Cytheroid ostracods (Crustacea) from South Korea, with description of a new species

ABSTRACT Living cytheroid ostracod fauna from South Korea is very poorly known, and so far only 12 species have been reported in the taxonomic literature with detail description. We describe one new species, Xestoleberis hujeongensis n. sp., and report three other cytheroid ostracods: X. setouchiensis Okubo, 1979; X. sagamiensis Okubo, 1976; and Hemicytherura kajiyamai Hanai, 1957 from the east coast of Korea. The new species is most closely related to Xestoleberis hanaii Ishizaki, 1968, a widely distributed and ecologically versatile species. The two species have a very similar carapace shape and soft body parts morphology. Nevertheless, the new species has a distinctively different carapace ornament (presence of wart-like structures on the male carapace), as well as hemipenis morphology. The other three species were known so far only from Japan, and the South Korean populations differ from the Japanese ones only by carapace size, while the carapace shape and all soft body parts are very similar to their original descriptions. This is also the first record of a living representative of the genus Hemicytherura Elofson, 1941 from Korea.


INTRODUCTION
The superfamily Cytheroidea has a cosmopolitan distribution, and is the most diverse ostracod group (Horne et al. 2002). The carapace is strongly calcified and therefore fossilizes easily. As a consequence, there are more fossil than Recent species described so far (Tanaka 2009). This is an ecologically versatile group that can be found in both marine and freshwater environments, from littoral to deep-sea regions ). However, this division encompassed only 13 species known from Japan. Nevertheless, the division is congruent with other important morphological characters, such as hingement structure, muscle scar imprints, and hemipenis morphology. According to the world register of marine species -WoRMS, (Brandão et al. 2018) there are 219 Xestoleberis species described so far. By far, the majority are known only as fossils or subfossils and therefore the morphology of their soft parts is unknown. The East Asian region accounts for 18 Recent species, almost all known only from Japan. Although, the majority of species can be considered as endemic, some as X. hanai , for example, are more widely distributed and have been reported from three East Asian countries: Russia (Schornikov 1974), Korea (Lee et al. 2000), and Japan (Sato & Kamiya 2007). No living species are known from China today. Here we describe one new species from Korea and briefly report X. setouchiensis Okubo, 1979 and X. sagamiensis Kajiyama, 1913, species previously known from Japan only (Okubo 1979;Kajiyama 1913 Hemicytherira kajiyamai Hanai, 1957 was so far known only from its type locality, i.e., the shore of the Imperial villa, Hayamamachi, Kanagawa Prefecture (Fig. 1).
The aim of this paper is to contribute to the better understanding of the cytheroid fauna of the Korean Peninsula, and also to add new data about the morphology of the soft parts of this diverse group of ostracods, since overwhelming majority of publications dealing with cytheroids rely on the carapace morphology only.

ColleCtions
Material was collected from intertidal-zone at Hujeong Beach: 37°04'12.3"N, 129°25'01.0"E, 0.1 m depth (Salinity: 32.7 part per thousand, Sediment temperature: 23.6°) (Fig. 1), which is situated in the southeast part of the South Korean east coast about 8 kilometers from Uljin. Samples were collected using rinsing method suggested by Giere (2009): After filling the bucket with algae, seawater was filtered through 38 μm mesh, poured over the algae and stirred by hand. After thoroughly stirring, the algae or sediment will settle down, and the seawater from the bucket was filtered through 63 μm hand-net and placed in a collection bottle. The process was repeated 5-6 times. All samples were fixed in 99% ethanol on site.
Drawing, seM anD analysis Sorting was done under a stereomicroscope (Olympus SZX12) in the laboratory of Hanyang University. Specimens were dissected on slides in CMC-10 mounting media (Masters Company, Inc., United States). All drawings were prepared using a drawing tube attached to a microscope Olympus BX51. Carapace and hemipenis measurement were performed according to the method presented on Figures 2 and 3    Antennule (Fig. 6F) Six-segmented. First segment without setae. Second segment with one bare seta situated postero-medially reaching one-third of fourth segment. Third and fourth segments each with one bare seta antero-distally; seta on third segment reaching end of fourth segment and seta on fourth segment reaching half of terminal segment. Fifth segment with two setae antero-distally: one seta more than 2 times longer than terminal segment; other seta 1.5 times longer than same segment. Same segment also with one posterodistal seta 2 times longer than terminal segment. Sixth segment with two short setae distally, one of which 2 times longer than terminal segment and other 2.5 times longer than same segment. Length ratios between six segments: 3: 3.2: 2.5: 2.8: 2.8: 1.
Antenna (Fig. 6H) Five-segmented. Exopod transformed into three-segmented spinneret seta. First endopodal segment without setae. Second segment with one bare and one plumose seta situated postero-distally; bare seta reaching half of third segment and plumose seta reaching two-third of the same segment. Third segment with two plumose setae postero-distally reaching half of fourth segment, and several setulae antero-proximally. Fourth segment with one bare seta situated medially reaching half L of the fourth segment; one plumose seta posteromedially not reaching distal margin of the fourth segment, and one short plumose seta antero-distally. Terminal segment with two strong, serrated claws and one short seta situated between claws. Appendage with several stiff seatulae groups along posterior margin of the first endopodal segment and on the anterior margin of the second endopodal segment. Length ratios between four segments: 1.9: 1.2: 1: 2.1.

