A New Species of Semicytherura (Crustacea: Ostracoda: Cytheroidea) from Obitsu River Estuary (Central Japan) and Its Microhabitat Author Nakao, Yuriko Corresponding author nakao.yuriko@nihon-u.ac.jp Author Tsukagoshi, Akira Department of Geosciences, Faculty of Science, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422 - 8529, Japan text Species Diversity 2020 2020-01-01 25 1 9 journal article 10.12782/specdiv.25.1 8942cb41-8781-4bd3-9d71-0050e088ff9f 2189-7301 3751609 6B3FBBFD-7DEB-43F7-B2AE-30FE3DD03B5D Semicytherura obitsuensis sp. nov. ( Figs 2–5 ) Pontocythere sp. 1: Yajima 1982 , pl. 10 7–8, text-fig. 13 5–6. Type series. Holotype : male ( SUM-CO-2461 ; length/ height of right valves 0.45/ 0.17 mm , of left valve 0.45/ 0.17 mm ). Paratypes : 9 males ( SUM -CO -2462–2470 ) and 10 females ( SUM-CO-2471–2480 ). Type locality. Obitsu River estuary, Tokyo Bay, Chiba Prefecture, Japan ( 35°25′04.9″N , 139°53′57.8″E ). Etymology. After the Obitsu River estuary, central Japan . Diagnosis. Carapace thin, weakly arched cylindroid, and elongated. Carapace surface covered with concentric muri and short muri, forming reticulation in anterior area. Concentric muri in antero-dorsal area showing like broken line. Inside valve, depressed triangular lamella of prismatic layer in anteroventral area and broad vestibules in anterior area in both male and female. Fourth and fifth podomeres of antennule fused. All terminal claws of thoracic legs bearing one small spinous process at 1/3 from proximal end. Male copulatory organ bearing sub-oval capsule and biramous distal lobe: one tip sharp and triangular, other one with round tip bent at almost right angle. Copulatory duct short and coiled. Description. Carapace ( Figs 2 , 3 , 4A, B ). Carapace small, thin, weakly arched cylindroid, and more elongated outline in lateral view in male than female. Greatest height somewhat behind mid-length; dorsal margin gently arched and ventral margin concave anterior mid-length. Anterior margin gently tapered-off downward. Posterior margin round without caudal process; lower part of posterior margin straight more prominently in male. Carapace surface with reticulation composed of concentric and short muri in anterior area; obscure reticulation in mid-lateral area; almost smooth in dorsal and posterior area. Concentric muri in antero-dorsal area showing like broken line. Sixty-three pore systems per valve. Eyes present without tubercle. Fig. 2. Semicytherura obitsuensis sp. nov. A, male right valve in external lateral view; B, male left valve in external lateral view; C, male left valve in internal lateral view; D, male right valve in internal lateral view; E, female right valve in external lateral view; F, female left valve in external lateral view; G, female left valve in internal lateral view; H, female right valve in internal lateral view; I, male carapace in dorsal view; J, female carapace in dorsal view; K, male carapace in ventral view; L, female carapace in ventral view. A–D, paratype, SUM-CO-3462; E–H, paratype, SUM-CO-2471; I, K, paratype, SUM-CO-2463; J, L, paratype, SUM-CO-2472. Fig. 3. Semicytherura obitsuensis sp. nov. A, male carapace in anterior view; B, male carapace in posterior view; C, female carapace in anterior view; D, female carapace in posterior view; E, adductor muscle scar of male right valve (arrow indicating anterior); F, adductor muscle scar of female right valve (arrow indicating anterior); G, hingement of male right valve; H, hingement of male left valve; I, hingement of female right valve; J, hingement of female left valve. A, B, paratype, SUM-CO-2463; C, D, paratype, SUM-CO-2472; E, G, H, paratype, SUM- CO-2462; F, I, J, paratype, SUM-CO-2471. Arrowheads indicate mandibular scars. Lamella of prismatic layer, depressed triangular shape with cleaved sides in antero-ventral area in both male and female; broad in posterior area in male. Vestibules developed in anterior, slightly narrow from ventro-median to posterior area in both male and female. Muscle scar ( Fig. 3E, F ). Vertical row of 4 adductor scars in ventro-median area. Uppermost circular; obscure in female. Lower 3 somewhat large; upper one elongate; middle one square; lower one semi-circular. Large circular fulcral point in front of row of adductor muscle scars. Two large frontal scars in front of fulcral point; anterior one circular and posterior one kidney-shaped. Two obscure mandibular scars on lamella of prismatic layer. Hingement ( Fig. 3 G–J). Lophodont. Hinge line weakly arched. Groove and weakly developed