The material provided here was collected as a part of an ongoing project that collaborates with the Marine Global Earth Observatory (MarineGEO) program and in partnership with the Korea Institute of Ocean Science and Technology (KIOST) on Jeju Island, Republic of Korea (ROK). The MarineGEO is under the umbrella of the Smithsonian’s global Tennenbaum Marine Observatories Network (TMON) with its headquarters in Washington, D.C., USA (https://marinegeo.si.edu/). It is the first long-term research program with partners from all over the world, applying a standardized method of sampling (ARMS, Fig. 1) to study coastal marine biodiversity. Its main aim is to understand the role of biodiversity in sustaining resilient marine ecosystems. As such, a thorough taxonomic knowledge of the current species is indispensable.

Figure 1. 

Autonomous Reef Monitoring Structure (ARMS). A. Assembled ARMS; B. ARMS after one year of deployment at Seongsan (representative photo); C–E. ARMS installation by SCUBA (representative photos).

Specimens were collected either from an Autonomous Reef Monitoring Structure (ARMS) (Fig. 1A, B, E), or from the sediment scoop samples (i.e., surface sediments) near the ARMS in August, 2019. The ARMS are standardized, three-dimensional samplers of marine biodiversity that are non-destructive to the environment. They are shelf-like units made of stacks of plates with different sized spacers in between the plates. Plate by plate, the units are assembled under water and left there, passively collecting the sample (i.e., surface and sedimentation-derived sediments) (Fig. 1C–E). For this study, ARMS were deployed by scuba at 19 m at Seongsan, Jeju Island, South Korea (Fig. 2) on the 3^rd of October, 2018 and left under the water until retrieval on August 20^th, 2019. On board the boat, the ARMS unit was immediately transferred to a recovery bin and submerged in the filtered seawater. Upon arrival on shore, the unit was disassembled inside the recovery bin and anything on and in between the plates was fixed with 99% ethanol. Living and dead ostracods were wet-picked in 99% ethanol from the 100–500 µm size fractions of the material collected by the ARMS unit and the scoop sample, respectively. Five of the 7 specimens presented here were recovered from the ARMS sample. Two specimens were recovered from a sediment scoop sample. The scoop sample was collected by SCUBA from the sea floor next to the ARMS structure during retrieval. A 50 mL conical tube was used to scoop up the sediment. Picking and sorting were done using a stereomicroscope (Olympus SZX12) in the Laboratory of Animal Systematics and Phylogeny at Hanyang University, ROK. Each disarticulated valve is counted as one specimen. One articulated male adult carapace was found and opened for the scanning electron microscope (SEM) imaging (LV: # 109_1 (lost during imaging); RV: 109_2). The SEM images were taken at Eulji University, ROK, with a Hitachi S-4700 scanning electron microscope after platinum coating.

Figure 2. 

Map of sampling location. A. Overview of Korea including adjacent countries and seas; B. Overview of Jeju Island with sampling location in yellow. Yellow star denotes the deployment site of the ARMS; red location pin icon points at the location of the Korea Institute of Ocean Science and Technology (KIOST) on Jeju Island; grey shades denote land masses, blue denotes seas. Coordinates given in decimal degrees.

The type material of Semicytherura kiosti sp. nov. (holotype: one male ARV, # 109_2; paratypes: five RVs (# 68 female A, # 108 male A, # 177 female A-1, # 239 female A-2, # 240 female A) and our specimen of S. kazahana, are deposited in the National Institute of Biological Resources (NIBR) in Incheon, ROK (deposition number S. kiosti: DSEVIV0000003716; S. kazahana: DSEVIV0000003720). The new species is registered with ZooBank (LSID. urn:lsid:zoobank.org:act:DAA1DAD2-32A5-4F02-8143-D0D158F8568B).

Sampling location and geographic distribution of selected species were mapped with the software QGIS (version 3.16.8 Hannover; 1989, 1991, Free Software Foundation, Inc.). Continent shapefiles were acquired through open sources at https://www.igismap.com, country specific shapefiles through https://gadm.org/download_country_v3.html. Map depicting trans-Arctic interchange through time were generated with MapCreator (version 2.0; personal edition) and edited with vector graphics software Inkscape (0.92.1 version 3; 2007, Free Software Foundation, Inc.). Length/height plot was generated with SigmaPlot (version 10.0; 2006, Systat Software, Inc.) and edited with Inkscape 0.92.

ALV Adult left valve;

ARV Adult right valve;

A-1 A-1 instar (last juvenile molt before adult);

A-2 A-2 instar (second-last molt before adult).

Specimens are deposited at the National Institute of Biological Studies (NIBR, Incheon, South Korea) under the deposition numbers DSEVIV0000003716 (S. kiosti sp. nov.) and DSEVIV0000003720 (S. kazahana). This published work and the nomenclatural acts it contains, have been registered in ZooBank (LSID. urn:lsid:zoobank.org:act:DAA1DAD2-32A5-4F02-8143-D0D158F8568B).

