Vestalenula gravata, 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 : 536-539

Vestalenula gravata sp. nov.

( Figs 3A–H , 4 )

Etymology. From the Latin gravatus, meaning loaded or weighed down, and referring to the large, long keel on the right valve.

Diagnosis. Small species of Vestalenula (length ca. ≤ 0.41 mm), with six to eight adductor muscle scars on each valve. Keel (external ridge near postero-ventral margin) on right valve very long (39–45% of valve length) and well-developed. No pigmented eye. Antennule with one seta on third segment. Antenna with one sub-apical seta on first endopodal segment. Terminal antennal segment with short GM claw, about 45% length of Gm. Mandible with long z seta extending beyond distal end of terminal segment, setae w and x much shorter, less than 50% length of z, seta y very small, about 15% length of z. Caudal ramus consisting of sub-triangular base supporting one short claw. Body terminating with small, rounded end, but without post-abdomen. No dorsal seta.

Type locality. Streams in Higashizato Kamiagata-machi Sago , Tsushima, Japan (34.62168º N, 129.37206º E, altitude 45 m) ( Figs 1 (sample 190830-04), 2A & B, Table 1 ) .

Type material.

Holotype. JAPAN — Nagasaki Prefecture, Tsushima • 1 ♀, with soft parts dissected in glycerine and sealed in a slide, valves stored dry in a micropalaeontological slide; Higashizato Kamiagata-machi Sago ; 34.62168º N, 129.37206º E; alt. 45 m; 30 Aug. 2019; Robin J. Smith, Cheon Y. Chang, Jimin Lee leg.; small stream in steep gully in forest, substate of poorly sorted mud, sand, pebbles, organic detritus; sample 190830-04, LBM 1430009555 .

Paratypes. JAPAN — Nagasaki Prefecture, Tsushima • 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 1430009556 , 1430009558 , 1430009560 . • 1 ♀, whole carapace, stored dry in a micropalaeontological slide; same collecting data as holotype; LBM 1430009559 . • 1 ♀, right valve, stored dry in a micropalaeontological slide; same collecting data as holotype; LBM 1430009557 .

Other material. JAPAN — Nagasaki Prefecture, Tsushima • 4 ♀♀, preserved in 70% EtOH; same collecting data as holotype. • 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. • 1 ♀, with soft parts dissected in glycerine and sealed in a glass slide, valves stored dry in a micropalaeontological slide; Mitsushima-machi Sumo ; 34.26246º N, 129.26264º E; alt. 58 m; 31 Aug. 2019; Robin J. Smith, Cheon Y. Chang, Jimin Lee leg.; stream in forest, cobbles, pebbles, gravel, interstitial sample; sample 190831-01 .

Description. Carapace length 377–389 µm, height 176–184 µm, height/length = 0.47–0.48 (n=4). Left valve overlaps right ventrally ( Fig. 3G ). Lateral view of carapace with dorsal and ventral margins both straight and parallel, anterior and posterior margins equally rounded, but maximum curvature of anterior margin lower than that of posterior margin ( Fig. 3A ). Dorsal view with relatively blunt anterior end, slightly more angular than posterior end, sides almost straight to slightly concave ( Fig. 3E ). Adductor muscle scars with six to eight scars arranged in rosette ( Fig. 3H ). Right valve with large and well-developed keel near postero-ventral margin, length about 45% of length of valve ( Fig. 3C, D, F & G ). Left valve with antero-ventral internal tooth ( Fig. 3B ). Surface of valves smooth. In transmitted light microscopy with granular appearance. Colour translucent white.

No pigmented eye.

Antennule with six segments ( Fig. 4A ). First segment with short, subapical-dorsal seta. Second segment with one long and one medium-length subapical-ventral setae. Third segment with apical-ventral seta. Fourth segment with short apical-ventral seta and tiny subapical seta. Fifth segment with two medium-length, stout apical-ventral setae, two longer apical-dorsal setae, and tiny subapical seta. Terminal segment with one long, one medium-length and one tiny setae, and aesthetasc ya.

Antenna with small hook h on first segment ( Fig. 4B ). Third segment with one robust seta on apical-ventral corner. Fourth segment with short z1 claw, approximately 50% as long as G2 and G3 claw. Claw G1 and seta z2 both very short, approximately half length of claw z1. Terminal segment with short GM claw, about 45% length of Gm.

Mandible with long z seta extending beyond distal end of terminal segment, setae w and x much shorter, less than 50% length of z. Seta y, very small, about 15% length of z ( Fig. 4C ). Terminal segment of palp with short c seta and five claws.

