Callistocythere ryukyuensis, Ha, Tran Manh & Tsukagoshi, Akira, 2015

Ha, Tran Manh & Tsukagoshi, Akira, 2015, First records of interstitial leptocytherids (Crustacea, Ostracoda): two new species and a redescription of Callistocythere ventricostata Ruan & Hao, 1988 collected from the Okinawa Islands, southern Japan, Zootaxa 4006 (1), pp. 83-102 : 89-94

Callistocythere ryukyuensis sp. nov.

( Figs 6–10 )

Type locality. Interstitial environment at northwest coast of Sesoko Beach ( Fig. 1 C), the Okinawa Islands, Japan, 26°39'4.10"N, 127°51'27.00"E on 16 May 2002 and 30 June 2014.

Type series. All specimens were collected from type locality on 16 May 2002 and 30 June 2014. Holotype: male (SUM-CO-2350). Paratypes: 5 males (SUM-CO-2351; SUM-CO-2352; SUM-CO-2353; SUM-CO-2354; SUM-CO-2355), and 3 females (SUM-CO-2356; SUM-CO-2357; SUM-CO-2358).

Etymology. After the type locality, in recognition of new records of interstitial Callistocythere species from the Ryukyu Islands, a traditional name of the Okinawa Islands.

Diagnosis. Carapace outline sub-reniform in lateral view and surface with unclear blunt reticulation in anterior and posterior areas. Posteroventral projection obscure. Single and celate sieve type pore canals sparsely distributed, number of lateral pore systems 67 per valve. Hingement modified entomodont with smooth bar and smooth groove of median element in right and left valves, respectively. Proximal half of first podomere of fifth to seventh limbs each with 5 transversal rows of setulae. In male, copulatory organ with oval-shaped capsule, distal lobe transparent, triangular-shape with blunt end, 3 stout clasping apparatus, extending posteriorly and forming pointed tip. Copulatory duct very long and complicated, coiling 6–7 times in capsule.

Description. Carapace ( Figs 6–8 ). Carapace slightly compressed. Outline of lateral view sub-reniform, right and left valves almost symmetrical, sexual dimorphism slight; male relatively elongate and lower than female. Both valves slightly tapering towards posterior; greatest height at anterior-most third. Dorsal margin almost straight and gradually sloping downwards to posterior. Anterior margin rounded downward, ventral margin sinuate and slightly concave at middle. Surface covered with unclear blunt reticulation in anterior, posterior and dorsal marginal areas and small fine pits in central area. Posteroventral projection obscure. Vestibulum well developed along anterior and posterior margins. Marginal pore canals ( Fig. 6 ) both un-furcated and poly-furcated in anterior area, and un-furcated in ventral and posterior areas. Four adductor muscle scars somewhat slanting; large and elongated frontal scar in front of adductor scars, 2 small divided rounded mandibula scars in front of lowest adductor scar, fulcral point rounded. Hingement modified entomodont; in right valve composed of prominent, somewhat serrated tooth at both anterior and posterior ends. Median element forming smooth bar in right valve and smooth groove in left valve. Snap structure ( Figs 6 , 8 E, F) existing in middle ventral inside, right valve bearing small knob interlocking with corresponding socket of left valve and almost obscure.

Antennula ( Fig. 9 A). Five articulated podomeres, length ratios from proximal to distal 10:9:3:5:4. First podomere without seta. Second podomere oblong, covered with numerous setulae along anterior distal margin, bearing 1 setulous seta at posterior distal corner, 2 bunches of fine setulae on one third from anterior proximal and anterior distal ends. Third podomere sub-quadrangular covered with numerous setulae along anterior margin and bearing 1 long setulous seta at anterior distal end. Fourth podomere fusing of two podomeres, covered with numerous setulae along anterior margin, bearing 1 stout serrated claw, 1 long and 1 medium-length simple seta on ledge of anterior margin, 2 long simple slender setae and 1 stout serrulated claw at anterior distal end, 1 mediumlength setulous seta on ledge of posterior margin, and 1 long, slender simple at posterior distal corner. Fifth podomere slenderer than others, equipped with 1 long slender, 1 short simple setae, 1 aesthetasc and stout terminal serrated claw at distal end.

