1. INTRODUCTION

The genus Desmoscolex Claparède, 1863 is characterized by a body cuticle composed of large main rings separated by narrower or equally wide interzones. This morphological feature makes it the most species- rich genus among the three genera (Timm 1970;Decraemer 1983, 1985;Decraemer and Rho 2014). The genus Desmoscolex is further divided into four subgenera-Pareudesmoscolex Weischer, 1962, ProtricomoidesTimm, 1970, Desmolorenzenia Fredenhammer, 1975, and Desmoscolex Claparède, 1863- each defined by unique morphological characteristics. The subgenus Pareudesmoscolex is distinguished from Desmoscolex by the structure and ornamentation of its body cuticle, which exhibits homonomous cuticular annulations adorned with distinct “warts” or “papillae,” distinguishing it from other subgenera. Similarly, the subgenus Protricomoides features homonomous annulations in the body cuticle but is further characterized by the presence of hairy spines or pores on each annule, giving it a unique textured appearance. Desmolorenzenia is defined by desmens with a triangular outline and an abrupt inversion of orientation of these rings. In contrast, the subgenus Desmoscolex is notable for its distinct morphological traits, particularly the cephalic setae. These setae exhibit significant variation in length, thickness, and shape, serving as key diagnostic features. The subgenus Desmoscolex comprises free-living nematodes found in marine, freshwater, and terrestrial environments, with a broad geographic distribution across diverse marine habitats worldwide (Timm 1970;Freudenhammer 1975;Decraemer 1985;Decraemer and Rho 2014). The two new species described in this study, Desmoscolex (Desmoscolex) jejuensis sp. nov. and Desmoscolex (Desmoscolex) longispiculatus sp. nov., also belong to this subgenus.

The Pacific Ocean harbors significant diversity within the genus Desmoscolex, particularly in intertidal and subtidal habitats, which remain underexplored. To date, 33 of the 99 known species in this genus have been recorded in the Pacific region, highlighting its ecological richness (Timm 1970;Gerlach and Riemann 1973;Freudenhammer 1975;Decraemer and Rho 2014;Jung et al. 2024;Nemys eds. 2024). The first discovery of Desmoscolex in the Pacific was described by Allgén (1932), who identified D. (D.) cristatus in the coastal region of Perseverance Harbour, Campbell Islands. Subsequent efforts revealed additional species, such as D. (D.) keiensisKreis, 1934 in the Kai Islands (Kreis 1934) and D. (D.) grandiamphisInglis, 1967 in New Caledonia (Inglis 1967).

Timm (1970) significantly expanded knowledge of Pacific Desmoscolex diversity by describing seven new species, including D. (D.) californicus, discovered in diverse intertidal and subtidal habitats such as sandy beaches, coral sands, and deep-sea environments extending to depths of 6,000 m. Decraemer (1974c, 1975b) further enriched this knowledge with the addition of 16 species from various habitats, ranging from reef flats and sandy gullies to subtidal sands at depths of 35 m. Among these, D. (D.) asetosusDecraemer, 1975 and D. (D.) membranosusDecraemer, 1974 were particularly noteworthy for their adaptations to shallow marine environments.

More recent studies have extended the known diversity of Desmoscolex to deeper waters. For instance, Bussau (1993) described five new deep-sea species, including D. (D.) brachyrhynchus, from depths of 4,147-4,178 m in the Peruvian Trench. Similarly, Decraemer (1996) reported D. (D.) parvospiculatus from coral sands and lagoons in Papua New Guinea.

The subgenus Desmoscolex has been relatively understudied in Korea, with only a few species documented to date. Lim and Chang (2006) reported D. (D.) cosmopolitesTimm, 1970, from various coastal areas, including Jejudo Island, in algal beds and sublittoral sands. More recently, Lee et al. (2022) recorded D. (D.) maxTimm, 1970, from subtidal coarse sediments at a depth of 68 m near Uljin. Furthermore, Jung et al. (2024) described two new species, D. (D.) lanceosetatusJung, Khim, and Rho, 2024, and D. (D.) rotundicephalusJung, Khim, and Rho, 2024, from subtidal benthic habitats in Korea within the Northwestern Pacific Ocean. These findings highlight a potentially broader distribution of Desmoscolex in Korean coastal ecosystems, although the nematode diversity in the region remains largely unexplored.

