Stenocypris sketi Petkovski & Meisch, 1996

Moonchaisook, Khattiya & Savatenalinton, Sukonthip, 2020, Redescriptions of Stenocypris major major (Baird, 1859) and Stenocypris major sketi Petkovski & Meisch, 1996 (Ostracoda: Crustacea), with discussion on their taxonomic position, Zootaxa 4786 (4), pp. 497-515 : 505-511

Stenocypris sketi Petkovski & Meisch, 1996

( Figs 5–8 , 9 E–H)

1996 Stenocypris major sketi n. ssp. — Petkovski & Meisch: 65–66, Fig. 2

Material examined. Six females (soft parts dissected in glycerine on a sealed glass slide, valves stored dry in a micropalaeontological slide) from two localities (three samples): 1) Canal, Sanam Chai Khet district , Chachoengsao Province, coordinates: 13° 39.174ʹ N and 101°26.166ʹ E, 4 May 2018 ( MSU-ZOC 288–289 ) , 14 October 2016 ( MSU-ZOC 290 ) ; 2) Rice field, Muang Sam Sip District , Ubon Ratchathani Province, coordinates: 15° 23.604ʹ N, 104° 48.907ʹ E, 25 October 2010 ( MSU-ZOC 291–293 ) .

Diagnosis. Carapace in lateral view elongated (length c. 2.7–2.8 times of height), LV overlapping RV. LV and RV with distinct postero-dorsal expansion. Septa present on all free margins, slightly widened at mouth zone. Posterior inner margin weakly sinuated curve or ripple-like. A1 with dorsal seta on third segment very long (reaching beyond tip of terminal segment), Wouters organ not observed. Natatory setae on A2 long (far beyond end of terminal claws), claw GM longer than claw G3. Md-palp penultimate segment with long ventro-subapical seta (c. 2/3 of length of terminal segment). Two large bristles on Mx1-third endite gently serrated. T1 with a, b and d-setae. T2 with d1 and d2 setae, length of h2 slightly longer than that of penultimate segment, h3 seta of similar length as h1 seta. CR obviously asymmetrical, right ramus with robust spines on posterior margin, seta Sa long (c. 3/4 of that of Ga), left ramus with few small spines on posterior margin, length of Ga c. 0.4 times that of ramus in both CR, length of Gp c. half that of Ga in both rami.

Measurements (μm). LV (n=3), L= 1785–1877, H= 651–688; RV (n=3), L= 1762–1866, H= 644–677; Carapace (n=3), L= 1785–1825, H= 651–700.

Description of female. Carapace in lateral view elongated (length c. 2.7–2.8 times the height), anterior margin rounded, posterior margin slightly more narrowly rounded, LV slightly overlapping RV, dorsal margin gently curved, greatest height situated slightly behind mid-length, ventral margin sinuated slightly in front of mid-length, valve surface smooth in medium magnification, but shallowly lineated in high magnification with tiny rimmed-pore setae.

LV in interior view ( Fig. 5 A–C, G) and RV in interior view ( Fig. 5 D–F, H) elongated, anterior and posterior margins subequally rounded, dorsal margin gently curved, greatest height situated slightly behind mid-length, postero-dorsal margin with expansion ( Fig. 5 G–H), ventral margin sinuated slightly in front of mid-length, calcified inner lamella relatively wide anteriorly, without inner list, posteriorly narrower, posterior inner margin with weakly sinuated curve (inner margin parallel to valve margin) or ripple-like (inner margin not parallel to valve margin), septa along anterior, ventral and posterior margin slightly widened at mouth zone.

A1 ( Fig. 6A ): first segment with one dorso-subapical seta of intermediate length (almost reaching tip of segment) and two long ventro-apical setae, Wouters organ on A1 not observed. Second segment slightly wider than long, with one very short dorso-apical seta and a short Rome organ. Third segment bearing two setae: one very long dorso-apical seta, reaching tip of terminal segment, and one short ventro-apical seta, reaching tip of fourth segment. Fourth segment with two long dorsal setae and two subequal, shorter ventral setae (reaching beyond tip of terminal segment). Fifth segment with two groups (one dorsally and one ventrally) of two setae, all setae very long. Penultimate segment with four long setae and one short dorsal seta (reaching tip of terminal segment). Terminal segment with three (two long, one shorter) apical setae and an aesthetasc Ya, the latter of about similar length as shorter apical seta.