Mandibule (Fig. 6G)
Coxa with one row of setula on distal margin, and with one seta. Palp 3 segmented. First segment with one bare seta antero-medially, and three bare setae on antero distal margin. Second segment with four setae postero-distally, and two setae antero-distally, all bare. Terminal segment with three setae on distal margin. Setae on all segments very strong, almost claw-like. Length ratio between three segments: 5.5: 1.5: 1.

Maxillule (Fig. 7D)
Palp present and 2-segmented. First segment with four bare setae on distal margin. Second segment with two bare setae on distal margin. Masticatory organ with three endites. First endite with four bare setae on distal margin. Second endite with four bare setae on distal margin. Third endite with two bare setae on distal margin. Exopodite with 12 plumose and two bare setae ("aberrant" setae) in vibratory plate.
Leg 5 (Fig. 7A) Four-segmented. First segment with one plumose seta anteromedially reaching distal end of the same segment, two plumose setae on antero-distal margin reaching half L of the second segment, one plumose and one bare seta on posterior margin not reaching end of the first segment. Second segment with one plumose seta antero-distally not reaching end of the third segment; row of setulae present medially along anterior to distal margin. Third segment with one row of setulae medially along anterior to distal margin. Terminal segment with setulae medially along anterior to distal margin, and most distally with one claw. Length ratio between four segments: 2.7: 1.4: 1: 1.2. Leg 5 (Fig. 8A) Four-segmented. First segment with one plumose seta antero-proximally reaching end of the first segment, two plumose setae antero-distally reaching half of the second segment; one plumose and one bare seta on posterior margin not reaching end of the first segment. Second segment with one plumose seta antero-distally not reaching end of the third segment; setulae present medially along anterior margin. Third segment with setulae situated medially along anterior and distal margins.
Leg 6 (Fig. 8B) Four-segmented. First segment with two plumose setae antero-medially reaching slightly beyond the first segment, one plumose seta antero-distally reaching one-third of the second segment; one bare and one plumose seta posteroproximally reaching end of the first segment. Second segment with one plumose seta antero-distally reaching half of terminal segment, setulae present along antero-medial and distal margins. Third segment with setulae along anterior margin. Terminal segment with setulae along anterior margin, one strong claw on distal margin. Length ratios between four segments: 2.3: 1.7: 1: 1.3.
Leg 7 (Fig. 8C) Four-segmented. First segment with one plumose seta anterodistally reaching 1/6 of the second segment, one bare seta postero-distally also reaching 1/6 of the same segment. Second segment with one plumose seta antero-distally reaching one-third of terminal segment; setulae present along anteromedial and distal margins. Third segment with setulae on antero-medial and distal margins. Terminal segment with claw on distal margin, setulae present on antero-medial and distal margins. Length ratios between four segments: 1.9: 2.1: 1: 1.3.
Antennule (Fig. 10E) Six-segmented. First segment with setulae on postero-proximal margin. Second segment with setulae antero-medially and antero-distally, one bare seta postero-distally not reaching end of third segment. Third segment with one plumose seta antero-distally not reaching end of fourth segment. Fourth segment with two bare setae antero-distally, twice longer than fifth segment, and one bare seta on distal margin not reaching end of fifth segment. Fifth segment with three bare setae on distal margin, twice longer than fifth segment. Terminal segment with two setae on distal margin, twice longer than same segment. Length ratios between six segments: 4.8: 4.1: 1.7: 1.9: 2.1: 1.
Antenna (Fig. 10C) Four-segmented. Exopod transformed into two-segmented spinneret seta. First endopodal segment with setulae posteromedially. Second segment with setulae present on anterior margin and with one plumose seta posterior-distally reaching half of the third segment. Third segment with two bare setae antero-medially not reaching end of the same segment, two plumose setae postero-medially not reaching end of the same segment, and one strong plumose seta postero-distally; setulae present on antero-proximal margin and postero-distal margin.
Fourth segment with two claw-like setae on distal margin, setulae present on distal margin. Length ratios between four segments: 13.3: 3: 10.6: 1 Mandibule (Fig. 10F, F', H) Coxa with five teeth on distal margin, one bare seta posterodistally, and two bare setae antero-distally. Palp 4-segmented. First segment bearing exopodite composed of two long setae, with one plumose seta antero-medially reaching end of the second segment, and one plumose seta on central margin reaching end of terminal segment. Second segment with two plumose setae medio-distally exceeding distal margin of the third segment; one long plumose seta antero-distally, and one bare seta postero-distally. Third segment with two sub-equally long plumose setae antero-distally, accompanied by one half of long bare seta; same segment with one claw-like seta mediodistally. Terminal segment with three claw-like setae, all serrated.
Maxillule (Fig. 10G) Palp present, 2-segmented. First segment with two setae, second segment with two claws, one fused with segment. First and second endites with three to four teeth-like setae respectively. Vibratory plate with 16 setae.
Leg 5 (Fig. 11E) Four-segmented. First segment with one plumose seta postero-proximally not reaching end of the same segment, two plumose setae antero-distally reaching half of the second segment, and two plumose setae antero-medially: one plumose seta reaching end of the first segment, another not reaching end of the same segment. Second segment with setulae along anterior margin and one plumose seta antero-distally reaching 1/3 of the terminal segment. Third segment with setulae on distal margin. Terminal segment with setulae along anterior and distal margins, one claw on distal margin. Length ratios between four segments: 3.4: 2.1: 1: 1.2. Leg 6 (Fig. 11F) Four-segmented. First segment with one plumose seta posteroproximally not reaching end of first segment; two plumose setae antero-medially not reaching end of the same segment; one plumose seta antero-distally reaching half of the second segment. Second segment with one plumose seta antero-distally not reaching end of the third segment, setulae present along anterior margin. Third segment with setulae along anterior margin. Terminal segment with one claw on distal margin and setulae present on distal margin. Length ratios between four segments: 2.9: 2.2: 1: 1.5.
Leg 7 (Fig. 11G) Four-segmented. First segment with one plumose seta postero-proximally not reaching end of the first segment; one plumose seta antero-medially reaching end of the same seg-ment and one plumose seta antero-distally reaching 2/3 of the second segment. Second segment with one plumose seta antero-distally reaching end of the third segment and setulae present along anterior margin. Third segment with setulae present on distal margin. Terminal segment with one claw on distal margin and setulae present along anterior margin and distal margin. Length ratios between four segments: 2.6: 2.5: 1: 1.6.
Hemipenis (Fig. 12A, B) Conspicuous cuticle present along the rim. Hemipenis with strongly asymmetrical lobes. Lobes foot-like, but more robust on right than on left Hp. Ejaculatory duct terminated on distal lobe. Three bare setae present near the upper central margin and one small seta called caudal process on between left and right (Fig. 12B). Antennule (Fig. 11D) Same as in male, with exception that all setae are thicker than in male.

