Bibliographic revision of Mesacanthion Filipjev, 1927 (Nematoda: Thoracostomopsidae) with description of a new species from Jeju Island, South Korea

Sampling and morphological study

A series of sampling took place in June 2018, during a survey of natural beaches of Jeju Island, South Korea (Fig. 1). Two sub-samples of the sediments from the intertidal zone were obtained using a 10 cm² acryl sampling tube. Sediments were fixed in 5% neutralized formalin solution and brought back to the laboratory. Meiofauna were extracted using the Ludox method (Burgess, 2001), and post-fixed with 70% ethanol dyed with Rose bengal. Nematodes were counted and individual specimens of interest were picked to a Petri dish filled with 10% glycerin. The dish was placed in a drying oven set at 40 °C for a day or two to be completely dehydrated as conferred in the glycerin-ethanol method (Seinhorst, 1959). A single or as many as five specimens (depending on their size) were mounted in a single drop of anhydrous glycerin on a glass slide using the wax-ring method (Hooper, 1986). Mounted specimens were identified under Olympus BX51, Leica DM5000B and DM2500 microscopes. All morphometric measurements were done manually using IC measure v.2.0.0.161 software. For scanning electron microscopy, specimens were placed in a drop of glycerin and gradually mixed with drops of distilled water to be washed from any remnant of glycerin. Hydrated specimen were treated to ethanol series for dehydration (20%, 40%, 50%, 70%, 80%, 90%, 95%, 100%, for 10 min each) and then placed in hexamethyldisilazane (HMDS). Specimens bathed in HMDS were placed in a drying oven to be dried. Once dried, specimens were mounted on a stub to be splutter coated, and observed with COXEM EM-30 microscope.

Revision of the genus

The Bremerhaven Checklist of Aquatic Nematodes by Gerlach & Riemann (1974) was used as primary referral when collecting original descriptions/references and additional information on their distribution. Any updates and changes made to the genus subsequent to 1974 were checked using NeMys, World Database of Nematodes. Once all references had been collected; (1) tabular key consisting of diagnostic characters of all valid species were compiled, (2) distribution of species were determined, (3) validity of each species were determined via comparison and examination, (4) diagnosis of the genus was updated. To construct a pictorial key, original depictions were collected from respective papers and their heads, tails and spicules (if available) were resized and oriented using Adobe Photoshop CS6 for optimum comparison between species. The original drawings were retraced using Wacom Intuous Pro Pen Tablet and Adobe Photoshop CS6.

Nomenclatural acts

The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub: 989DF431-166A-4534-9A37-9AC408194DE7. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.

List of valid species

1. Mesacanthion africanthiforme Warwick, 1970 (Warwick, 1970: 142–145, fig. 2A–E; three males and three females, Exe estuary, England).

2. Mesacanthion africanum Gerlach, 1957 (Gerlach, 1957b: 4, fig. 3A–C; description based on one male, Atlantic at Congo mouth, plankton net from above muddy ground).

3. Mesacanthion agubernatus Vitiello, 1971 (Vitiello, 1971: 860, fig. 1A–E; description based on one male, Mediterranean, terrigenous coastal muds, 60 m deep).

4. Mesacanthion alexandrinus Nicholas, 1993 (Nicholas, 1993: 163, 165, fig. 1A–E, 2A–D; four males and three females, sand at water edge of fresh-water Lake Alexandrina, South Australia).

5. Mesacanthion arabium Warwick, 1973 (Warwick, 1973: 114–116, fig. 14A–G; three males and three females, Arabian Sea, fine sand, 49 m deep).

6. Mesacanthion arcuatile Wieser, 1959 (Wieser, 1959: 16–17, Pl. 11 fig. 11A–B; description based on one female, Alki Beach, Washington, US, 6.5 ft, lapsus arcuatilis).

7. Mesacanthion armatum Timm, 1961 (Timm, 1961: 32, fig. 5A–C; more than one male and one female, Bay of Bengal, on Siphonocladus, lapsus armatus).