This study describes on two newly discovered species of Desmoscolex from intertidal habitats on Jejudo Island, Korea-Desmoscolex (Desmoscolex) jejuensis sp. nov. and Desmoscolex (Desmoscolex) longispiculatus sp. nov.-which are distinguished by their long and slender cephalic setae, with D. (D.) jejuensis sp. nov. exhibiting a unique arrangement of 18 main rings and D. (D.) longispiculatus sp. nov. notable for its elongated spicules. Morphological analyses using scanning electron microscopy (SEM) and differential interference contrast (DIC) microscopy were performed to describe these two species and to compare them with closely related taxa, contributing to the documentation of Desmoscolex diversity in the Pacific region.

2. MATERIALS AND METHODS

Intertidal sand sediment samples containing marine nematodes were collected in 2024 from Sinyang Beach, a sandy shore located on the eastern coast of Jejudo Island, Korea. In the field, meiobenthic organisms were initially separated by decantation through a 67 μm mesh sieve, followed by a quick rinse with freshwater to reduce sediment adherence (Kristensen and Higgins 1989). The samples were then preserved in 5% neutralized formalin prepared with seawater.

In the laboratory, meiobenthos were further isolated using the Ludox flotation technique with Ludox^® (Grace, Columbia, MD, USA) HS 40 (Burgess 2001). The concentrated samples were again preserved in 5% neutralized formalin for subsequent analysis. Marine nematode specimens were selected under high magnification using a LEICA M205 C stereomicroscope (LEICA, Wetzlar, Germany). For detailed morphological examination under a light microscope, specimens were gradually dehydrated in a glycerin series (Seinhorst 1959) and mounted on HS slides for observation (Shirayama et al. 1993). Observations and photographs were taken using a LEICA DM2500 LED microscope equipped with a LEICA K5C color CMOS camera (LEICA, Wetzlar, Germany), which was also used for sketching and measuring the specimens.

For scanning electron microscopy (SEM), specimens preserved in 5% buffered formalin were rinsed twice with distilled water to remove formalin residues. They were then freeze-dried on an FDU-1200 cooling stage (EYELA, Tokyo, Japan). The dehydrated specimens were mounted on aluminum stubs, sputter-coated with gold/palladium using a high-vacuum evaporator, and examined with a SNE-3200M Desktop Mini SEM (SEC, Gyeonggi-do, Korea) (Lee et al. 2021).

Terminology and Abbreviations. All measurements are provided in micrometers along the arc. Ratios a, b, and c are calculated using de Man’s ratios. The following abbreviations are used:

3. SYSTEMATIC ACCOUNTS

Phylum Nematoda Potts, 1932

Class Chromadorea Inglis, 1983

Order Desmoscolecida Filipjev, 1929

Family Desmoscolecidae Shipley, 1896

Subfamily Desmoscolecinae Shipley, 1896

Genus Desmoscolex Claparède, 1863

Subgenus Desmoscolex (Desmoscolex) Claparède, 1863

Type species. Desmoscolex (Desmoscolex) minutus Claparède, 1863

3.1. Desmoscolex (Desmoscolex) jejuensis sp. nov. (Figs. 1-5, 10; Table 1)

Type Locality. Marine nematodes were collected from Sinyang Beach, a sandy shore situated on the eastern coast of Jejudo Island, Korea (33°26ʹ09.6ʺN, 126° 55ʹ29.3ʺE), on May 30, 2024.

Type Material. The holotype male (MABIK NA00 158744), mounted in glycerin on an HS slide, is preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, one paratype male (KIOST NEM-1- 2787) and six paratype females (KIOST NEM-1-2788 to KIOST NEM-1-2793), also mounted in glycerin on HS slides, are deposited in the nematode collection housed in the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN) at the East Sea Research Institute, Korea Institute of Ocean Science and Technology (KIOST), Korea.

Diagnosis.Desmoscolex (Desmoscolex) jejuensis sp. nov. is characterized by 18 main rings tapering towards both terminal ends. The head is rounded, tapering to a truncate end, and covered with concretion particles. The cephalic setae are very long and slender, with a broad base narrowing distally. The amphidial fovea are vesicular, extending to the first main ring, with an amphidial pore located posteriorly on the head. In males, the somatic setae arrangement is defined by the absence of the 8th, 10th, and 14th subventral setae and a lateral shift of the 2nd and 15th subventral setae. In females, the additional absence of the 6th subventral seta further differentiates the somatic setae pattern from the typical arrangement seen in Desmoscolex species. The subdorsal setae are bent at a swollen terminal section, with wrinkling observed along the sheath. The terminal ring is conical, with the posterior tapering part bearing small, round phasmata, clearly separated from the anterior concretion.

