Siamopsis sudzukii ( Okubo, 1992 ) Smith, 2023

Smith, Robin James, 2023, Descriptions of two Cypridopsinae (Ostracoda, Crustacea) species from the Nansei Islands, Japan, with the first records of non-marine ostracods from the Daito Islands, Zootaxa 5293 (2), pp. 294-316 : 305-309

Siamopsis sudzukii ( Okubo, 1992) nov. comb.

( Figs 6C, D , 7 & 8 )

1992 Potamocypris sudzukii sp. nov. —Okubo in Okubo & Terauchi: 104–105, fig. 1g –m, pl. 1n–q.

2002 [ Potamocypris ] sudzukii Okubo, 1992 —George & Martens: 14.

2004 Potamocypris sudzukii Okubo, 1992 —Okubo: 39, figs 19n–q.

2011 Potamocypris sudzukii Okubo, 1992 —Martens & Savatenalinton: 65.

2012 P. sudzukii Okubo 1992 —Karanovic: 418, 421.

2018 Potamocypris sudzukii Okubo & Terauchi, 1992 —Smith et al.: Appendix.

2019 Potamocypris sudzukii Okubo, 1992 —Meisch et al.: 91.

Material examined. Paratype: JAPAN • 1 ♀ with soft parts and valves mounted on a glass slide in Canada balsam; Okinawa Prefecture, Yaeyama District, Kuroshima ; 24.2371º N, 124.0122º E; 18 Jul. 1991; M. Sudzuki leg.; “ Potamocypris sudzukii 28 [eligible character] 8907” topside of slide, “787” underside of slide, LBM 1430009616 .

The appendages of the paratype are generally faint and somewhat difficult to observe, especially lightly sclerified parts, such as the palps of the fifth limbs, branchial plates and distal sections of setae. The appendages contain no traces of musculature.

Diagnosis

Dorsal margin strongly arched, maximum height at about mid-length, ventral margin sinuous. Selvage marginal. Postero-ventral edge of left valve with row of five setae noticeably more robust than other setae. Surface of valves with smooth appearance. Antenna, longest seta on exopodite reaching beyond end of first endopodite segment. Aesthetasc Y relatively long and slender, about 28% length of anterior margin of first endopodite segment. Natatory setae long, reaching beyond end of claws by about one-third of their total length. Gm-claw approximately 60% length of GM-claw. Mandibular gamma seta just over twice length of terminal segment, very slender and with setules on distal half. Maxillular tooth-bristles (Zahnborsten) on third endite finely serrated towards distal end, second palp segment very elongate and curved. Fifth limb with approximately 12 apical setae on endite, and two a-setae on basis. Walking leg terminal segment with very robust, curved h2-claw. Caudal ramus with elongate, cylindrical base.

Description

Left valve, length 551 µm, height 334 µm, anterior margin evenly curved, posterior margin more tightly curved than anterior, with apex below mid-height ( Fig. 6C , 7A ). Dorsal margin strongly arched, maximum height at about mid-length, ventral margin sinuous. Calcified inner lamella wide anteriorly, narrower posteriorly. Selvage marginal, forming hyaline flange along anterior and ventral margins. Postero-dorsal margin with slight thickening (interpreted as postero-dorsal plate, marked with black triangle on Fig. 7A ). Right valve, length 556 µm, height 342 µm, generally similar outline to left valve, but with more angular postero-dorsal margin ( Fig. 6D , 7B ). Line (groove?) running along calcified inner lamella (marked with black triangle on Fig. 7B ). Adductor muscle scars consisting of group of 5–6 scars in pattern consistent with family ( Fig. 7B ). Valves sparsely covered in normal pore systems, postero-ventral edge of left valve with row of five setae noticeably more robust than other setae. External surfaces of valves with smooth appearance (but see Remarks section below).

Antennule with seven articulated segments ( Fig. 7C ). First segment with seta on dorsal margin, and two long sub-apical setae on ventral margin. Second segment with apical-dorsal seta. Third segment with one short apical-dorsal seta and one short apical-ventral seta. Fourth segment with two long apical-dorsal setae, and two short apical-ventral setae. Fifth segment with two long apical-dorsal setae, one medium-length and one short apical-ventral setae. Sixth segment with four long apical setae and one tiny seta, less than half length of terminal segment (latter seta difficult to observe). Terminal segment apically with one short and two long setae and long ya aesthetasc, over four times longer than terminal segment. Rome and Wouters organs not visible.

Antenna, longest seta on exopodite reaching beyond end of first endopodite segment ( Fig. 8A ). Aesthetasc Y relatively long and slender, about 28% length of anterior margin of first endopodite segment. Natatory setae long, reaching beyond end of claws by about one-third of their total length. Two t-setae, one slightly longer than other ( Fig. 8B ). G1-claw large and robust, G3-claw shorter than G1 and much thinner, G2-claw similar length to G3 and seta-like ( Fig. 8B, C ). Gm-claw approximately 60% length of GM-claw.

Mandibular palp, first segment with small, alpha seta, large setulous s1 and s2 setae and thinner and shorter smooth seta ( Fig. 7D ). Second segment with 3+1+beta setae on inner edge, and three setae on outer edge; beta seta setulous, elongate ( Fig. 7E ). Third segment with group of four sub-apical setae on outer edge, gamma + 3 setae along distal edge, and one long and one very short setae on inner sub-apical edge; gamma seta just over twice length of terminal segment, very slender and with setules on distal half. Terminal segment with three stout claw-like setae and three thinner setae. Mandibular coxa with slender endite with seven teeth of various sizes, main part with small protrusion near mid-length (marked with black triangle of Fig. 7F ), forming contact point with palp endoskeleton.