8. Mesacanthion audax (Ditlevsen, 1918) Filipjev, 1927 [Ditlevsen, 1918: 208–209, pl. 14 fig. 4, 7, pl. 15 fig. 5 (=Enoplolaimus audax); description based on one male, Øresund, off Aalsgaarde. Filipjev, 1927: 143; transfer Enoplolaimus audax to subgenus Mesacanthion. Gerlach, 1958b: 73; (as Mesacanthion audax), Kiel Bay, Sand and silt, 6 m deep. Riemann, 1966: 186; three males, North Sea, sand].

9. Mesacanthion banale (Filipjev, 1927) Gerlach & Riemann, 1974 [Filipjev, 1927: 147, Pl. 7 fig. 40A, B; (=Enoplolaimus (Mesacanthion) banalis), description based on three females, Barents Sea, muddy sand, 25 m deep. Gerlach & Riemann, 1974: 531; transfer Enoplolaimus banale to genus Mesacanthion].

10. Mesacanthion breviseta (Filipjev, 1927) Gerlach & Riemann, 1974 [Filipjev, 1927: 150–151, pl. 7 fig. 43A–C; (=Enoplolaimus (Mesacanthion) breviseta) description based on one male and a juvenile male, Barents Sea, sand with shells and stones, 83 m deep. Gerlach & Riemann, 1974: 531; transfer Enoplolaimus (Mesacanthion) breviseta to genus Mesacanthion].

11. Mesacanthion cavei Inglis, 1964 [Inglis, 1964: 313–314, fig. 76–78; description based on two males (one in poor condition) and one damaged juvenile, South Africa, coarse sand and broken shells, 26–27 m deep].

12. Mesacanthion ceeum Inglis, 1964 (Inglis, 1964: 313, fig. 74–75; description based on one male and one juvenile, South Africa, coarse sand and broken shells, 26 m deep, lapsus ceeus).

13. Mesacanthion conicum (Filipjev, 1918) Filipjev, 1927 [Filipjev, 1918: 105–107, Table 3, fig. 16A–B; (=Enoplolaimus conicus), description based on one female, Black Sea. Filipjev, 1927: 143; transfer Enoplolaimus conicus to subgenus Mesacanthion].

14. Mesacanthion cricetoides Wieser, 1959 (Wieser, 1959: 17–18, fig. 13A–B; description based on one female, Richmond Beach, Washington, 2.5 ft deep).

15. Mesacanthion diplechma (Southern, 1914) Filipjev, 1927 [Southern, 1914: 55–56, fig. 25A–J; (=Enoplus diplechma), two males and two females, Clew Bay, sandy bottom, 25–31 m deep. Filipjev, 1927: 143; transfer Enoplus diplechma to subgenus Mesacanthion. Gerlach, 1958: 72; as Mesacanthion diplechna, Kiel Bay, silt, 8 m deep. Riemann, 1966: 186 North Sea, sand. Boucher, 1977: 741–743, Figs. 4A–4E; as Mesacanthion diplechma Southern, 1914, one male, three females and six juveniles, Pierre Noire (Western Channel), infralittoral sands].

16. Mesacanthion ditlevseni (Filipjev, 1927) Gerlach & Riemann, 1974 [Filipjev, 1927: 148, pl. 5 fig. 41A–D; (=Enoplolaimus (Mesacanthion) ditlevseni), three males and one female, Barents Sea, silt with stones, 36–280 m deep. Ditlevsen, 1928: 210–213, fig. 8–13; (=Enoplolaimus angustignathus), one male and one female, Greenland, mud, clay, 100–200 m deep, De Coninck and Stekhoven, 1933: 38. Allgén, 1954: 22; (as Enoplolaimus (Mesacanthion) angustignathus), five males and nineteen females, Jan Mayen, Greenland, black sand, 23 m deep. Gerlach & Riemann, 1974: 532; transfer Enoplolaimus ditlevseni to genus Mesacanthion].

17. Mesacanthion fricum Inglis, 1966 (Inglis, 1966: 87, fig. 10–12; description based on one male, South Africa, sand, lapsus frica).

18. Mesacanthion heterospiculum Sergeeva, 1974 (Sergeeva, 1974: 123, fig. 4A–4B; description based on 14 males, Black Sea, various depths and sediments).