Etymology. The species name Desmoscolex (Desmoscolex) jejuensis sp. nov. is derived from the locality where the species was discovered, Sinyang Beach, located in the eastern part of Jejudo Island. The epithet ‘jejuensis’ is a Latinized form referencing Jejudo Island, reflecting the geographical origin of the species. Measurements.

Holotype male: L=277, hd=15×13, cs=19, sd₁=18, sd₃=16, sd₅=15, sd₇=15, sd₉=15, sd₁₁=15, sd₁₃=20, sd₁₆=23, sd₁₇=26, sv₂=8, sv₄=8, sv₆=8, sv₁₂=8, sv₁₅=8, mbd =29, (mbd) =23, oes=44, spic=27, gub=8, abd=26, t=49, tmrw=9, tmr=7, a=9.5, b=6.2, c=5.6 Paratype male (n=1): L=237, hd=13×13, cs=19, sd₁=16, sd₃=16, sd₅=16, sd₇=15, sd₉=15, sd₁₁=16, sd₁₃=21, sd₁₆=24, sd₁₇=32, sv₂=9, sv₄=10, sv₆=7, sv₈=7, sv₁₂=9, sv₁₅=8, mbd=26, (mbd)=22, oes=34, spic=27, gub=8, abd=25, t=44, tmrw=9, tmr=6, a=9.1, b=6.9, c=5.3

Paratype females (n=6): L=259-302, hd=13- 14 ×9-12, cs =22-26, sd₁ =17-19, sd₃ =15-18, sd₅=15-16, sd₇=15-16, sd₉=15-17, sd₁₁=16- 17, sd₁₃=20-23, sd₁₆=21-22, sd₁₇=25-31, sv₂=9- 10, sv₄=10-11, sv₁₂=10-11, sv₁₅=9-11, mbd=26- 30, (mbd)=23-26, oes=40-51, abd=23-27, t=42- 52, tmrw=9-10, tmr=6-9, a=8.9-10.9, b=5.9-6.7, c=5.4-6.2, V=54.2-56.6%

Description. Males. The body comprises 18 main rings, gradually tapering towards both terminal ends. Each main ring is covered with fine foreign particles measuring 3-4 μm in width. These main rings are separated by narrower intermediate zones consisting of 3-4 secondary rings, which measure 2-3 μm in width (Figs. 1A, 2A, 3A, B).

The head is rounded and tapers anteriorly to a truncate end, covered with concretion particles. The cuticle extends further backward on the lateral sides compared to the dorsal and ventral sides, excluding the regions surrounding the amphidial pore and labial area. The amphidial groove begins at the labial region, narrows in the middle, and widens again towards the posterior.

The cephalic setae are inserted midway along the head and feature a broad base that gradually tapers into a long, fine, and slender distal portion (Fig. 3E). The amphidial fovea are vesicular, measuring 11 μm in width and 19 μm in length, extending from the labial region to the first main ring. An amphidial pore is positioned on the posterior part of the head (Figs. 1C, 2B, 3C).

The pharynx is narrow and cylindrical, extending anteriorly, with the nerve ring encircling it at the level of the second main ring. The pharyngeal-intestinal junction is located at the third main ring, and the intestine extends through the 16th main ring. Paired ocelli are situated between the 4th and 5th main rings.

The somatic setae are symmetrically paired on the left and right sides:

The subdorsal setae are enclosed within a sheath, which features wrinkles extending along up to twothirds of its length. Beyond the sheath’s boundary, the subdorsal setae bend distinctly, with a slightly swollen section at the point of the bend. All subdorsal setae are similar in length, except for the longer first seta and the progressively elongating setae from the 13th to the 17th subdorsal setae (Fig. 3F).

The absence of the 8th, 10th, and 14th subventral setae deviates from the typical arrangement observed in 17-ring species. Additionally, the 2nd and 15th subventral setae are laterally shifted. In one paratype male, the 8th subventral seta was present on the left side but absent on the right, suggesting individual variability rather than a consistent species-wide pattern. SEM observations further confirmed the absence of the 8th subventral seta, supporting the conclusion that this variation is attributable to individual differences rather than a defining characteristic of the species. The shorter subventral setae are also enclosed within a sheath that extends up to two-thirds of their length, maintaining a consistent thickness without tapering at the ends. They are uniform in length throughout (Fig. 3G).