Antenna, Mandibule
Same as in male.
Maxillule (Fig. 11C) Palp present. 2-segmented. First segment with three bare setae antero-distally. Second segment with two plumose setae distal margin. Masticatory process with three endites. First to third endites with four bare setae on distal margin. Vibratory plate same as in male.
Genital field (Fig. 11H) Bean-shaped ovary. Genital lobes with total of five plumose setae situated on distal margin. ( Fig. 13D). Ventral margin straight. Ornament smooth with few sieve type setae. Scar print consisting of four prints in vertical row and one situated in front of vertical row (Fig. 13C).

Xestoleberis sagamiensis
Hemipenis (Fig. 14 Hemipenis (Fig. 16) Same as in original description (Okubo 1979  , one of the most widely distributed recent species of the genus. The shape of the carapace is almost identical between the two species, and the differences are very subtle. The major difference is presence of the wart-like structures on the anterior end of the male carapace in the new species (Fig. 9A), the shape of the distal lobe and the number of setae at the central part of Hp. Left lobe in the new species has a less pointed distal lobe while the right lobe has longer most distal extension and the entire distal lobe is more rectangular than in X. hanaii. The new species also has three bare setae at the central part (Fig. 12B) but X. hanaii has four (Schornikov 1974: fig. 26). Other soft parts are very similar between the two species, with the exception of the expodite of the Md. Namely there are two long bare setae (Fig. 10F) in the new species, and in X. hanaii three (Schornikov 1974: fig. 26). Hemicytherura kajiyamai, X. sagamiensis and X. setouchiensis collected from South Korea are very similar to the Japanese populations. Korean and Japanese populations of Hemicytherura kajiyamai, X. sagamiensis are of a similar size, but Korean individuals of X. setouchiensis are slightly smaller than the Japanese ones (see Table 1).