Microloxoconcha semicircularis, Smith & Chang, 2022

Smith, Robin James & Chang, Cheon Young, 2022, Two new species of non-marine Ostracoda (Crustacea) of the genera Vestalenula Rossetti & Martens, 1998 (Darwinuloidea) and Microloxoconcha Hartmann, 1954 (Cytheroidea) from Tsushima, Japan, and a summary of the non-marine ostracod fauna of the island, Zootaxa 5150 (4), pp. 529-555 : 543-545

publication ID	https://doi.org/ 10.11646/zootaxa.5150.4.4
publication LSID	lsid:zoobank.org:pub:7D852735-1CC9-4FB4-B3F8-38C5D46E8B9E
DOI	https://doi.org/10.5281/zenodo.6626879
persistent identifier	https://treatment.plazi.org/id/03C687F9-FB22-A82E-5D86-FEEBFAD29AA6
treatment provided by	Plazi
scientific name	Microloxoconcha semicircularis
status	sp. nov.

Treatment

Microloxoconcha semicircularis sp. nov.

( Figs 6E–J View FIGURE 6 & 7 View FIGURE 7 )

Etymology. Latin for semicircular, referring to the two small semicircular depressions on the postero-ventral margin of each valve, as well as the semicircular carapace shape of the female in lateral view.

Diagnosis. Carapace dorsal margin strongly arched, ventral margin slightly concave, anterior and posterior margins similar in size. Line of concrescence strongly deviating from outer margin along posterior and anterior margins, in places forming inner margin. Very edge of postero-ventral corner of both valves with two small, semicircular depressions. Male sexual organs symmetrical, basal capsule quadrate, inner and outer margins sub-parallel, distal lobe very lightly sclerotized, large and triangular, terminating with tightly rounded tip. Upper ramus elongate irregular oval. Copulatory duct with straight inner edge and bulbous outer edge. Copulatory duct slender and straight. Female caudal process large and rounded.

Type locality. Tidal part of a small river in Kamitsushima-machi Kawachi, Tsushima, Japan, 34.67306º N, 129.42726º E ( Figs 1 View FIGURE 1 (sample locality 190830 - 03A), 2C–E, Table 1 View TABLE 1 ) GoogleMaps .

Type material.

Holotype. JAPAN — Nagasaki Prefecture, Tsushima • 1 ♂, with soft parts dissected in glycerine and sealed in a glass slide, valves stored dry in a micropalaeontological slide; Kamitsushima-machi Kawachi ; 34.67306º N, 129.42726º E; alt. 1 m; 30 Aug. 2019; Robin J. Smith, Cheon Y. Chang, Jimin Lee leg.; small tidal river with concrete banks, substrate flattened pebbles of sedimentary rock, low tide, salinity 0.3%; sample 190830-03A, LBM 1430009561 View Materials . GoogleMaps

Allotype. JAPAN — Nagasaki Prefecture, Tsushima • 1 ♀, with soft parts dissected in glycerine and sealed in a glass slide, valves stored dry in a micropalaeontological slide; same collecting data as holotype; LBM 1430009562 View Materials . GoogleMaps

Paratypes. JAPAN — Nagasaki Prefecture, Tsushima • 1 ♂, 3 ♀♀, with soft parts dissected in glycerine and sealed in a glass slide, valves stored dry in a micropalaeontological slide; same collecting data as holotype; LBM 1430009563 View Materials , 1430009564 View Materials , 1430009565 View Materials and 1430009566 GoogleMaps .

Other Material. JAPAN — Nagasaki Prefecture, Tsushima • 7 ♂♂ ♀♀, preserved in 70% EtOH; same collect - ing data as holotype GoogleMaps .

Description.

Carapace length female 315–327 µm, height 147–152 µm, height/length = 0.45–0.48 (n=2), male 305–310 µm, height 125–129 µm, height/length = 0.41–0.42 (n=2). Lateral view roughly semicircular, with strongly arched dorsal margin and slightly concave ventral margin, anterior and posterior margins similar in size, with maximum curvature towards ventral margins ( Figs 6E, F, I & J View FIGURE 6 , 7A & B View FIGURE 7 ). Internal view of both valves with wide calcified inner lamellae. Postero-ventral inner margin of inner calcified inner lamella straight to slightly sinuous, and with three shallow grooves running sub-parallel to each other and outer margin. Line of concrescence strongly deviating from outer margin along posterior and anterior margins, in places forming inner margin. Very edge of postero-ventral corner with two small, semi-circular depressions ( Fig. 6H–J View FIGURE 6 ; marked with white triangles on Fig. 6G and I View FIGURE 6 ). Adductor muscle scars with row of four closely spaced scars, and one large frontal scar. Dorsal view laterally compressed with bulge in middle area. Surface of valves with smooth surface.

