, H. Blanc S.”, 23 ad., MNB (N º 18910), d) “ B. longimanus, Genfer See, 28.7.1900, Blanc G.”, numerous ad., MNB (N º 18910), e) “ B. longimanus, Genfer See, 9.10.1900, Blanc S.G.”, 12 ad., 1 juv., MNB (N º 18910); 2) Vierwaldstättersee, “ B. longimanus, 7.11”, 8 ad., 3 juv. (deformed), coll. Verestchagin, ZIN (N º 6855); 3) Lake Neuchâtel: a) “Neuchateler See, Februar, Fuhrmann”, 22 ad., 4 juv., MNB (N º 18910), b) “Neuchateler See, July, Fuhrmann”, 5 ad., MNB (N º 18910), c) “ Bythotrephes longimanus Leydig, Schweiz, Neuchateler See, October, Blanc I Fuhrmann G.”, 13 ad., MNB (N º 18910). Italy: 1) Lago di Como (Lago Lario, Comer See), “ Bythotrephes longimanus Leydig, 1860, Comer See, bis 72 m tief, leg. Besana, 23.9.1900, ded.Weltner”, some dried ad., MNB (N º 18909); 2) Lago di Garda (Gardasee), “ Bythotrephes longimanus Leydig, 1860, Gardasee, 0‒70 m, leg. Bullemer, 8.1910, ded. Weltner, 1.1918”, numerous ad., MNB (N º 18845). Austria: 1) Mondsee, 0 7.1993, 5 ad.., leg. H. Löffler and S. Gaviria-Melo; 2) Millstättersee, 0 8.11.2011, 2 ad., 3 juv., 1 male, leg. G. Santner; 3) Grundlsee, 18.07.2007, 20 ad., 31.08.2007, 15 ad., leg. M. Luger and R. Schabetsberger. Great Britain: 1) bottle (MNB (N º 18929)) with a label: “ Bythotrephes longimanus Leydig, England, Scourfield G.” and four tubes: a) “Windermere, English Lake District, Scourfield G.”, 4 ad., b) “Grasmere, English Lake District, Scourfield G.”, 1 ad., c) “Llyn Quellyn, North Wales, Scourfield G.”, 1 ad., 1 male, d) “Llyn Padarn, North Wales, Scourfield G.”, 1 ad., 1 male; 2) bottle (MNB (N º 18913) with a label: “ Bythotrephes longimanus Leydig, 1860, Schottland, ded. Thomas Scott and Scourfield / Weltner” and two tubes: a) “Loch Ness, Scotland, Scourfield G.”, 3 ad., b) “Loch Lochy in Inversness, Scotland ”, 14 gam., 3 males.   
Data on body and body parts measurements of two populations are presented in Table 1. The main part of the description is based on a population from Bodensee. Data from other populations have been taken into consideration for comparison.  
 Description. Female. General body appearance and segmentation. Body elongated and divided into four parts: head, thorax, abdomen, and postabdomen with long caudal process (Fig. 2 A). Its longitudinal axis is conspicuously incurved when head is located at almost right angle to the thorax. Also highly movable abdomen can be either at straight line with the thorax or stays at different angles to it. Head large with rounded anterior part filled by the enormously developed compound eye and bearing small antennules ventrally. Posterior part of head bears long swimming antennae and mouth parts consisting of mandibles, maxillules (mx I), and upper lip (labrum). Thorax with strongly developed muscular ventral side bearing four pairs of thoracic limbs of different size directed antero-ventrally. Dorsally, thorax bears sack-like carapace transformed into brood pouch sometimes reaching large size. Abdomen (metasome) is elongated, cylindrical, inconspicuously three-segmented and flexible, connected with small postabdomen, bearing ventrally a pair of straight claws and posteriorly very long straight caudal process with a pair of similar claws proximally. General body length of females (without caudal process) may reach 3.0 mm or slightly more (in the examined specimens it ranges from 1.62 to 2.66 mm) while the length of caudal process may surpass the body length considerably.   