A2 ( Fig. 6 B–C): exopodite with three (one long, two short) setae, the long one reaching beyond tip of first endopodal segment. First endopodal segment with five long (reaching beyond tip of terminal claws) and one short natatory setae, length of the shortest seta almost reaching half way the penultimate segment, aesthetasc Y long, ventro-apical seta of intermediate length, reaching c. 2/3 of penultimate segment. Penultimate segment undivided, distally with three serrated claws, aesthetasc y2 long (reaching tip of terminal segment), z1–z3 setae unequally long, longest one reaching slightly beyond tip of apical claws; this segment medially with two subequal dorsal setae and four ventral setae (t1–t4) of unequal length; one of these four setae large, reaching slightly beyond tip of terminal segment. Terminal segment with two serrated claws (GM and Gm), a very short g-seta, an aesthetasc y3 and two (one apical, one subapical) short setae, apical one of similar length as g-seta); length of Gm c. half of that of GM, length of aesthetasc y3 c. 2/3 of that of accompanying seta, the latter c. 2/3 of GM.

Md ( Fig. 6 D–E): first segment of md-palp with two large setae (s1 and s2), one slender, long seta and a long, smooth α-seta. Second segment dorsally with three unequally long apical setae, length of the shortest c. half of that of the longest; ventrally with a group of three long hirsute setae, one shorter hirsute seta and the β-seta, the latter plumose, cone-shaped and with pointed tip. Penultimate segment consisting of three groups of setae: dorsally with a group of four unequally long, subapical setae; laterally with an apical γ-seta and three further smooth apical setae, the former stout, hirsute, long (length c. 1.6 times of that of terminal segment); ventrally with two (one long, one shorter) subapical setae, the latter reaching c. 2/3 of length of terminal segment. Terminal segment bearing three claws and two setae.

Mx1 ( Fig. 7A ) with two-segmented palp, three endites and a large branchial plate; basal segment of palp with a group of five long, unequal, apical setae and one short subapical seta, the latter reaching tip of segment, terminal segment elongated (length = 1.6 times of width), apically with three claws and two setae. Two large bristles on third endite gently serrated. Sideways-directed bristles on first endite subequally long.

T1 ( Fig. 7B ): protopodite with two a-setae, long b and d-setae, b-seta of similar length as d-seta, distally with 10 apical and four subapical hirsute setae of unequal length. Endopodite a weakly built palp with three unequal apical setae.

T2 ( Fig. 7C ) with setae d1 and d2, both setae thin and almost of the same length. Second segment with long eseta (reaching half of penultimate segment). Penultimate segment divided, proximal segment (a) bearing long f-seta (reaching beyond tip of segment), distal segment (b) with a pair of apical setae (long g-seta, one spine-like seta). Terminal segment with two (one dorsally, one ventrally) apical h1 and h3 setae and a long serrated claw (h2), length of h2 c. 1.2–1.3 times of that of penultimate segment, h3 and h1 setae of similar length.

T3 ( Fig. 7 D–E) a cleaning limb. First segment with long d1, d2, dp setae. Second segment with long apical eseta (reaching c. 2/3 of next segment). Third segment with medially long f-seta (reaching tip of segment). Terminal segment with an apical pincer (h2), one short h1 seta and one reflexed subapical h3 seta, length of the latter c. 5/6 of that of third segment.

CR ( Fig. 8 A–B) well-developed and asymmetric with right ramus slightly longer and thicker than left ramus. Right ramus ( Fig. 8A ) with posterior margin strongly serrated, bearing robust spines reducing in size along margin. Claws Ga and Gp long and strongly serrated, length of Ga c. 0.4 times of that of ramus, length of Gp c. half of that of Ga, claw Ga with large spines on c. 2/3 of Ga length, claw Gp with large spines all along its length. Seta Sa long (length c. 3/4 of that of Ga), seta Sp absent.

