Leptodora richardi, Korovchinsky, Nikolai M., 2009

Korovchinsky, Nikolai M., 2009, The genus Leptodora Lilljeborg (Crustacea: Branchiopoda: Cladocera) is not monotypic: description of a new species from the Amur River basin (Far East of Russia), Zootaxa 2120, pp. 39-52 : 41-49

Leptodora richardi sp. nov.

Data on body parts measurements are presented in Table 1 .

Etymology. The species is named in honour of well-known cladoceran researcher, Professor J. Richard who, together with his co-author S.-A. Poppe, supposed the presence of a new small-bodied species of Leptodora in China.

Female. Body strongly elongated, highly transparent and divided into four parts—head, thorax, threesegmented abdomen, and postabdomen ( Fig. 1 A). Head large (30.9–35.3% of body length), strongly elongated and narrowing anteriorly. Dorsally it bears a large saddle-shaped neck organ poorly visible in specimens preserved for several decades. Eye comparatively small, ocellus is absent. Head pore not visible, may be also due to long preservation of the material.

Antennules small, consisting of more or less cylindrical basal part slightly widened distally, bearing nine aesthetascs, and one small sensory seta on its end, which is shorter than aesthetascs ( Fig. 1 B). Aesthetascs are in three groups and seem to be two-segmented or constricted.

Swimming antennae long (50.4–62.5% of body length) and strong with massive basipodite which has small distal outer seta between bases of branches ( Fig. 1 E) and one small denticle dorsally in front of basal segment of upper antennal branch ( Fig. 1 F) (dorsal thin naked seta on its folded proximal part has not been found as sometimes in case of L. kindtii ). Basipodite narrows distally and bears two long branches, the upper of which (exopodite) is slightly longer than lower one (endopodite) (16.1–20.1% vs. 15.7–17.6% of body length). Upper branch is four-segmented and lower branch is five-segmented ( Fig. 1 C, D). Proximal-most segment of the lower branch ( Fig. 1 D, indicated by arrow) is rudimentary and clearly developed only dorsally, while the following segment of the branch articulates with the basipodite ventrally under the rudimentary basal segment. Small dorsal denticle on the end of second segment of upper antennal branch ( Fig. 1 G) and small apical spine shifted somewhat proximally on the fourth segment of the same branch ( Fig. 1 H). End of the last, fifth segment of the lower antennal branch is also armed by a small apical spine ( Fig. 1 I). Small proximal-most segments of both antennal branches lack setae, while other segments possess a row of twosegmented swimming setae of different size. General formula of antennal setae: 0 (6–10) (5–7) (7–10) / 0 (3–6) (7–12) (4–5) (5–7). Thus, total number of setae reaches 27 and 30 on upper and lower branches respectively, but normally their number is smaller, 18–24 setae on each branch. Intra- and interpopulational variability may be significant.

Mouth parts are represented by upper and lower lips and mandibles. The upper lip (labrum) looks like a thick plate inflated externally, having spade-like, wide distal margin and two papillae on inner surface ( Fig. 2 C, D). Under the labrum there is a large trilobed lower lip with a large median lobe and two smaller lateral lobes ( Fig. 2 C). The anterior part of the former one is armed by two to three rows of small prominences. Each lateral lobe of lower lip possesses a large anterior palpus-like outgrowth ( Fig. 2 C) and a row of about eight - ten flattened, lanceolate prominences along the external margin ( Fig. 2 C, E). Mandibles large with massive, widened proximal part and long saber-like distal part. The latter one is armed distally with three large denticles, the proximal-most of which is thinner and longer than others ( Fig. 1 J).

Body HL:BoL, % AbL: Bol, PL: BoL, CL: BoL, SL: BoL, AnL: BoL, TlIL:BoL, PSL: Bol, % length % % % % % %

L. richardi sp. nov., Lake Bolon', August 1933 (n = 15)

L. kindtii , the Volga Delta, June 1915 (n = 15)

Carapace is visible as a massive bag-like structure, attached to the postero-dorsal margin of thorax ( Fig. 1 A). It is rather variable in its size (24.0–32.2% of body length), reaching in adults the end of abdomen or proximal part of postabdomen ( Fig. 2 A, B).