 Head. Comparatively very large (36.8‒44.6 % of body length) and subdivided into two parts: rounded anterior part mostly filled by large compound eye (18.4‒22.8 % of body length) and posterior part bearing dorsally a large saddle-shaped neck organ, swimming antennae and mouth parts. Eye contains numerous ommatidia (~ 300 or> 200 ommatidia according to Miltz (1899) and Martin & Cash-Clark (1995), respectively) and have large pigment spot which occupies about one-third or at most a half of the eye’s volume (see also Ekman 1904). Ocellus (naupliar eye) is absent (Elofsson 1966).   
 Antennules. Small and situated on the ventral side of the anterior head part beneath the eye. They are bulbous (Fig. 2 C) and sit on the joined basis slightly split anteriorly. Terminally they bear five regular aesthetascs in two groups, in three and two in each one, and one shorter and thinner aesthetasc-like structure, situated in a group with two regular aesthetascs, and having slightly widened apical end with dark granular structure inside (“accessory simple seta” according to Scourfield (1896)).   
 Swimming antennae. Comparatively long (56.6‒77.3 % of body length), with elongated cylindrical basipodite (43.7 % of body length) (Fig. 2 A) which has dorsal thin naked seta on its folded proximal part (Fig. 2 D) and tiny distal sharp outer denticle located closer to base of lower branch (Fig. 2 E, arrowed) (this denticle is not always well visible, possibly sometimes it may be absent). Of two antennal branches, the lower three-segmented one (endopodite), sitting on the apical basipodital prominence, is slightly longer than upper branch (33.3 % vs. 29.8 % of body length). The upper branch is four-segmented (length of segments from proximal to distal: 8.7 % (first rudimentary one + second segment), 9.1 %, and 12.3 % of body length, respectively) and lower branch is threesegmented (10.3 %, 7.9 % and 15.1% of body length, respectively). Proximal most segment of the upper branch is rudimentary and clearly visible only externally, all other segments of both branches are much more developed. Internally, two proximal segments seem fused rather firmly while distally their junction probably is more flexible. Integument of antennal basipodite and branches are covered by tiny spinules, which are situated in rows (Fig 3 A). Small denticle with a saw-like row of surrounded minute prominences on the end of second segment of upper antennal branch (Fig. 3 A), similar prominences on the end of third segment of the same branch (Fig. 2 F), and small apical denticle on the distal segment of the same branch (Fig. 2 G). The distal segment of the lower branch is also armed by a small apical spine (Fig. 2 H). Small proximal-most segment of upper branch lacks setae, while other segments possess a row of two-segmented swimming setae of more or less similar size except distal of them which are shorter (the comparative lengths of setae of three segments of upper branch from proximal to distal ones are as follows: (32.1), (32.9, 35.3), (32.5, 32.5, 28.6, 23.4, 17.5) % of body length; the same for the four segments of lower branch: (31.3), (35.3), (34.5, 33.7, 32.1, 31.0, 22.6) % of body length. The length of basal setae segments are about 50‒63 % of their total length. All setae are bilaterally armed with rows of uniform thin setules. General formula of antennal setae: 0‒1‒2‒5/ 1‒1‒5.   
 Mouth parts. They are represented by upper lip (labrum), mandibles, and maxillules (maxilla I). The upper lip (Fig. 3 E) is composed of two parts; the posterior thick and flattened slightly triangular lobe and anterior large proboscis-like outgrowth. The latter is separated from the former one by a deep indention of the cuticle. The triangular lobe bears numerous papillae (Fig. 3 F) along its posterior-internal (oral) margin while the outgrowth is armed with tiny spinules. Mandibles are bilobed and adapted for biting (Fig. 3 B), with a toothed, blade-like posterior lobe and small anterior lobe (mandibular process) armoured with a cluster of 12‒15 long prominences, bearing some tiny spinules distally (Fig. 3 C). Posterior lobe is strongly sclerotized and divided in two tooth-shaped parts, the larger (posterior) of which has a small additional tooth about midway along its border (Fig. 3 B). The inner surface of the mandible bears a small field of about 10 tiny spines (Fig. 3 D). Maxillules (mx I) look like two cylindrical structures situated posterior to mandibles (Fig. 2 I). Distally, they bear short central seta and some papillae near it. Maxillae (mx II) are absent; the openings of maxillar glands are situated near the bases of tl I laterally (see Olesen et al. 2003).   