terminal teeth in right valve. Bar and terminal socket in left valve. Antennule ( Fig. 4C ). Six articulated podomeres, with 4th and 5th podomeres fused. First podomere with minor assemblage of setulae on anterior-distal and distal margin. Second podomere with 1 very long setulose seta at middle of posterior side, assemblage of short setae on proximal and distal area, and setulae along distal half of anterior margin. Third podomere with 1 medium setulose seta on near distal end, one assemblage of short setae at middle, and setulae along anterior margin. Fourth and 5th podomeres almost fused. Fourth podomere with 2 long simple setae and 1 medium setulose seta on distal end, and row of numerous short setulae on anterior side. Fifth podomere with 2 setulose stout setae, 1 thin short and 1 simple setae on distal end. Sixth podomere with 2 long, 1 medium, and 1 spatulate setae on distal end. Fig. 4. Semicytherura obitsuensis sp. nov. , valves and appendages. A, internal view of male right valve; B, internal view of female right valve; C, antennule; D, antenna of male; E, antenna of female; F, mandibule. A, C, D, F, holotype, SUM-CO-2461; B, paratype, SUM-CO-2471; E, paratype, SUM-CO-2473. Antenna ( Figs 4D, E ). Five articulated podomeres. First podomere with 1 long stout exopodite (= spinneret seta) and assemblage of short setae on anterior-distal end. Numerous rows of setulae on outer lateral side. Second podomere with 1 setulose and 1 simple short setae on posterodistal end, assemblage of short setae on antero-proximal end and on outer lateral side, and setulae along anterior margin. Third podomere with 2 short setulose setae on postero-distal end, setulae along anterior margin, and short setae along proximal half of anterior margin. Fourth podomere with 1 long curved spatulate seta more developed in male near proximal end on outer lateral side, 1 thin seta at middle of anterior margin, 1 stout setulose seta on postero-distal end, and setulae along posterior margin. Fifth podomere with 1 claw-like seta on ledge of posterior margin, 1 short thin seta, and 1 claw-like seta possessing spinous process on distal end. Serrations along distal part of both of claw-like setae. Fig. 5. Semicytherura obitsuensis sp. nov. , appendages. A, maxillule; B, fifth limb; C, sixth limb; D, seventh limb; E, male copulatory organ. A–E, holotype, SUM-CO-2461. Mandibula ( Fig. 4F ). Five articulated podomeres, with 2nd to 5th podomeres fused. First podomere (coxa) with several stout irregular teeth on anterior margin and setulose seta at about 1/3 from dorso-distal end. Second podomere (basis) with bunch of short setae at middle of ventral margin, 1 long setulose seta on distal end, and exopodite reduced to 1 stout setulose seta. Third podomere (1st podomere of endopodite) with 1 long seta on dorso-distal end, 1 long seta on ventro-distal end, and 2 long setae on near ventral distal end, all setae setulose; fine setulae along dorsal margin. Fourth podomere (2nd podomere of endopodite), with 2 medium simple setae and 2 long simple setae on ledge at middle of dorsal margin, 1 stout setulose seta, and 1 simple seta on ventro-distal end. Fifth podomere (3rd podomere of endopodite) with 1 claw-like seta, 2 rather short claw-like setae, and 1 fine short simple seta on distal end. Maxillula ( Fig. 5A ). Extremely thin branchial plate (exopodite) consisting of 14 simple setae and 2 long reflexed setae. Basal podomere bearing palp (endopodite) and 3 endites. Palp consisting of 2 podomeres; first podomere with 4 long setae at dorso-distal end; second podomere with 4 setae on distal end and 1 short seta on middle of ventral margin. Each of three endites with 5 setae on distal end. Brush-shaped organ . Not found. Fissh limb ( Fig. 5B ). Four articulated podomeres. Costae (frame of podomere) especially developed along anterior and posterior margins of second podomere. First podomere with 1 seta at middle of anterior margin, 2 long setae on antero-distal end, and 2 setae on near postero-proximal end; all setae setulose. Second podomere with 1 long setulose seta on antero-distal end and setulae along distal half of anterior margin and distal margin. Third podomere with setulae along distal half of anterior margin and distal margin. Fourth podomere with setulae along distal half of anterior margin and distal margin and 1 large terminal claw bearing serrations; on anterior margin of claw, one small spinous process at 1/3 of from proximal end. Sixth limb ( Fig. 5C ). Four articulated podomeres. Costae (frame of podomere) especially