The two valves pictured clearly show the anterior longitudinal ridge extending past the anterior third and connecting with the dorsal margin at the posterodorsal corner, which is characteristic for specimens of S. balrogi (Fig. 4P). True, S. henryhowei does not have such a long, smooth anterior longitudinal ridge (Fig. 4F). Additionally, S. henryhowei has a thin ridge branching off the dorsal margin at two-thirds of the valve length, in front of the posterodorsal corner, connecting with the anterior subvertical ridge within the dorsal half (Fig. 4F). The imaged left valve (fig. 1; Irizuki, 1994) shows no posterior subvertical ridge, whereas the right valve shows an inconspicuous, short posterior subvertical ridge, not merging with the ventral ridge. Both, S. henryhowei and S. balrogi lack the posterior subvertical ridge.

Figure 3. 

Schematic drawing for terminology of major ridges (carinae) of the Semicytherura henryhowei group. Red: anterior ridge; orange: dorsal ridge; yellow: posterior ridge; green: ventral ridge; pink: posterior subvertical ridge; blue: anterior sublongitudinal ridge; turquoise: posterior subvertical ridge; Green triangles: anteroventral marginal denticles (crenulations); white dots: corners; AD: Anterodorsal; PD: posterodorsal; P: posterior; PV: posteroventral; AV: anteroventral; ADF: anterodorsal fossa; AVF: anteroventral fossa; PF: posterior fossa.

Figure 4. 

Illustrated guide of major ridge systems of the Semicytherura henryhowei species complex. A. Semicytherura kiosti sp. nov. Jöst and Karanovic male ARV; based on # 109_2 herein; B. Semicytherura kiosti sp. nov. Jöst and Karanovic male ALV; based on # 109_1 herein; C. Semicytherura robustundata Ozawa & Kamiya, 2008 ARV; based on Ozawa and Kamiya 2008; D. Semicytherura slipperi Yamada, Tsukagoshi & Ikeya, 2005 ALV; based on Yamada et al. 2005; E. Semicytherura subslipperi Ozawa & Kamiya, 2008 ARV; based on Ozawa and Kamiya 2008; F. Semicytherura henryhowei Hanai, 1977 ALV; based on Yamada et al. 2005; G. Semicytherura leptosubundata Ozawa & Kamiya ARV; based on Ozawa and Kamiya 2008; H. Semicytherura kazahana Yamada, Tsukagoshi & Ikeya, 2005 ALV; based on Yamada et al. 2005; I. Semicytherura subundata (Hanai, 1957) ARV; based on Ozawa and Kamiya 2008; J. Semicytherura sasameyuki Yamada, Tsukagoshi & Ikeya, 2005 ALV; based on Yamada et al. 2005; K. Semicytherura tanimurai Ozawa & Kamiya, 2008 ALV; based on Ozawa and Kamiya 2008; L. Semicytherura undata (Sars, 1865) ARV; based on Cronin and Ikeya 1987; M. Semicytherura maxima Yamada & Tsukagoshi, 2010 ARV; based on Yamada and Tsukagoshi 2010; N. Semicytherura ikeyai Yamada & Tsukagoshi, 2010 ARV; based on Yamada and Tsukagoshi 2010; O. Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. 4) ALV; based on Irizuki 1994; P. Semicytherura balrogi Brouwers, 1994 ARV; based on Brouwers 1994; Q. Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. 5) ALV; based on Irizuki, 1994; R. Semicytherura sp. nov. sensu Cronin and Ikeya 1987 (S. sp. A) ARV; based on Cronin and Ikeya 1987; S. Semicytherura sp. nov. sensu Cronin 1989 (S. sp. cf. henryhowei) ALV; based on Cronin 1989; T. Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. 2) ARV; based on Irizuki 1994; U. Semicytherura simplex (Brady & Norman, 1889) ALV; based on Hu 1978; V. Semicytherura sp. nov. sensu Ozawa et al. 2008 (S. sp. 1) ARV; based on Ozawa et al. 2008; W. Semicytherura pseudoundata Irizuki & Yamada, 2004 ALV; based on Irizuki et al. 2004; X. Semicytherura sp. nov. sensu Ozawa et al. 2008 (S. sp. 2) ARV; based on Ozawa et al. 2008; Y. Semicytherura neosubundata (Ishizaki, 1966) ALV; based on Ishizaki 1966; Z. Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. aff. henryhowei) ARV; based on Irizuki 1994; AA. Semicytherura kaburagawensis Tanaka, 2013 RV; based on Tanaka and Hasegawa 2013; AB. Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. 1) ALV; based on Irizuki 1994; AC. Semicytherura usuigawensis Tanaka, 2013 RV; based on Tanaka and Hasegawa 2013; AD. Semicytherura quadraplana Allison & Holden, 1971 ALV; based on the original drawing in Allison and Holden 1971. Red arrows indicate whether anterior sublongitudinal ridge is horizontal or sloped; red numbers indicate the anteroventral marginal denticles.