 Carapace. It looks like a bag-like structure, strongly modified into closed brood pouch (Fig. 2 A). It is attached to the dorsal side of thorax and reaching sometimes rather big size, surpassing body length. The females of first generation possess the biggest brood pouch, containing up to 22 embryos which distend its walls greatly (Zozulja 1977). The postero-ventral region of the brood pouch near its junction with dorsal side of thorax always appears to be indented.   
 Thoracic limbs. Four pairs of strongly chitinized, stenopodous limbs are densely situated along the muscular ventral side of thorax and directed antero-ventrally (Fig. 2 A). All of them have complex and variously setaceous armament along their inner side. Limbs of three anterior pairs are five-segmented and those of the last fourth pair are three-segmented. Protopodites of all of them, covered by comparatively softer cuticle, are inconspicuously delimited into two parts (segments), coxa and basis, (Fig. 4 B, 4C) while the endopodites of limbs of three anterior pairs are composed of three well developed segments and those ones of the fourth pair are unisegmented (Figs. 2 J, 4D, 4G, 6A, 6B).   
Limbs of first pair (tl I) are especially long and strong, their length often surpasses body length (83.1–112.8 % of body length) (Fig. 2 A, 2J). Terminally, the inner side of their protopodite bears a small triangular lobe, “gnathobasic” process (the nature and homology of this structure will be discussed below), armed laterally and distally with two denticles and numerous spinules (Figs. 2 K, 4B). The external part of protopodite is longer than internal one and bears apically a small conical outgrowth, having at its tip a tiny spine (Fig. 4 B, arrowed, 4C). The first segment of endopodite is long (30.7–35.4 % of tl I length) and bears 4–7 (mostly 5–6) anterior lateral setae (their number on limbs of one individual can vary) (Fig. 2 K) of type “j” with rough spine-like setules (Fig. 4 J, 5A). Distally, this segment bears short thorn-like anterior seta of type “j” and long posterior finely setulated seta of type “k” (Figs. 2 M, 2N, 4K). The second segment of endopodite is conspicuously shorter and bears only two apical setae: very short anterior seta of type “j” (sometimes it may be reduced up to small prominence, see Fig. 2 P) and longer posterior one (Figs. 2 O–2R); both of them often seem naked. The terminal third segment of endopodite is also long (27.2–35.3 % of tl I length), almost equal or even surpassing the length of first segment (79.1–114.3 % of the latter), and bears apically four long roughly spinulated setae, two of them terminally and two subterminally (Fig. 2 J, 2L); the anterior subterminal of them is shorter than others. Basally, these setae are armed by a row of smaller spines (Fig. 4 F, 5F), while distally—by larger lanceolate spines directed terminally (Fig. 5 B).  
