1. INTRODUCTION

The order Desmoscolecida Filipjev, 1929 constitutes a small group of free-living marine nematodes distinguished by their characteristic body annulations, obscured by foreign particles, and the arrangement of somatic setae (Decraemer and Smol 2006;Lim and Chang 2006;Decraemer and Rho 2013). Within this order, three families, 24 genera, and approximately 270 species have been recorded, with over 90% of them being marine and distributed worldwide (Freudenhammer 1975;Decraemer and Smol 2006;Decraemer and Rho 2013;Bezerra et al. 2021).

The genus Tricoma is the second-largest taxon within the order Desmoscolecida, following Desmoscolex, and exhibits a remarkable diversity with over 100 valid species documented to date. Most of these species (98%) are confined to marine environments, except for one unidentified species discovered in a freshwater cave in France (Timm 1970;Decraemer and Smol 2006;Decraemer and Rho 2013). The genus Tricoma was initially established by Cobb in 1894 and currently includes two subgenus Tricoma (Tricoma) and Tricoma (Quadricoma) (Decraemer and Rho 2013). In 1985, Decraemer categorized the presence of a quadricomoid main ring and a distinct inversion ring in the subgenus Quadricoma as a single phylogenetic subgroup within the genus Tricoma. Nevertheless, the taxonomic position of the genus Tricoma remains debated. The subgenus Tricoma is characterized by the rounded or triangular outline of the desmen, the absence of an abrupt inversion ring, a more or less triangular head, and mainly cylindrical terminal ring (Decraemer and Rho 2013).

During a comprehensive faunistic survey focused on the biodiversity of free-living marine nematodes in the waters around Korea, our investigation yielded the discovery of four new and two previously unrecorded Tricoma nematodes. These findings emerged from the analysis of sediment washings obtained from both shallow subtidal areas and deep-sea environments, particularly within the East Sea region, encompassing the Ulleung Basin, the subtidal zone around Ulleungdo Island, and Hupo Bank.

The East Sea, situated as a semi-enclosed, mid-latitude marginal sea, with an average depth of approximately 1,684 meters, borders the Korean Peninsula, the Japanese Islands, and eastern Russia (Min et al. 2023). Up to the present time, a total of 36 species of free-living marine nematodes have been documented in the East Sea. Notably, only three species, namely Desmoscolex (Desmolorenzenia) pedunculusRho, Kim and Chang, 2007, Desmoscolex cosmopolitesTimm, 1970, and Tricoma (Quadricoma) unipapillataLee, Lee and Rho, 2022, belong to the order Desmoscolecida (Timm 1970;Rho et al. 2007;Lee et al. 2022).

In this study, we have compiled a comprehensive species inventory by thoroughly reviewing existing literature on the Tricoma subgenus species, outlining their geographical and bathymetric distribution. Furthermore, our research includes taxonomic descriptions for the four recently identified new species and two previously unrecorded species discovered in the East Sea, Korea.

2. MATERIALS AND METHODS

Sediment samples were gathered from the deep-sea benthic regions off Hupo Bank and the Ulleung Basin, obtained using a box corer at a depth of 2,500 meters, and from the subtidal zone around Ulleungdo Island in the East Sea, collected using a Smith-McIntyre grab at a depth of 20 meters (Fig. 1). In the field, the samples underwent osmotic shock with tap water for one minute, were then filtered through a 67 μm mesh-sieve, and promptly preserved in a 5% formaldehyde solution for long-term storage. In the laboratory, meiobenthos were roughly separated from detritus and sediments through flotation in Ludox HS40, screened through a 67 μm mesh-sieve, and subjected to three centrifugation cycles (Burgess 2001).

Nematodes were meticulously sorted from a mixed meiobenthos sample using a pasteur pipette, employing high magnification with a dissecting LEICA 205 C stereomicroscope (Leica, Wetzlar, Germany). Individual nematodes were carefully selected and placed in a 3% glycerin solution, allowing for slow evaporation at room temperature over a ten-day period. Following measurement and observation, they were mounted in glycerin between two cover slips on an HS slide using the standard wax-ring method (Shirayama et al. 1993).

All illustrations were produced under Nomarski differential interference contrast (DIC) using an Olympus BX 53 microscope (Olympus, Tokyo, Japan), equipped with a drawing tube. The drawings were crafted using a tracing technique in Adobe Illustrator 2022 software. Measurements and examinations were conducted with an Olympus BX53 microscope equipped with an Olympus DP26 digital camera, using Olympus CellSens imaging software (Olympus, Tokyo, Japan). Image quality enhancements were applied using Adobe Photoshop 2022 software.

Abbreviations used in the text are as follows: a, body length divided by maximum body diameter; b, body length divided by esophagus length; c, body length divided by tail length; V (%), vulva distance from the anterior end represented as a percentage of the total body length. Scale bars in the figures are provided in micrometers ( μm).

3. SYSTEMATIC ACCOUNTS

Order Desmoscolecida Filipjev, 1929

Family Desmoscolecidae Schepotieff, 1907

Subfamily Tricominae Lorenzen, 1969

Genus TricomaCobb, 1894

Subgenus TricomaCobb, 1894

Generic diagnosis. (Updated from Decraemer 1978;Decraemer 1985;Decraemer and Smol 2006;Decraemer and Rho 2013). Tricominae. The body cuticle of Tricominae is annulated, featuring raised rounded or triangular desmen for the secretion and retention of foreign particles. These desmen are separated by a narrow smooth interzone, constituting a single annule. Notably, there is an absence of an abrupt inversion ring. The end ring is predominantly cylindrical, anteriorly covered by a desmos, and slightly tapered posteriorly at the spinneret level. In longitudinal optical section, the head appears more or less triangular. The labial region may or may not have separate lips, with one or two crowns of labial papillae. There are four cephalic setae, which are pedunculate. The esophagus is mainly cylindrical. The male reproductive system is characterized by having two testes.

Type species.Tricoma (Tricoma) cinctaCobb, 1894.

Comments on the type species. In 1894, Cobb initially introduced the genus name Tricoma under the misconception that it possessed only three cephalic setae. The type species, Tricoma cinctaCobb, 1894, was described from memory using misplaced specimens, but the two drawings created by Cobb unequivocally confirm the validity of the genus. A reevaluation of the type species is necessary, with emphasis on the type locality (Timm 1970;Freudenhammer 1975).

Note on generic diagnosis. The taxonomic classification of the subgenus Quadricoma and subgenus Tricoma continues to be a matter of ongoing discussion. Baylis and Daubney (1926), Stammer (1935), Allgén (1942), and Lorenzen (1969) considered Quadricoma synonymous with Tricoma. In contrast, Timm (1970), Freudenhammer (1975), and Decraemer (1978) recognized it as a distinct and valid genus. Decraemer (1985) categorized Quadricoma as a subgenus of Tricoma based on morphological similarities, including head shape, end ring, and spinneret. The subgenus Quadricoma is characterized by distinctive features, such as a main ring with a triangular outline oriented posteriorly in the anterior body region and reversing posteriorly on the body. This reversal occurs at the level of a single ring, with a terminal ring featuring a spine-like spinneret and a head that is never widely triangular. Some species within the subgenus Tricoma exhibit traits such as a terminal ring with a spin-like spinneret and a relatively narrow triangular head. However, none of these species possess a completely Quadricoma-like main ring or a distinctly inverted ring.

Catalog of valid species within the subgenus Tricoma

1. Tricoma absidataTimm, 1970

Tricoma absidataTimm, 1970, p. 54, figs. 94-95, pl. 5 fig. 40, four males and three females, Crescent Beach, Florida, USA.

2. Tricoma absidata lizardiensis Decraemer, 1979

Tricoma absidata lizardiensisDecraemer, 1979c, p. 103, figs. 1-4, four males, five females, and four juveniles, Lizard Island, Great Barrier Reef, Australia, fine sandy bottom covered with algae on a small patch reef, 5 m deep.

3. Tricoma aculeataDecraemer, 1978

Tricoma aculeataDecraemer, 1978, p. 21, fig. 4, one female, Yonge Reef, Great Barrier Reef, Australia, reef flat.

4. Tricoma adelpha (Greeff, 1869)

Desmoscolex adelphusGreeff, 1869, p. 113, North Sea; Schepotieff, 1907, p. 143, Bergen, North Sea, sublittoral and deep-sea zone; Schepotieff, 1908, p. 191, Taf. 8, figs. 17-19, Bergen.

Tricoma adelpha: Stauffer, 1924, p. 62; Timm, 1970, p. 54, figs. 101-102, pl. 5 fig. 45, one female, Miller Park, Tomales Bay, California, USA, mud at 2 m deep.

5. Tricoma albimarisDecraemer and Tchesunov, 1996

Tricoma albimarisDecraemer and Tchesunov, 1996, p. 120, fig. 3, three males and two females, Velikaja Salma Strait, Karela Shore, Kandalaksha Bay, White Sea, 12 m deep, medium sand.

6. Tricoma allgeniDecraemer, 1978

Tricoma allgeniDecraemer, 1978, p. 23, figs. 5-6, six males and one female, Yonge Reef, Great Barrier Reef, Australia, reef flat.

7. Tricoma amydramphidaTimm, 1970

Tricoma amydramphidaTimm, 1970, p. 55, figs. 103- 104, pl. 6 fig. 48, three males and one female, Indian Ocean, 80 m deep.