Antenna ( Fig. 9 B). Four articulated podomeres, length ratios from proximal to distal 26:13:31:4. First podomere stout with very long, stout two-segmented (but almost fused) exopodite (= spinneret seta) at anterior distal end. Second podomere short broad with bunch of fine seta at anterior proximal end and 1 setulous seta at posterior distal end. Third podomere oblong, covered with numerous setulae along anterior proximal half, 2 long, simple slender setae at middle of anterior margin, 1 medium-length simple, 1 short setulous seta, and aesthetasc at middle of posterior margin, 1 simple and 1 stout serrated seta at one fourth from posterior distal end. Fourth podomere very small, with two strong claws of sub-equal length.

Mandibula ( Fig. 9 C). Five articulated podomeres. Coxa stout and bearing developed teeth and aesthetasc-like setae on ventral edge. Basis sub-trapezoid with 2 simple setae on middle of ventral margin, and exopodite as one large plumose seta on dorsal margin. First podomere of endopodite with 1 medium-length simple seta on middle of distal margin and 2 long stout plumose setae (annulated at distal half) at ventro distal corner (lower ventro-distal one only setulous along ventral margin). Second podomere of endopodite sub-pentagonal, bearing 1 long simple and 2 medium-length, annulated setulous setae on middle of dorsal margin, 1 medium-length, annulated setulous seta at anterior distal end, 1 medium-length, simple seta and 1 long stout plumose seta (annulated at distal half) at anterior ventral corner. Third podomere of endopodite very small sub-quadrangular, equipped with 1 annulated setulous seta and 3 simple setae at distal end.

Maxillula ( Fig. 9 D, E). Thin branchial plate with 14 long feather-like setae and 2 short simple setae. Basal podomere bearing palp and 3 endites. Palp consisting of 2 podomeres, proximal one with 2 simple setae at dorsodistal end, distal podomere with 1 annulated setulous, 1 simple and 1 claw-like seta distally. Dorsal endite (endite 1) bearing 3 setulous setae, middle endite (endite 2) bearing 3 simple setae, ventral endite (endite 3) bearing 1 setulous and 2 simple setae.

Fifth limb ( Fig. 9 F). Four articulated podomeres, length ratios from proximal to distal 4:3:2:2. First podomere adorned with 5 transversal rows of setulae on proximal half, and bearing 1 setulous seta on middle of anterior margin, 2 setulous setae at anterior distal end, and 1 setulous seta on posterior proximal corner. Second podomere covered with numerous setulae along anterior margin and distal end, respectively and possessing 1 setulous seta at anterior distal end. Third podomere covered with numerous setulae along anterior margin and distal end. Fourth podomere covered with numerous setulae along anterior margin and distal end, bearing 1 strong terminal claw with serrations arranged in 1 row along anterior margin near tip.

Sixth limb ( Fig. 9 G). Four articulated podomeres, length ratios from proximal to distal 5:3:2:2. First podomere adorned with 5 transversal rows of setulae on proximal half, and bearing 1 setulous seta on middle of anterior margin, 1 setulous seta antero-distally, and 1 setulous seta at posterior proximal corner. Second podomere covered with numerous setulae along anterior margin and distal end, and possessing 1 setulous seta at anterior distal end. Third podomere covered with numerous setulae along anterior margin and distal end. Fourth podomere covered by short setulae along anterior margin and distal end, and bearing 1 strong terminal claw with serrations arranged in 1 row along anterior margin near tip.

Seventh limb ( Fig. 9 H). Four articulated podomeres, length ratios from proximal to distal 5:4:2:2. First podomere adorned with 5 transversal rows of setulae on proximal half, and bearing 1 setulous seta on middle of anterior margin, 1 setulous seta antero-distally, and 1 setulous seta at posterior proximal corner. Second podomere covered with setulae along anterior margin and distal end, and possessing 1 setulous seta at anterior distal end. Third podomere covered with setulae along anterior margin and distal end. Fourth podomere covered with setulae along anterior margin and distal end, and bearing 1 strong terminal claw with serrations arranged in 1 row along anterior margin near tip.