19. Mesacanthion hirsutum Gerlach, 1953 (Gerlach, 1953: 536–537, fig. 9A–E; description based on one male and one female, Mediterranean. Gerlach, 1967: 26, fig. 10A–E; two males, two juveniles and one male, Sarso Island, Red Sea, Saudi Arabia).

20. Mesacanthion infantile (Ditlevsen, 1930) De Coninck & Schuurmans Stekhoven, 1933 [Ditlevsen, 1930: 205–208, fig. 8–10; (=Enoplolaimus infantilis), one male and one female, Stewart Island, Halfmoon Bay, sand, 5–7 fms. Allgén, 1951: 322–323, fig. 33A–B; (=Enoplolaimus mortenseni), description based on one female, Australia see Mawson, 1956: 65–66 (re-examination of type specimen=Mesacanthion infantilis), op Wieser, 1953: 75. Allgén, 1951: 323–324, fig. 34A–B; (=Enoplolaimus philippinensis), description based on one juvenile, Australia, op Mawson, 1956: 65–66 (re-examination of type specimen=Mesacanthion infantilis). De Coninck & Schuurmans Stekhoven, 1933: 38; (as Mesacanthion infantile). Wieser, 1953: 76, fig. 39A–Btwo females, Chile. Mawson, 1956: 65–66, fig. 29A–C; two juveniles, Antarctica].

21. Mesacanthion karense (Filipjev, 1927) Gerlach & Riemann1974 [Filipjev, 1927: 152, pl. 7 fig. 45A–C; (=Enoplolaimus (Mesacanthion) karensis), one juvenile male and three females, Kara Sea, sand, 15 m deep. Gerlach & Riemann, 1974: 533; transfer Enoplolaimus (Mesacanthion) karensis to genus Mesacanthion].

22. Mesacanthion kerguelense Mawson, 1958 (Mawson, 1958: 338–339, fig. 22A–D; five males, two females and three juveniles, Kerguelen Island, Heard Island, Macquarie Island).

23. Mesacanthion longispiculum Gerlach, 1954 (Gerlach, 1954: 228–229, fig. 1A–B; one male and one female, Mediterranean. Gerlach, 1957a: 421; Brazil. Gerlach, 1958a: 352–353, fig. 4A–C; (as cf. longispiculum), one male, Mananjary, Madagascar, muddy sand).

24. Mesacanthion longissimesetosum Wieser, 1953 (Wieser, 1953: 78–79, fig. 42A–E; two males, one female and thirteen juveniles, Chile, littoral exposed and sheltered sand, sublittoral secondary substratum and soft bottom, lapsus longissimesetosus).

25. Mesacanthion lucifer (Filipjev, 1927) Gerlach & Riemann, 1974 [Filipjev, 1927: 149–150, pl. 7 fig. 42A–C; (=Enoplolaimus (Mesacanthion) lucifer), one male and two females, Barents Sea, Kara Sea, sand and sandy silt, 18–83 m deep. (Gerlach & Riemann, 1974): 533; transfer Enoplolaimus (Mesacanthion) lucifer to genus Mesacanthion.]

26. Mesacanthion majus (Filipjev, 1927) Gerlach & Riemann, 1974 [Filipjev, 1927: 151–152, pl. 7 fig. 44A–C; (=Enoplolaimus (Mesacanthion) major), three females, Kara Sea, Barents Sea, sand and gravel, 15–36 m deep. Wieser, 1953: 78, fig. 41A–D; (as Mesacanthion major (Filipjev, 1925b), four males, two females and 15 juveniles, Arctic Sea, Chile, sublittoral, secondary substratum and coarse bottom, lapsus major. Gerlach & Riemann, 1974: 533; (as Mesacanthion majus Filipjev, 1927).]

27. Mesacanthion marisalbi Galtsova, 1976 (Galtsova, 1976: 261–263, fig. 7; two males, one female and one juvenile, White Sea, littoral zone in slightly silted sand).

28. Mesacanthion monhystera Gerlach, 1967 (Gerlach, 1967: 27–28, fig. 11A–F; one male and one juvenile female, Red Sea, sandy beach and littoral subsoil water).

