Urosomoida sejongensis Jung, Baek, Kim & Choi, 2016, n. sp.

Urosomoida sejongensis n. sp.

(Fig. 1–2; Table 1)

Diagnosis. Size in vivo 75–130 × 20–35 Μm; slender to elongated shape; flexible but not contractile; grayish under low magnification. Contractile vacuole slightly above left of mid-body. 1 micronucleus between two macronuclear nodules. Groups of cortical granules sparsely distributed. Ring-shaped structures in cytoplasm. 17 frontal-ventraltransverse cirri composed of 3 frontal, 1 buccal, 4 frontoventral, 3 postoral ventral, 2 pretransverse ventral, and 4 transverse cirri. 1 left and 1 right marginal cirral row. On average 28 adoral membranelles with undulating membranes in Oxytricha pattern. 4 kineties composed of 3 dorsal and 1 dorsomarginal kineties. 3 caudal cirri.

Type locality. Freshwater near the King Sejong Station, King George Island, Antarctica (62°14′22.8′′S, 58°44′40.1′′W).

FIGUIRE 1. Morphology of Urosomoida sejongensis n. sp. in vivo (A–C) and after protargol impregnation (D, E). A. Ventral view of representative specimen. B, C. Dorsal views showing contractile vacuole (CV) and cortical granules (CG), respectively. Dorsal (D) and ventral (E) view of holotype specimen. AZM—adoral zone of membranelles; BC—buccal cirrus; DK—dorsal kineties; DM—dorsomarginal kinety; EM—endoral membrane; FC—frontal cirri; FVC—frontoventral cirri; LMR—left marginal cirral row; Ma—macronuclear nodules; Mi—micronucleus; PM—paroral membrane; PTVCpretransverse ventral cirri; PVC—postoral ventral cirri; RMR—right marginal cirral row; TC—transverse cirri. Scale bars: 50 µm.

All measurements in Μm. CV—coefficient of variation (%); Max—maximum; Min—minimum; N—number of specimens investigated; SD—standard deviation; SE—standard error of arithmetic mean.

Type slides. One holotype slide (NIBRPR0000106568) and one paratype slide (ACNS 000272) of protargolimpregnated specimens have been deposited in the National Institute of Biological Resources and the Korea Polar Research Institute (KOPRI) in South Korea, respectively. Relevant specimens including holotype have been marked with circles on the bottom of the slides.

Etymology. The name “ sejongensis ” is derived from the name of the King Sejong Station (Korea Antarctic Research station) because the species was discovered near the station.

Gene sequence. SSU rDNA sequence was deposited in the GenBank under the accession number KT723011.

Morphological description. Size 75–130 × 20–35 Μm in vivo (Fig. 1A; 2A–G), on average 95 × 32 Μm in protargol preparation (Fig. 1D, E; 2K, L). Body slender to elongated shape, slightly concave on right margin of mid-body (Fig. 1A; 2A, B), dorsoventrally flattened (width:thickness = 1:0.75); flexible but not contractile; cell color grayish under low magnification. Invariably, on left of mid-body, two macronuclear nodules and one micronucleus (Fig. 1A, E; 2G, I, L). Macronuclear nodules ellipsoid, 10–17.5 × 6–12.1 Μm (stained); spherical micronucleus located between the nodules, 3.0–3.8 × 2.5–3.5 Μm (stained). One contractile vacuole slightly above left of mid-body, 10 Μm in diameter, lacking conspicuous collecting canals (Fig. 1B; 2B, E). Cortical granules spherical and colorless, 0.4 Μm in diameter; groups of granules formed patchy distribution on dorsal side (Fig. 1C; 2J). Spherical ring-shaped structures distributed in cytoplasm, 3–7 Μm in diameter (Fig. 1A; 2G); the shape left intact from burst cell. Feed on bacteria and diatoms.

Cirri 10–15 Μm long in vivo; usually 17 cirri, on ventral side, composed of 3 frontal, 1 buccal, 4 frontoventral, 3 postoral ventral, 2 pretransverse ventral, and 4 transverse cirri (Fig. 1A; 2F, L); very rarely 1 pretransverse ventral cirri (1 of 21 cells). Two marginal cirral rows composed of one left and one right row, terminated at level of posteriormost transverse cirrus, marginal rows never connected at posterior body end. Anterior part of right marginal row commenced on dorsal side (Fig. 1D; 2K). Invariably four dorsal kinety rows composed of 3 dorsal and 1 dorsomarginal kineties without any fragmentations. Dorsal bristles about 4 Μm in vivo. One caudal cirrus at the end of each of dorsal kineties, resultant invariably 3 caudal cirri. Leftmost dorsal kinety anteriorly shortened; dorsomarginal row about 1/3 of cell length.

Adoral zone composed of 27–30 membranelles with continuous arrangement; about 1/3 of cell length. Undulating membranes nearly parallel, slightly curved (= Oxytricha pattern); one buccal cirrus located anterior right end of paroral membrane (Fig. 1E; 2L).

Molecular analysis of Urosomoida sejongensis n. sp. (Fig. 3). The SSU rDNA sequence of U. sejongensis is 1,578 bp in length and had 97.3% and 96.4% nucleotide similarity with U. agilis (type of this genus) and U. sp. (FN429124), respectively. In Fig. 3, Urosomoida sejongensis clustered with family Uroleptidae, not with the type species U. agilis or U. sp. The three SSU rDNA sequences of Urosomoida were split into each clade in the tree showing a non-monophyletic relationship. Urosomoida agilis clustered with the species having the undulating membranes in Cyrtohymena pattern. Urosomoida sp. showed a sister relationship with the clade Hemiurosomoida longa (formerly U. longa) and had a SSU rDNA sequence similarity of 96.9%.