Actinonema pachydermatum Cobb, 1920

Actinonema pachydermatum Cobb, 1920 View in CoL

Original description ( Cobb, 1920: 338-339, fig. 117a- b, Florida, USA)

Other descriptions ( Wieser, 1954: 81, fig. 142a- d, Chile. Lorenzen, 1971: 290, fig. 5a- d, North Sea)

Synonymised names [ Actinonema amphidiscatum (Schuurmans Stekhoven & Adam, 1931) Wieser, 1954 ; Actinonema fragile (Allgén, 1929) Wieser, 1954 ; Adeuchromadora megamphida Boucher & Bovée, 1972 ; Euchromadora fragilis Allgén, 1933 ; Pareuchromadora amphidiscata Allgén, 1940 ; Pareuchromadora amphidiscata Schuurmans Stekhoven & Adam, 1931 ; Pareuchromadora fragilis (Allgén, 1934) ; Spiliphera fragilis Allgén, 1929 ]

Locality. The coast of Samsan-myeon , Goseong-gun , Gyeongsangnam-do: sub-tidal Eastern part of South Sea, Korea ( 34°56 ʹ 2.1 ʺ N, 128°15 ʹ 20.6 ʺ E), at a depth of 25 m, in the clay sediment GoogleMaps .

Material. Three males ( MABIK NA00158042 , MABIK NA00158708 - MABIK NA00158709 ) and One female ( MABIK NA00158710 ) collected by Jung-Ho Hong at the coast of Goseong on 2nd May 2023, from Research Vessel Haeyanghwnangkyung 2 .

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

Description. Males ( Fig. 1 View Fig , Table 1). Body cylindrical, uniform in thickness, gradually narrowing toward both ends; anterior apex blunt. Cuticle heterogeneous and complex, with irregular minute dots beginning at the anterior border of amphids. Transverse dots present in the anterior part of the pharynx; lateral differentiation starts anterior to the nerve ring (~50-60 μm from the anterior apex) with a reversed ‘V’ shape. Double reversed ‘V’ shaped lateral differentiation present from mid-body to anus, transitioning to irregular differentiation from near the anus to the posterior tail. Lateral differentiation diminishes in the distal two-thirds of the tail. Somatic setae inconspicuous, minute, and scattered. Labial region with six inner labial papillae and six outer labial setae (~3 μm) in a single circle, accompanied by four cephalic setae (~2 μm). Six solid cones surround the labial region. Stoma conical, bearing a sharply pointed dorsal tooth. Amphideal fovea conspicuous, transversely oval with a double contour, 10-11 μm wide (75-80% of cbd), located posterior to the cephalic setae circle. Pharynx cylindrical, 130-135 μm long, with a distinct posterior bulb. Cardia inconspicuous. Excretory pore located just posterior to the nerve ring, 80-85 μm from the anterior body end. Reproductive system monorchic; single testis situated on the left side of the intestine. Spicules thin, curved, measuring 25-30 μm; talamons 27-31 μm long. Gubernaculum 20-23 μm, without apophyses. Precloacal supplements absent. Tail conical, tapering to a pointed tip, with three caudal glands and a well-developed spinneret.

Females ( Fig. 1 View Fig , Table 1). Overall features similar to males. Reproductive system amphidelphic. Ovaries located to the left of the intestine, reflexed. Vulva slightly posterior to mid-body, positioned at 51.5% of body length.

Remarks. To date, the genus Actinonema Cobb, 1920 contains 12 valid species: A. celtica Boucher, 1976 , A. diplobulba Chu, Hao and Huang, 2023 , A. dolichurum Gagarin and Long, 2017 , A. falciforme Shi, Yu and Xu, 2018 , A. fidatum Vitiello, 1970 , A. grafi Jensen, 1991 , A. longicaudatoides Gagarin, 2015 , A. longicaudatum (Stein- er, 1918) Wieser, 1954, A. pachydermatum Cobb, 1920 , A. paraceltica Muthumbi and Vincx, 1998 , A. parvum Gagarin, 2015 and A. smolae Muthumbi and Vincx, 1998 ( WoRMS Editorial Board, 2024; Nemys eds., 2024). Specimens from the eastern part of the South Sea, Korea, generally match the original description by Cobb (1920), with the exception of differences in body width (33-35 μm vs. 27-30 μm) and slight variation in body length (817- 850 μm vs. 830-900 μm). These differences are considered intraspecific variations.