Redescription of Limnosida frontosa Sars, 1862 (Crustacea: Cladocera: Sididae), with evidence of possible incipient species differentiation Author Korovchinsky, Nikolai M. text Zootaxa 2014 3764 1 61 80 journal article 36903 10.11646/zootaxa.3764.1.4 10711c82-e354-42ae-a83e-99d828b60d13 1175-5326 285541 E1CA4935-5A1D-4BDD-9847-830FE4570369 Limnosida frontosa Sars, 1862 ( Figs. 1–7 ) Sars, 1861 (1993): 18–20, Pl. 6, 1–5. Sars, 1862 : 259–261; Sars, 1865 : 37–41, Tab. II, fig. 1–15; Richard, 1895 : 341–343, Pl. 15, fig. 6, 10, 17; Lilljeborg, 1901 : 29–34, Tab. II, fig. 4–11, Tab. III, fig. 1–5; Rylov, 1935 : 105, Taf. 14, Fig. 135a, b; Šramek-Hušek, 1962 : 182–184, Obr. 63; Manuilova, 1964 : 108, Fig. 26, 1–7; Flössner, 2000 : 39–41, Abb. 15a–i; Korovchinsky, 1992 : 5, Figs. 18–25; Korovchinsky, 2004 : 256, Fig. 81, 1–7. Body measurements of specimens of seven populations are presented in Table 1. Description. Parthenogenetic female . Body oval-elongated with comparatively large head (length 34.6–45.3 % and height 29.4–40.7 % of body length) of peculiar triangular shape ( Fig. 1 A–C), narrowing frontally and ventrally, and with no rostrum. Large eye (7.7–12.7 % of body length), situated in the narrowed ventral part of head. Small black ocellus near the eye. Antennules small ( Fig. 1 D) with 9 aesthetascs with constrictions and short sensory seta in apical position, situated in two groups: four aesthetascs and sensory seta on the anterior small apical prominence of the antennular base and five aesthetascs positioned posterior to them. Swimming antennae long (61.4–80.6 % of body length) with massive basipodite (40–46 % of body length), having long setulated seta on dorsal side of its base ( Fig. 1 G). Distal end of antennal basipodite bears stout spine on its outer middle side, small bifurcated prominence dorsally in front of upper branch, and small dorsal spine-like seta better seen from the inner side ( Fig. 1 E, F, L). The upper three-segmented antennal branch (exopodite) is longer (29–32 % of body length) with segments of different size, the proximal of which is smallest (5 % of body length) and lacks setae while the following two are longer (10 and 16 % of body length) and setulated with three (6.7, 10.0 and 13.6 % of body length) and seven (14.2, 14.2, 14.2, 13.6, 13.0, 11.5, 11.5 % of body length from proximal to distal) long, two-segmented setae, respectively. Two latter segments bear apically one small spine each ( Fig. 1 H, I; 4B). In lower shorter (19–22 % of body length) three-segmented antennal branch (endopodite), proximal and distal segments are small while second one is most developed (2.5, 13 and 4.5 % of body length) bearing apically one long seta (13.3 % of body length) and strong spine ( Fig. 1 J). Distal segment of this branch bears four setae (14.2, 14.2, 12.4, 13.6 % of body length, counting from ventral to distal side) and one spine ( Fig. 1 J, K). Proximal segments of both branches clearly articulate with neighboring ones externally while internally they seem completely fused with the latter ( Fig. 1 L). Formula of antennal setae: 0–3–7 / 0–1–4. All setae of swimming antennae armed uniformly with stout setules of swimming type . Mouth parts consist of two maxillules (mx I), each with 9–11 densely situated feathered setae incurved forward, two small maxillae (mx II) situated just near base of tl I ( Fig. 2 F) and having short setulated subapical seta, and two mandibles. Molar surface of right mandible ( Fig. 4 D) is oval-elongated, with a group of one large thorn and few smaller ones near it (pegs—P) posteriorly. A row of some comb-like prominences (pales—Pa) goes along dorsal margin while a row of sharpened prominences (fishbones—Fe) and one large rounded outgrowth are situated along ventral margin. Central part of molar surface with 10–11 diagonals (D) supplied with numerous pores. Left mandible ( Fig. 4 E) seems somewhat wider, with a group of pegs, consisting of two large and some small ones near them, at its dorso-posterior angle. Pales form a practically fused row, lowering anteriorly. A group of two-three large fishbones near pegs, the outer of which is clearly branched while inner one(s) looks like distally widened plate with comb-shaped margin. Another prominent group of fishbones with