29. Mesacanthion obscurum Gagarin & Klerman, 2006 (Gagarin & Klerman, 2006: 533–535, fig. 1A–E; twelve males and eight females, Mediterranean Sea off the Israeli coast near Hadera, sandy sediment, 30–35 m deep).

30. Mesacanthion pali Wieser1959 (Wieser, 1959: 16, fig. 10A–B; description based on one male, Puget Sound, subterranean water, medium fine to coarse sand).

31. Mesacanthion pannosum Wieser, 1959 (Wieser, 1959: 17, fig. 12A–D; one female and one female, Puget Sound, medium fine to coarse sand, 2.5 ft deep).

32. Mesacanthion propinquum Gagarin & Klerman, 2006 (Gagarin & Klerman, 2006: 536–538, fig. 2A–E; twelve males and eleven females, Mediterranean Sea off the Israeli coast near Hadera, sandy sediment, 30–35 m deep).

33. Mesacanthion proximum Gerlach, 1957 (Gerlach, 1957a: 427–429, fig. 5G–5M; one male and one juvenile, Santos, Brazil, fine sand).

34. Mesacanthion rigens Gerlach, 1957 (Gerlach, 1957a: 427, fig. 5C–5F; one male and one female, Bertioga, Brazil. Gerlach, 1956: 204; Brazil, nomen nudum).

35. Mesacanthion southerni Warwick, 1973 (Warwick, 1973: 111–114, fig. 12A–C, 13A–C; six males, three females and two juveniles, Arabian Sea, fine sand and fine muddy sand, 48–49 m deep).

36. Mesacanthion studiosum Inglis, 1964 (Inglis, 1964: 315–316, fig. 79–90; two males, two females and two juveniles, South Africa, coarse white sand, 27 m deep, lapsus studiosa).

37. Mesacanthion tenuicaudatum (Ssaweljev, 1912) De Coninck & Schuurmans Stekhoven, 1933 [Ssaweljev, 1912: 111–112; (=Enoplolaimus tenuicaudatus), both sex but number of specimen not specified, White Sea, lapsus tenuicaudatus. De Coninck & Schuurmans Stekhoven, 1933: 39; transfer and correct name from Enoplolaimus tenuicaudatus to Mesacanthion tenuicaudatum].

38. Mesacanthion virile (Ditlevsen, 1930) De Coninck & Schuurmans Stekhoven, 1933 [Ditlevsen, 1930: 208–211, fig. 11–14; (=Enoplolaimus virilis), description based on one male, Stewart Island; Halfmoon Bay, New Zealand, Sand, 9.1–12.8 m(converted from fathom). De Coninck & Schuurmans Stekhoven, 1933: 39; transfer and correct name Enoplolaimus virilis to Mesacanthion virile. Allgén, 1959: 48–50; 8 females and twelve juveniles, Falkland Islands, South Georgia, Graham Land].

Species Inquirenda

1. Mesacanthion brachycolle Allgen, 1959 [Allgén, 1959: 50, fig. 32A, B; two females and two juveniles, Falkland Islands, sandy bottom with algae, 40 m deep, Graham Island, mud, 125 m deep. Allgén, 1960: 479, fig. 3; (as Enoplolaimus (Mesacanthion) brachycollis), lapsus brachycollis, one female and one juvenile, Falkland Islands]. Species Inquirenda. This species is placed as species inquirenda due to the following reasons: (1) substandard quality of the original text and figures making it impossible to understand, to which genus this species should be referred as; (2) ambiguity of the material, where females and juveniles are indicated as the only materials yet a male tail is given on fig. 32B.

2. Mesacanthion donsitarvae (Allgen, 1935) Wieser, 1953 (species inquirenda) [Allgén, 1935: 47; (=Enoplolaimus donsitarvae) Norway, lapsus (donsi)-tarvae. Wieser, 1953: 76; transfer Enoplolaimus donsitarvae to genus Mesacanthion and opinionates the fact that Allgén provided no figures and description was based on erroneous data of Ditlevsen on wrong number of cephalic setae].