Male copulatory organ ( Fig. 10 ). General outline of capsule oval. Right and left hemipenes almost symmetrical. Basal capsule with clear costae. Distal lobe transparent, triangular shape with blunt tip, 3 clasping apparatus stout, distal end extending posteriorly; proximal one forming pointed tip and distal one with blunt tip. Copulatory duct very long and complicated, coiling 6–7 times.

Dimension. See Table 1 .

Remarks. The optimal habitat for Callistocythere ryukyuensis sp. nov. is the mid to high tide zone in areas dominated by coarse sand (particle size range from 1.1–1.9 mm) from 13–60 cm below the surface of the sediment. The morphology of its carapace is similar to Callistocythere minor Hanai, 1957 , but this new species can be easily distinguished by certain characteristics, such as the indistinct, blunt reticulation on the carapace and the obscure posteroventral projection, compared to the irregularly undulating carapace ridges and V-shaped posteroventral projection in Callistocythere minor Hanai, 1957 . Moreover, the hinge structure of C. ryukyuensis sp. nov. is identical to that of C. minor Hanai, 1957 , although specimens of this species have only been recovered and described from sandy beaches as empty (or dead) carapaces. The occurrence of this new species in an interstitial environment strongly suggests that C. minor is also interstitial. No brush-shaped organ has been observed in any of the specimens that have been examined. This description is the first to include the soft part morphology of the C. minor group.

Figures

FIGURE 1. Maps showing the location of the study area. A, map of Japan; B, map of the Okinawa Islands; C, sampling site of this study (Sesoko Beach).

FIGURE 6. Callistocythere ryukyuensis sp. nov. Internal view of male carapace. A, left valve; B, right valve (holotype, SUM- CO- 2350). Scale: 50 µm.

FIGURE 7. Callistocythere ryukyuensis sp. nov. (SEM images). A, B, external lateral view of female right and left valves, respectively (paratype, SUM-CO- 2356); C, D, external lateral view of male right and left valves, respectively (holotype, SUM- CO- 2350); E, F, internal lateral view of female left and right valves, respectively (paratype, SUM-CO- 2357); G, complete male carapace in dorsal view (paratype, SUM-CO- 2351); H, complete male carapace in ventral view (paratype, SUM-CO- 2352). Scale 50 µm.

FIGURE 8. Carapace of Callistocythere ryukyuensis sp. nov. (SEM images). A, B. posterior and anterior socket of hingement on left valve, respectively; C, D, posterior and anterior teeth of hingement on right valve, respectively; E, F, snap knob on right valve and snap pit on left valve, respectively; G, scars of male right valve; H, celate sieve type pore; A – F, female (paratype, SUM-CO- 2357); G, male (paratype, SUM-CO- 2353); H, male (holotype, SUM-CO- 2350). Scale: 20 µm for E, F; 10 µm for A – G; 1 µm for H.

FIGURE 9. Callistocythere ryukyuensis sp. nov. A, antennula; B, antenna; C, mandibula; D, maxillula palp with distal parts figured at higher magnification; E, branchial plate of maxillula; F, fifth limb; G, sixth limb; H, seventh limb. A, B, F, G, H, male (holotype, SUM-CO- 2350); C, male (paratype, SUM-CO- 2354); D, male (paratype, SUM-CO- 2355); E, female (paratype, SUM-CO- 2358). Scale: 20 µm for A, B, C, E, F, G, H and 10 µm for D.

FIGURE 10. Male copulatory organ of Callistocythere ryukyuensis sp. nov. (holotype, SUM-CO- 2350). Abbreviations: ca, clasping apparatus; dl, distal lobe; cd, copulatory duct; oc, opening of copulatory duct. Scale 20 µm.