The male reproductive system is characteristic of the subfamily, comprising a single, outstretched testis with a germinal zone that transitions into a finely granular vas deferens. The spicules are nearly straight, featuring a slender distal portion that broadens towards the base and terminates in a capitulum. A gubernaculum lies parallel to the spicules. The cloacal tube is positioned centrally within the 15th main ring, terminating in a 3 μm high papilla (Figs. 1E, 2C).

The terminal main ring comprises an anterior concretion ring, similar in length to the preceding rings, with terminal setae inserted at this point. Ventrally, the concretion ends at the insertion site, while dorsally, it extends approximately twice as far. The posterior tapering section, which bears small, round phasmata, is distinctly separated from the anterior concretion (Fig. 3D, H).

Females. The female exhibits most of the same morphological features as the male, including 18 main rings that taper toward both ends, separated by intermediate zones consisting of 3-4 secondary rings (Figs. 1B, 4A, 5A, D). The head tapers to a truncate end, with cephalic setae characterized by a broad base and a long, slender distal portion. The amphidial fovea are vesicular and include an amphidial pore (Figs. 1D, 4B, 5B, C).

The somatic setae are symmetrically paired on the left and right sides:

Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17=9

Subventral: 2, 4, (-), (-), (-), 12, (-), 15=4

Similar to the male, the absence of the 8th, 10th, and 14th subventral setae deviates from the typical 17- ring arrangement, with the additional absence of the 6th subventral seta being unique to females. The morphology of the subdorsal and subventral setae is consistent with that of the male. The female reproductive system is didelphic-amphidelphic, consisting of two outstretched ovaries with immature oocytes. The vulva is located between the 10th and 11th main rings, with spermathecae containing globular sperm positioned near the vulval region. The anal tube is situated on the 15th main ring, and the terminal main ring shares the same morphology as that of the male (Figs. 1F, 4C).

Remarks.Desmoscolex (Desmoscolex) jejuensis sp. nov. represents a distinct new species within the genus, distinguished by a unique combination of morphological features, including 18 main rings, a distinctive somatic setae arrangement, characteristic head morphology, vesicular amphidial fovea, and subdorsal setae enclosed within a wrinkled sheath. This section provides a comprehensive comparison of the diagnostic differences between D. (D.) jejuensis sp. nov. and its closely related species (D. (D.) curvespiculatumDecraemer, 1984;D. (D.) deconinckiDecraemer, 1974;D. (D.) decraemeraeSoetaert, 1989;D. (D.) longisetosusTimm, 1970;D. (D.) longispiculatus sp. nov.; D. (D.) sievertiFreudenhammer, 1975; and D. (D.) vanoyeiDe Coninck, 1943) based on critical taxonomic characters.

Desmoscolex (D.) curvespiculatumDecraemer, 1984. Desmoscolex (D.) jejuensis sp. nov., characterized by 18 main rings, is distinct from D. (D.) curvespiculatum, which has 17 main rings. In males of D. (D.) jejuensis sp. nov., the absence of the 8th, 10th, and 14th subventral setae, combined with the lateral shift of the 2nd and 15th subventral setae, contrasts with D. (D.) curvespiculatum, where somatic setae placement is influenced by the presence of an additional partial ring between the 12th and 13th rings. Furthermore, the subdorsal setae of D. (D.) jejuensis sp. nov. are enclosed in a sheath with wrinkles extending along two-thirds of their length and bend distinctly beyond the sheath, whereas the subdorsal setae of D. (D.) curvespiculatum are fine, tapered, and lack such wrinkling. The head of D. (D.) jejuensis sp. nov. is rounded and tapers to a truncate end covered with concretion particles, contrasting with the head of D. (D.) curvespiculatum, which is longer, features a naked labial region, and has subterminal cephalic setae. Additionally, the amphidial fovea of D. (D.) jejuensis sp. nov. are vesicular and extend from the labial region to the first main ring, whereas in D. (D.) curvespiculatum, the amphidial fovea are large, bipartite structures that extend halfway down the first main ring (Decraemer 1984).