3. Mesacanthion gracilisetosum (Allgen, 1930) Wieser, 1953 (species inquirenda) [Allgén, 1930: 189–191, Figs. 1–3; (=Enoplolaimus gracilisetosus), one male, two females and one juvenile, Macquarie Island. Wieser, 1953: 76; transfer Enoplolaimus gracilisetosus to genus Mesacanthion, lapsus gracilisetosus ].

4. Mesacanthion hawaiiense (Allgen, 1951) Wieser, 1953 (species inquirenda) [Allgén, 1951: 274–275, fig. 5A–5B; (=Enoplolaimus hawaiiensis), description based on one female, Honolulu, Hawaii. Wieser, 1953: 75; transfer Enoplolaimus hawaiiensis to genus Mesacanthion and opinionates description is insufficient, lapsus hawaiiensis ].

5. Mesacanthion pacificum (Allgen, 1947) Wieser, 1953 (species inquirenda) [Allgén, 1947: 212, fig. 76A–B; (=Enoplolaimus pacificus), description based on one female and one juvenile, Bay of Panama, Perlas Island. Allgén, 1951: 275, 277, Figs. 6A–6D; one male, one female and three juveniles, Coast of Honolulu. Wieser, 1953: 66, 76; transfer Enoplolaimus pacificus to genus Mesacanthion and opinionates it resembles Oxyonchus more. Allgén, 1959: 48; (as Mesacanthion pacificus), two juveniles, Falkland Islands, sand and small stones with algae, 40 m deep, lapsus pacificus].

6. Mesacanthion paradentatum (Allgen, 1932) Wieser, 1953 (species inquirenda) [Allgén, 1932: 111–112, fig. 8A–B; (=Enoplolaimus paradentatus), description based on one juvenile, Campbell Island. Wieser, 1953: 76; transfer Enoplolaimus paradentatus to genus Mesacanthion, lapsus paradentatus].

7. Mesacanthion primitivum (Allgen, 1929) Wieser, 1953 (species inquirenda) [Allgén, 1929: 441, fig. 6A–B; (=Enoplolaimus primitivus), Skagerrak. Wieser, 1953: 76; transfer Enoplolaimus primitivus to genus Mesacanthion, lapsus primitivus].

8. Mesacanthion ungulatum (Wieser, 1953) Wieser, 1953: 78, fig. 40A–B; description based on two juveniles, Seno Reloncavi proper, Chile, exposed littoral algae, lapsus ungulatus). Species inquirenda. Further discussed in the discussion.

Nomen nudum

1. Mesacanthion microsetosus Allgen, 1932 (nomen nudum –Bezerra et al., 2019) [Allgén, 1932: 110–111, fig. 7A–B; (=Enoplolaimus microsetosus) description based on one juvenile, Campbell Island 40 m deep. Allgén, 1959: 48; (transfer Enoplolaimus microsetosus to genus Mesacanthion) nine females and five juveniles, South Georgia, Antarctica, clay with sparse stones, 125 m deep, lapsus microsetosus, nomen nudum]. Only female or juvenile used for description and according to Wieser, 1953, Allgén stating four labial and four cephalic setae makes his description doubtful, Wieser, 1953: 82; moved to Paramesacanthion.

Type locality: Intertidal zone at coast of Jeju Island, South Korea (33°26′05″N 126°55′15″E), in sandy beach

Type material: All specimen deposited in National Institute of Biological Resources (South Korea). Holotype 1♂ (NIBRIV00008488276) on one slide, Allotype 1♀ (NIBRIV00008488277) on one slide, Paratypes 2♂♂, 1♀ on two different slides (NIBRIV00008488278–NIBRIV00008488279), 1♂ and 1♀ dried, mounted on two separate stubs and coated with gold for SEM (NIBRIV00008488280–NIBRIV00008488281) from coast of Jeju Island, South Korea (33°26′05″N 126°55′15″E) collected on 17 June 2018.