The limbs of second pair (tl II) are considerably shorter, their protopodite, again externally, is conspicuously longer and bears a conical outgrowth (Fig. 4 A, 4C). The first, basal segment of their endopodite, bears a row of 4– 7 (mostly 5–6) rather long anterior lateral setae of type “l” (their number also can vary in one individual) (Table 2) (Fig. 4 D, 4E, 4L). Sometimes there is one posterior lateral seta of the same type on this segment (Table 2). The terminal setae of the segment are of the same type “l” but may be longer, especially posterior one. Internally, this segment bears stout cylindrical “gnathobasic” process, possessing some prominences of different size, one small, thin seta, and a pore apically (Figs. 4 D, 5D). The second segment is short with only two setae, the anterior of which is of “j”- type, but comparatively thicker with more rough setules than it was in respective setae of tl I (Fig. 4 D, 4J). The distal, third segment of endopodite of the limb bears four setae, two terminal and two subterminal ones (Fig. 4 D). Of the latter, the anterior seta is of type “m”; it is comparatively short, thick and armed with a number of thin lateral denticles, while its distal part is naked and slightly hooked apically (Fig. 4 D, 4M). Its neighboring posterior subterminal seta is considerably longer and similar with subterminal and terminal setae of tl I, having sharp straight apex (Fig. 4 D). The anterior terminal seta is of type “n”; thick and comparatively short with longitudinal ribs, few thin lateral denticles, and slightly hooked apically as well (Figs. 4 D, 4N). The posterior terminal seta similar to the respective subterminal one but shorter, has fewer denticles and slightly hooked apical end (Fig. 4 D).  
The limbs of the third pair (tl III) are generally similar to those of the previous ones, differing in some details. The external outgrowth of their protopodite is conspicuously larger (Fig. 4 C) and lateral anterior setae of first segment of endopodite are fewer (3–4) while posterior ones were not observed at all (Table 2) (Fig. 4 G, 4H). These setae bear fewer proximal spinules. The “gnathobasic” process is similar to that one of tl II (Fig. 5 E). Of setae of second segment, the anterior one is thinner, longer, and more finely setulated than the respective seta of tl II. Terminal and subterminal setae of third segment are similar to those of tl II but shorter and bear fewer denticles (Figs. 4 G, 4I, 5C).  
The limbs of the fourth pair (tl IV) are considerably reduced; their protopodite bears externally and posteriorly distally spinulated seta sited on a short cylindrical base (Figs. 5 G, 6A, 6B, 6E). The only segment of endopodite has two rows of comparatively short spine-like setae. The external row always consists of two setae, and the internal row of 5–7 (usually 5–6) setae, which differ in their appearance. Most of them (both external setae and four internal setae, located closer to protopodital seta, are stronger and armed with few proximal spinules (Figs. 6 A, 6B, 6D), while two anteriormost setae are slightly thinner and more feathered (Fig. 6 C). Almost the whole internal part of the endopodital segment is occupied by the reduced “gnathobasic” process armed by some denticles and thin seta (Figs. 5 G, 5H, 6B). The apex of the process probably has a pore covered by membrane (Fig. 5 H). The bulbous structure is situated just after the tl IV (Fig. 2 A).  
 Abdomen (metasome) (Fig. 2 A, 2S) is often deformed, compressed and for this reason it may be hardly measured (approximately its length is about 15–20 % of body length). It is inconspicuously delimited in two parts (segments?), short proximal and longer distal with prominent fold in the middle dorsal side. Due to softness of the abdomen’s integument, some additional smaller folds disguise its “segmentation”.   
 Postabdomen is comparatively small (6.3–9.1 % of body length), the anal opening is situated between postabdominal claws. The latter are large and more or less straight, directed downwards or slightly curved posteriorly (6.6–10.9 % of body length) (Figs. 2 A, 2S, 6F, 6G). Sometimes the line divided postabdomen in two longitudinal parts, proximal and distal ones, may be observed, to the latter of whose the claws are attached (Figs. 2 S, 6G, arrowed).   
 Caudal process is directly connected with postabdomen either invisibly (Fig. 6 F) or being separated from it by a conspicuous border (Fig. 6 G) and then proceeds as a very long and straight spine-like structure variable in its length (186.0–301.0 % of body length), thus surpassing the body length in 1.8–3.0 times (Fig. 2 A, 2B). Basally it bears a pair of claws similar to those of postabdomen (6.0–13.2 % of body length) (Fig. 6 F, 6G), and apically two minute setae arose from common base (Fig. 2 T). Two pairs of claws sit closely (distance between them 7.5–14.9 % of body length). Between them, the thickness of the structure is considerable, reaching 40.0–123.0 % of the interclaw distance (Table 1). The caudal process is strongly chitinized and its surface is covered by numerous minute spinules (Fig. 5 I).   