Six pairs of strongly chitinized, stenopodous limbs are situated along the triangular massive muscular ventral part of thorax and directed antero-ventrally ( Fig. 1 A). Limbs of the anterior four pairs have a much enlarged shoe-like basis adopted for the attachment of strong musculature ( Fig. 2 F). All limbs are foursegmented and have complex and variously setaceous armament along their inner side ( Table 2). Setae of the limbs are also strongly chitinized, sit on an elevated basis, and most of them lack setules. On the outer side, limbs of tl I - tl V have very small spine-like setae on the distal margin of their basal segment ( Fig. 2 I, 3E). Limbs of the first pair (tl I) are especially long and strong (28.9 - 36.9% of body length) ( Fig. 2 F) with very long basal segment (13.8 - 19.6% of body length), having in the middle of its inner side one (rarely two) short seta ( Fig. 2 F, G) and one short setae distally (sometimes it may be absent) ( Fig. 2 H). The second segment of the limb bears one fairly long seta (rarely absent) in the middle part and two very long setae distally, which reach or sligntly exceed the end of the limb. One of these setae with slightly bent apical end, bearing few very fine setules ( Fig. 2 F, K, ad(s)), while another one bears more numerous stout setules distally ( Fig. 2 F, L, pd(s)). The third segment normally bears two or three lateral setae ( Fig. 2 F, al, pl) and two long setae distally; one of the latter bears denticles, another one setules ( Fig. 2 F, ad(d) and pd(s), respectively; Fig. 2 J). The fourth distal segment of the limb with two to three pairs of lateral setae and three distal setae, most of which, excluding the proximal-most ones, bear denticles ( Fig. 2 F). There is a small outgrowth of gnathobase (processus maxillaris) near the basis of basal segment of tl I, bearing one long and one short seta and tiny prominence between them ( Fig. 2 F, M). Basal segment of limbs of the second pair (tl II) bears four to eight comparatively short lateral setae along each side and three median setae, two in the middle part and one distally ( Fig.3 A). Two subsequent segments of tl II bear two, rarely three, long setae along each lateral side and one median seta distally, which differs from those on the basal segment, being slender and setulated ( Fig. 3 A, B, C). The fourth distal segment usually posseses one or two lateral setae on each side (sometimes only on one side) and three distal setae which seem smooth. Limbs of the third and fourth pairs (tl III, tl IV) have a similar structure and setae armament (Fig. D, F), however, their setae are less numerous and the basal segment of tl IV bears four median setae. The structure and armament of limbs of the fifth pair (tl V) mostly resemble those of the previous ones but their basal segment is inflated, having four median setae ( Fig. 3 G, H). Distal median setae of the second and third segments lack setules. Limbs of the sixth pair (tl VI) are small, twosegmented and always bear seven stout setae distally ( Fig. 3 I, J).

Abdomen long (29.4–34.3% of body length), flexible and composed of three segments, the second of which is the shortest, while the first and third are of almost equal size ( Fig. 1 A). Postabdomen straight, comparatively long (20.2–26.7% of body length). Two small, two-segmented and setulated setae natatoriae in the antero-dorsal position ( Fig. 1 M). Postabdomen terminates in a pair of postabdominal claws, which are long (11.4–16.5% of body length) and more or less straight with a dorsal row of 10–22 denticles ( Fig. 1 K, L) and numerous groups and combs of spinules all over their surface.

Body length 2.6–6.1 mm.

population each (data for both limbs of one pair; numbers of limb segments from basal to distal; lateral setae on each

limb side in parentheses; d—distal setae; m—median setae; dominating type of setation in bold)

Limb Leptodora richardi sp. nov. Leptodora kindtii

pair Lake Bolon' (n = 12) The Volga Delta (n = 3)

I Limb segments Limb segments

1 2 3 4 1 2 3 4 (1)-0d (1)-2d (1-2)-2d (2-3)-3d (1)-1d (1)-2d (2-3)-2d (3-4)-3d (1)-0d (1)-2d (1-2)-2d (2-2)-3d (0)-1d (1)-2d (2-3)-2d (4-4)-3d II (5-7)- 3m (2-2)- 1m (2-2)- 1m (1-2)-3d (6-8)- 3m (3-3)- 1m (3-3)- 1m (1-2)-3d (4-6)- 3m (2-2)- 1m (2-3)- 1m (0-1)-3d (7-8)- 3m (3-3)- 1m (3-3)- 1m (1-2)-3d to be continued.

Limb Leptodora richardi sp. nov. Leptodora kindtii

pair Lake Bolon' (n = 12) The Volga Delta (n = 3) (5-7)- 3m (1-2)-3d (5-7)- 3m (1-2)-3d (5-7)- 3m (1-2)-3d (5-6)- 3m

(5-8)- 3m

III (5-6)- 3m (2-2)- 1m (2-2)- 1m (1-1)-3d (6-7)- 3m (2-3)- 1m (2-3)- 1m (2-2)-3d (5-7)- 3m (2-2)- 1m (2-2)- 1m (1-1)-3d (6-7)- 3m (3-3)- 1m (2-3)- 1m (1-2)-3d (5-6)- 3m (2-2)- 1m (1-1)-3d (5-6)- 3m (3-3)- 1m (3-3)- 1m (1-2)-3d (4-6)- 3m (1-2)- 1m (1-2)-3d (6-7)- 3m (3-3)- 1m (3-3)- 1m (1-2)-3d (5-7)- 3m (2-3)- 1m (0-1)-3d (6-7)- 3m (2-2)- 1m (1-1)-3d (6-6)- 3m