comb-shaped apical ends inclined anteriorly is situated on large outgrowth between pegs and pales. Diagonals, also with pores, are poorly delineated. Shell with more or less arched dorsal side, connecting with rather high posterior valve margin through emarginated dorso-posterior angle. More or less prominent longitudinal fold separates upper and lower parts of shell ( Fig. 1 A, B). Posterior valve margin with prominent emargination in its middle part, the form ( Fig. 1 M) and depth (1.5–5.2 % of body length) of which vary among specimens. Row of tiny spinules runs marginally and submarginally along its lower part. Ventral valve margin is almost straight, with numerous small uniform denticles along it. Comparatively large denticle, naked or spinulated, is situated on ventro-posterior corner of both valves ( Fig. 2 B–E). Six pairs of thoracic limbs, all with comparatively small oval epipodites. Their structure and armament are shown in Table 2 . Exopodites of tl I–tl V are widened terminally and show signs of segmentation ( Fig. 5 A, C). Endopodites (this is a conditional term according to Smirnov and Kotov (2010)) of these limbs are more or less clearly subdivided in four segments, the proximal of which is largest, bearing numerous filtering setae, while three terminal segments bear two-four such setae. Terminal segment from tl I to tl V and subterminal segment from tl II to tl V bear also outer setae each, similar to those of exopodite, of which the former one is especially long ( Fig. 5 A, B, D, indicated by arrows). Large proximal segment of endopodite of tl I also possesses terminally one short naked seta ( Fig. 5 A: “ns”). A row of comparatively long rough setules along inner side of endopodite and a group of spines on inner anterior basal side of tl I ( Fig. 5 A, C). Gnathobase of tl I ( Fig. 5 A, B) has outer row of six-eight finely setulated filtering setae; inner long, two-segmented setae (I) on distal corner with setules along its length, small spine-like seta near its base, and another smaller curved setulated seta (c), between these there are two soft, sensilla-like prominences ( 2i ) of different sizes ( Fig. 5 B). Two groups of long spinules situated just proximal to them ( Fig. 5 A). Gnathobases of tl II to tl V ( Fig. 5 C) with row of 14–23 finely setulated filtering setae, ending proximally with one naked seta (p), and distally with two setae of different types , one of which is long and twosegmented (I), resembling that one of tl I but differing in presence of stout spinules distally, and the other one strongly modified, being hooked terminally and denticulated laterally (J) ( Fig. 5 C, E). Tl VI ( Fig. 5 F) is small and strongly modified. Its exopodite ( Fig. 5 G) has been reduced to a terminal plate, armed with five terminal and one lateral setae and some long spinules. Endopodite is almost entirely fused with exopodite, bearing seven similar setae with long setules and one thorn between the proximal two and distal five. Gnathobase with two long setae and three thorns of differing shapes. Surface of endopodite and gnathobase with rows of long setules. TABLE 2. Data on structure and armament of thoracic limbs of Limnosida frontosa (specimens from the Neva River estuary (Finnish Gulf, August 8, 2006). Limb pairs Exopodite (apical + Endopodite Gnathobase Epipodite lateral setae) I 7 + 4 ( n 3+1)+( n 3)+( n 3)+( n 18−21) ( n 6−8)+(b+I+ 2i +c) + Postabdomen comparatively massive, short (12.2–17.1 % of body length), tapering distally, high (height in its proximal part 8.3–12.2 % of body length) ( Fig. 2 A) with long two-segmented postabdominal setae (58.7–63.6 % of body length) sitting on two separate bases. Their longer distal parts bear very thin and sparsely situated setules of “sensory” type . Small triangular prominence situated anterior to the setae on the dorsum of abdomen. Lateral and dorsal sides of postabdomen with groups of small spinules ( Fig. 4 C). Postabdominal claws rather long (9.7–16.4 % of body length) and massive with 7–16 long spines, diminishing in size proximally and varying considerably in size and shape ( Figs. 2 H–R, 3A–X, Z). Claw outer lateral sides with long rows of spinules of smaller or larger size ( Fig. 4 F, G) while on dorsal and inner sides these are less numerous ( Fig. 4 H).