The pictured left valve shows a conspicuous, long, arching anterior longitudinal ridge, running from the mid-height at the anterior margin to the posterodorsal corner without prominent ridges connecting it with the ventral ridge, which is typical of S. balrogi (Fig. 4P).

The pictured right valve has four anteroventral marginal denticles, a long anterior longitudinal ridge, and a ventral ridge that starts off anteriorly at the height of the third anteroventral marginal denticle, slightly arching toward the ventral margin, then both merging within the posterior third of the valve, before ascending toward the posterior ridge. All these characteristics are distinct patterns found in S. balrogi (Fig. 4P).

The pictured left valve is characterized by a continuous anterior longitudinal ridge that connects with the dorsal margin at the posterior half, in front of the posterodorsal corner, which is typical for S. ikeyai (Fig. 4N).

The pictured right and left valves show an anterior longitudinal ridge ascending in a nearly straight manner from the anterior margin at mid-height of the valve to the end of the anterior third above mid-height, where the anterior subvertical ridge crosses in the posteroventral direction and a thin ridge splits from the anterior half of the anterior subvertical ridge, and merging, as dorsal continuation of the anterior longitudinal ridge, with the dorsal margin in front of the posterodorsal corner. The ventral ridge is comparably thin. These are typical morphological traits of S. ikeyai (Fig. 4N). In case of the specimens of Whatley and Boomer (1995), the ridge connecting the anterior subvertical ridge with the dorsal margin is rather inconspicuous.

The pictured right valve has a short anterior longitudinal ridge, which, with the anterior subvertical ridge, splits the anterior half of the valve into two fossae. The posterior ridge connecting the ventral ridge with the dorsal ridge is distinct. The ventral half of the anterior subvertical ridge is wider than the dorsal half. There appears to be five marginal anteroventral denticles, although the anterior edge of the specimen is not clear. All these characteristics resemble S. kazahana (Fig. 4H).

The imaged right and left valves have the ventral ridge typical for S. undata (Fig. 4L). It starts from, or close to, the anteroventral margin and runs in a slightly convex curve above the ventral margin until the anterior third to half of the valve length. From there, it descends in a straight, diagonal line toward the ventral margin until merging with it and terminating at or behind the posterior third of the valve length. The subvertical dorsal ridge, however, is lacking in some specimens [pl. 3, figs 9–11 (Irizuki 1994)], which is uncharacteristic for S. undata. One pictured left valve [pl. 3, fig. 8 (Irizuki 1994)] has a short subvertical dorsal ridge, as typical for specimens of S. undata (Fig. 4L). The dorsal ridge of S. undata is characterized by a prominent, very short, “comma-shaped” ridge in the posteroventral area, which all specimens here show, although to a varying extent (Irizuki 1994).

The pictured left valves (pg. 383, text-figs 2C, E) are characterized by a short anterior longitudinal ridge, steeply running upward in a straight fashion, thickening toward its posterior end at the anterior third of the valve-length (Fig. 4D). The posterior subvertical ridge is lacking. The ventral part of the anterior subvertical ridge is not a smooth continuation of the slightly curved, prominent dorsal part, but connecting to the ventral ridge in a rather inconspicuous zig-zag manner. Four anteroventral marginal denticles are present (Yamada et al. 2004).

The pictured left valve (pg. 383, text-fig. 2A) shows the typical ridge pattern of S. kazahana [pg. 247, fig. 2C, pg. 253, fig. 6; (Yamada et al. 2005)] and S. sasameyuki [pg. 247, fig. 2D; pg. 255, fig. 8; (Yamada et al. 2005)]. The anterior subvertical ridge smoothly connects to the anterior longitudinal ridge with its dorsal part, and to the ventral ridge with its ventral part. The posterior subvertical ridge is smooth, tapering toward ventral and connecting with the ventral ridge. The four comparably large, acuminate, anterior marginal denticles can indicate the specimen’s association to both, S. sasameyuki (Fig. 4J), which always shows four marginal denticles, as well as to S. kazahana (Fig. 4H), which may either show four or five marginal denticles, or none at all (see pg. 243, fig. 6A–D, four denticles; E, zero denticles; F, five denticles; Yamada et al. 2005). For a definite distinction between the two species, soft-body analysis is necessary.

The pictured adult left and juvenile right valves (pg. 93, pl. 11 figs 7, 8) show the typical ridge pattern of S. kazahana and S. sasameyuki (Yamada et al. 2005). However, the adult valve shows the presence of four anteroventral marginal denticles (pg. 93, pl. 11, fig. 7), whereas they are absent in the juvenile valve (pg. 93, pl. 11, fig. 8 in Yasuhara and Irizuki 2001). This variation in number of anteroventral denticles indicates that the specimens of Yasuhara and Irizuki (2001) belong to S. kazahana (Fig. 4H), unless they present juvenile and adult of separate species. However, although the geographical distribution of S. kazahana and S. sasameyuki overlaps, their microhabitats differ with the former being a phytal species typical of rocky shore environments, and the latter living on the silty sands of inner bays (Yamada et al. 2005). This speaks for the presence of just one of the two species, here S. kazahana, due to the varying number of anteroventral marginal denticles.