8. Tricoma antarcticaTimm, 1970

Tricoma antarcticaTimm, 1970, p. 56, figs. 105-106, pl. 5 figs. 43-44, one female, Gauss Station, Antarctica, deep-sea dredging, 3,423 m deep.

9. Tricoma apophysisSoetaert and Decraemer, 1989

Tricoma apophysisSoetaert and Decraemer, 1989, p. 224, fig. 1, four males and eleven females, Corsica, Mediterranean, ranging from 150 m to 990 m deep.

10. Tricoma atlanticaFreudenhammer, 1975

Tricoma atlanticaFreudenhammer, 1975, p. 53, figs. 7, 10-11, one male and one female, Portugal, sublittoral sediment, 59 m deep.

11. Tricoma auritaChitwood, 1936

Tricoma auritaChitwood, 1936, p. 15, fig. 4M-R, one female, Bogue sound, North Carolina, USA, marine (shallow); Timm, 1970, p. 56, fig. 107.

12. Tricoma bathycolaFreudenhammer, 1975

Tricoma bathycolaFreudenhammer, 1975, p. 54, figs. 12-15, three males and one female, Iberian, deepsea, 4,354 m deep, foraminiferal mud.

13. Tricoma beataFreudenhammer, 1975

Tricoma beataFreudenhammer, 1975, p. 54, figs. 16- 18, one female, continental slope off Portugal, 568 m deep, coarse silt.

14. Tricoma bipapillataDecraemer, 1987

Tricoma bipapillataDecraemer, 1987, p. 232, fig. 1, two males, Papua New Guinea, eastern reef flat, green algae Chlorodermis fastigata.

15. Tricoma blomeiDecraemer, 1996

Tricoma blomeiDecraemer, 1996, p. 108, fig. 1E-I, one male and two females, Sylt Island, North Sea.

16. Tricoma brevispiculaDecraemer, 1978

Tricoma brevispiculaDecraemer, 1978, p. 35, fig. 9, one male, Yonge Reef, Great Barrier Reef, Australia, reef flat.

17. Tricoma bullapophysaDecraemer, 1983

Tricoma bullapophysaDecraemer, 1983, p. 7, pl. 2 fig. A-F, one male and two females, South-east of Iles Glorieuses, Mocambique channel, 615-625 m deep.

18. Tricoma capitataDecraemer, 1987

Tricoma capitataDecraemer, 1987, p. 235, fig. 2, one male and ten females, Papua New Guinea, east side of reef flat, low tide at 20 m from high-water level, sand with silt and small calcareous algae.

19. Tricoma coralicollaDecraemer, 1987

Tricoma coralicollaDecraemer, 1987, p. 238, fig. 3, one male and one female, Papua New Guinea, east side of reef flat, overgrown dead coral.

20. Tricoma corsicanaSoetaert and Decraemer, 1989

Tricoma corsicanaSoetaert and Decraemer, 1989, p. 230, fig. 3, two males and one female, deep-sea transect off Calvi, Corsica, Mediterranean, ranging from 820-1,220 m deep.

21. Tricoma curvespiculataDecraemer, 1983

Tricoma curvespiculataDecraemer, 1983, p. 9, pl. 3 fig. A-D, one male and three females, West of Iles Glorieuses, Mocambique Channel, 250 m deep.

22. Tricoma cylindricauda (Chitwood, 1936)

Tricoma adelpha cylindricaudaChitwood, 1936, p. 14, fig. 4F-K, one male and one female, Bogue Sound, North Carolina, USA, marine (shallows).

Tricoma cylindricauda: Chitwood, 1951, p. 643; Timm, 1970, p. 57, fig. 110.

23. Tricoma demanema (Timm, 1970)

Demanema cobbiTimm, 1970, p. 31, figs. 43-44, pl. 3 fig. 20, one female, San Andros Island, Bahamas. Tricoma demanema: Decraemer, 1985, p. 278, fig. 8A.

24. Tricoma denticulataTimm, 1970

Tricoma denticulataTimm, 1970, p. 58, figs. 111-115, pl. 6 fig. 47, two males and one female, Bermuda, Bahamas.

25. Tricoma deuterum (Timm, 1970)

Demanema deuterumTimm, 1970, p. 31, figs. 45-46, pl. 3 fig. 19, one female and one juvenile, San Andros Island, Bahamas.

Tricoma deuterum: Decraemer, 1985, p. 278, fig. 8B-D.

26. Tricoma dimorphaDecraemer, 1978

Tricoma dimorphaDecraemer, 1978, p. 37, figs. 10-11, three males and one female, Yonge Reef, Great Barrier Reef, Australia, reef flat and outside the reef.

27. Tricoma dimorpha papuensisDecraemer, 1987

Tricoma dimorpha papuensisDecraemer, 1987, p. 240, fig. 4, four males and ten females, Papua New Guinea, east side of reef flat, polychaeta tubes of sand and mucus.

28. Tricoma duopapillataSoetaert and Decraemer, 1989

Tricoma duopapillataSoetaert and Decraemer, 1989, p. 239, fig. 6, five males and nine females, deep-sea transect off Calvi, Corsica, Mediterranean, ranging from 150-1,220 m deep.

29. Tricoma fisheriTimm, 1970

Tricoma fisheriTimm, 1970, p. 58, figs. 116-119, two males and four females, Myonga Beach, Australia; Decraemer, 1978, p. 44, fig. 12, two males and two females, between One Tree Is. and Wistari Reef, Australia.

30. Tricoma globocapitataTimm, 1970

Tricoma globocapitataTimm, 1970, p. 59, figs. 120- 123, pl. 6 fig. 46, two females, Egregia holdfasts, Shell Beach, La Jolla, California, USA.

31. Tricoma globulosaKreis, 1934

Tricoma globulosaKreis, 1934, p. 116, fig. 3, one male and one female, Kai Islands, Indonesia.

32. Tricoma gloriosaDecraemer, 1983

Tricoma gloriosaDecraemer, 1983, p. 12, pl. 4 fig. A-D, three males and two females, South-east of Iles Glorieuses, Mocambique Channel, 335-390 m deep.

33. Tricoma goldeniDecraemer, 1978

Tricoma goldeniDecraemer, 1978, p. 48, fig. 13, two males, Yonge Reef, Australia, outside the reef.

34. Tricoma gracilisSteiner, 1916

Tricoma gracilisSteiner, 1916, p. 345, fig. 17, Gulf of Guinea, 9 m deep.

35. Tricoma hoplostomaDecraemer, 1978

Tricoma hoplostomaDecraemer, 1978, p. 51, figs. 14- 15, five males and one female, Yonge Reef, Australia, outside the reef.

36. Tricoma hopperiTimm, 1970

Tricoma hopperiTimm, 1970, p. 60, figs. 124-127, pl. 6 fig. 51, one male and five females, Bermuda.

37. Tricoma hopperi australiensisDecraemer, 1978

Tricoma hopperi australiensisDecraemer, 1978, p. 56, figs. 16-18, five males, ten females, and one juvenile, Yonge Reef, Australia, reef flat.

38. Tricoma incompositaFreudenhammer, 1975

Tricoma incompositaFreudenhammer, 1975, p. 55, figs. 19-20, one male, Iberian, deep sea, 1,311 m deep, foraminiferal mud.

39. Tricoma islandicaKreis, 1963

Tricoma islandicaKreis, 1963, p. 24, fig. 13A-C, one male, Iceland; Decraemer, 1979b, p. 310, fig. 3, one male and two females, Pierre Noire, France.

40. Tricoma latispiculaSoetaert and Decraemer, 1989

Tricoma latispiculaSoetaert and Decraemer, 1989, p. 227, fig. 2, three males and five females, deep-sea transect off Calvi, Corsica, Mediterranean, ranging from 150-990 m deep.

41. Tricoma lobataJuario, 1974

Tricoma lobataJuario, 1974, p. 278, fig. 2, one male and one female, southwest of Helgoland, German Bight, Germany, sublittoral zone of 39 m deep.

42. Tricoma longirostris (Southern, 1914)

Desmoscolex longirostrisSouthern, 1914, p. 62, fig. 29A-D, two males, Clare Island, Clew bay, bottom of sand and shells.

Tricoma longirostris: Steiner 1916, p. 339; Decraemer, 1983, p. 15, pl. 6 fig. A-C, two males, Mocambique Channel, 330-625 m deep; Platt and Warwick, 1988, p. 484, fig. 229; Ansari, Lyla and Ajmal Khan, 2015, p. 762, fig. 10, sixteen males and nineteen females, Bay of Bengal continental shelf, southeast coast of India, 30-176 m deep, sandy silt sediments.

43. Tricoma longispiculaDecraemer, 1978

Tricoma longispiculaDecraemer, 1978, p. 64, fig. 20, one male, Yonge Reef, Australia, reef flat.

44. Tricoma lucidaTimm, 1970

Tricoma lucidaTimm, 1970, p. 61, Figs. 128-130, pl. 6 fig. 50, four males and one female, 4 miles SE of mouth of N. Edisto River, South Carolina, USA, 12 m deep; Decraemer, 1987, p. 248, fig. 7.

45. Tricoma manganicolaBussau, 1993

Tricoma manganicolaBussau, 1993, p. 446, fig. 190, three males, Peru-beckens, south eastern Pacific, 4,157-4,175 m deep.

46. Tricoma mauretaniaFreudenhammer, 1975

Tricoma mauretaniaFreudenhammer, 1975, p. 56, figs. 21-22, one male, Iberian, deep-sea, 4,354 m deep, foraminate mud.