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

Description: Male (Fig. 2). Cuticle smooth above cephalic capsule, finely striated posterior to cephalic capsule until tail tip. Three lips well developed; edges of lips narrowed and distally pointy curving outwards, each lips carrying two inner labial setae. Six inner labial setae, stout and conical 12 µm long. Six longer outer labial setae and four shorter cephalic setae sharing one crown, situated at midlevel of cephalic capsule. Cephalic capsule vaguely set off at mid-level, anterior part narrow, and posterior part gradually thicker. Buccal cavity funnel shaped, wide at anterior end, gradually narrowing to the base. Coffee-bean shaped epidermal glands distributed along dorsal plane from anterior end of body until posterior end. Buccal cavity armed with three mandibles and three teeth. Mandible consisting of two rods distancing from one another anteriorly joined by anterior rod. Lateral edges of each rod with teeth or denticle pointing to the lumen (Fig. 3E). ∼5–6 short cervical setae in singles at level of posterior end of cephalic capsule. Modified cervical setae, a flap, inverse triangular, just posterior to a single lateral outer labial setae at posterior end of cephalic capsule, observable in all four males on both lateral body sides (Fig. 4B). Amphid ambiguously present below the cervical flap, pouch-shaped. Two pores observed diagonally below cervical flap and amphid (Fig. 4B). Cervical somatic setae in 8 groups of 2–3 around pharyngeal region a, roughly two cephalic capsule lengths below level of cephalic capsule end (Figs. 4A and 4B). Some cervical setae partly possessing irregular lateral and terminal processes, resembling penicillus or plumule (Figs. 4A and 4B). Somatic setae scarcely distributed along the body in singles until tail region. Pharynx fairly long and annulated with plasmatic lens-like interlayers and sinuous external contours, cardia triangular and going into the middle of intestine. Metanemes not visible. Testes paired opposed, both ends situated to the right of the intestine. Thick supplement, 18 µm long, 165 µm above from cloacal opening. Spicules paired, bipartite, symmetrical, curved slightly and thick. Each spicule with distinct transverse, oblique seam, dividing it distal and proximal portions (Figs. 5A and 5B). Distal portion shorter than proximal portion. Distal portion slightly curved towards cloacal opening, anterior end with one denticle just above and/away from its round pointy end. Proximal portion rather straight, posterior end with a knob/neck-like constriction. Gubernaculum embracing spicules, shaped like irregular triangle, lateral end which lies lateral to the spicule, almost perpendicular to axis of the anus, even extending beyond distal end of spicule, and the other end arching off at an angle towards the tail. Tail elongated and papilliform. five somatic setae in tail region. Caudal gland protruded anterior to the anus, their nucleus-containing bodies located along the posterior midgut. Spinneret well developed. 1 short caudal (terminal) setae (with porous) just above distal end of tail.

Female (Fig. 3). Female generally longer and larger in size. Three lips higher in female, edges of lips noticeably stronger in female, distal end aggressively curved, each lips carrying two inner labial setae. No subtle sexual dimorphism found in setae in the head region, other than shorter length outer labial and cephalic setae compared to male. Short knobs on each anterior end of mandible. Female lacking cervical setae flap on cephalic capsule end. Amphid not observed. Groups of cervical setae found in esophageal region in males are in singles as opposed to doubles/trios (Fig. 5D). Vulva located at 55% of total body length with protruding lips. Reproductive system didelphic amphidelphic, both ends flexed inwards. Both ovaries positioned left of the intestine, antidromously reflexed. Tail conico-cylindrical, three somatic setae in tail region with no apparent caudal setae.

Diagnosis: Mesacanthion. Body length 2700–4630 µm. Cuticle finely striated along the body, smooth only in cephalic capsule region, head set off with cephalic capsule. Metanemes not visible. Six inner labial setae 8–15 µm. Six longer outer labial setae 36–59 µm, four shorter cephalic setae 18–34 µm long sharing one crown. Buccal cavity armed with mandible and three teeth. Mandible consisting of two rods distancing from one another anteriorly joined by anterior rod. Lateral edges of each rod with teeth or denticle pointing to the lumen. Buccal cavity 37–61 µm long. 8–9 groups of cervical setae in groups of two to three at stoma region. Cervical setae in single groups in females. Males with testis paired and opposed. Spicule paired, symmetrical, slightly curved, divided into two portions by a seam. Distal portion shorter than proximal. Proximal portion with knob/neck-like end. Gubernaculum paired, shaped like an irregular triangle with caudal apophysis, distal end extending beyond spicules and ventrally towards cloacal opening. Precloacal supplementary organ present. Three to four somatic setae distributed along the tale. Tail conico-cylindrical, c’ 4–5.7, cylindrical portion of the tail constituting about 30% of the entire tail length.