The pictured right valve (pg. 1076, pl. 1, fig. 12) shows a prominent posterior subvertical ridge, which is absent in S. henryhowei (Fig. 4F). Additionally, S. henryhowei has small, rounded, anterior marginal denticles, whereas this specimen shows four distinct and pointy anterior denticles. The ridge pattern, and number and shape of anteroventral denticles, marks this specimen as, either S. sasameyuki or S. kazahana. Whereas S. sasameyuki, so far, has only been reported from extant sediments of Japan, S. kazahana has a Pleistocene record from Korea (Lee 1990, unpublished), an extant Japanese distribution (Yamada et al. 2005; living specimens collected 1977–2000), as well as an extant Korean distribution (this study; valves collected 2019). Our species re-assignment indicates a Japanese Miocene existence of either of the species.

The pictured right valve exhibits a prominent anterior subvertical ridge with a posterior branch at its dorsal half (pg. 122, pl. 1, fig. 16; Ozawa and Kamiya 2008), typical of S. tanimurai (Fig. 4K). Although this upward-curved branch is sharper, more like a V-shape in S. tanimurai (Fig. 5A), whereas this specimen shows a rounder U-shape (Fig. 5B). Additionally, the ventral ridge is slightly different at its posterior half, as it continues to the posterior tip of the ventral margin (anteroventral corner) (Fig. 5B), whereas in S. tanimurai it discontinues after merging with the ventral margin ahead of the anteroventral corner (Fig. 5A). Hence, our species re-assignment to S. tanimurai is tentative.

The pictured right valve (pg. 138, fig. 17A) shows all distinct carapace characteristics of S. kiosti sp. nov.: three small anteroventral marginal denticles, a prominent posterior subvertical ridge starting at the posterodorsal corner and merging with the ventral ridge at around mid-length, a posterior ridge connecting posterodorsal and posteroventral corners in a straight line, and a horizontal, straight, and short sublongitudinal anterior ridge (Fig. 4B, AA).

The following three species were not included in the list of members of the S. henryhowei complex by Yamada and Tsukagoshi (2010), however, based on their prominent ridge systems, they also belong to this species group (Fig. 4AA, AC, AD).

The species is characterized by a broad, sigmoid ventral ridge that starts as a thin ridge at anteroventral margin, running parallel to ventral margin toward posterior, growing broader and forming a sigmoid curve at posterior third of carapace length at merging point with subvertical dorsal ridge (pg. 144, fig. 4.2 in Tanaka and Hasegawa 2013). Subvertical dorsal ridge prominent, starting from posterodorsal corner in a bifurced manner (Fig. 4AA). Posterior ridge branch thin, merging with ventral ridge at posteroventral corner; subvertical dorsal ridge branch thick, merging with ventral ridge at posterior third of carapace length. Posterior ridge, subvertical dorsal ridge, and ventral ridge form a rectangular posterodorsal fossa. Anterior sublongitudinal ridge short, mildly arched, slightly ascending, terminating at anterior third of carapace length. Posterior half of subvertical dorsal ridge running from anterodorsal corner (eye tubercle according to type description) ventrally, in a straight, horizontal line, merging with dorsal end of anterior sublongitudinal ridge. Anterior half of subvertical dorsal ridge not observed.

The species is characterized by a long, sigmoid subvertical dorsal ridge, running from posterodorsal corner ventrally through anterior sublongitudinal ridge, and merging with ventral ridge at posterior third of carapace length (pg. 144, fig. 4.3 in Tanaka and Hasegawa 2013). Posterior subvertical ridge very short. Posterior ridge broad, terminalizing at mid-height. Elliptical anterodorsal fossa (Fig. 4AC).

The species is characterized by four anteroventral marginal denticles, a downward arching anterior sublongitudinal ridge that fuses with the ventral half of its anterior subvertical ridge, which, in turn, connects to the ventral ridge within the anterior half of the carapace length (pg. 194, fig. 20b in Allison and Holden 1971). The dorsal half of the anterior subvertical ridge is very short, the posterior subvertical ridge bifurcate, forming an elliptical posterodorsal fossa (Fig. 4AD).