 Female of first generation hatched from resting eggs. Three such specimens were found, one (deformed) in the sample from Bodensee taken in the beginning of May 2009 and other two in old sample from the same lake stored in MNB (Nº 9596). One of the latter had body length 1.89 mm, head length—38.1 %, length of tl I—90.7 %, and length of caudal process –180.5 % of body length. Characteristically, all studied females had three pairs of comparatively small claws (3.3–4.3 % of body length), one on postabdomen and two on comparatively thick caudal process (Fig. 6 N, 6O), distance between whose is considerably larger (16.9–19.5 % of body length) than in females described above. Furthermore, in these females the apical setae of first and second endopodital segments of tl I are long (Fig. 6 L, 6M) and lateral seta of proximal endopodital segments of tl II–tl III are fewer (5 and 3, respectively) than in females of later generations.   
 Juvenile females. The investigation of few juvenile females (with only a pair of postabdominal claw) from Bodensee has revealed that they differ from adults in presence of larger head (42.9–46.6 % of body length) and eye (24.5–26.0 % of body length), comparatively shorter third distal segment of endopodite of tl I (75.0–82.1 % of length of first proximal segment of endopodite), comparatively longer caudal process (347.2–437.1 % of body length) and postabdominal claws (13.7–18.6 % of body length). Also the juveniles have fewer lateral setae on first segment of endopodite of tl I–tl III and only one external terminal seta in tl IV. The apical setae of tl I–tl III are comparatively shorter and their armament is undeveloped.   
 Male. Only one partly deformed adult male was encountered from an old sample (MNB, № 9596) from Bodensee (Fig. 6 H). It had body length 1.94 mm, head length—38.8 %, length of tl I—105.0 %, length of caudal process—295.0 %, and interclaw distance—9.5 % of body length. The distal segment of tl I is slightly swollen proximally and bears on its inner side a small strongly chitinized hook with two inner denticles, a field of tiny prominences is situated near it (Fig. 6 I, 6J). The apical setae of first and second segments of tl I are comparatively longer than in females (Fig. 6 H, I). The copulatory appendages, set just after tl IV, are small and armed with numerous minute spinules terminally (Fig. 6 H, 6K). According to Ischreyt (1930), who studied rather abundant material, males from Bodensee had an average body length 2.4 mm, length of tl I 97.7 %, and length of caudal process 283.8 % of body length. They were slightly smaller than females, had slightly longer caudal process, shorter tl I and tl II, swimming antennae, and smaller interclaw distance.   
 Type material. The original specimens of the species under consideration described by Leydig (1860) were undoubtedly lost because of their poorly preserved original condition and absence of the tradition to designate and store the type materials at that time. Absence of the type material and of its adequate description has resulted in long confusion of the idea of Bythotrephes longimanus s. str. The situation required the detailed redescription of the species and establishment of its type. For this reason the neotype of Bythotrephes longimanus was designated from specimens collected in the type locality (a parthenogenetic female with body length 2.26 mm, Bodensee, main basin—Fischbach-Uttwil, 0 7.09.2009, coll. H.-B. Stich) and deposited in Zoological Museum of Moscow State University (ZMMU, № Ml 133). Also this was made for clarifying the taxonomic status of different populations of the species all over its range which have appeared to be morphologically rather variable.   
 Remarks. The studied specimens of B. longimanus s.str. have appeared to be quite similar to those originally described by Leydig (1860), judging from their long tl I with very long and thin terminal segment and short apical setae of endopodital segments, straight caudal process, and presence of only two pairs of large and straight claws situated close one to another. Later, the better drawing of general view of the species with its characteristic diagnostic features was presented by Weismann (1876). With consideration of these data, it is possible to be sure that the same species occurs in Bodensee for all period of its investigation. P.E. Müller (1870), Pengo (1880), and Weltner (1888) also studied specimens of Bythotrephes from this lake but they either not presented at all or presented very limited data, which are generally congruous with those obtained above.   
