Thalassironus koreanus, Jeong & Tchesunov & Lee, 2019

Jeong, Raehyuk, Tchesunov, Alexei V. & Lee, Wonchoel, 2019, A new species of the genus Thalassironus (Nematoda: Enoplida: Ironidae) from the coasts of South Korea, Zootaxa 4563 (3), pp. 516-530 : 519-524

Thalassironus koreanus sp. nov.

Figs. 2 , 3 , Table 1

Type locality. Sub-littoral at East Sea of Korea (35°17'05"N 129°42'14"E), depth 143 m, in muddy sediment .

Type material. All specimens deposited in National Institute of Biological Resources ( Republic of Korea). Holotype 1♂ (NIBRIV0000834113) on one slide, from East Sea , Korea (35°17'05"N 129°42'14"E) collected 15 August 2016 . Paratypes: 2♂♂ (NIBRIV0000834114, NIBRIV0000834115) on two separate slides, 2♀♀ (NIBRIV0000834116) on one slide from East Sea , Korea (35°17'05"N 129°42'14"E), depth 143 m, collected 15 August 2016 by Raehyuk Jeong and Somin Lee

Additional Localities. Locality 1, Sub-littoral at Yellow Sea, Korea (36°55'10.8001"N 124°34'14.4998"E), depth 76 m, in muddy sediment. Locality 2, sub-littoral at East Sea, Korea (37°23'55.0000"N, 129°14'57.0001"E), depth 54 m, in muddy sediment.

Additional material. 8♂♂ (NIBRIV0000834117–NIBRIV0000834121), five on five separate slides, three dried, mounted on a stub and coated with gold for SEM (NIBRIV0000834125), 3♀♀, one on a slide (NIBRIV0000834120), two dried, mounted on a stub and coated with gold for SEM (NIBRIV0000834126) from Locality 1, Yellow Sea, Korea (36°55'10.8001"N 124°34'14.4998"E), depth 76 m, collected 7 August 2015 by Jungho Hong ; 1♂ (NIBRIV0000834122) on one slide, 2♀♀ (NIBRIV0000834123, NIBRIV0000834124) on two separate slides from Locality 2, East Sea, Korea (37°23'55.0000"N, 129°14'57.0001"E), depth 54 m, collected 21 June 2016 by Raehyuk Jeong and Jisu Yeom .

Measurements. See Table 1 for detailed measurements and morphometric ratios.

Description. Male ( Fig. 2 ). Body elongate spindle-shaped (fusiform). Cuticle thick and smooth. Thirteen loxometanemes of type II distributed in spiraling manner along the lateral fields of the body per lateral side: two metanemes in the pharyngeal region, 11 in intestinal region, no evident metanemes in caudal region ( Fig. 2A ). Mouth opening surrounded by three (one dorsal and two ventrosublateral) triangular inner microlabia spread wide in the open mouth and set off by a shallow circular groove, and six labia bearing small inner labial papilla ( Fig. 4A, B, D ). Outer labial and cephalic setae in one crown. Head not set off by constriction. Amphideal aperture slit-like vaguely visible under the light microscope (~7 µm in width) just below single lateral labial seta; it running perpendicular to the lateral field. Two cervical papilla distributed along level of buccal cavity, one immediately below the median of the amphid and one near the posterior end of buccal cavity lateroventrally ( Fig. 2B, D , 4B ). Buccal armament consists of three equal (one dorsal and two lateroventral) protrusible, strong, solid teeth. Each tooth composed of wide base and two claw-like curved processes equal in shape but unequal in size. Dorsal tooth consists of left large and right small processes while right lateroventral—of large lateral and small ventral, and left lateroventral tooth—of large ventral and small lateral processes, respectively. Thus, large and small processes alternate in a circular fashion. Each double tooth flanked from outer side by an arch of about ten minute denticles ( Fig. 4A ). Buccal cavity consisting of two parts: anterior part with cheilostome with big tooth-complexes and posterior part with thick tubular cuticularized walls. Pharynx with distinct transversal muscular striations running across its entire length. Nerve ring situated approximately midway through pharynx. Ventral gland not observed. Cardia embedded in the intestine and round. Testis monorchic and bent near the anterior end, situated to the right of intestine ( Fig. 2A ). Twenty-three very prominent, oblique bursal muscles near caudal region running diagonally. Paired somatic setae distributed sparsely along the body until cloacal opening. Spicules very strong and slightly arcuate with clear and slightly bent proximal knobs. Ventral distal end, blunt-tipped; ventral edge of spicule close to the distal end with two clear dents. Gubernaculum consists of paired lateral bars with curvature in the middle. Supplementary papilla just anterior to the cloacal opening vaguely visible under light microscope but evident in SEM ( Fig. 5A ). Two pairs of pre-cloacal setae positioned one before and one after level of supplementary papilla. Tail conico-cylindrical. Conical portion of the tail bearing approximately nine to ten very minute paired subventral setae sparsely distributed along the body. Caudal gland vaguely evident below distal end of spicule, running until spinneret. Inverse triangular spinneret vaguely visible on the tail tip.