The following nine species did not fit the ridge pattern of any of the known species of the group, hence are undescribed species of the henryhowei group:

Semicytherura sp. 1 is too poorly preserved to make any definite statements. However, it appears to exhibit a long anterior longitudinal ridge, arching over the entire length of the valve exterior and terminating when merging with the dorsal ridge below the caudal process (Fig. 4AB). This is a unique feature, not observed in any of the other species of the S. henryhowei complex. Semicytherura sp. 2 has a conspicuous anterior subvertical ridge shaped like a large “3” (Fig. 4T). Additionally, it sports two, horizontal dorsal ridges, the one closer to the dorsal margin is curved, and the one below is straight and forking at its dorsal end. Semicytherura sp. 4 (Fig. 4O) sports a long anterior longitudinal ridge, as do S. balrogi, S. robustundata, and S. ikeyai among the described species of the S. henryhowei group. Unlike the three described species, however, its dorsal branch at the anterior third of the anterior longitudinal ridge is short and does not merge with the dorsal margin. Additionally, its anterior subvertical ridge that connects the anterior longitudinal ridge with the ventral ridge is not straight as in S. balrogi (Fig. 4P) and S. ikeyai (Fig. 4N), but more of a zig-zag line, similar to S. robustundata (Fig. 4C). However, whereas in S. robustundata this connection is described as a “U-shape” and a “T-shape” (upside-down T) (Fig. 6B), here, it is a hook-shape and upside-down T or Y (Fig. 6A). Semicytherura sp. 5 (Fig. 4Q) is characterized by a long anterior longitudinal ridge ascending from mid-height at the anterior margin to the posterodorsal corner, parallel to the likewise ascending ventral ridge, which is situated above the ventral margin (Fig. 4Q). This main pattern of two long, parallel ridges running across the valve is very conspicuous and characteristic of this undescribed species.

The pictured left valve has a very prominent, thick posterior subvertical ridge, tapering toward its ventral tip (Fig. 4R). The ventral ridge is straight, horizontal, and tapering toward posterior. The anterior longitudinal ridge starts below the mid-height of the valve at the ventral margin, and ascends as a curve toward the anterior third of the valve length, where it connects with the anterior subvertical ridge above the mid-height of the valve. Behind the anterior subvertical ridge, it continues in a thin, zig-zag fashion, merging with the dorsal margin/ridge at the mid-length of the valve. The caudal process is broadly triangular, inconspicuous, above the mid-height. The carapace surface is covered in fine pits and thin reticulation.

The pictured right and left valves are characterized by the absence of the anterior and posterior subvertical ridges (Fig. 4T). The valves are less ornamented by thick ridges, but rather show near-circular inflations in their posterior and anterior halves. The anterior longitudinal ridge starts from the anterior margin at the mid-height of the valve, and ascends toward the end of the anterior third of the valve length, on top of the anterior inflation. A deep, median sulcus separates the anterior longitudinal ridge from its continuation, which merges with the dorsal ridge slightly in front of the posterodorsal corner. The carapace is ornamented by fine pits and thin reticulation.

The pictured right and left valves are characterized by comparably thick ridges (Fig. 4V). The ventral ridge is distinct, rising from the anterior to the end of the anterior third of the valve length, where the anterior subvertical ridge connects with it at its peak. It then drops in a straight line to the ventral margin, from where it rises again in a curved fashion to the dorsal corner. The anterior sublongitudinal ridge is long, thick, but its posterior end is either thinner and lower where it connects with the posterodorsal corner (pg. 167, pl. 2, fig. 14; Ozawa et al. 2008), or it terminates as a thick ridge without connecting to the posterodorsal corner (pg. 167, pl. 2, fig. 15; Ozawa et al. 2008). Both valves have four anteroventral marginal denticles.

The pictured left valve lacks a prominent posterior subvertical ridge, and the anterior sublongitudinal ridge continues after the anterior subvertical ridge as a short, upward-curved extension (Fig. 4X). The carapace is ornamented by fine pits and reticulation.

Figure 5. 

Shape details of posterodorsal branch of anterior subvertical ridge in Semicytherura sp. 2 sensu Yamada et al. 2002 proposed S. tanimurai Ozawa & Kamiya, 2008 and S. tanimurai original type. A. Semicytherura tanimurai Ozawa & Kamiya, 2008. B. Semicytherura sp. 2 sensu Yamada et al. 2002 proposed S. tanimurai Ozawa & Kamiya, 2008. Turquoise highlights the specific shape caused by the different angles, the dorsal branch forks off the anterior subvertical ridge in both specimens.

The pictured left valve shows an anterior sublongitudinal ridge that continues as a thinner ridge after the crossing of the anterior subvertical ridge. The posterior sublongitudinal ridge is present, but inconspicuous (Fig. 4Z). True, S. henryhowei has a short anterior sublongitudinal ridge, terminating when crossing with the anterior subvertical ridge at the end of the anterior third of the valve length, and the posterior subvertical ridge is absent (Fig. 4F). Additionally, S. henryhowei has either four or five anteroventral marginal denticles, whereas this species has three (Fig. 4Z).