II	7 + 4	( n 4+1)+( n 3+1)+( n 3)+( n 18−21) ( n 17−23)+(I+J)+p	+
III	7 + 4	( n 4+1)+( n 3+1)+( n 3)+( n 19–20) ( n 16)+(I+J)+p	+
IV	7 + 4	( n 4+1)+( n 3+1)+( n 3)+( n 16) ( n 14)+(I+J)+p	+
V	6 + 3	( n 3–4+1)+( n 2–3+1)+( n 14–15) ( n 14)+(I+J)+p	+
VI	5 + 1	7 + one thorn 2 + three thorns	+

Juvenile females. They have small body size ( 0.56–0.90 mm ), fewer antennal setae (0–2–7 / 0–1–4), which sometimes are accompanied by rudiment of proximalmost seta on second segment of upper antennal branch (larger juveniles often have full number of antennal setae), and fewer number of spines on postabdominal claws (5–8) ( Fig. 3 Y). Gamogenetic female. They are similar to parthenogenetic females with respect to all diagnostic features, differing from them only in presence of dark widely-oval resting eggs ((0.21–0.26) x 0.18 mm ) in brood pouch ( Fig. 2 G). Male. General body shape ( Fig. 6 A) is subtler than in females with low dorsal side of shell, having prominent dorso-posterior angle. Head with rounded anterior part and slightly larger eye (8.7–13.2 % of body length). It bears very long (57.3–82.2 % of body length), more or less S-curved antennules with massive proximal part ( Fig. 6 B, C), bearing two outgrowths and nine aesthetascs situated on a small prominence on inner side of antennules in two groups of four and five ( Fig. 6 D). Proximal outgrowth large, cone-shaped with slightly widened apical end directed backwards; distal one, smaller and usually rounded, quite variable in size and shape ( Fig. 7 A–F, H–V), occupies anterior semiinternal position. A transparent stick-like solid structure ( Sars 1865: Tab II, 14, 14a ), which probably fulfils a supportive function, lies inside the basal parts of the antennules. Distal portion of antennules long, narrow, tapering to cone-shaped or finely-pointed terminus, and covered with numerous densely situated fine setules ( Figs. 4 A, 6E, F). Swimming antennae (64.3–75.0 % of body length) identical to those of female ( Fig. 6 I). First thoracic limbs with large clasping organ ( Fig. 6 J–L), consisting of a large, probably movable, hook, a group of tiny denticles adjacent to it, and a prominent thorn located among them external to the hook ( Fig. 6 L). Three filtering setae and one massive two-segmented naked seta are attached to small outgrowth situated internally near clasping hook ( Fig. 6 J, K). All other setae as in female, including small naked seta (“ns”) near distal end of proximal row of filtering setae. Juvenile males. In smaller males, antennules somewhat like that of females ( Fig. 6 G) but the sensory seta seems thicker and stays separate from aesthetascs. In larger males, antennule base enlarges, stretches and fuses with modified and enlarged sensory seta ( Fig. 6 H). Size. Parthenogenetic female body length: 1.0– 2.03 mm ; up to 2.40 mm according to Sokolova (2005) . Male body length: 1.0– 1.87 mm . Individual and interpopulation variability. In adult females, measured structures usually showed moderate variability among individuals, but variablility of diameter of eye, height of postabdomen, length of postabdominal claws, and especially depth of embayment of posterior valve margin was high (see Table 1). Number of filtering setae of endopodite and gnathobase of thoracic limbs varied as well (see Table 2 ). Differences between mean values (M) of features, both significant (p <0.001; p <0.01) and insignificant, of specimens from different regions showed random distribution within the range of L. frontosa . FIGURE 1. Limnosida frontosa Sars, 1862 , females. (A, Lake Stensrudvann, Norway; B, L, Lake Konnevesi, Finland; C, D, Cheboksarskoe reservoir; E, F, H–K, Neva River estuary; G, Lake Toros, Karelia; M, from left to right: Lake Gassi, Gor’kovskoe reservoir, Neva River estuary). A, B, general lateral view. C, head, frontal view. D, antennule. E, distal end of basipodite, outer side. F, the same, dorsal side. G, seta on the dorsal basal part of antennal basipodite. H, distal end of proximal segment of upper antennal branch. I, distal end of upper antennal branch. J, distal end of lower antennal branch, lateral side. K, the same, dorsal side. L, distal end of basipodite, inner side. M, posterior valve margin. FIGURE 2 . Limnosida frontosa Sars, 1862 , females (A, E, Neva River estuary; B, Sheksninskoye