47. Tricoma megamphidaTimm, 1970

Tricoma megamphidaTimm, 1970, p. 62, figs. 131- 132, pl. 6 fig. 49, three males and two females, 4 miles SE of mouth of N. Edisto River, South Carolina, USA, 12 m deep.

48. Tricoma meridionalisKreis, 1934

Tricoma meridionalisKreis, 1934, p. 118, fig. 4A-C, one male and one female, Banda and Sunda Sea, Indonesia.

49. Tricoma meteoraFreudenhammer, 1975

Tricoma meteoraFreudenhammer, 1975, p. 56, figs. 23-24, one male and three females, one male from continental slope or Atlantic coast of Morocco, 823 m deep, and three females from Iberian, deep-sea, 4,354 m deep, foraminiferal mud.

50. Tricoma multiannulataKreis, 1937

Tricoma multiannulataKreis, 1937, p. 176, fig. 6A-D, two males and one female, Kai Islands, Indonesia, 263 m deep.

51. Tricoma oblitaBlome, 1982

Tricoma oblitaBlome, 1982, p. 46, fig. 7F-G, twelve males, ten females, and twelve juveniles, sandy beach, Sylt Island, North Sea.

52. Tricoma pachycephalaDecraemer, 1978

Tricoma pachycephalaDecraemer, 1978, p. 67, figs. 21- 22, one male, one female, and two juveniles, west of Lizard Island, Australia.

53. Tricoma parabrevirostrisDecraemer, 1996

Tricoma parabrevirostrisDecreamer, 1996, p. 112, fig. 2F-J.

Tricoma brevirostris: Decraemer, 1978, p. 29, figs. 7-8, two males and one female, Yonge Reef, Great Barrier Reef, Australia, reef flat; Decraemer, 1983, p. 15, pl. 5 fig. A-C, one male, Iles Glorieuses, Mocambique Channel, 330 to 550 m deep.

54. Tricoma paracapitataDecraemer and Tchesunov, 1996

Tricoma paracapitataDecraemer and Tchesunov, 1996, p. 122, fig. 4, one male, Velikaja Salma Strait, Karela Shore, Kandalaksha Bay, White Sea, 12 m deep, medium sand.

55. Tricoma paralucidaDecraemer, 1987

Tricoma paralucidaDecraemer, 1987, p. 243, figs. 5-6, two males, three females, and one juvenile, Papua New Guinea, north of east side of reef flat, 10 m from high-water level, polychaeta tubes.

56. Tricoma parasitifera Kreis, 1934

Tricoma parasitifera Kreis, 1934, p. 120, fig. 5A-E, one male and one female, Indonesia.

57. Tricoma paratimmiDecraemer, 1987

Tricoma paratimmiDecraemer, 1987, p. 249, fig. 8, two males, four females, and one juvenile, Papua New Guinea, east of reef flat, calcareous algae, sand and silt.

58. Tricoma parvaspiculataDecraemer, 1987

Tricoma parvaspiculataDecraemer, 1987, p. 252, fig. 9, one male and one female, Papua New Guinea, eastern reef flat, polychaeta tubes.

59. Tricoma pedunculataTimm, 1970

Tricoma pedunculataTimm, 1970, p. 65, figs. 135-136, pl. 6 fig. 52, one male and one female, Pacific, deepsea dredging, 4,500 m.

60. Tricoma perparvulaTimm, 1970

Tricoma perparvulaTimm, 1970, p. 65, figs. 137-138, pl. 7 fig. 57, one male, holdfasts of Egregia, Dillon Beach, California, USA.

61. Tricoma peruvensisNichols and Musselman, 1979

Tricoma peruvensisNichols and Musselman, 1979, p. 456, fig. 5, six males and one female, upwelling region off the coast of Peru, subtidal zone of 40 m deep.

62. Tricoma platapophysaDecraemer, 1978

Tricoma platapophysaDecraemer, 1978, p. 72, fig. 23, two males, Yonge Reef, Australia, reef flat.

63. Tricoma platycephalaFilipjev, 1922

Tricoma platycephalaFilipjev, 1922, p. 156, fig. 27a-c, one male, Black sea.

64. Tricoma polydesma (Southern, 1914)

Desmoscolex polydesmusSouthern, 1914, p. 64, fig. 31A-C, one male, Clare Island, Clew bay, bottom of sand and shells.

Tricoma polydesma: Steiner, 1916, p. 339; Decraemer, 1979b, p. 313, fig. 4, one female, Pierre Noire, Brittany, France; Platt and Warwick, 1988, p. 486, fig.230.

65. Tricoma pygmaeaSoetaert and Decraemer, 1989

Tricoma pygmaeaSoetaert and Decraemer, 1989, p. 233, fig. 4, five males and seven females, deep-sea transect off Calvi, Corsica, Mediterranean, depths ranging from 150-1,220 m deep.

66. Tricoma riemanniDecraemer, 1978

Tricoma riemanniDecraemer, 1978, p. 76, figs. 3E, 24- 26, four males and ten females, Yonge Reef, Australia, reef flat, behind reef.

67. Tricoma rostralisDecraemer, 1978

Tricoma rostralisDecraemer, 1978, p. 82, figs. 27-28, four males and one female, Lizard Island, Australia.

68. Tricoma rostrataTimm, 1970

Tricoma rostrataTimm, 1970, p. 66, figs. 139-141, pl. 7 fig. 56, one male and one female, 4 miles SE of mouth of N. Edisto River, South Carolina, USA, 12 m deep.

69. Tricoma secundaBlome, 1982

Tricoma secundaBlome, 1982, p. 48, fig. 8A-B, three males and three juveniles, sandy beach, Sylt Island, North Sea.

70. Tricoma septentrionalisTimm, 1978

Tricoma septentrionalisTimm, 1978, p. 233, fig. 4EH, two females, Scott Base and Cape Royds, Ross Island, Mcmurdo sound, Antarctica, 457-540 m deep.

71. Tricoma septuagintaSchuurmans Stekhoven, 1942

Tricoma septuagintaSchuurmans Stekhoven, 1942, p. 261, fig. 33A-F, Coral, Bay of Mallorca; Timm, 1970, p. 67, figs. 142-143, pl. 7 fig. 55, one male, Isla Santa Gruz, Galapagos Islands; Decraemer, 1978, p. 87, fig. 29, one male and one female, Yonge Reef, Australia, reef flat.

72. Tricoma setosaSoetaert and Decraemer, 1989

Tricoma setosaSoetaert and Decraemer, 1989, p. 242, fig. 7, two males, deep-sea transect off Calvi, Corsica, Mediterranean, 990 m deep.

73. Tricoma similisCobb, 1912

Tricoma similisCobb, 1912, p. 482, figs. 1-2, one male, one female, and one undefined specimen, Larat, India; Timm, 1970, p. 67, figs. 144-145, pl. 7 fig. 60, one male and three females, Bermuda; Decraemer, 1978, p. 91, figs. 3A, F, G, 30-35, nine males, ten females, four 2nd stage juveniles, and two 3rd stage juveniles, Yonge Reef, Australia, reef flat; Decraemer, 1983, p. 18, pl. 7 figs. A-C, one male, Iles Glorieuses, Mocambique Channel; Decraemer and Tchesunov, 1996, p. 124, one female, Velikaja Salma Strait, Kandalaksha Bay, White Sea, 12 m depth in medium sand.

74. Tricoma spinosaChitwood, 1936

Tricoma spinosaChitwood, 1951, p. 643, fig. 8F-H, one male, Mud Island, Aransas Bay, Texas, depth of 1.2 m.

75. Tricoma spinosoidesChitwood, 1951

Tricoma spinosoidesChitwood, 1951, p. 643, fig. 8CE, one male, Mud Island, Aransas Bay, Texas, USA, depth of 1.2 m.

76. Tricoma spuriaInglis, 1968

Tricoma spuriaInglis, 1968, p. 61, figs. 68-73, five males, two females, and two juveniles, Vincent’s bay, New Caledonia; Decraemer, 1986, p. 356, fig. 1.

77. Tricoma steineri de Man, 1922

Tricoma steineride Man, 1922a, p. 133, Zuidersee; de Man, 1922b, p. 259, fig. 49a-c, one male, North sea; Timm, 1970, p. 68, figs. 146-147; Decraemer, 1979b, p. 317, fig. 5, one male and one female, Pierre Noire, Brittany, France.

78. Tricoma strandi (Allgén, 1939)

Desmoscolex strandiAllgén, 1939, p. 319, fig. 1, one male, Norway; Allgén, 1940a, p. 277, one male two juveniles, Norway; Allgén, 1940b, p. 503, one male three juveniles, Norway; Allgén, 1943, p. 302, fig. 18, one male and one juvenile, Norway, 150-200 m deep.

Tricoma strandi: Gerlach, 1958, p. 82, Kiel Bay, 16-26 m deep.

79. Tricoma tenuisSteiner, 1916

Tricoma tenuisSteiner, 1916, p. 345, fig. 16, two females, Gulf of Guinea.

80. Tricoma tertiaBlome, 1982

Tricoma tertiaBlome, 1982, p. 50, fig. 8C-E, two males, three females, and one juvenile, sandy beach, Sylt Island, North Sea; Decraemer, 1996, p. 107, fig. 1A-D, Sylt Island, North Sea.

81. Tricoma timmiDecraemer, 1978

Tricoma timmiDecraemer, 1978, p. 103, figs. 3B, 36- 37, three males and two females, Lizard Island, three males and two females, Yonge Reef, Australia.