Antennule with five segments ( Fig. 7C View FIGURE 7 ). First and second segments elongate. Third segment short, with one short apical-dorsal seta. Fourth segment elongate with one short dorsal seta at mid-length position and one short apical-dorsal seta. Fifth segment elongate, apically with one short seta, and two aesthetasc-like setae fused together at base, one with widened bulbous end, and one thinner with rounded end.

Antenna basis elongate, supporting three-segmented endopod and long, jointed exopod ( Fig. 7D View FIGURE 7 ). First endopodal segment with one short apical-ventral seta. Second endopodal segment with one seta on ventral margin, near mid-length, and one apical-ventral seta. Third endopodal segment small, with two short, curved claws. Exopodite reaching to end of claws of endopodite.

Mandible palp four-segmented, but division between second and third segments weakly defined ( Fig. 7E View FIGURE 7 ). First segment with one short seta on apical inner edge. Second segment elongate with two setae on inner apical corner, and one sub-apical seta on outer edge. Third segment elongate with one stout, long and one much smaller sub-apical setae on inner edge, and one stout, long apical seta. Terminal segment elongate with two medium-length apical claw-like setae. Coxa small with about five teeth and thin, relatively long seta on outer edge ( Fig. 7F View FIGURE 7 ).

Maxillula palp with very indistinct segmentation between first and second segments ( Fig. 7G View FIGURE 7 ). First segment with one stout apical seta on outer edge. Second segment with two long, curved apical setae. First, second and third endites with about four, three and four apical setae respectively.

Fifth limb first segment with one apical seta ( Fig. 7H View FIGURE 7 ). Second segment very elongate, with small antero-apical seta. Third and fourth segments approximately equal in length, with fourth segment supporting small, curved claw.

Brushed-shaped organs of male consisting of small elongate base supporting numerous, fine setae (not figured).

Sixth limb first and second segments thin and very elongate, each with antero-apical seta ( Fig. 7I View FIGURE 7 ). Third segment shortest of limb, with no setae. Fourth segment longer than third, and supporting small, curved claw, slightly smaller than on fifth limb.

Seventh limb large and robust ( Fig. 7J View FIGURE 7 ). First segment with wide base tapering distally. Second segment supporting thick seta tapering to thin distal part. Third segment slightly longer than fourth, both much shorter than second segment (about 40% of length), fourth segment with large, robust, curved claw.

Male sexual organs symmetrical, basal capsule quadrate, inner and outer margins sub-parallel, distal lobe very lightly sclerotized, large and triangular, terminating with tightly rounded tip ( Fig. 7K View FIGURE 7 ). Upper ramus elongate irregular oval. Clasping apparatus with straight inner edge and bulbous outer edge. Copulatory duct hooked-shaped, distally slender and straight.

Posterior of female with large, angular genital lobe, caudal ramus consisting of small base and two flexible claws, and large, rounded caudal process fringed with long setules (tending to clump together in dissection slides) ( Fig. 7L View FIGURE 7 ).

Remarks. Of the 12 other species in the genus Microloxoconcha , all differ from Microloxoconcha semicircularis sp. nov. in the shape of the hemipenes, with the closest being Microloxoconcha dimorpha Higashi et al., 2011 , from Shizuoka and Kanagawa Prefectures. However, the hemipenes of M. dimorpha are asymmetrical, and the carapace, although also elongate, is noticeably different in shape; in lateral view the apex of the curvature of the anterior and posterior margins are shallower and positioned higher than those of Microloxoconcha semicircularis sp. nov., which has a noticeable angle at the postero-ventral corner. The chaetotaxy of the appendages of Microloxoconcha semicircularis sp. nov. is most similar to Microloxoconcha kikaijimaensis Higashi & Tsukagoshi, 2008 , which is so far only known from Kikaijima Island, Kagoshima Prefecture, Japan, about 650 km south of Tsushima. Both species have a large, robust seta on the second segment of the seventh limb, two robust and one tiny setae on the third segment of the mandible, and similar caudal rami and large caudal processes in the females.

Distribution and ecology. This species was recovered from a shallow hole dug into the bed of a tidal part of a small river, with a salinity of 0.3% at time of collecting (low tide), and about 150 m from the sea. The substrate consisted mostly of flattened pebbles of sedimentary rock with a little fine sand and mud ( Fig. 2C–E View FIGURE 2 ).