Desmocolex (D.) deconinckiDecraemer, 1974. The somatic setae arrangement in D. (D.) jejuensis sp. nov. is notably distinct, with males lacking the 8th, 10th, and 14th subventral setae, and females additionally lacking the 6th subventral seta. In contrast, D. (D.) deconincki lacks only the 10th subventral seta, with several subventral setae on the 2nd, 6th, 8th, 12th, and 14th rings shifted sublaterally, while the setae on the 4th ring are shifted laterally. The subdorsal setae of D. (D.) jejuensis sp. nov. are bent and feature a swollen terminal section, whereas those of D. (D.) deconincki are longer, thicker than the subventral setae, and have a curved sheath at their base. Additionally, the head of D. (D.) jejuensis sp. nov. is rounded and covered with concretion particles, contrasting with the head of D. (D.) deconincki, which is as long as it is wide and has a narrow, sclerotized anterior region terminating in six minute papillae (Decraemer 1974b).

Desmoscolex (D.) decraemeraeSoetaert, 1989.Desmoscolex (D.) jejuensis sp. nov. can be distinguished from D. (D.) decraemerae by its somatic setae arrangement and subdorsal setae morphology. In males of D. (D.) jejuensis sp. nov., the absence of the 8th, 10th, and 14th subventral setae, coupled with the lateral shift of the 2nd and 15th subventral setae, are distinctive features. In contrast, D. (D.) decraemerae exhibits the typical 17-ring somatic setae pattern without these modifications. The subdorsal setae of D. (D.) decraemerae are spatulate, differing from the wrinkled and distinctly bent subdorsal setae observed in D. (D.) jejuensis sp. nov. Furthermore, the head morphology differs significantly: D. (D.) jejuensis sp. nov. has a rounded head with a truncate end, while D. (D.) decraemerae displays a head that is slightly wider than long, with a broad, rounded posterior and a truncated anterior region (Soetaert 1989).

Desmoscolex (D.) longisetosusTimm, 1970. In males of D. (D.) jejuensis sp. nov., the absence of the 8th, 10th, and 14th subventral setae, along with the lateral shift of the 2nd and 15th subventral setae, contrasts with D. (D.) longisetosus, which lacks only the 10th and 14th subventral setae but retains a different lateral setae arrangement. The head morphology also differs significantly; D. (D.) jejuensis sp. nov. has a rounded head, whereas D. (D.) longisetosus features a long, narrow head with long, thin, and pointed cephalic setae. Furthermore, the transverse rows of fine spines observed on the secondary rings of D. (D.) longisetosus are absent in D. (D.) jejuensis sp. nov. (Timm 1970;Decraemer 1974b).

Desmoscolex (D.) longispiculatus sp. nov.Desmoscolex (D.) jejuensis sp. nov., characterized by 18 main rings, is distinctly differentiated from D. (D.) longispiculatus sp. nov., which possesses 17 main rings. In males of D. (D.) jejuensis sp. nov., the absence of the 8th, 10th, and 14th subventral setae further distinguishes it from D. (D.) longispiculatus, which retains a typical arrangement of subventral setae. The subdorsal setae in D. (D.) longispiculatus are broader at the base, with prominent wrinkles and striations, whereas in D. (D.) jejuensis sp. nov., the subdorsal setae are bent with a swollen terminal section and are enclosed by a wrinkled sheath. Moreover, the spicules of D. (D.) jejuensis sp. nov. are nearly straight, broad at the base, and terminate in a capitulum. In contrast, the spicules of D. (D.) longispiculatus are elongated, narrow, and strongly arcuate, with a slender distal portion ending in a capitulum.

Desmoscolex (D.) sievertiFreudenhammer, 1975. The somatic setae arrangement in D. (D.) jejuensis sp. nov., characterized by the absence of the 6th, 8th, 10th, and 14th subventral setae in females, is distinct from the nine subdorsal setae present in both sexes of D. (D.) sieverti. In D. (D.) sieverti, males lack subventral setae on the 10th and 14th rings, while females lack subventral setae on the 4th, 8th, 10th, and 14th rings. Additionally, the head of D. (D.) jejuensis sp. nov. is truncate and rounded, contrasting with the lip region of D. (D.) sieverti, which forms a narrow rim around the mouth opening (Lorenzen 1969;Freudenhammer 1975).