reservoir; C, D, Lake Bolon’, Far East of Russia; F, L, O, Lake Kilometrovoe; G, Lake Nidjili, Yakutia; H, Lake Stensrudvann, Norway; I, J, Lake Rostajarvi, Karelia; K, N, Lake Bol’shoi Harbei; M, Kunovatskiy sor; P, Lake Toros, Karelia; R, Pregel River). A, postabdomen, lateral side. B–E, posterior part of ventral valve margin. F, maxilla (mx I) and maxillula (mx II) near the base of tl I. G, resting eggs in brood pouch. H–R, postabdominal claws. FIGURE 3 . Limnosida frontosa Sars, 1862 , females (A, Ob’ River basin; B, C, Kunovatskiy sor; D, S, Lake Nijili, Yakutia; E, Lake Beloe, Yakutia; F, Nadymsky gorodok–4; G, H, Lower Ob’ River; I, Cheboksarskoye reservoir; J, Lake Usun-Kuel’, Yakutia; K, L, Lake Taiylar, Yakutia; M, N, roadside pond, Yakutia; O, Lake Petropavlovskoye; P, Q, Lower Amur River near Leninskoye; R, Lake Chlja; T, Lake Bolon’; U, Lake Gassi; V, River Amur near mouth of River Hungari (Gur); W, Gor’kovskoye reservoir; X, channel under the bridge, Lower Amur River; Y, Lake Jingpo, China; Z—Lake Khanka). A–Z, postabdominal claws and distal parts of the claws (S, T, W—malformed claws). FIGURE 4 . Limnosida frontosa Sars, 1862 , females (Cheboksarskoye reservoir). A, setules of male antennule. B, apical end of upper antennal branch. C, armament of lateral side of postabdomen. D, molar part of right mandible. E, molar part of left mandible. F, G, armament of lateral side of postabdomenal claw, outer side. H, the same, inner side. Scale bars—2.5 µm. FIGURE 5 . Limnosida frontosa Sars, 1862 , females (Lake Konnevesi, Finland) (most filtering setae of endopodites (en) not shown; terminal soft, non- segmented setae on the end of endopodite are indicated by unlabeled arrows; naked seta of tl I is indicated as “ns”). A, thoracic limb of first pair (tl I), inner view. B, soft sensilla-like outgrowths of gnathobase. C, thoracic limb of second pair (tl II), inner view. D, scheme of attachments of two soft, not segmented setae of tl II, outer view (setulae not shown). E, modified thorn-like, naked, hooked seta on distal corner of gnathobases of tl III–tl V (J). F, Thoracic limb of sixth pair (tl VI), inner view. G, exopodite of thoracic limb of sixth pair. FIGURE 6. Limnosida frontosa Sars, 1862 , males (Lake Kilometrovoe, Komi Republic, European Russia). A, general lateral view. B, antennule. C, proximal part of antennule, outer side. D, aesthetascs. E, distal end of antennule, inner view. F, the same, lateral view. G, H, antennules of juvenile males with body length 0.75 and 0.83 mm, respectively. I, distal end of antennal basipodite, outer side. J, endopodite of the thoracic limb of first pair (tl I) with clasping hook, inner view (terminal soft setae on the end of endopodite is indicated by arrow; naked seta of tl I is indicated as “ns”, setules of filtering setae not shown). K, apical end of endopodite of the same limb, frontal view. L, the same, outer view. FIGURE 7 . Limnosida frontosa Sars, 1862 , antennules of males (A, Neva River estuary; B, Lake Toros, Karelia; C, D, E, Lake Kilometrovoe, Komi Republic; F, I–L, Kunovatskiy sor; G, Lake Bol’shoi Harbei, Komi Republic; M, Lake Nidjili, Yakutia; N, O, Lake Beloe, Yakutia; P, Q, Lake Usun-Kuel’, Yakutia; R, S, Lake Chlja; T, Lake Pir; U, Verhne-Olinskaja channel; V, River Amur near the mouth of River Hungari (Gur)). A–F, H–V, proximal part of antennule and distal outgrowth of antennule (F, antennule with malformed proximal outgrowth). G, malformed antennule. Spines of postabdominal claws, especially distal ones, differed in size and degree of inclination to claw. Specimens of some populations had comparatively short and weakly inclined spines ( Fig. 2 H–J, Fig. 3 O, R), while others had both less inclined and more inclined spines ( Fig. 2 K, L, N, O), or had only strongly inclined spines ( Fig. 3 A–G, I–M, Q, U, V, X, Z). While the proximal outgrowth of male antennules demonstrated morphological stability, the distal one varied considerably in size and shape ( Fig. 7 A–F, H–V). Conspicuous intra- and interpopulation variability of this feature was frequently observed ( Fig. 7 C–E, N–O, P–Q, R–S); this was especially prominent in specimens from Kunovatsky sor (Lower Ob’ River basin, Western Siberia). In some specimens, malformed postabdominal claws, spines, and antennules ( Figs. 3 S, T, W; 