Description of the group’s characteristics

Superfamily Cytheracea Baird, 1850

Family Cytheruridae Müller, 1894

Subfamily Cytherurinae Müller, 1894

Genus Semicytherura Wagner, 1957

The Semicytherura henryhowei Hanai & Ikeya, 1997 species group

Revised diagnosis of the species group. External view. Thick carapace, sub-rectangular to sub-trapezoid in lateral view (right valves more sub-trapezoid, left valves more sub-rectangular). External ornamentation of prominent ridges including anterior ridge from anteroventral to anterodorsal corner (Fig. 3, red), anterior longitudinal ridge (short or long, horizontal or arched; Fig. 3, blue), ventral ridge from anteroventral to posteroventral or posterior corner, depending on species-specific length of ridge (Fig. 3, green), posterior ridge from posterior to posterodorsal corner (Fig. 3, yellow), dorsal ridge from posterodorsal to anterodorsal corner (Fig. 3, orange), anterior subvertical ridge from anterodorsal corner to ventral ridge (depending on species-specific length of ridge) at anterior half of valve (Fig. 3, turquoise), and posterior subvertical ridge from posterodorsal corner to ventral ridge (depending on species-specific length of ridge) at posterior half of valve (Fig. 3, purple). Ridges vary in length and degree of conspicuousness. Not all ridges are present in all species. In species where anterior subvertical ridge is connected with dorsal and/or ventral ridge(s), anterior fossa(e) form (Fig. 3). Anterior fossa(e) elliptic/rectangular (S. subslipperi, Fig. 4E, ventral only; S. usuigawensis, Fig. 4AC, dorsal only; S. kazahana, Fig. 4H; S. sasameyuki, Fig. 4J; S. simplex, Fig. 4U) to triangular/trapezoid (S. kiosti sp. nov., Fig. 4A, B, dorsal only; S. henryhowei, Fig. 4D) in shape. Likewise, prominent posterior ridge, as well as distinct posterior subvertical ridge connecting with ventral ridge, form a posterior fossa (e.g., S. kiosti sp. nov., Fig. 4A, B; S. kazahana, Fig. 4H; S. sasameyuki, Fig. 4J; S. kaburagawensis, Fig. 4AA; S. quadraplana, Fig. 4AD). Anterior margin with zero up to five denticles ventrally (Fig. 3, green triangles). Horizontal caudal process with varying degree of conspicuousness; generally, at or above mid-height.

Figure 6. 

Connection details between anterior sublongitudinal ridge and ventral ridge in Semicytherura robustundata Ozawa & Kamiya, 2008 and Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. 4). A. Semicytherura sp. nov. sensu Irizuki 1994 (S. sp. 4); B. Semicytherura robustundata Ozawa & Kamiya, 2008. Blue denotes the anterior sublongitudinal ridge; green denotes the ventral ridge; turquoise denotes the connecting ridge system.

Internal view. Recurved inner not present in all species of the group; lacking in at least the following species: S. balrogi, S. simplex. Recurved inner subject to sexual dimorphism in at least the following species: S. kiosti sp. nov., S. slipperi, S. maxima, S. ikeyai.

Description of species

Semicytherura kazahana Yamada, Tsukagoshi & Ikeya, 2005

Fig. 7

Note

First published record of the species from Korean waters. NIBR deposition number: DSEVIV0000003720.

Synonymy

(Semicytherura henryhowei sensu Yajima 1988: pg. 1076, pl. 1, fig. 12); (This specimen is either S. kazahana or S. sasameyuki, see Discussion). Semicytherura quadrata sensu Ishizaki 1968: pg. 20, pl. 4, figs 11, 12. Semicytherura sp. B sensu Lee 1990 (unpublished): pl. 27, figs 12, 13. Semicytherura sp. 1 sensu Yasuhara and Irizuki 2001: pg. 93, pl. 11, fig. 8. Semicytherura kazahana Yamada, Tsukagoshi & Ikeya, 2005: pgs 247, 253, 254, figs 2C, 6, 7.

Diagnosis

Juvenile. Sub-rectangular to sub-trapezoid lateral outline, as typical for species of the S. henryhowei group. Anteroventral margin here without crenulations [there are records of specimens showing four or five anteroventral marginal denticles; see Yamada et al. (2005)]. Carapace surface finely pitted. Dorsal margin weakly descending toward posterior. Acute caudal process at mid-height. Ventral margin weakly descending toward anterior. Thick, prominent ridges on carapace, as typical for the S. henryhowei group.

Anterior longitudinal ridge short, slightly ascending in straight line until posterior end of anterior one-third of valve length. Prominent, continuous ventral ridge running horizontally in antero-posterior direction, curving at posterior third of valve length. Two elliptical fossae in anterior half; sub-trapezoid fossa in posterior half.

Material

Juvenile, right valve # 70, from ARMS sediment (i.e., surface and sedimentation-derived sediments).

Locality and age

Seongsan (성산), Jeju Island, ROK (33°27'13"N, 126°56'45"E) (Fig. 2), 19 m water depth. Extant (collection date 2019; surface and sedimentation-derived sediments, valve only, no soft body).