82. Tricoma uncinataGerlach, 1955

Tricoma uncinatum Gerlach, 1955, p. 301, fig. 31a-d, one male, Libertad, San Salvador, El Salvador.

Tricoma uncinata: Timm, 1970, p. 69, figs. 148-149, pl. 7 fig. 59, one male, Bermuda.

83. Tricoma vincxaeSoetaert and Decraemer, 1989

Tricoma vincxaeSoetaert and Decraemer, 1989, p. 236, fig. 5, two males and eight females, deep-sea transect off Calvi, Corsica, Mediterranean, depths ranging from 280-990 m deep.

Geographical and bathymetric distribution

Species within the subgenus Tricoma have been documented across all oceans (Fig. 2). Out of the 83 recognized taxa, 28% (23) were originally documented in connection with coral reefs or green algae. Additionally, 25% (21) are found in the deep-sea at depths exceeding 200 meters, with six of these taxa classified as abyssal. Eleven taxa (13%) have been recorded in tidal or upper subtidal zones at depths surpassing 10 meters. The remaining taxa are found in beach or coastal habitats with limited detail available. Among the 83 recognized taxa, 34% (28) are known from Europe, while 17 and 7 have been described in Australia and Papua New Guinea, respectively. Furthermore, 16 taxa have been identified in both South and North America, 5 in Africa, 5 in Asia, and an additional 5 in Antarctica, the Pacific, and the Indian Ocean.

Tricoma (Tricoma) breviseta sp. nov. (Figs. 3-4; Table 1)

Type locality. The designated type locality is the deep-sea region of the Ulleung Basin (37°37ʹ42.16ʺN, 131°19ʹ39.00ʺE), Ulleungdo Island, Gyeongsangbuk- do, Korea. The specimen was collected on April 26, 2023, by W. I. Jung and H. J. Lee. Nematodes were acquired from benthic sediments of the deep-sea, obtained at a depth of 2,327 meters using a box corer.

Type material. The holotype male (MABIK NA001 58129), preserved in glycerin on a HS slide, is deposited in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), located in Seochun, Korea. Additionally, one paratype male (KIOST NEM- 1-2679) and two paratype females (KIOST NEM-1- 2680, KIOST NEM-1-2681) have been deposited in the nematode collection within the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN) at the East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Diagnosis.Tricoma (Tricoma) breviseta sp. nov. is characterized by the following combination of features: (1) relatively small body length ranging from 228 to 255 μm in males; (2) cuticles feature 38-39 tricomoid main rings; (3) the first main ring is not covered by a desmos; (4) the head is globular, with a broadly truncated anterior end and slightly protruding lips; (5) cephalic setae are relatively short and stumpy; they are flanked by a narrow membrane along their entire length and inserted on low peduncles; (6) somatic setae include 7 pairs of subdorsal setae and 8 pairs of subventral setae; (7) spicules exhibit an offset capitulum; (8) gubernaculum is distally hooked in shape; (9) males have a tail composed of 6-7 main rings, while females have 5 main rings; and (10) the terminal ring includes an elongated spinneret measuring 6 μm length. These distinctive characteristics collectively define Tricoma (Tricoma) breviseta sp. nov.

Etymology. The suggested specific name “breviseta” is derived from the Latin words “brevis,” signifying short, and “seta,” referring to setae. This name is chosen to highlight the distinct shortness of both cephalic and somatic setae.

Measurements. Refer to Table 1 for details on measurements and morphometrics.

Description. Males (holotype and paratype). The body length of males ranges from 228 to 255 μm, depicting a relatively diminutive and slender physique that tapers, particularly towards the tail region (Fig. 3A). The maximum body diameter at the mid-body level measures 22-23 μm in width. The cuticle exhibits 39 tricomoid main rings in the holotype and 38 main rings in the paratype (Fig. 4A). Each cuticular zone of the main rings is coated with secretion and fine foreign material. The first main ring is devoid of a desmos, and the intervals between main rings are relatively broad (Fig. 3B). The main rings at the anterior (1 to 4) and the tail rings assume a slightly quadricomoid shape, while the remaining main rings are round (Fig. 4B, D). The head is distinct from the rest of the body, tapering anteriorly from cephalic setae with a truncated end. The head cuticle is entirely covered with a thin layer of fine granules, with the border of the head cuticles notably thickened and sclerotized (Fig. 4B). The labial region exhibits six slightly protruding lips and papilliform external labial sensilla (Fig. 3C). Cephalic setae are relatively short, measuring 4 μm in length, accounting for 44-46% of the head diameter (Fig. 3C). They are inserted on a broad and low peduncle in the middle of the head. The amphideal fovea is vesicular, with a slight constriction at the middle region (Fig. 3B). Its anterior end extends just before the protruding lips, while the posterior end covers the first naked main ring (Figs. 3B, 4C). The stoma is small and cylindrical. The esophagus, also cylindrical, measures 56-58 μm in length and corresponds to 21-22 μm in diameter at the level of the cardia. The nerve ring surrounds the esophagus at the level of main rings 6-7, and the esophagus-intestinal junction occurs at the level of 10-11 main rings (Fig. 3A). Ocelli are dark yellowish and oval-shaped, measuring 6×7 μm in the holotype (Fig. 4A). They are positioned opposite main ring 11 (in the holotype) or rings 12-13 (in the paratype). Smaller pigment spots are present along the esophagus. Somatic setae, ranging from 3 to 7 μm, are short and stumpy. They are inserted with a row and a small peduncle, increasing in length from the anterior to the mid-body region and then shortening towards the posterior end (Fig. 3E). The somatic setae comprise 7 pairs of subdorsal setae and 8 pairs of subventral setae, although the arrangement may vary somewhat among individuals (Fig. 3A). The disposition of somatic setae is outlined as follows (numbers in parentheses signify variable locations in the paratypes):

The reproductive system follows a typical pattern, with the anterior testis extending up to the main ring 18. Only one prominent ejaculatory gland is observed in the posterior part. The spicules, measuring 18-20 μm, are of equal length and straight, featuring a distally tapered, sharp tip (Fig. 3D). Proximally, they exhibit an offset capitulum. The gubernaculum, measuring 14 μm, possesses a thin, sclerotized wall. It is anteriorly directed and posteriorly features hooked apophyses at one-third of its end (Fig. 3D). The tail comprises 7 main rings in the holotype and 6 main rings in the paratype. The terminal ring is 22 μm long, constituting 37-44% of the total tail length (Fig. 4D). It consists of a conical anterior part and a cylindrical posterior part with a terminal spinneret. The anterior part of the terminal ring covers 58-62% of the region with cuticles coated in foreign material, while the remaining part is naked (Fig. 4D). Phasmata is not visible.

Females. Females are akin to males in most aspects but vary in the number of tail rings and sexual characteristics. The body length ranges from 223 to 240 μm, with a maximum body diameter of 23 μm (Fig. 3F). The body cuticle features 38 main rings, and notably, the first main ring is devoid of a desmos (Fig. 4E). The somatic setae consist of 7 pairs of subdorsal setae and 8 pairs of subventral setae (Fig. 4E). The disposition of somatic setae is outlined as follows (numbers in parentheses indicate variable locations in the paratypes):

The reproductive system is didelphic-amphidelphic, with both branches extended. The vulva is positioned between main rings 19 to 20 or 20 to 21 (Fig. 3F). The anal tube, measuring 3-4 μm in length, protrudes from the medioventral main ring 33 (Fig. 3F). The tail is comprised of five main rings and measures 45-47 μm in length. The terminal ring is 23-25 μm long and 6-7 μm wide. It consists of a conical anterior part covered with foreign materials and an uncovered spinneret, measuring 6 μm in length (Fig. 3F).

Remarks.Tricoma (Tricoma) breviseta sp. n. closely resembles T. (T.) apophysis Soeraert and Decraemer, 1989, in sharing several characteristics such as a small body length (223-255 μm vs. 185-260 μm), head shape, and the presence of relatively short cephalic setae (4-6 μm vs. 5-7 μm), as well as the structure of the male copulatory system. However, the new species differs from T. (T.) apophysis in the number of main rings (38-39 vs. 39-42), the low number of tail rings in males (6-7 vs. 7-8), and females (5 vs. 6-8). Moreover, T. (T.) breviseta sp. nov. possesses 7 pairs of subdorsal setae and 8 pairs of subventral setae, while T. (T.) apophysis has 4-5 subdorsal setae and 8-9 subventral setae. The new species is also closely related to T. (T.) corsicana Soeraert and Decraemer, 1989, but can be distinguished by the number of main rings (38-39 vs. 40), the number of somatic setae (8 pairs of subventral setae, whereas T. (T.) corsicana has 9 pairs of subventral setae), the number of tail rings in females (5 vs. 7 in T. (T.) corsicana), and the shape of the gubernaculum (strongly curved apophyses in T. (T.) breviseta sp. nov.).

Tricoma (Tricoma) donghaensis sp. nov. (Figs. 5-6; Table 2)

Type locality. The nematodes were collected from the subtidal zone of the East Sea (36°49ʹ14.15ʺN, 130°04ʹ39.64ʺE) in Hupo-myeon, Uljin-gun, Gyeongsangbuk- do, Korea, on May 29, 2014, by J. H. Lee. These nematodes were extracted from benthic sediments in the deep sea, obtained at a depth of 2,500 meters using a box corer.