Left ramus ( Fig. 8B ) with posterior margin bearing few small spines. Claws Ga and Gp long and strongly serrated, length of Ga c. 0.4 times of that of ramus, length of Gp c. half that of Ga, claw Ga with large spines being on c. 2/3 of Ga length, claw Gp with large spines being on all its length. Seta Sa long (length c. 5/6 of that of Ga), seta Sp absent.

CR attachment ( Fig. 8C ) stout, with triangular basal loop, db and vb well-developed.

Male unknown.

Ecology. In the Eastern part of Thailand, S. sketi was recorded only from one locality: a canal in Chachoengsao Province. It occurred at a pH range of 6.92–7.44, a temperature range of 29.0–32.5°C and a DO range of 7.0– 8.3 mg /l. However, in the Northeastern part of the country, it was found in a rice field.

Remarks. The morphology of Thai S. sketi is generally congruent to the original description based on specimens from Sri Lanka ( Petkovski & Meisch 1996), with the presence of the obvious postero-dorsal expansion in particular. However, some slightly different aspects occurred, e.g., the carapace size and the morphology of the CR. The carapace size of S. sketi from Thailand was slightly smaller than that from Sri Lanka, namely c. 1.79–1.83 mm and 1.88–1.95 mm in Thai and Sri Lankan specimens, respectively. Slightly more robust spines of the right ramus were recognized in the Thai specimens. The number of spines on the Ga and Gp claws of the right ramus was also slightly different. There were 13 spines on each claw in the Sri Lankan samples while 14 and 11 spines were found on the Ga and Gp claws, respectively, in the Thai specimens.

The presence of S. sketi in this study is the first report in Thailand and Southeast Asia. Since S. sketi was first discovered from Sri Lanka, it has never been reported again, although it was assumed that its possible distribution would cover tropical and subtropical areas ( Petkovski & Meisch 1996).

Figures

FIGURE 2. Stenocypris major (Baird, 1859), female (MSU-ZOC.279). A. A1. B. A2. C. Terminal segment of A2. D. Md-coxa. E. Md-palp. Scale bars: 100 μm for A, C–D, 115 μm for B, 50 μm for E.

FIGURE 5. Stenocypris sketi Petkovski & Meisch, 1996, female (MSU-ZOC.288). A. LV, interior view. B. anterior part of LV, interior view. C. posterior part of LV, interior view. D. RV, interior view. E. anterior part of RV, interior view. F. posterior part of RV, interior view. G. posterior-dorsal expansion on LV. H. posterior-dorsal expansion on RV. I. muscle scars of LV. Scale bars: 200 μm for A, D, 100 μm for B–C, E–H, 50 μm for I.

FIGURE 6. Stenocypris sketi Petkovski & Meisch, 1996, female (MSU-ZOC.288). A. A1. B. A2. C. Terminal part of A2. D. Md-coxa. E. Md-palp. Scale bars: 100 μm for A–D, 50 μm for E.

FIGURE 7. Stenocypris sketi Petkovski & Meisch, 1996, female (MSU-ZOC.288). A. Mx1. B. T1. C. T2. D. T3. E. Terminal part of T3. Scale bars: 100 μm for A–B, E, 200 μm for C–D.

FIGURE 8. Stenocypris sketi Petkovski & Meisch, 1996, female (MSU-ZOC.288).A. right CR. B. Left CR. C. CR attachment. Scale bars: 200 μm.

FIGURE 9 Valves (internal view) of Stenocypris major (Baird, 1859) (A–D) and Stenocypris sketi Petkovski & Meisch, 1996 (E–H).A. LV (MSU-ZOC.280). B. RV (ditto). C. LV (MSU-ZOC.284). D. RV (ditto). E. LV (MSU-ZOC.288). F. RV (ditto). G. LV (MSU-ZOC.289). H. RV (ditto). Scale bars: 200 μm.