 The following and much more detailed data on Bythotrephes from Bodensee were presented by Ischreyt (1930, 1934a, 1935, 1936,1939) who made numerous body measurements and drew selected morphological features of adults and juveniles. Again he marked the characteristic features of the species namely large length of tl I and presence of short apical setae on its endopodital segments. It should be noted that specimens described by this researcher were larger (average body length 2.6 mm), having longer tl I (av. 116.7 % of body length) with slightly more numerous setae, than it was found in the present study. Later only Flössner (2000) presented further limited data on topotypic population and correctly showed the diagnostic significance of armament of tl I of the species. He noted again a large body length of the studied specimens (1.94–3.18 mm), having long tl I (88–122 % of body length) with long terminal segment of endopodite (92–122 % of length of first proximal segment of endopodite).   
 Specimens from other lakes. Lakes of Switzerland and Italy. The local specimens sometimes possessed relatively larger head: up to 46.1 % (Lake Neuchâtel) and 49.6 % (Lago di Garda) of body length. Specimens from lakes Vierwaldstättersee and Garda had especially long tl I (up to 131.7 % of body length) with long and thin distal segment (Fig. 7 A–7C). The relative length of caudal process might be smaller (160.2–172.3 % of body length in Lake Geneva) than that one in Bodensee, while claws of both pairs were larger (up to 14.0–15.0 % in lake Lago di Como) (Fig. 7 H). Also the distance between pairs of claws was slightly larger in specimens from Italian lakes (up to 17.1–17.8 % of body length) (Fig. 7 F, 7H). It is especially characteristic that apical spine-like setae of first and second endopodital segments of tl I of the specimens might be very short or even rudimentary (Fig. 7 I–7W). Sometimes more numerous anterior lateral setae of the first proximal segment of tl I were observed in specimens from Lake Geneva.   
 Austrian lakes. Numerous specimens from Grundlsee possess relatively large head (42.5–49.5 % of body length), long tl I (up to 129.6 % of body length) (Table 1), but distal segment of the latter was of moderate length (80.0–97.6 % of first proximal segment of endopodite). Also they had more numerous setae on thoracic limbs, in particular, the posterior lateral setae were almost always present on tl II–tl III (Table 2). Specimens from Mondsee had comparatively large distance between claws of postabdomen and those of caudal process (17.0–19.5 % of body length) (Fig. 8 D). At the same time, in Millstättersee, specimens seem to possess relatively short tl I (82.8–96.6 % of body length). Sometimes there were observed relatively long apical setae on the second endopodital segment (Fig. 8 N, 8Q, 8R, 8S). Small size of claws of postabdomen and caudal process was the most pronounced common feature of all Austrian specimens (Fig. 8 D–8F).Terminal setae of caudal process (Fig. 8 W, 8X) look similar to those ones in specimens from Bodensee.   
 Scottish lakes. Specimens with long tl I (95.1–124.1 % of body length), having moderately long distal segment (86.0–98.0 % of first proximal segment of endopodite) (Fig. 8 B), and bearing one or two small or strongly reduced apical setae of first and second endopodital segments (Fig. 8 L, 8T, 8U; Fig. 9 N, 9P, 9Q). Claws might be larger (5.5–9.0 % of body length in Loch Lochy) or smaller (3.7–5.9 % in Loch Ness). Population from Loch Lochy was presented by only gamogenetic individuals. Males were similar to those from Bodensee in all important respects (Fig. 9 B, 9D–9F), but their tl I were shorter (86.5 % of body length) and seta on antero-dorsal side of antennal basipodite looked feathered (Fig. 9 C). Gamogenetic females do not differ from parthenogenetic ones besides presence of large globular dark eggs (diameter 0.45–0.48 mm) in brood pouches (Fig. 9 A). In general, Scottish and Austrian specimens seem most similar.   