(5-6)- 3m

(5-6)- 3m

(5-6)- 3m

(4-7)- 3m

(5-6)- 3m

IV (5-6)- 4m (2-2)- 1m (2-2)- 1m (1-1)-3d (5-5)- 4m (2-3)- 1m (2-2)- 1m (1-2)-3d (5-6)- 4m (2-2)- 1m (2-2)- 1m (1-1)-3d (5-6)- 4m (2-2)- 1m (2-2)- 1m (1-2)-3d (4-6)- 4m (1-2)- 1m (2-2)- 1m (1-1)-3d (5-6)- 4m (2-2)- 1m (2-3)- 1m (5-6)- 4m (2-2)- 1m (1-2)- 1m (0-1)-3d (5-6)- 4m (2-2)- 1m (2-3)- 1m (5-6)- 4m (0-1)-3d (6-6)- 4m (4-5)- 4m (0-1)-3d (5-6)- 4m (6-6)- 4m

(5-6)- 4m

(5-5)- 4m

(5-6)- 4m

(4-6)- 4m

(4-4)- 4m

V (3-4)- 4m (1-1)- 1m (1-1)- 1m (0)-3d (4-4)- 4m (1-1)- 1m (1-1)- 1m (1)-3d (3-4)- 4m (1-1)- 1m (1-1)- 1m (0)-3d (4-4)- 4m (1-1)- 1m (1-1)- 1m (1)-3d (4-4)- 4m (1)-3d (3-4)- 4m (0)-3d (4-4)- 4m

(4-4)- 4m

(4-5)- 4m

(4-4)- 4m

VI 0 7 0 7 0 7 0 7

Male. General body shape and structure as in female but carapace is undeveloped and looks like a postero-dorsal thoracic outgrowth reminiscent of juvenile females. Antennules long (19.2–28.7% of body length) with thickened basal part, bearing a group of nine aesthetascs. The elongated part of the antennule bears up to its apical end a long row of 26–28 aesthetascs ( Fig. 4 A). Thoracic limbs mostly as in females, except for the presence of the clasping organ on tl I. This organ is composed of a large bud-like structure on the inner, proximal-most part of the distal segment, and two (sometimes one) prominences on the apical end of the previous, third, segment, they may be accompanied by one or two tiny additional prominences ( Fig. 4 B); the number of these prominences on limbs of one individual may be different. The bud-like organ is movable, being supplied with a muscle and bears tiny spinules apically. In fixed specimens it is usually retracted beneath the margin of the previous segment.

Body length 2.5–3.2 mm.

Morphological variability. In adult specimens, the body length, length of postabdomen, postabdominal claws, swimming antennae, length of tl I and of its basal segment were the most variable structures ( Table 1 ). The same was true for the number of antennal setae, denticles of postabdominal claws, and number of setae in thoracic limbs, which were most variable in the proximal segments of tl II–tl V and in distal segments of tl I and tl II ( Table 2).

The wide overlapping of body length of adult and juvenile specimens is of special interest. The latter ones may be identified according to their undeveloped shell, reaching not more than just the proximal part of second abdominal segment, and ovaries, which are invisible or poorly developed. Judging from these features, the body length of juvenile females varied from 1.12 to 3.8 mm while that of adult females from 2.6 to 6.1 mm. The same was probably true for males, the juveniles of which, judging from their undevelopped antennules, had body length 2.5–2.9 mm, while the adult males 2.5–3.2 mm. For comparison, similar overlapping in body size in L. kindtii seems smaller (body length was 5.7 mm in largest juvenile female and 4.9 mm in smallest adult female, see Boikova 2005).

Remark. In a previous publication on Leptodora ( Korovchinsky & Boikova 2008, p. 2846), the presented data of body measurements for small adult individuals from Lake Bolon', having in fact strongly deformed abdomens, were incorrect. In the course of this study, they were corrected with measurements of a new set of less deformed individuals (see Table 1 ).

Differential diagnosis. The new species differs from L. kindtii in the small body size of both juvenile and adult individuals. Body sizes for both species are as follow: 1.12–3.8 mm and 1.5–5.7 mm for juvenile females, 2.5–2.9 mm and 2.0– 4.7 mm for juvenile males, 2.6–6.1 mm and 4.9–21.0 mm for adult females, and 2.5–3.2 mm and 4.3–7.4 mm for adult males, respectively ( Vijverberg & Koelewijn 2004; Boikova 2005; Korovchinsky & Boikova 2008; present study). Thus, the difference in minimal body sizes of juveniles for both species reaches about 0.4 mm and in adults about 2.0 mm, while their difference in maximal body sizes is much bigger. The only known exception is shown by several unusually small specimens of L. kindtii from the Volga Delta but they differ from L. richardi sp. nov. in other features described below.