Type material. The holotype male (MABIK NA0015 8128), mounted in glycerin on an HS slide, is deposited in the nematode collection at the Marine Biodiversity Institute of Korea (MABIK) in Seochun, Korea. Additionally, two paratype males (KIOST NEM-1-345, KIOST NEM-1-346) and one paratype female (KIOST NEM-1-350) have been placed in the nematode collection within the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN) at the East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Diagnosis.Tricoma (Tricoma) donghaensis sp. nov. is distinguished by the following set of characteristics: (1) having 37-39 tricomoid main rings in males; (2) the absence of a desmos covering the first main ring; (3) a globular head 1.0-1.2 times longer than its width; (4) cephalic setae that taper into a fine open tip and a cuticular groove; (5) somatic setae consisting of 8-9 subdorsal setae and 11-13 subventral setae on each side; (6) somatic setae distally exhibiting a groove with a seemingly split tip; (7) spicules proximally featuring a capitulum, and gubernaculum proximal 1/3 with hooked apophyses; and (8) the male tail composed of six to seven main rings, and the female tail having five main rings; the terminal ring with an elongated spinneret (13-18 μm long).

Etymology. The chosen specific name, ‘donghaensis’, is derived from the type locality.

Measurements. Refer to Table 2 for details on measurements and morphometrics.

Description. Males (holotype and paratypes). Body length ranges from 426 to 595 μm, displaying a slender and elongated form that tapers towards the posterior end. The maximum body diameter at the mid-body level measures 46-73 μm. The cuticle is characterized by 37 tricomoid main rings in the holotype and 38- 39 main rings in paratypes (Fig. 6A). Each main ring̓s cuticular zone exhibits secondary annulation and is covered with secretions and fine foreign material, with the first main ring lacking desmos coverage (Figs. 5B, 6B). The head is slightly globular, longer than its width, narrowing from cephalic setae towards both sides with an offset labial region (Fig. 5C). The head cuticle is entirely covered with a thin layer of fine granules, except for the labial region, where the edges of the head cuticles are notably sclerotized (Fig. 6B). The labial region is slightly protruding and features six external labial sensilla (Fig. 5C). Cephalic setae have a broad and cylindrical basal part, tapering into a fine open tip, and the entire setae are flanked by a cuticular groove (Fig. 5B). These cephalic setae measure 12-13 μm in length, accounting for 67-76% of the head diameter, and are inserted on a high and narrow peduncle in the middle of the head. The amphideal fovea is vesicular; the anterior end extends just before the protruding lips, while the posterior end covers the first naked main ring (Figs. 5B, 6C). The esophagus is surrounded by a nerve ring at the level of main ring 5 (Fig. 5A). They measure 97- 117 μm in length and correspond to a body diameter of 44-66 μm. The esophagus-intestinal junction occurs at main ring 9 in the holotype and main rings 11 and 13 in paratypes (Fig. 5A). Oval-shaped dark-yellowish ocelli (6-9 μm wide, 7-13 μm long) are situated at the posterior part of the esophagus, opposite main rings 8-12 (Fig. 6A). Somatic setae, ranging from 10-21 μm, have a distal groove with a seemingly split tip and are inserted almost directly into cuticular rings with a peduncle (Figs. 5E, 6E). The arrangement of somatic setae includes 8-9 subdorsal setae and 12-13 subventral setae on each side, although the arrangement may vary somewhat among individuals. The arrangement of somatic setae is as follows (the number in parentheses denotes the variable location in paratypes):

The reproductive system, characteristic of the subgenus, features two testes. The spicules are relatively short, straight distally with a pointed tip, and proximally bent with a capitulum (Fig. 5D). They measure 35-45 μm in length, approximately 1.0-1.1 times the diameter of the cloacal opening. The gubernaculum is 32-41 μm long, parallel to the spicules, with hooked apophyses in the proximal 1/3 (Fig. 5D). The tail consists of 6-7 main rings, with the terminal ring measuring 52-60 μm in length, constituting 46-54% of the total tail length (Fig. 6D). The terminal ring includes a relatively cylindrical anterior part and an elongated spinneret (16-18 μm long). Approximately 54-62% of the terminal ring is covered with foreign materials, and phasmata are not visible (Fig. 6D)

Female. Resembling males in most aspects, female exhibit variations in the pattern of somatic setae and sexual characteristics (Fig. 6F, H). The body has a length of 375 μm and a maximum diameter of 37 μm. The body cuticle consists of 38 main rings, adorned with secretions and fine foreign material, excluding the first main ring (Fig. 6H). Somatic setae are organized into 9 pairs of subdorsal setae and 11 pairs of subventral setae, arranged as follows:

The reproductive system is didelphic-amphidelphic, featuring both ovaries outstretched. The vulva is positioned between main rings 25 and 26, lacking protruding lips (Fig. 5F). The anal tube protrudes from the medioventral main ring 33 (Fig. 6G). The tail is comprised of 5 main rings, with the terminal ring measuring 41 μm in length. It consists of a cylindrical anterior part covered with foreign materials and an elongated spinneret (13 μm long) (Fig. 6G).

Remarks.Tricoma (Tricoma) donghaensis sp. nov. is categorized among species with fewer than 49 body rings within the subgenus Tricoma. Among them, only five species are recognized for having a ‘naked’ first main ring: T. (T.) apophysis Soetaret and Decraemer, 1989, T. (T.) corsicanaSoetaert and Decraemer, 1989, T. (T.) latispiculaSoetaert and Decraemer, 1989, T. (T.) dimorpha papuensisDecraemer, 1987, and the newly identified species T. (T.) breviseta sp. nov. Tricoma (T.) dimorpha papuensis is notably distinguished by its 48-55 main rings, setting it apart from other species with fewer than 40 main rings. In comparison, T. (T.) apophysis, T. (T.) corsicana, T. (T.) latispicula, and T. (T.) breviseta sp. nov. are easily differentiated from the new species due to their notably short body length (< 300 μm), relatively short cephalic and somatic setae (<10 μm), and a pattern of 8-9 subventral setae (compared to 11-13 subventral setae in T. (T.) donghaensis sp. nov.). The newly identified species shares similarities with T. (T.) bathycola in terms of body length (375-595 μm vs. 400-515 μm) and the arrangement of somatic setae (8-9 subdorsal setae and 11-13 subventral setae vs. 8-9 subdorsal setae and 11-12 subventral setae in T. (T.) bathycola). However, T. (T.) donghaensis can be distinguished by the presence of a naked first ring, somatic setae featuring a distally split appearance, a gubernaculum with strongly hook-shaped apophyses in the proximal 1/3 (as opposed to relatively smooth apophyses in T. (T.) bathycola), and a conical terminal ring that is anteriorly covered with desmos to the extent of 54-62% (in contrast to the cylindrical terminal ring of T. (T.) bathycola, which is anteriorly 60-75% entirely covered with desmos, and the posterior presence of a pointed spiny spinneret).

Tricoma (Tricoma) longicauda sp. nov. (Figs. 7-8; Table 3)

Type locality. Collected on May 23, 2023, by W. I. Jung, H. J. Lee, and H. S. Rho, the nematodes were sourced from intertidal benthic sediments in the subtidal zone of the East Sea (37°32ʹ27.28ʺN, 130°50ʹ34.78ʺE), Ulleungdo Island, Gyeongsangbuk-do, Korea, using a Smith-McIntyre grab at a depth of 20 meters.

Type material. The holotype male (MABIK NA0015 8130), mounted on a HS slide with glycerin, has been placed in the nematode collection at the Marine Biodiversity Institute of Korea (MABIK) in Seochun, Korea. Additionally, two paratype males (KIOST NEM-1- 2683, KIOST NEM-1-2684) and one paratype female (KIOST NEM-1-2685) have been deposited in the nematode collection within the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN) at the East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Diagnosis.Tricoma (Tricoma) longicauda sp. nov. is predominantly characterized by the following set of features: (1) possessing 37 tricomoid main rings; (2) cephalic setae longer than the head diameter, tapering into a fine open tip, and the entire setae flanked by a cuticular groove; (3) amphids exhibiting anterior circular and posterior cylindrical shapes, extending to main ring 3 or 4; (4) somatic setae consisting of 6-8 subdorsal setae and 9-10 subventral setae on each side; (5) spicules with a proximal capitulum, and a gubernaculum with the upper one-third region bent upward with a capitulum; (6) the tail comprising six main rings in males and five main rings in females; the terminal ring being relatively long and cylindrical, making up 56-65% of the total tail length, with a spinneret measuring 9-10 μm in length.

Etymology. The specific name originates from the Latin words “longus” (meaning long) and “cauda” (meaning tail), highlighting the elongated nature of the terminal tail ring.

