Diaphanosoma elongatum, Korovchinsky, Nikolai M. & Sanoamuang, La-Orsri, 2008

Korovchinsky, Nikolai M. & Sanoamuang, La-Orsri, 2008, Overview of Sididae (Crustacea: Cladocera: Ctenopoda) of Northeast and East Thailand, with description of a new species of the genus Diaphanosoma, Zootaxa 1682, pp. 45-61 : 50-53

Diaphanosoma elongatum sp. nov.

( Figs 5–25 )

Material: 18 adult parthenogenetic females, 1 gamogenetic female, 2 adult males, and 7 juvenile females. One adult female and one juvenile female were dissected during this study.

Locality: Yam River, Akat Amnuai dt., Sakon Nakhon Province, Song Khram River basin, Thailand, 31.08.2004, coll. Sawitri Nachai.

Etymology: The species is named " elongatum " ("elongated" in Latin) due to its unusually low and elongated body shape.

Description: Parthenogenetic female. Body measurements are shown in Table 2 .

Body in general is low and elongated ( Fig. 5 ). Head comparatively small (length 28.2–33.3% and height 17.1–18.2% of body length), roundish-rectangular, its dorsal margin clearly sloping to the front and then smoothly joining the frontal margin. Eye large (8.6–11.7% of body length) and occupies almost all frontal part of head. Antennules ( Fig. 6 ) comparatively large with unusually long sensory papillae which are only slightly shorter than sensory seta (1:1.5 respectively). Swimming antennae short (57.5–69.7% of body length) and weak, not far reaching the posterior valve margins. Antennal branches long (81.3–96.2% and 56.3–71.2% of basipodital length in upper one and lower one respectively). Long seta on dorsal side of basipodital basal part ( Fig. 12 ). Large sharp spine on the basipodital distal outer end ( Fig. 7 ). Proximal segment of two-segmented upper antennal branch (exopodite) with small sharp spine ( Fig. 9 ). Similar short spine with one small inner outgrowth near its base on the end of the exopodital distal segment ( Fig. 10 ). Small spines on the ends of two distal segments of lower antennal branch (endopodite) ( Fig. 11 ). Distal segment of upper antennal branch with only seven long swimming setae (formula of antennal setae 4–7 / 0–1–4) ( Fig. 8 ). Armament of antennal setae as usual for genus: setules of the longest apical setae of both branches are differentiated into thick proximal ones (row type) and thin distal ones (sensitive type); all other setae uniformly possess only thick row setules (see Korovchinsky 1987). Ventral valve margins form unusually short and moderately wide fold shifted far anteriorly which connects smoothly with posterior part of ventral margin ( Figs 14, 15 ). This fold bears 10–12 long and very finely feathered setae, the proximal ones sit on inner fold's side near margin. Numerous very small denticles along posterior part of ventral and postero-ventral valve margins which sit partly in two rows — full marginal and intermittent submarginal ones ( Figs 14, 16, 17 ). Posterior valve margins comparatively low with one inner small spiny thorn ( Figs 14, 16, 18 ).

BL—body length, HL—head length, DE—diameter of eye, LSA—length of swimming antennae, BaL— length of antennal basipodite, LUAB—length of upper antennal branch, LLAB—length of lower antennal branch

Armament of the thoracic limbs is shown in Table 3. They do not possess specific features but the number of their filtering setae is comparatively small, corresponding to the small body size of the species.

Postabdomen with groups of small setules and spinules on its lateral sides, dorsalmost of which may be larger and stouter ( Fig. 13 ). Setae natatoriae long. Terminal claws comparatively long and massive with three basal spines proximal of which is noticeably smaller than others. Small denticles along distal dorsal part of claws.

Limbs Exopodite (apical + lateral setae) Endopodite Gnatobase I 6 + 4 (n5+1) + (n3) + (n3) + (n25+1) n5 + I + i

II 7 + 4 (n7+1) + (n3+1) + (n3) + (n23) n20 + I + i III 7 + 4 (n7+1) + (n3+1) + (n3) + (n22) n19 + I + i IV 6 + 4 (n7+1) + (n3+1) + (n3) + (n22) n19 + I + i V 6 + 3 (n5+1) + (n3+1) + (n3) + (n14) n18 + I + i VI 4 + 1 6 + i 2 + 3 outgrowths Body length 0.54–0.69 mm. Most of females with 1– 3 eggs or embryos in their brood pouches.