Most of all other features of general body structure and appendages of females and males are similar in both species, except the relative size of head and abdomen, which in L. richardi sp. nov. is longer and shorter, respectively (30.9–35.3%, av. 33.6% vs. 25.3–33.6%, av. 28.8% in L. kindtii and 29.4–34.3%, av. 31.8% vs. 30.3–39.7%, av. 35.4% in L. kindtii , respectively) (see Figs. 5 , 6 , p <0.01 for Lake Bolon’ and the Volga Delta; p <0.001 for Lake Bolon’ and other populations). In the species, due to presence of comparatively shorter abdomen, shell also seems sometimes comparatively longer, reaching the proximal part of postabdomen ( Fig. 2 B).

The only meristic difference between the two species is the number of small prominences on the distal end of third segment of tl I of males, which seems not to show age-related variability. In few studied males of L. richardi sp. nov., there were two (rarely one) of such prominences, while in L. kindtii , they are usually more numerous (three–four, rarely up to six–seven), though sometimes there are only one or two. This difference should be studied in more detail due to small number of available males of L. richardi sp. nov.

Another small qualitative difference of L. richardi sp. nov. from L. kindtii is the presence of very fine setules on the end of anterior distal seta of the second segment of tl I ( Fig. 2 F, K, ad(s)) which is naked in the latter species.

Type material. Holotype: adult female with body length 4.38 mm contained in a small jar with formalin deposited in Zoological Museum of Moscow State University; Cat. No. Ml 89.

Paratypes: 5 adult females in a small jar with formalin deposited in Zoological Museum of Moscow State University, Cat. No. Ml 90.

All other specimens (paratypes) have been deposited in a personal collection of NMK.

Type locality. Large Lake Bolon’ in the Lower Amur River basin.

Figures

FIGURE 1. Leptodora richardi sp. nov., adult female, Lake Bolon’. A, general lateral view. B, antennule. C, upper antennal branch (exopodite). D, lower antennal branch (endopodite) (proximal rudimentary segment is indicated by arrow). E, apical end of antennal basipodite. F, denticle on dorsal side of apical end of antennal basipodite near the base of upper branch. G, apical end of second segment of upper antennal branch. H, apical end of upper antennal branch. I, apical end of lower antennal branch. J, distal part of mandible. K, postabdominal claw. L, denticles of postabdominal claw. M, seta natatoria.

FIGURE 2. Leptodora richardi sp. nov., adult female, Lake Bolon’ (see also the " Abbreviations " after the Introduction). A – B, shell, dorsal view. C, mouth parts. D, labrum, ventral view. E, lateral lobe of lower lip. F, tl I. G, setae on basal segment of tl I. H, inner apical seta of basal segment of tl I. I, outer apical spine-like seta of basal segment of tl I. J, apical end of posterior distal seta (pd (s )) of third segment of tl I. K, apical end of anterior distal seta (ad (s )) of second segment of tl I. L, apical end of posterior distal seta (pd (s )) of second segment of tl I. M, processus maxillaris.

FIGURE 3. Leptodora richardi sp. nov., adult female, Lake Bolon’. A, tl II (median setae are indicated by arrows). B – C, distal median setae of second and third segments of tl II. D, tl III (median setae are indicated by arrows). E, outer apical spine-like seta of basal segment of tl III. F, tl IV (median setae are indicated by arrows). G, tl V (median setae are indicated by arrows). H, scheme of setae disposition of tl V. I, tl VI, lateral view. J, tl VI, frontal view.

FIGURE 4. Leptodora richardi sp. nov., adult male, Lake Bolon’. A, antennule. B, clasping organ of tl I.

FIGURE 5. Head length and body length relationships in Leptodora richardi sp. nov. from different water bodies of the Lower Amur River basin (1) and L. kindtii from Lake Khanka (2), Gulf of Finland (3), and the Volga Delta (4).

FIGURE 6. Length of abdomen and body length relationships in Leptodora richardi sp. nov. and L. kindtii (indications of localities are the same).

Tables

TABLE 1. Data on body measurements of the representatives of Leptodora spp. from two populations (in each column from top to down: range, mean, SD, CV).

3.5–4.6	30.9–35.3	29.4–34.3	20.2–26.7	11.4–16.5	24.0–32.2	50.4–62.5	28.9–36.9	13.8–19.6
4.0	33.6	31.8	22.8	13.4	27.7	55.1	32.6	16.8
0.35 8.8	1.4 4.0	1.0 3.1	1.5 6.6	1.3 9.9	1.3 4.8	3.5 6.3	2.3 6.9	1.3 7.5