Description. Males (holotype and paratypes). The body exhibits a length ranging from 380 to 514 μm, characterized by its slender and elongated form, tapering towards the posterior end (Fig. 7A). The maximum body diameter at the mid-body level measures 34-42 μm. The cuticle is comprised of 37 main rings, with the anterior 1 to 4 main rings and tail rings exhibiting a quadricomoid shape, while the other main rings display a tricomoid shape (Fig. 8A). Each main ring’s cuticular zone features secondary annulation and is coated with secretions and fine foreign material. The head has a diameter of 10-14 μm, being 1.0-1.2 times wider than longer (Fig. 8B). It tapers anteriorly from the cephalic setae, ending in a truncated manner, and has a more or less cylindrical neck region towards the posterior end. The anterior margin of the head cuticle is significantly thickened and sclerotized, forming a distinct rim-shaped border (Fig. 7C). The labial region does not protrude and is characterized by six labial papillae. Cephalic setae, measuring 14-18 μm in length, are longer than the width of the head and are inserted on a broad and low peduncle in the middle of the head. These setae have a broad and cylindrical basal part, tapering into a fine open tip, with the entire setae flanked by a cuticular groove (Fig. 7C). The amphids are long and vesicular, presenting a circular shape at the anterior end, almost covering the lateral side of the head. Posteriorly, they take on a relatively narrow cylindrical shape, extending to the main ring 3 or 4 (Figs. 7B, 8C). The stoma is small and cylindrical. The esophagus is cylindrical, measuring 64-73 μm in length, which constitutes about 14-18% of the total body length. The corresponding body diameter is 30-41 μm. The esophagus is encircled by a nerve ring at the level of main rings 4-5, with the esophagus-intestinal junction occurring at main ring 8 (Fig. 7A). Oval-shaped dark-yellowish ocelli, measuring 6-7 μm in width and 9-15 μm in length, are situated at the posterior part of the esophagus, between main rings 9-10 (Fig. 8A). Somatic setae, ranging from 8 to 30 μm, have a distal groove with a seemingly split tip and are inserted almost directly into cuticular rings with a peduncle (Fig. 7E). There are 6-8 subdorsal setae and 9-10 subventral setae on each side of the somatic setae, although some setae are presumed to be cut or damaged. The arrangement of somatic setae in the holotype male is as follows (numbers in parentheses indicate variable locations in paratype):

The reproductive system is characteristic of the subgenus, featuring two testes (Fig. 7A). The spicules are 20-29 μm in length, gently arched, with a distal tapering towards a pointed tip, and a proximal capitulum (Figs. 7D, 8F). The gubernaculum measures 26-31 μm, slightly longer than the spicules, and runs parallel to them. The gubernaculum tapers distally with a pointed tip, and the proximal one-third region bends upward with a capitulum (Figs. 7D, 8F). The tail is 102- 137 μm long, comprising 6 main rings. The terminal ring, accounting for 58-77 μm or 56-58% of the total tail length, consists of a relatively long and cylindrical anterior part, and an elongated spinneret (9-10 μm long). Approximately 83-87% of the terminal ring is covered with foreign materials (Fig. 8G). Phasmata are not visible.

Females. Similar to males in most aspects, females differ with smaller tail body rings and distinct sexual characteristics (Fig. 8D, E). The body measures 387 μm in length, with a maximum diameter of 39 μm. The body cuticle displays 37 main rings, adorned with secretions and fine foreign material (Fig. 8I). Somatic setae, consisting of 7-8 subdorsal setae and 10 pairs of subventral setae, are organized in the following manner:

The reproductive system is didelphic-amphidelphic, featuring both ovaries outstretched. The vulva is located between main rings 25 and 26, without protruding lips (Fig. 8I). The anal tube protrudes from the medioventral main ring 32 (Fig. 8H). The tail is comprised of 5 main rings, with the terminal ring measuring 58 μm in length, constituting 65% of the total tail length (Fig. 8H). It consists of a relatively long cylindrical anterior part covered with foreign materials and an elongated spinneret (9 μm long). Phasmata are not visible.

Remarks. The newly identified species, Tricoma (Tricoma) longicauda sp. nov., can be distinguished from all other species within the genus by its unique features. These include the relatively elongated and cylindrical terminal ring, constituting 56-65% of the total tail length, and featuring a pointed spiny spinneret. Additionally, the new species possesses long amphids extending to the 3-4 main rings. In terms of body characteristics, the new species shares similarities with T. (T.) bathycola, such as comparable body length (380-514 μm vs. 400-515 μm), the number of body rings (37 vs. 36-37 in T. (T.) bathycola), and the presence of a relatively long and cylindrical terminal ring with a pointed spiny spinneret.

However, T. (T.) longicauda sp. nov. distinguishes itself in several aspects, including the number of somatic setae [6-8 subdorsal setae and 9-10 subventral setae compared to 8-9 subdorsal setae and 11-12 subventral setae in T. (T.) bathycola], the length of cephalic setae (122-147% as opposed to 60-80% of the head diameter at the level of the cephalic setae), and the presence of amphids that extend to the 3-4 main rings (whereas amphids extend only to the first main ring in T. (T.) bathycola). Tricoma (T.) pygmaea bears some resemblance to the new species, particularly with amphids extending to the third to fourth main ring. However, it is distinctly characterized by its shorter body length [135-230 μm compared to 380-514 μm in T. (T.) longicauda sp. nov.], a higher number of main rings [42-43 vs. 37 in T. (T.) longicauda sp. nov.], the arrangement of somatic setae [4-5 subdorsal setae and 7 subventral setae in contrast to 6-8 subdorsal setae and 9-10 subventral setae in T. (T.) longicauda sp. nov.], and the presence of a medioventral papilla on the terminal ring—features absent in T. (T.) longicauda sp. nov.

Tricoma (Tricoma) ulleungensis sp. nov. (Figs. 9-10; Table 4)

Type locality. Collected on May 23, 2023, by W. I. Jung, H. J. Lee, and H. S. Rho, the nematodes were gathered from intertidal benthic sediments in the subtidal zone of the East Sea (37°32ʹ27.28ʺN, 130°50ʹ34.78ʺE), Ulleungdo Island, Gyeongsangbuk- do, Korea. The sampling was conducted at a depth of 20 meters using an SM grab.

Type material. The holotype male (MABIK NA0015 8131), preserved in glycerin on a HS slide, is archived in the nematode collection at the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, one paratype male (KIOST NEM-1-2687) and two paratype females (KIOST NEM-1-2688, KIOST NEM-1-2689) have been deposited in the nematode collection within the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Diagnosis.Tricoma (Tricoma) ulleungensis sp. nov. is distinguished by a unique combination of features, including: (1) cuticles with 54-57 tricomoid main rings; (2) a triangular head, 1.6-1.7 times wider than long; (3) slender cephalic setae flanked by a narrow membrane, inserted on elevated peduncles; (4) somatic setae comprising 6-7 subdorsal setae and 9-12 subventral setae on each side; (5) slightly curved spicules with a proximal offset capitulum; (6) dorsocaudally apophysis gubernaculum with a knob-like enlarged apical end; (7) a tail consisting of 8-9 main rings, with a conical terminal ring, where the anterior 21-31% is covered by desmos, and phasmata are present in a round configuration.

Etymology. The chosen specific name, ‘ulleungensis’, directly denotes the species̓ type locality, Ulleungdo Island in Korea.

Description. Males (holotype and paratype). Body length ranges from 409 to 415 μm, exhibiting tapering towards the extremities, particularly in the tail region. The maximum body diameter at the mid-body level is between 22 and 24 μm (Fig. 9A). In the holotype, the cuticle displays 54 tricomoid main rings, while in the paratype, there are 55 main rings. Each cuticular zone of the main rings consists of secretion and relatively fine foreign material, along with secondary annulation (Fig. 10A). In lateral view, the head is typically triangular, being approximately 1.6-1.7 times wider than long and gradually tapering anteriorly to a relatively broad truncated end (Fig. 10B). The head cuticles are covered with a thin layer, with the exception of a short labial region, which is especially sclerotized and thickened at the base of the peduncles of the cephalic setae (Fig. 9D). The labial region lacks division into separate lips and features a single crown of very fine papillae. Cephalic setae, measuring 20-22 μm, are slightly shorter than the head diameter at the level of the cephalic setae (Fig. 9D). They are inserted on elevated peduncles protruding from the posterior part of the head and are characterized by their slender, tapered structure, flanked by a narrow membrane along their entire length (Fig. 9C). The amphids are vesicular and circular, nearly entirely covering the head. They extend anteriorly to the labial region and posteriorly reach just before the border of the first main ring (Figs. 9C, 10C). The stoma is small and cylindrical. The esophagus is short, swollen anteriorly, then tapers at the posterior end of the head, widening again from the anterior end of the second main ring. The nerve ring encircles the esophagus opposite main ring 6, while the esophago-intestinal junction is positioned opposite the posterior end of main ring 10 (Fig. 9A). The intestine is broad and cylindrical. Ocelli, measuring 6-7 μm in width and dark yellowish, are located at the 14th or 15th main ring in the holotype and the 22nd or 24th main ring in the paratype (Fig. 10A). Somatic setae, ranging from 6 to 22 μm, are slender, tapering to a pointed end, and are inserted on relatively high peduncles that hardly protrude from the desmen. The anterior pair of subventral setae is mainly on main ring 3, displaced sublaterally (Fig. 9C). Somatic setae consist of 6 or 7 subdorsal setae and 10-12 subventral setae, with some setae cut or destroyed. The arrange ment of setae is somewhat variable depending on individuals. Somatic setae are organized as follows (numbers in parentheses denote variable location observed in paratype):

The reproductive system exhibits typical characteristics, with ejaculatory glands present along each side of the terminal part of the vas deferens. A small glandular cell is positioned above the spicule capitulum (Fig. 9A). The spicules, measuring 22-24 μm, are slightly curved, with a corpus usually featuring parallel walls, distally tapering to a sharp tip, and proximally possessing a slightly offset capitulum (Fig. 10D). The gubernaculum, measuring 11-13 μm, has an anterior portion with a relatively thin sclerotized wall parallel to the spicules. Posteriorly, it exhibits an obliquely dorsocaudally sclerotized apophysis with a knob-like enlarged apical end (Fig. 9E). The tail consists of 8 main rings in the holotype and 9 main rings in the paratype. The terminal ring is more or less conical, measuring 32-33 μm in length and constituting 35-38% of the total tail length (Figs. 9E, 10D). The cuticles of the anterior 27-29% are covered by desmos, while the posterior portion is naked, thickened, and sclerotized. Phasmata, rounded and 2 μm in diameter, are located at the posterior end of the desmos of the terminal ring (Fig. 9E).