Gamogenetic female ( Fig. 23 ). All diagnostic features as in parthenogenetic females. One large (0.18 x 0.12 mm) oval resting egg with small prominences in brood pouch ( Figs 23–25 ). Body length 0.85 mm.

Male ( Fig. 19 ). Diagnostic features as in females. Antennules long (51–60% of body length) with small thin spinules along their distal part ( Figs 19, 20 ). Spine on the distal end of antennal basipodite small, reminiscent of that of female. Small denticulated hooks on the end of endopodites of first pair of thoracic limbs ( Fig. 21 ). Copulatory appendages tube-like and rather thick, not narrowing distally ( Fig. 22 ).

Body length 0.51–0.57 mm.

Differential diagnosis. This new species is quite different from all other congeners, having a combination of some unusual diagnostic features: low and elongated body, large antennules with long sensory papillae, short swimming antennae with only seven setae on distal segment of upper antennal branch, very short ventral flap of shell valves shifted much anteriorly, presence of only long rows of very small denticles on ventral and ventro-posterior valve margins, and one inner thorn near posterior valve margins.

Type specimens. Holotype— adult female with body length 0.68 mm contained in a small jar with formalin deposited in Zoological Museum of Moscow State University; Cat. No. Ml 61.

Paratypes— 10 adult females, two males, and six juvenile females in a small jar with formalin deposited in Zoological Museum of Moscow State University, Cat. No. Ml 62.

Seven other adult females (paratypes) in similar jar have been deposited in a personal collection of NMK.

Distribution. This species is to date known only from its type locality — Yam River in Song Khram River basin, Thailand.

Figures

FIGURES 5 – 13. Diaphanosoma elongatum sp. nov., female. Yam River, Akat Amnuai dt., Sakon Nakhon Province, Thailand, 31.08. 2004. 5) general lateral view, 6) antennule, 7) distal part of antennal basipodite, outer side, 8) upper antennal branch (exopodite), 9) distal part of proximal segment of upper antennal branch, 10) apical end of upper antennal branch, 11) distal part of lower antennal branch, 12) dorsal proximal seta of antennal basipodite, 13) postabdomen, lateral view. Scale bars in mm.

FIGURES 14 – 22. Diaphanosoma elongatum sp. nov., Yam River, Akat Amnuai dt., Sakon Nakhon Province, Thailand, 31.08. 2004. Female: 14) shell valve, ventral view, 15) shell valve, lateral view (setae of ventral inflexion not shown), 16) postero-ventral and posterior valve margins, 17) denticles of postero-ventral valve margin, 18) spine near posterior valve margin. Male: 19) general lateral view, 20) armament of distal part of antennule, 21) distal part of thoracic limb of first pair, 22) copulatory appendage. Scale bars in mm.

FIGURES 23 – 29. Diaphanosoma elongatum sp. nov., gamogenetic female, Yam River, Akat Amnuai dt., Sakon Nakhon Province, Thailand, 31.08. 2004. 23) general lateral view, 24) resting egg, 25) the same, enlarged. Diaphanosoma macrophthalma, female, marsh in Hui Lam Sebai, Loeng Nok Tha dt., Yasothon Province. 26) general lateral view, 27) distal part of antennal basipodite, outer side, 28) distal part of proximal segment of upper antennal branch, 29) posteroventral valve margin. Scale bars in mm.

Tables

TABLE 2. Measurements of body parts' proportions of adult females of Diaphanosoma elongatum sp. nov. (n = 15) (in each column from top to down: Range, Mean, SD, CV)

HL: BL, %	DE: BL, %	LSA: BL, %	LUAB: BaL, % LLAB: BaL, %
28.2–33.3	8.6–11.7	57.5–69.7	81.3–96.2 56.3–71.2
30.4	10.4	61.6	88.4 65.2
1.79	0.71	3.36	4.54 4.41
5.89	6.83	5.45	5.13 6.76