Description

Heavily calcified, thick valve, sub-rectangular in lateral view. Maximum height at anterior third. Dorsal margin is weakly ascending towards posterior. Ventral margin is weakly descending towards posterior. Anterodorsal margin is smooth, obliquely rounded; anteroventral margin without visible marginal denticles, but partially broken; posterior margin is slightly curved above the caudal process. Acute posterior caudal process at mid-carapace.

The carapace surface is covered in fine pits and pores with sensory hairs. Valve with thick ridges; ventral ridge prominent; slightly ascending toward posterior third; then describing a steep drop toward the edge of the ventral margin and ascending again in a straight line toward the posterior margin, resembling the lower loop of a large S-shape; the upper loop of the S-shape is the thick posterior end of the dorsal ridge at the posterodorsal corner; interconnection between posterodorsal and posteroventral loops by a thin descending posterior subvertical ridge, completing the S-shape; anterior longitudinal ridge short; starting at mid-height from anterior margin, slightly ascending until terminating at posterior end of anterior third of valve length; dorsal half of anterior subvertical ridge steeply running upward and bending backward to anterodorsal corner, forming elliptical, anterodorsal fossa; ventral half of anterior subvertical ridge less prominent, zig-zag course, connecting to ventral ridge at mid-length of carapace, forming elliptical, anteroventral fossa.

Reticulation. Surface finely pitted (Fig. 7D, E) with prominent, species-specific ridge system.

Pores. Some simple pores with sensory hairs (Fig. 7D).

Hingement. Merodont hinge with a socket at each end of a ridge structure in the right valve (Fig. 7C) [not collected, but complementary negative structures in the left valve (tooth at each end of a groove)]. Typical for genus. Hingeline arched.

Adductor muscle scars. Not observed.

Recurved inner. Absent in juveniles [see e.g., S. subundata in Ozawa and Kamiya (2008), pg. 143, fig. 4; S. kiosti Jöst & Karanovic sp. nov., Fig. 12 herein].

Dimensions

Carapace dimensions: length: 0.3064 mm, height: 0.166 mm.

Occurrence

Extant sediments, Korea (this study; surface and sedimentation-derived sediments collected 2019), and Japan (Yamada et al. 2005; living specimens, surface sediment collection: 1977–2000); Pleistocene sediments, Korea (Lee, 1990; unpublished); (potentially) Miocene sediments, Japan (Yajima 1988) (after species re-assignment, this study; uncertain whether S. kazahana or S. sasameyuki, see Discussion).

Remarks

Our specimen greatly resembles the juvenile specimen of Semicytherura sp. 1 sensu Yasuhara and Irizuki 2001 (pg. 93, pl. 11, fig. 8), which was assigned to S. kazahana herein. The specimen of Yasuhara and Irizuki displays small, rounded anterior marginal denticals, which are absent in our specimen. However, S. kazahana has shown to display differing numbers of crenulations along its anterior margin (Yamada et al. 2005).

Figure 7. 

Semicytherura kazahana external and internal lateral views. A–E. Specimen # 70 A-1RV; A–B, D–E. Lateral external view; D. Details of pore with sensory hair and pitted carapace surface; E. Details d anterior subvertical ridge; C. Lateral internal view. Scale bars: 50 µm.

Semicytherura kiosti Jöst & Karanovic, sp. nov.

https://zoobank.org/DAA1DAD2-32A5-4F02-8143-D0D158F8568B

Figs 8, 9, 10, 11, 12

Note

NIBR deposition number: DSEVIV0000003716.

Synonymy

Hemicytherura sp. 3 sensu Ikeya and Itoh 1991: pg. 138, fig. 17A.

Etymology

After the collaborating institution, Korea Institute of Ocean Science and Technology (KIOST), who provided the samples and funding for the MarineGEO project in Korea.

Diagnosis

Sub-rectangular lateral outline (especially LV), as typical for species of the S. henryhowei group. Anteroventral margin with three crenulations (denticles). Carapace surface roughly pitted with finer, smaller pits at marginal areas. Dorsal margin straight, horizontal. Broadly acute caudal process above mid-height. Ventral margin straight, horizontal, but posterior half obscured by ventral ridge. Thick, prominent ridges on carapace, as typical for the henryhowei group. Prominent posterior subvertical ridge forming large, subtriangular fossa in posterior half of valve. Prominent, horizontal, straight anterior longitudinal ridge forming large, subtrapezoid fossa with dorsal half of anterior subvertical ridge.

Holotypes

Two valves: adult male left valve, # 109_1 (lost, only SEM) and right valve, # 109_2 from ARMS sediment (i.e., surface and sedimentation-derived sediments).

Paratypes

Five valves: # 68 (female ARV) and # 108 (male ARV) from ARMS sediment (i.e., surface and sedimentation-derived sediments), and # 177 (female A-1RV), # 239 (female A-2RV), # 240 (female ARV) from scoop sediment (i.e., surface sediment).