Differential diagnosis: Total of 23 species of Mesacanthion with spicules shorter than 2 abd were examined. Species such as M. arcuatile, M. conicum and M. cricetoides were omitted from examination due to the fact that only female was ever described. Also, species with asymmetrical spicules (anisomorphic and anisometric) were omitted even if the shorter spicule is shorter than 2 abd, since Gagarin & Klerman (2006) already provided a key for those group of species. M. tenuicaudatum which most likely does have spicules shorter than 2 abd, is also omitted from examination and pictorial key as there are no depiction of the specimen available. Description also lacks information regarding gubernaculum, measurements of anal body diameter and length of all setae, making it not feasible for comparison. Lastly, M. virile is included for analysis, despite its lack of information on abd. Although it cannot be confirmed that it possesses spicules which are shorter than 2 abd, given the length of spicule and other relative body measurements, it is likely that this species belongs to this group.

The new species is most similar to M. pannosum as they both share striking resemblance in overall morphology. They both have spicules shorter than 2 abd with presence of both supplementary organ and gubernaculum. Index value of both species (a and c) are within range to each other as well. Most interesting character shared by the two species is the presence of modified/transformed cervical setae seen as a flap. This flap-like appendage is seen as inverse triangle just below a single outer labial seta (lateral seta), at the end of cephalic capsule. It was first observed by Wieser (1959) in males of M. pannosum and it has been a character unique to the species until it now. Like M. Pannosum, the flap is also observed only in males of the new species, and the morphology is in line with those previously seen in M. pannosum. The only difference concerning the flap between the two species is that M. pannosum had two flaps next to each other, but only a single flap is seen in the new species (Fig. 4B).

The new species differs from P. pannosum by having stronger (stout) inner labial setae; longer outer labial setae (43–59 µm vs. 24–25 µm, respectively); difference in the index value of b and c’ (4.5–5 vs. 6.3 and 4.0–5.7 vs. 3, respectively); type of spicules (unipartite vs. bipartite with seam, respective).

The validity of diagnostic value in presence or absence of a seam (bipart spicule), can be questionable considering it is an ambiguous character and could be mistaken from a diffraction caused by a large gubernaculum. Due to this reason, the seam was not given a significant importance in the diagnosis of species within the genus, but the character and species which bear it are still discussed for reference.

Total of four species within the genus Mesacanthion have paired bipartite spicules: M. audax, M. ditlevseni, M. infantile, and M. jejuensis sp. nov. They all have spicules which are shorter than 2 abd, but M. audax is most easily distinguished from the other three species by its lack of supplementary organ. While the three species have supplementary organ, the new species resembles M. ditlevseni the most. They both have stout inner labial setae with presence of both supplementary organ and triangular gubernaculum. Their index value (a, b, and c’) are also within range from one another. The new species can be distinguished by its proportionally longer cephalic setae (in which is double the length of cephalic setae observed in M. ditlevseni); presence of cervical flap in new species; dense distribution of cervical setae in groups of doubles/trios below at stoma region in male; differing details to its spicules and gubernaculum. The spicule of the new species is different from M. ditlevseni in that distal portion of the spicule is shorter than proximal portion where vice versa is true for M. ditlevseni. This is peculiar characteristic unique to the new species, as all bipartite spicules found in Mesacanthion species have longer distal portion over proximal portion. We believe that diagnostic value of modified cervical setae flap is greater than bipartite spicule (spicule with seam) in terms of ambiguity. Therefore, the species most similar to the new species is considered to be M. pannosum.

Etymology: The species name jejuensis is given as the species was discovered from coast Jeju Island, South Korea.

Pictorial key to species with spicules shorter than 2 anal body diameter within the genus Mesacanthion and morphometric values for valid species of Mesacanthion Figs. 6–8, Table 2

Key to species with spicules shorter than 2 anal body diameters within the genus Mesacanthion