7F, G) were observed. Remarks. Sars (1861 (1993), 1862) found and originally described L. frontosa only from Lake Koldbjørnsvigsjønen in south-eastern Norway . The type series ( syntypes ) is stored in ZMUO (M. Christiansen, pers. comm.) and was not accessible for this study. Norwegian specimens used in this study, initially studied and labeled by G.O. Sars, originate from another Norwegian location, Lake Stensrudvann. The original description of L. frontosa was brief and incomplete, lacking figures and including only females. (Although figures were present in the unpublished manuscript ( Sars, 1861 (1993)), some of them were later lost). In addition, comparison of the present data with those by Sars revealed some inaccuracies. Initially, he counted eight antennular aesthetascs in two groups of four, and later, in his more detailed description ( Sars 1865 ), seven aesthetascs were incorrectly noted, three in the anterior group and four in the posterior one. Thoracic limbs were poorly described and sketchily drawn: in the female only tl III and tl VI and in the male only tl I. TABLE 3. Regional distribution of two variable morphological features of Limnosida frontosa (populations with juvenile specimens only not included) (NW—north-west of Europe, VR—basin of Middle Volga River, NE—north-east of European Russia, WS—north of Western Siberia, ES—Eastern Siberia (Yakutia), FE—Far East of Russia). Region and number Spines of postabdominal claws Distal outgrowth of male’s antennules of locality short and weakly inclined long and strongly inclined short long NW 1 + NW 2 + NW 3 + NW 4 + NW 5 + + NW 6 + NW 7 + + NW 8 + + NW 10 + VR 11 + + + VR 12 + VR 14 + + NE 15 + + + NE 16 + + NE 17 + + + WS 18 + WS 20 + + WS 21 + + WS 22 + + ES 23 + + ES 24 + + + ES 25 + ES 26 + ES 27 + + + ES 29 + + ES 30 + + FE 33 + FE 34 + FE 35 + FE 37 + FE 38 + FE 41 + + FE 42 + + FE 43 + + FE 44 + FE 46 + + FE 47 + FE 48 + Lilljeborg (1901) was even less detailed than Sars and almost fully repeated his mistakes. He described “7−8 aesthetascs” of the antennules and presented sketches only of selected thoracic limbs (tl I and tl VI in female and tl I in male). Shortly thereafter, Scourfield (1905) first described the antennules of Limnosida correctly. Some variable morphological features described above, i.e. spines of postabdominal claws, distal outgrowth of male’s antennules, denticle on postero-ventral angle of shell valves, and emargination of posterior shell margin, seemed likely to have potential taxonomic significance. However, the latter three features proved in general to be randomly distributed over the different regions of the continent which is also characteristic for all meristic features presented in Table 1. Although from Table 3 , data suggest that males with a long distal outgrowth of the antennules are absent from the Far East, too few males were collected from this region to make this a certainty. On the other hand, distribution of types of spinulation of the postabdominal claws shows, in general, a more regular trend (see Table 3 ). Specimens with comparatively short and weakly inclined distal spines (“north-western type ”) are distributed mostly in the north-west of Eurasia (Fennoscandia, including the north-west of Russia ) (populations 1–10 in “Material and Methods”) ( Fig. 2 H–J, P), while specimens with comparatively long and strongly inclined spines (“eastern type ”) dominate in Eastern Siberia (Yakutia) (populations 23–30) and the Far East (populations 33–48) ( Fig. 3 D, E, J–Q, T–V, X, Z) as well as in the north of Western Siberia (populations 18– 22) ( Fig. 3 A, C). Further, similar specimens were encountered from the middle part of the Volga River basin (populations 11–14) ( Fig. 3 I). In addition, specimens from two populations (15 and 16) from the north-east of European Russia (Komi Republic) ( Fig. 2 K, N) and from one population from Western Siberia (17—Kunovatskiy sor) ( Figs. 2 M, 3B, C) demonstrated presence of claw spines of both types . Also, specimens from two localities from the Lower Amur River (Far East—41, 42) ( Fig. 3 O, R) have the claw spines of “north-western type ” and specimens from the Pregel River (Kaliningrad area, Russia ) possess long and rather inclined spines ( Fig. 2 R) of “eastern type ” (but here only two juveniles were present).