Fifth limb endopodite leg-like, with three segments ( Fig. 4D ). First segment with two subapical setae. Second segment with one apical seta. Terminal segment with one subapical seta, one apical seta and small claw.

Sixth limb five-segmented with very short d1 seta and two d2 setae on first segment ( Fig. 4E ). Second segment with two e setae and long pd seta, latter extending to about end of third segment. Third segment with robust f seta. Fourth segment with short, but robust g seta. Terminal segment with robust h1 seta and h2 claw, and short h3 seta.

Seventh limb five-segmented with two d2 setae on first segment ( Fig. 4F ). Second segment with medium length e seta. Third segment with long f seta, extending beyond end of fourth segment. Fourth segment with well-developed g seta. Terminal segment supporting very elongate claw h2 (approximately twice as long as claw of sixth limb), short but robust h1 seta, and very long h1 seta, approximately 40% as long as h2 claw.

Caudal ramus consisting of sub-triangular base supporting one short claw ( Fig. 4G ). Body terminating with small, rounded end, but without post-abdomen. No dorsal seta.

Males unknown.

Remarks. The long keel (an external ridge near the postero-ventral margin on the right valve) indicates that this species belongs to the danielopoli -group of Vestalenula (species with a short keel belong to the boteai -group; see Rossetti & Martens 1998), which consists of seven previously described species ( Table 3). The length of the keel of Vestalenula gravata sp. nov., at 39–45% the length of the right valve, is proportionally much longer than most other species of the group. In this respect, the Brazilian Vestalenula carinata ( Pinto et al., 2013) , with a keel about 40% the length of the right valve, is the most similar to Vestalenula gravata sp. nov., but Vestalenula gravata sp. nov. is smooth, while V. carinata has a lightly pitted carapace, and V. carinata is slightly more elongate (height/length = 0.45 vs. 0.47–0.48).

No pigmented eye was observed in any of the specimens, while most other Vestalenula species found in Japan, Vestalenula cornelia Smith et al., 2006 , Vestalenula cylindrica ( Straub, 1952) , Vestalenula lundi ( Neale & Victor, 1978) and Vestalenula molopoensis (Martens & Rossetti, 1997) , have pigmented eyes, albeit rather small examples. An eyeless species of Vestalenula was previously reported from a cave in Susono City, Japan, but this remains unnamed and is not figured ( Kaji & Tsukagoshi 2010). For some other species of Vestalenula , the presence or absence of an eye was not noted in descriptions, but one species, Vestalenula carveli Artheau, 2007 , was reported as not possessing an eye (although Fig. 4C of Artheau 2007 appears to show an eye in this species). Vestalenula carveli can be distinguished from Vestalenula gravata sp. nov. by the shorter keel and the presence of a post-abdomen in the former.

Distribution and ecology. Vestalenula gravata sp. nov. was found at two sites during our survey. The type material was collected from a small, very shallow stream in a steep gully in a forest. The substrate was sedimentary rock, with small areas covered with poorly sorted mud, sand, pebbles and organic detritus. In places branches and logs were infilling the gully.A trowel was used to dig out some of the sediment to a depth of about 10–20 mm, which was then washed through the sieves (see Methods and Material section above). The second site was similar to the first; a small, shallow stream in a forest on the lower slopes of a mountain. The sediment was very poorly sorted and angular, consisting of cobbles, pebbles and gravel, with specimens found interstitially.

The lack of a pigmented eye and small size suggests that this species may be a stygobiont, and it is possible that specimens were washed from groundwater habitats during the heavy rains that occurred a couple of days before collection ( Japanese Meteorological Agency 2019; see Material and Methods section).

Figures

FIGURE 1. Location map of Tsushima and sampling localities.

FIGURE 3. A–H, Vestalenula gravata sp. nov. A, LV, ext. (LBM 1430009558). B, LV, int. (LBM 1430009558). C, RV, int. (LBM 1430009557).D, RV, oblique int.(LBM 1430009557).E, Cp, dor., anterior to right (LBM 1430009559). F, RV vent. (LBM 1430009557). G, Cp, vent., anterior to left (LBM 1430009559). H, adductor muscle scars, anterior to left (LBM 1430009557). I–K, Vestalenula sp. I, LV, ext. J, RV, int. K, RV, vent. Scale bar = 200 µm for A–G, I–K, 46 µm for H.

FIGURE 4. Vestalenula gravata sp. nov. A, antennule (LBM 1430009556). B, antenna (LBM 1430009555). C, mandible (LBM 1430009555). D, fifth limb endopodite (LBM 1430009556). E, sixth limb (LBM 1430009555). F, seventh limb (LBM 1430009555). G, posterior of body (LBM 1430009560).