Female ( Fig. 3 ). Body elongate, fusiform. Cuticle rather thick and optically smooth. Loxometanemes distributed along the lateral field of the body, however very indistinct. Mouth surrounded by 6 lips, each lip bearing a small inner labial papilla on the tip. Outer labial and cephalic setae clearly visible, in one crown. Amphid slit-like (~7 µm in width), just below single labial setae. Cervical papilla below the middle of the amphid. Somatic papilla not seen. Buccal cavity consisting of two parts: anterior part with cheilostome with big tooth-complexes, consisting of three strong and solid claw-like teeth of about equal size. Each tooth consists of wide base and curved process which summit directed anteriorly and out. Posterior region of the buccal cavity cylindroid with strong cuticularized walls. Pharynx with transversal muscular striation along its entire length (apparent, more clear along posterior end after region of nerve ring); only anterior end of pharyngeal tissue of the cylindroid buccal cavity without transversal striation. Cardia embedded in intestine and trapezoid. Ventral gland not observed. Reproductive system didelphic, amphidelphic, antidromously reflexed. Both ovaries (anterior and posterior) positioned right of the intestine. Vulva located approximately midway of body length with protruding lips, a transverse slit, vagina 5–6 µm long, almost perpendicular to longitudinal body axis. Tail conico-cylindrical. Subventral setae not observed along conical part of the tail. Caudal glands vestigially present below level of anus with inverse triangular spinneret evident on the tail tip.

Diagnosis. Thalassironus . Body length 2160–2840 µm. Cuticle appears smooth under light microscope. Head not set off. Metanemes (loxometanemes) present. Outer labial setae 5–8 µm, cephalic setae 3–5 µm long. Mouth opening surrounded by three prominent triangular microlabia. Buccal armament present three equal teeth, each tooth apically with two unequal claw-like processes. Buccal cavity (stoma) 21–39 µm long, its termination not very distinct. Males with single anterior testis. Spicules arcuate, anteriorly knobbed and posteriorly blunt-tipped, 96–110 µm long. Gubernaculum with paired lateral bars with curvature. Precloacal midventral supplementary papilla present. Nine to ten minute subventral setae sparsely present along the tail only in males. Tail conico-cylindrical, c’ 3.5–4, posterior cylindrical portion constitutes about 20% of the entire tail length.

Differential diagnosis. T. koreanus sp. n. is closely related to both T. bipartitus and T. bohaiensis sharing characters as cephalic end not set off, short anterior setae, and conico-cylindrical tail with posterior cylindrical portion constituting less than 50% of the entire tail (see key for species identification). Relationships between these three Pacific species are defined by more fine characters (see the Table 2). Concerning conventional discriminating characters, T. koreanus differs from both T. bipartitus and T. bohaiensis by possessing longer spicules (96–110 µm vs. 52 and 62–67 µm, respectively) and by proportion (spicule length as arc / anal body diameter = 1.8 vs. 1.2 and 1.3–1.4 respectively). The body length of the new species is longer than both species (at most by 700–800 µm, which is roughly 40% the average body length). Pattern of cervical papillae, shape of spicules and gubernaculums as well as number of metanemes described for T. koreanus might also have diagnostic value in differentiation of related species of Thalassironus , but this requires more detailed examination.

Etymology. The species name koreanus is given because the species was found from two of three coasts (East Sea and Yellow Sea) of South Korea.

Figures

FIGURE 2. Thalassironus koreanus sp. nov. (♂) A. habitus showing 13 metanemes distributed along the body; B. head, lateral view, showing teeth and buccal cavity; C. tail, with spicule and gubernaculum apophysis; D. head to cardia, lateral view. Scale bars: 100 µm (A) and 20 µm (B, C, D).

FIGURE 3. Thalassironus koreanus sp. nov. (♀) A. head, lateral view, showing teeth and buccal cavity; B. caudal region, showing anus; C. female, lateral view, reproductive system; D. habitus; E. vulva (paratype). Scale bars: 20 µm (A, B, E) and 100 µm (C, D).

FIGURE 4. Scanning electron micrographs of Thalassironus koreanus sp. nov. A. male, face view; B. male mouth obstructed by unknown object, cervical papillae visible near the left-plane (arrow); C. female mouth and six lips and six inner labial papillae (arrow); D. female mouth, lateral view with outer labial setae and cephalic setae with slit for amphid ambiguously showing as well as cervical setae below.

FIGURE 5. Scanning electron micrographs of Thalassironus koreanus sp. nov. A. male, cloacal opening with distal part of spicule visible, supplementary organ anteriorly above it (asterisk); B. male, surface with metanemes visible (arrow); C. male, tail with pairs of subventral setae (arrow); D. female, tail lacking subventral setae.