Rake-shaped organ with eight distal teeth ( Fig. 7H ).

Maxillula, first segment of palp with group of five long setae on outer apical edge, and one short sub-apical seta located offset from outer edge ( Fig. 7G ). Second segment very elongate and curved, apically with five setae. Tooth-bristles (Zahnborsten) on third endite finely serrated towards distal end.

Fifth limb (not drawn, both very faint in dissection slide), with approximately 12 apical setae on endite, and two a-setae on basis. Palp terminating with at least two setae, one longer than other.

Sixth limb (walking leg) well-developed and robust, with five segments ( Fig. 8D ). First segment with short d2- seta. Second segment with e-seta shorter than second segment. Third segment with f-seta slightly longer than e-seta, extending beyond distal end of fourth segment. Fourth segment with short g-seta. Fifth segment with very robust, curved h2-claw, only lightly serrated, relatively long h1-seta, about 1/4 length of h2, and very short, thin h3-seta.

Seventh limb, first segment with d2- and dp-setae both long, sub-equal in length, d1-seta slightly shorter ( Fig. 8E ). Second segment with e-seta about half length of third segment. Third segment with f-seta relatively short, not reaching to end of third segment. Pincer with h3-seta long, similar length as third segment, h2-seta curved and relatively long, about half length of h3, h1-seta short.

Caudal ramus with elongate, cylindrical base, supporting one long apical seta and one short sub-apical seta ( Fig. 8F ).

Remarks. The Rome and Wouters organs of the antennule are not visible in the paratype, but this could be caused by the mounting medium; all lightly sclerified parts are faint and musculature is absent, suggesting some degradation of the specimen. The valves of the paratype examined appear smooth, but Okubo & Terauchi (1992) stated that “surface smooth at a glance, covered with a number of minute hollows and normal pores”. The wide hyaline edges of the valves are not visible in the photographs provided by Okubo & Terauchi (1992: Plate 1N & O), but are depicted in their line drawings of the valves (Okubo & Terauchi op. cit.: Fig 1G & H ) and are visible in the paratype. A specimen figured in Okubo (2004) (specimen 783) is listed as a juvenile in Okubo & Terauchi (1992) (as FO-783).

Examination of the appendages revealed that this species does not belong to the genus Potamocypris and it is transferred to the genus Siamopsis , based on the carapace features and the claws of the antenna. The valves have a lateral outline generally similar to other Siamopsis species, and the left valve has a probable postero-dorsal plate (see Okubo & Terauchi 1992), but it is not easily observable in the paratype due to the mounting of the valves in Canada balsam. The antenna has a reduced G2 claw, more like a seta (homologous claw labeled as G 3 in Savatenalinton 2017b) and a very robust G1 claw, similar to other Siamopsis species. Other appendages also show a strong similarity with those of Siamopsis species, e.g., only two t-setae on the antenna, elongate, cylindrical terminal segment of the maxillular palp, very robust claw on the walking leg, and setae of similar lengths.

Although S. sudzukii is generally similar in lateral view to the other five Siamopsis species, it is higher than Siamopsis suttajiti Savatenalinton, 2017b and Siamopsis conspecta Savatenalinton, 2017b , and less angular along the postero-dorsal margin than Siamopsis renateae Savatenalinton, 2017b , Siamopsis khoratensis Savatenalinton, 2017b and Siamopsis planitia Savatenalinton, 2017b . Discrimination of species within the genus relies in part on the morphology of the postero-dorsal plate of the left valve, which is not clearly known in S. sudzukii .

The five robust setae on the postero-ventral margin of the left valve of S. sudzukii are similar to those in other species of the genus figured by Savatenalinton (2017b).

Okubo & Terauchi (1992) noted that this species was abundant in the Yaeyama Islands and Okinawa, with a large number of specimens on Kuroshima (a small island of the Yaeyama Islands) ( Fig. 1 ). Okubo & Terauchi (1992) did not mention the habitat, but Okubo (2004) listed ponds and rice fields.

Figures

FIGURE 1. Map of the Nansei Islands. Numbers 1–6 correspond to the locations in Table 2. The exact locations of 5 and 6 are unknown.

FIGURE 6. A, Potamocypris kusuokai sp. nov., male, whole specimen, right lateral view (dark field). B, Potamocypris kusuokai sp. nov., Zenker organ (LBM 1430009610). C, Siamopsis sudzukii, internal view of left valve (paratype 787, LBM 1430009616). D, Siamopsis sudzukii, internal view of right valve (paratype 787, LBM 1430009616, female). E, Stenocypris major, external view of female left valve. F, Stenocypris major, external view of female right valve. Black triangles mark the edges of the posterior part of the calcified inner lamellae.

FIGURE 7. Siamopsis sudzukii (paratype 787, LBM 1430009616, female). A, left valve, internal view. The five robust setae on the postero-ventral margin are drawn, but the other thinner setae scattered over the valves are not included. B, right valve, internal, black triangle indicates probable groove. C, antennule. D, mandibular palp. E, alpha, beta and gamma setae of mandibular palp. F, mandibular coxa, black triangle indicates protrusion. G, maxillula palp and endites (setae on endites not drawn). H, rake-shaped organ.

FIGURE 8. Siamopsis sudzukii (paratype 787, LBM 1430009616, female). A, antenna. B, antenna, detail of terminal two segments. C, claws G1–3 of antenna. D, sixth limb. E, seventh limb, F, caudal ramus.