Females. Most characteristics closely resemble those of males (Figs. 9B, 10E). The body is slim, ranging from 462 to 567 μm in length, and features 55-57 tricomoid main rings. Somatic setae consist of 6 or 7 pairs of subdorsal setae and 9 or 10 pairs of subventral setae. Somatic setae are organized as follows (numbers in parentheses indicate variable locations observed in paratype):

The reproductive system is didelphic-amphidelphic, with both branches outstretched. The vulva is situated in the naked medioventral part of the main ring 31, relatively protruding. An anal tube, measuring 4 μm in length, protrudes from the medioventral main ring 47 or 48 (Fig. 9B). The tail is composed of 8-9 main rings, measuring 86-99 μm in length. The terminal ring is 35-37 μm long and 9-10 μm wide, with the anterior one-fourth covered with foreign materials, leaving the posterior end uncovered. Phasmata, rounded and 2 μm in diameter, are located at the posterior end of the desmos of the terminal ring (Fig. 9B).

Remarks.Tricoma (Tricoma) ulleungensis sp. nov. is classified within a species group characterized by 50- 60 main rings. This group comprised eight species reported to date: T. (T.) absidataTimm, 1970, T. (T.) atlanticaFreudenhammer, 1975, T. (T.) bipapillataDecraemer, 1987, T. (T.) capitataDecraemer, 1987, T. (T.) coralicollaDecraemer, 1987, T. (T.) fisheriTimm, 1970, T. (T.) goldeniDecraemer, 1978, T. (T.) secundaBlome, 1982) and two subspecies T. (T.) absidata lizardiensis Decraemer, 1979, T. (T.) dimorpha papuensisDecraemer, 1987. The new species distinguishes itself from this group by its triangular head, which is wider than longer. Regarding the male copulatory system, T. (T.) absidata is the most morphologically similar species. However, they are clearly differentiated by the number of main rings (54-57 vs. 57-60), the arrangement of somatic setae (6-7 subdorsal setae and 10-12 subventral setae vs. 11-12 subdorsal setae and 17-19 subventral setae), and smaller spicule length (22-24 μm vs. 42-45 μm). Tricoma (T.) ulleungensis sp. nov. bears similarities to T. (T.) coralicolla in terms of the number of main rings (54-57 vs. 56-58), the arrangement of somatic setae (6-7 subdorsal setae and 10-12 subventral setae vs. 9 subdorsal setae and 11-12 subventral setae), and the shape of the head and terminal ring. However, it differs from T. (T.) coralicolla in having a longer body length (409-567 μm vs. 185-220 μm) and the presence of two dorso-caudal apophyses on the gubernaculum.

Tricoma (Tricoma) paralucidaDecraemer, 1987 (Figs. 11-12; Table 5)

Tricoma paralucidaDecraemer, 1987, p. 243, figs. 5-6.

Locality. Collected on May 23, 2023, by W. I. Jung, H. J. Lee, and H. S. Rho, the nematodes were retrieved from intertidal benthic sediments in the subtidal zone of the East Sea (37°32ʹ27.28ʺN, 130°50ʹ34.78ʺE), Ulleungdo Island, Gyeongsangbuk-do, Korea, at a depth of 20 meters using an SM grab.

Material examined. One male (MABIK NA00158132), preserved in glycerin on a HS slide, is archived in the nematode collection at the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, two males (KIOST NEM-1-2691, KIOST NEM- 1-2692) and one female (KIOST NEM-1-2693) have been deposited in the nematode collection within the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Description. Males. The body is relatively slender, measuring 993-1,087 μm in length, tapering towards the extremities, especially in the tail region. The maximum body diameter at the mid-body level is 39-48 μm wide (Fig. 11A). The cuticle is composed of 86 tricomoid main rings, each featuring secondary annulation, and the desmen consists of secretion along with fine and coarser foreign material (Fig. 12A). In side view, the head appears slightly hexagonal, tapering from the cephalic setae towards both sides with a broad truncated end in the labial region (Fig. 12B). The head cuticles are covered with a thin layer, except in the labial region, which is notably thick and sclerotized at the edge of the head (Fig. 12B). The labial region is slightly protruding and bears six minute papillae (Fig. 11C). The cephalic setae are slender, tapering to a pointed tip, wider along the proximal part, and flanked throughout their entire length by a membrane (Figs. 11B, 12C). Each seta is inserted on a high peduncle in the middle of the head. The amphids are rounded and large, almost entirely covering the head, extending anteriorly to the labial region and posteriorly to the border of the head (Fig. 12C). The stoma is shallow, covering the protruding end of the esophagus. The esophagus is cylindrical, measuring 104-115 μm in length, with a diameter of 36-44 corresponding to the body diameter at the level of the cardia. The esophagus is typically surrounded by the nerve ring opposite main ring 4, and the esophago-intestinal junction occurs at 9-10 main rings (Fig. 11A). The intestine is broadly cylindrical, featuring spherical inclusions of various dimensions. Ocelli, which are dark yellowish and oval shaped, have a diameter ranging from 7-12 μm and are situated opposite between main ring 11-13 (Fig. 12A). Somatic setae, measuring 8-21 μm in length, are fine and taper to a pointed tip. They are inserted on a row and a small peduncle. The somatic setae consist of 10-13 subdorsal setae and 19-25 subventral setae on each side, with the anterior pair of setae (on main ring 2) inserted laterally. The arrangement of somatic setae is as follows (numbers in parentheses indicate variable locations):

The reproductive system exhibits typical features, with only one large ejaculatory gland observed on each side of the posterior part (Fig. 11A). The spicules measure 38-39 μm in length, are slightly curved, and possess an offset capitulum proximally (Figs. 11D, 12D). The gubernaculum has a distal part with a sclerotized wall and proximally features two apophyses, each composed of a short and thin cuticular rod (Figs. 11D, 12D). The tail is conical, gradually tapering posteriorly, and consists of 13 main rings. The terminal ring is 34-38 μm long, with cuticles covered anteriorly in the region of 23-37%, posteriorly naked, and thickened and sclerotized except for the terminal spinneret (Fig. 12E). Phasmata, rounded and 3 μm in diameter, are situated posteriorly in the desmos of the terminal ring (Fig. 12E). Additionally, three well-developed caudal glands extend beyond the gubernaculum.

Female. In most aspects, it is identical to males but differs in the number of tail rings and sexual characteristics (Fig. 12J). The body is 1,207 μm long, with a maximum body diameter of 59 μm. The body cuticle has 86 main rings, covered by a desmos. Somatic setae include 11 and 12 subdorsal setae, and 27 and 28 subventral setae on each side (Fig. 11E). The anterior- most pair of setae on main ring 2 (left side) and 3 (right side) is inserted sublaterally. The arrangement of somatic setae is as follows:

The head and digestive system exhibit the same characteristics as in the male (Fig. 12F, G). The anal tube protrudes medioventrally from the body wall at the main ring 74 (Fig. 12H). The ocelli are rounded and irregularly shaped between main ring 10-11. The reproductive system is typical, with the vulva slightly protruding from the naked medioventral part of the body wall at main ring 45 (Fig. 11E). The tail consists of 12 main rings, measuring 160 μm in length. The terminal ring is 43 μm long, with cuticles covered anteriorly in demos, posteriorly naked and thickened and sclerotized, except for the terminal spinneret (Fig. 12I). Three consecutive well-developed caudal glands extend anteriorly just above the anus.

Remarks.Tricoma (Tricoma) paralucida, as described by Decraemer in 1987, is primarily characterized by the following combination of features: (1) cuticles with 86-89 tricomoid main rings; (2) a broad triangular head with a truncated end; (3) the labial region exhibiting noticeable circular spaces in the cuticle at the level of the labial papillae, resembling forked structures; (4) somatic setae comprising 9-10 subdorsal setae and 19-21 subventral setae; (5) spicules featuring a proximal offset capitulum, and a gubernaculum with two apophyses proximally. The Korean specimen of T. (T.) paralucida presented here closely aligns with Decraemer’s original description, particularly in terms of the broad triangular head, 86 tricomoid main rings, labial region with forked structure, and the copulatory apparatus featuring two dorsocaudally oriented apophyses of the gubernaculum. However, the Korean specimens of T. (T.) paralucida do not entirely conform to Decraemer’s original description in terms of body length (993-1,087 μm vs. 730-740 μm in males, 1,207 μm vs. 725-790 μm in females) and the number of subventral somatic setae. Specifically, the Korean specimens exhibit 10-13 subdorsal setae and 19-28 subventral setae, whereas the original description mentions 9-10 subdorsal and 19-21 subventral setae.

Distribution. Papua New Guinea and Korea.

Tricoma (Tricoma) similisCobb, 1912 (Figs. 13-15; Table 6)

Tricoma similisCobb, 1912, p. 482, figs. 1-2;Timm, 1970, p. 67, figs. 144-145, pl. 7 fig. 60; Decraemer, 1978, p. 91, figs. 3A, F, G, 30-35; Decraemer, 1983, p. 18, pl. 7 fig. A-C; Decraemer and Tchesunov, 1996, p. 124.