developed along anterior and posterior margins of second podomere. First podomere with 2 setae on anterior margin, 1 seta on antero-distal end, and 1 seta on near postero-proximal end; all setae setulose. Second podomere with 1 long setulose seta on antero-distal end and setulae along distal half of anterior margin and distal margin. Third podomere with setulae along distal half of anterior margin and distal end. Fourth podomere with setulae along anterior margin and distal margin and 1 large terminal claw bearing serrations; on anterior margin of claw, one small spinous process at 1/3 of from proximal end. Seventh limb ( Fig. 5D ). Four articulated podomeres. First podomere with one short setulose seta on antero-distal end, setulae along anterior margin, and assemblage of setulae on postero-proximal area. Second podomere with 1 long setulose seta on anterto-distal end and setulae along distal half of anterior margin and distal margin. Third podomere with setulae along distal half of anterior margin and distal margin. Fourth podomere with setulae on distal half of anterior margin and distal margin and 1 large terminal claw bearing serrations; on anterior margin of claw, one small spinious process at 1/3 from proximal end. Male copulatory organ ( Fig. 5E ). Basal capsule sub-oval. One clasping apparatus, thin and broad with round top. Distal lobe with biramous structure on distal end: one tip sharp and triangular, other one with round tip bent at almost right angle. Pectinate structure and parallel lines on proximal area of distal lobe. Copulatory duct short and coiled. Dimensions. See Table 1 . Remarks. In the morphology of soft part, the present new species is appropriately assigned to the genus Semicytherura . Semicytherura obitsuensis sp. nov. is distinguished from other Semicytherura species based on its carapace morphology, i.e. , relatively smooth surface with weak reticulation, elongated outline, the round posterior part without caudal process, and broad vestibules in the anterior area. Semicytherura obitsuensis sp. nov. resembles S. sabula from Tateyama Bay. However, S. obitsuensis sp. nov. has more tapered anterior and posterior margins and shows a much clearer reticular ornamentation. Moreover, only males of S. obitsuensis sp. nov. have developed “lamella of prismatic layer” ( Yamada et al. 2004: 384 ) in the posterior and ventral areas, whereas in S. sabula both males and females have it. Their carapace size is also different: S. obitsuensis sp. nov. ( 0.45–0.48 mm male length) is more extensive than S. sabula ( 0.31–0.33 mm male length, Frydl 1982 ). Semicytherura obitsuensis sp. nov. also resembles S. hiberna Okubo, 1980 from Seto Inland Sea. However, S. obitsuensis sp. nov. has a broad vestibule in anterior area whereas S. hiberna has a broad lamella of prismatic layer in that area. Moreover, the shape of the male copulatory organ is different, and the 4th and 5th podomeres of antennule are not fused in S. hiberna . The carapace morphology and size of S. obitsuensis sp. nov. are similar to Pontocythere sp. 1 reported by Yajima (1982) (0.43/ 0.16 mm , length/height). Based sp. nov. on carapace morphology, Pontocythere sp. 1 seen in SEM images of Yajima (1982) are male specimens. They differ from the new species by the absence of the lamella of prismatic layer. However, the lamella of prismatic layer was not observed or was unclear in some male specimens of S. obitsuensis sp. nov. on microscopic observation. Moreover, it was sometimes destroyed after washing our specimens with sodium hypochlorite in preparation for SEM observation. Given those situations, it is reasonable to assume that the lamella of prismatic layer was not preserved in the fossil specimen of Yajima (1982) . Thus, we conclude that Pontocythere sp. 1 should be recognized as a Pleistocene fossil record of S. obitsuensis sp. nov. from the Boso Peninsula, central Japan . Table 1. Dimensions (mm) of Semicytherura obitsuensis sp. nov.

Length	Height	Width
Object
Av	OR	n	Av	OR	n	Av	OR	n
Male	RV	0.46	0.45–0.48	5	0.18	0.17–0.18	5
LV	0.47	0.45–0.48	5	0.18	0.17–0.18	5
C	0.16	0.15–0.17	2
Female	RV	0.45	0.43–0.48	4	0.19	0.18–0.21	4
LV	0.45	0.43–0.48	4	0.19	0.18–0.21	4
C	0.18	0.16–0.19	6

Abbreviations: Av, average; C, carapace; LV, left valve; n , number of specimens; OR, observed range; RV, right valve. Nakao and Tsukagoshi (2002) reported 21 species belonging to 17 genera from the Obitsu River estuary intertidal zone. This new Semicytherura species is the twenty-second species reported from the area.