Type deposition

Specimens are deposited at the National Institute of Biological Studies (NIBR, Incheon, South Korea) under the deposition number DSEVIV0000003716.

Type locality and age

Seongsan (성산), Jeju Island, ROK (33°27'13"N, 126°56'45"E) (Fig. 2), 19 m water depth. Extant (collection date 2019; surface and sedimentation-derived sediments; valves only, no soft bodies).

Description

Heavily calcified, thick valve, sub-rectangular (LV) to ovoid (RV) in lateral view. Right valve larger than left valve (see Fig. 8, length/height plot, # 109_1 and # 109_2). Maximum height at anterodorsal corner. Dorsal margin is horizontal, straight, parallel to ventral margin. Ventral margin is obscured by overhanging ventral ridge at posterior half. Anterodorsal margin is smooth, obliquely rounded; antero-ventral margin with three small, acute marginal denticles; posterior margin is ascending in a straight line above the caudal process. Broadly acute posterior caudal process above mid-carapace.

The carapace surface is covered in rough pits, and pores with sensory hairs. Finer, smaller pits at marginal areas. Valve with thick ridges; anterior sublongitudinal ridge short, horizontal, straight line, terminating at anterior third of valve length; prominent posterior subvertical ridge connecting posterodorsal corner with ventral ridge at mid-length of valve; posterior ridge branching off posterodorsal corner and running in a straight, vertical manner to posterior corner; posterior subvertical ridge, posterior ridge and posterior half of ventral ridge form large, subtriangular fossa; anterior longitudinal ridge, dorsal half of anterior subvertical ridge, and anterodorsal margin form large, subtrapezoid fossa. Right valves slightly higher and longer than left valves. Females longer and slightly higher than males (see length/height plot, Fig. 8).

Reticulation. Surface covered in large pits with prominent, species-specific ridge system.

Pores. Some simple pores with sensory hairs.

Hingement. Merodont hinge with a socket at each end of a ridge structure in the right valve (Figs 9A, C, 10B, F, I). Complementary negative structures in the left valve (tooth at each end of a groove) (no SEM). Typical for genus. Hingeline arched.

Adductor muscle scars. Vertical row of 4 adductor scars in ventro-median area (Fig. 9). Uppermost and lowermost scar less elongate in comparison with the middle two scars. At least 8 dorsal scars in upper half of valve (Fig. 11B). 2 mandibular scars slightly below, in front of adductor scars (Fig. 11F, blue arrows). 2 frontal scars in front of two uppermost adductor scars (Fig. 11F, white arrows).

Recurved inner. Strongly recurved in male adult (#109_2); absent in female adult (#240); not developed at juvenile stages (# 239) yet (Fig. 12).

Dimensions

Carapace dimensions of holotype: female ARV (# 240): length: 0.330 mm, height: 0.197 mm. Carapace dimensions of paratypes: female ARV: (# 68): length: 0.329 mm, height: posterior margin obscured by glue, ~ 0.196? mm; male ALV (# 109_1): length: 0.306 mm, height: 0.162 mm; male ARV (# 109_2): length: 0.308, height: 0.182 mm; male ARV (# 108): length: 0.310 mm, height: 0.185 mm; female A-2RV (# 239): length: 0.262 mm, height: 0.153 mm; female A-1RV (#177): length: 0.291 mm, height: 0.175 mm (Fig. 8).

Occurrence

Extant sediments, Korea (Jeju Island; surface and sedimentation-derived sediments collected 2019) and Japan (Sendai Bay, Matsushima Bay, Pacific Ocean; surface sediments collected 1985, 1986, 1988).

Remarks

The present species is characterized by a short, straight, horizontal anterior longitudinal ridge (Fig. 4B, AA). Of all known members of the henryhowei group, Semicytherura kiosti shares this trait only with S. leptosubundata (Fig. 4G) and S. neosubundata (Fig. 4Y). Morphological distinction between the three species based on prominent ridge patterns is straightforward. Semicytherura kiosti has an obvious anterior subvertical ridge, which is lacking in the other two species. Hemicytherura sp. 3 sensu Ikeya and Itoh 1991 is identical to our specimens of S. kiosti (fig. 17A in Ikeya and Itoh 1991). Hemicytherura Elofson, 1941 can easily be differentiated from the other members of the Semicytherura henryhowei group by the carapace surface features. Species of Hemicytherura, do exhibit something like ridges, but these do not resemble the characteristic ridge system of thick carinae of the S. henryhowei group. Rather, Hemicytherura is categorized by the characteristic fossa reticulation units that form around pores (Tanaka et al. 2011). Species of the S. henryhowei complex also show a less coarsely pitted surface ornamentation when compared to the Hemicytherura species.

Figure 8. 

Length/height plot of Semicytherura kiosti sp. nov. Jöst & Karanovic. Circles indicate female, squares indicate males. Adult valves depicted in red, juveniles in blue: light blue: A-1 stage, dark blue: A-2 stage; LV indicates left valve, RV indicates right valve.