Locality. Collected on May 23, 2023, by W. I. Jung, H. J. Lee, and H. S. Rho, the nematodes were extracted from benthic sediments in the shallow subtidal zone of the East Sea (37°32ʹ27.28ʺN, 130°50ʹ34.78ʺE), Ulleungdo Island, Gyeongsangbuk-do, Korea, at a depth of 20 meters using an SM grab.

Material examined. One male (MABIK NA00158133), mounted in glycerin on a HS slide, is archived in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, three males (KIOST NEM-1-2695, KIOST NEM-1-2696, KIOST NEM-1-2697) and one female (KIOST NEM-1- 2698) have been placed in the nematode collection at the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

Description. Males. The body is elongated, measuring 979-1,110 μm, tapering towards the extremities, particularly in the tail region. The maximum body diameter at the mid-body level is 46-54 μm (Fig. 13E). The cuticle exhibits 79-80 tricomoid main rings, each characterized by secondary annulation and covered with secretion and fine foreign material (Fig. 14A). In side view, the head appears triangular, tapering anteriorly from the cephalic setae to a truncated end (Figs. 13B, 14B). The cuticle is notably thickened and sclerotized along the border, excluding the labial region (Fig. 13B). The labial region is comprised of six lips with small papillae. Cephalic setae, measuring 29-38 μm in length, are slender and taper to a pointed tip, flanked by a membrane over their entire length. They are inserted on high peduncles that protrude from the posterior part of the head (Figs. 13B, 14B). The amphids are rounded, almost entirely covering the head, extending anteriorly to the labial region and posteriorly to the head border (Fig. 14C). The stoma is shallow, encompassing the protruding end of the esophagus. The cylindrical esophagus measures 115-119 μm in length, with a diameter of 40-47 μm at the level of the cardia. The typical esophagus is surrounded by the nerve ring opposite the main ring 5, and the esophago- intestinal junction occurs between the main rings 9-10 (Fig. 13E). The dark-yellowish ocelli are long and ellipse-shaped, located opposite between the main rings 10-13 (Fig. 14A). Somatic setae, ranging from 12-33 μm in length, are fine and tapering, flanked by a membrane over their entire length, and inserted on very row peduncles. The somatic setae consist of 12-17 subdorsal setae and 23-30 subventral setae on each side. The anterior pair of setae (on main ring 2) is inserted laterally (Fig. 13A). Some setae are cut or destroyed, and the arrangement of setae varies somewhat among individuals. Somatic setae are organized as follows (numbers in parentheses indicate variable locations):

The reproductive system is typical, featuring a large ejaculatory gland on both sides flanking the posterior vas deferens. Spicules measure 59-71 μm, exhibit an arched shape, taper distally to a pointed tip, and possess a proximal offset capitulum (Figs. 13C, 14E). The gubernaculum is 33-41 μm long, with a distal part featuring a strongly sclerotized wall, tapering to a blunted end, and proximally exhibiting weakly sclerotized apophyses with a knob-like enlarged apical end (Figs. 13C, 14E). Two short and curved medioventral genital setae (8-10 μm long) are situated between main rings 54 to 58 (Fig. 14D). The tail is conical, composed of 11-12 main rings. The terminal ring, 32-40 μm long, has cuticles covered with anteriorly 31-40% in the region, posteriorly naked, except for the terminal spinneret, and is thickened and sclerotized (Fig. 14E). Phasmata, rounded with a diameter of 3 μm, are located posteriorly in the desmos of the terminal ring.

Female. In most aspects, females closely resemble males but differ in the number of cuticular body rings, tail rings, and sexual characteristics (Fig. 15A-C). The body measures 1,011 μm in length, with a maximum diameter of 52 μm. The ventral part of the body cuticle consists of 77 main rings, while the dorsal part has 79 main rings (Fig. 15A). The head and digestive system are similar to those of males (Fig. 15B, C). The anal tube protrudes medioventrally from the body wall at main ring 67 (Fig. 15E). The ocelli are long and ovalshaped, situated between main rings 11-12. Somatic setae comprise 12 or 14 subdorsal setae and 11 or 16 subventral setae on each side, with some setae likely cut or damaged, and the anterior pair of setae on main ring 3 is inserted sublaterally. Somatic setae organized in the following manner:

The reproductive system is typical, with the vulva slightly protruding from the exposed medioventral part of the body wall at main ring 45 (Figs. 13D, 15D). Two large spermathecae contain globular spermatozoids. Both branches of the genital system overlap and extend beyond the vagina in opposite directions (Fig. 13D). The tail consists of 10 main rings, measuring 133 μm in length. The terminal ring is 42 μm long, with cuticles covered anteriorly in a demos, and posteriorly naked, thickened, and sclerotized, except for the terminal spinneret (Fig. 15E).

Remarks.Tricoma (Tricoma) similis Cobb, 1920, is a species with a broad distribution. Originally discovered in the East Indies (Cobb 1912), it has been subsequently documented in various locations such as Bermuda (Timm 1970), the Great Barrier Reef (Decraemer 1978), the Isles Glorieuses, the Mozambique Channel (Decraemer 1983), and the White Sea (Decraemer and Tchesunov 1996). Tricoma (T.) similis can be distinguished from other species within its genus by several key characteristics: (1) possessing 77-84 main rings, (2) somatic setae consisting of 14-16 subdorsal setae and 19-27 subventral setae on each side, (3) exhibiting a distinctive triangular head, (4) featuring arched spicules with a terminally offset capitulum, (5) having a gubernaculum with a weakly apophysis and a knoblike enlarged apical end, and (6) bearing two short, setiform medioventral setae. The unusual variant of the species discovered in the Mozambique Channel was characterized based on a solitary male specimen, exhibiting 126-128 main rings, 11-12 subdorsal setae, 19 subventral setae, and spicules measuring 36 μm, slightly shorter than previously documented. The Korean specimens of T. (T.) similis closely match the description provided by Decraemer (1978), particularly in terms of the head and tail morphology and the male copulatory system. These Korean specimens also possess a key diagnostic feature of T. (T.) similis - the presence of two short male genital setae. However, deviations from previous descriptions are noted in the Korean specimens of T. (T.) similis due to their extended body length (500-870 μm compared to 979-1,110 μm in Korean specimens) and the arrangement of somatic setae (14-16 subdorsal setae and 19-27 subventral setae compared to 12-17 subdorsal setae and 11- 30 subventral setae in Korean specimens).

Distribution. East Indies (Cobb 1912), Bermuda (Timm 1970), Australia (Decraemer 1978), Mozambique Channel (Decraemer 1983), White Sea (Decraemer and Tchesunov 1996), and Korea (this study).

4. DISCUSSION

The genus Tricoma was initially introduced by Cobb in 1894, using the type species Tricoma cinctaCobb, 1894, discovered in the sea sand of the Bay of Naples. Cobb (1894) assigned the genus name Tricoma under the misconception that the species possessed only three cephalic setae. Although the description of the type species was based on recollection and a lost specimen, two earlier drawings undeniably validate the existence of the genus. The genus Tricoma underwent an initial revision in 1922 by Filipjev, followed by subsequent revisions conducted by Timm (1970), Gerlach and Rieman (1973), Freudenhammer (1975), and Decraemer (1978). These revisions aimed to reevaluate poorly described species, designate some as inquirenda, and synonymize others. The outcomes of these revisions have contributed valuable morphological descriptions and systematic identification keys that continue to be utilized in contemporary research. Since the last revision by Decraemer in 1978, an additional 29 species have been described globally, bringing the total number to 80 valid species and three valid subspecies. Among the Tricoma genus, 34% (28 taxa) are documented in Europe, 28% (24 taxa) in Australia and Papua New Guinea, and 20% (16 taxa) in North or South America. The remaining 18% (15 taxa) are distributed across Africa, Asia, Antarctica, the Pacific and Indian oceans. Notably, 25% (21 taxa) of the reported species thrive in deep-sea marine environments beyond 200 meters, contributing to the exploration of deep-sea biodiversity.

The taxonomic challenges within the subgenus Tricoma exceed those in other genera, stemming from the abundance of species, inadequate information on internal organs and the labial region, descriptions relying on single specimens observed in dorsal-ventral optical view, and species identification based solely on the number of main rings without considering interspecific variability (Decraemer 1979a). Within the Tricoma subgenus, 40% (33 taxa) are documented based on one or two specimens, with 14 taxa described from only a single specimen. The combination of high species diversity and low species richness poses challenges in sampling, and discerning species-level variability from a single specimen is inherently challenging. Specifically, the diagnostic value of the subgenus Tricoma in terms of somatic setae pattern and main ring number is more limited compared to the genus Desmoscolex (Decraemer 1979a). Certain species within the subgenus Tricoma display significant variability in their main ring count. For instance, Tricoma timmiDecraemer, 1978, and Tricoma blomeiDecraemer, 1996 differ by 12 to 14 rings between their maximum and minimum counts. In contrast, Tricoma species with fewer than 40 main rings exhibit a smaller difference of one to three. It's essential to acknowledge that somatic setae may be truncated or destroyed, contributing to a broader range of variability. In Tricoma peruvensisNichols and Musselman, 1979, the subdorsal setae range from 11 to 20, and the subventral setae range from 21 to 34. While identifying species can be facilitated by examining the main ring or somatic setae patterns, their variation in most species is not well-established and should not be overly emphasized.