 Lakes of England and North Wales. Specimens from these lakes differ from all previous ones in comparatively short tl I (79.4–97.5 % of body length) with short distal segment (73.0–92.3 % of first proximal segment of endopodite) (Fig. 8 C). At the same time, two proximal segments of endopodite of tl I bear characteristic small setae, which was especially pronounced in individuals from Lake Gracemere (Figs. 8 V, 9L, 9M, 9O, 9R–9T). Claws of all specimens were small (4.9–7.1 % of body length) and either densely situated (lakes of North Wales) (Fig. 9 I, 9J) or stayed distantly (England lakes) (Fig. 8 H, Fig. 9 K). Tl I of one male from Llyn Quellyn was conspicuously short (60.7 % of body length).   
 In his monograph Lilljeborg (1901) imaged tl I of specimen from Mycklaflon See (Småland, Sweden) with very long and thin distal segment and small apical setae of first and second segments which much remind those ones of B. longimanus s. str. So there is a possibility of occurrence of this species in Sweden which requires further verification.   
 Intra- and interpopulation variability. The morphological variability of population of B. longimanus from Bodensee was studied in most detail (Ischreyt 1930, 1934a, 1935, 1939; this study). The degree of variability (CV) enlarges in the following row: relative lengths of distal segment of tl I; caudal process; claws of postabdomen and caudal process; thickness of interclaw distance (see Table 1). The same sequence is in specimens from Grundlsee but values of variables are lower (Table 1). The correlation between length of tl I and number of their setae is absent (Ischreyt 1939). In representatives from Bodensee, the number of lateral setae of the first segment of endopodite of tl I is comparatively low and little variable, posterior setae are rarely present on tl II and are absent on tl III (Table 2). On the contrary, in Grundlsee, these setae are more numerous: they are always present on tl II and rarely absent on tl III (Table 2).   
In specimens from Bodensee and Scotland, tl I and their distal segment are comparatively long but in specimens from Switzerland and Italy the length of tl I is larger (up to 131.7 % of body length) (Fig. 7 A–7C). At the same time, in specimens from England and North Wales, these structures are comparatively short (Fig. 8 C). The size of apical setae of first and second segments of tl I of specimens from Bodensee, lakes of Switzerland and Italy varies considerably both in different individuals and on thoracic limbs of one individual; these setae may be very small or even reduced (Fig. 7 I–7W). The latter was characteristic for specimens from Lake Geneva and Lake Neuchâtel, while in lakes Lago di Garda and Lago di Como these setae are often relatively long which was also recorded by Ischreyt (1939).  
The size of claws of postabdomen and caudal process varies significantly as well, being especially large in specimens from Vierwaldstättersee (up to 15. 2 % of body length), while in Austrian and Great Britain specimens (probably besides those from Loch Lochy) they are comparatively small (Figs. 7 G, 8B, 8D–8F, 8H, 9G–9K). The interclaw distance in specimens from English lakes Windermere and Grasmere is larger than in specimens from other populations (Fig. 8 H, 9K), while those in specimens from North Wales are situated very closely (Fig. 9 I, 9J).  
 Differential diagnosis. Before the completion of full revision of the genus, only preliminarily diagnosis of the species can be done.   
 The adult females of B. longimanus possess medium body size (~ 1.4–3.2 mm), long tl I (80–132 % of body length) with comparatively long distal segment (80–122 % of proximal segment of endopodite of tl I). The anterior distal seta of proximal segment of endopodite and both apical setae of the following second segment of endopodite of tl I are small, reduced up to minute thorns or sometimes may be absent completely. Caudal process is straight and fairly long (180–302 % of body length). Adults always have only two pairs of relatively large (~5.0–13.0 % of body length) and straight claws directed downwards or/and slightly backwards and situated close one to another (interclaw distance ~5–25 % of body length, 6.0–13.2 % in topotypic specimens).
description: Published as part of Korovchinsky, Nikolai M., 2015, 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, pp. 1-44 in Zootaxa 3955 (1) on pages 6-21, DOI: 10.11646/zootaxa.3955.1.1, http://zenodo.org/record/288438