Desmoscolex (D.) vanoyeiDe Coninck, 1943. Both D. (D.) jejuensis sp. nov. and D. (D.) vanoyei share the characteristic of having 18 main rings. However, notable differences exist in the somatic setae arrangement of males. In D. (D.) vanoyei, only the 14th subventral seta is absent, whereas in D. (D.) jejuensis sp. nov., the 8th, 10th, and 14th subventral setae are absent. The head morphology also differs significantly; D. (D.) jejuensis sp. nov. has a truncate head covered with concretion particles, while D. (D.) vanoyei features a rounded head with a small, naked labial region devoid of cuticular structures (De Coninck 1943).

3.2. Desmoscolex (Desmoscolex) longispiculatus sp. nov. (Figs. 6-10; Table 1)

Type locality. Marine nematodes were collected from Sinyang Beach, a sandy shore situated on the eastern coast of Jejudo Island, South Korea (33°26ʹ09.6ʺN, 126° 55ʹ29.3ʺE), on May 30, 2024.

Type material. The holotype male (MABIK NA00 158743), mounted in glycerin on an HS slide, is preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seocheon, South Korea. In addition, three paratype males (KIOST NEM-1-2781 to KIOST NEM-1-2783) and two paratype females (KIOST NEM-1-2784 to KIOST NEM- 1-2785), also mounted in glycerin on HS slides, are deposited in the nematode collection housed at the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science and Technology (KIOST), South Korea.

Diagnosis.Desmoscolex (Desmoscolex) longispiculatus sp. nov. is distinguished by 17 main rings that taper towards both terminal ends. The head is broad and rounded, narrowing to a truncate end, with an anterior non-sclerotized region. The cephalic setae are slender and emerge midway along the head. The amphidial fovea are longitudinally oval, extending to the first main ring, with a circular amphidial pore located posteriorly. The somatic setae exhibit a standard arrangement for species with 17 rings, with distinct deviations where the 14th subventral seta is laterally shifted and the 15th subventral seta is dorsally displaced, resembling a subventral configuration. The subdorsal setae are bent at the swollen terminal section and feature prominent wrinkles and striations along their length. The spicules are elongated, narrow, and arcuate, ending in a well-defined capitulum. The terminal ring is conical, curving ventrally, and terminates in a short spinneret.

Etymology. The species name Desmoscolex (Desmoscolex) longispiculatus sp. nov. is derived from the Latin terms longi, meaning “long,” and spiculatus, referring to “having spicules,” reflecting the characteristic elongated spicules of this species. The name succinctly highlights the defining morphological feature of the species.

Measurements.

Holotype male: L=168, hd=10×11, cs=15, sd₁=12, sd₃=12, sd₅=12, sd₇=12, sd₉=13, sd₁₁=13, sd₁₃=14, sd₁₆=17, sd₁₇=22, sv₂=9, sv₄=9, sv₆=9, sv₈=9, sv₁₀=8, sv₁₂=8, sv14=8, sv₁₅=8, mbd=23, (mbd)=17, oes=29, spic=40, abd=21, t=38, tmrw=10, tmr=15, a=7.3, b=5.7, c=4.4

Paratype males (n =3): L =182-206, hd =11- 12×11-12, cs=13-15, sd₁=13, sd₃=13-14, sd₅=13- 14, sd₇=12-14, sd₉=12-14, sd₁₁=12-14, sd₁₃=14-15, sd₁₆=17-20, sd₁₇=21-23, sv₂=8-9, sv₄=8-9, sv₆=8- 9, sv₈=8-9, sv₁₀=8-9, sv₁₂=8-9, sv14=8, sv₁₅=8-9, mbd=22-23, (mbd)=15-18, oes=32-37, spic=38- 42, abd=20-22, t=35-41, tmrw=8-9, tmr=16-20, a=8.2-9, b=5.3-6.2, c=5-5.2

Paratype females (n=2): L=183-199, hd=12- 14 ×12-13, cs=13-14, sd₁=11, sd₃=11, sd₅=11, sd₇=11, sd₉=11, sd₁₁=11, sd₁₃=14-16, sd₁₆=17- 19, sd₁₇ =21-25, sv₂ =8, sv₄ =8, sv₆ =8, sv₈ =8, sv₁₀=8-9, sv₁₂=8-9, sv14=8-9, sv₁₅=10, mbd=22- 29, (mbd) =19-21, oes =33, abd =18-21, t =35- 43, tmrw=9-10, tmr=16-19, a=6.8-8.3, b=5.5-6, c=4.6-5.2, V=54.7-55.1%

Description. Males. The body is composed of 17 main rings that taper gradually toward both terminal ends. Each main ring is densely covered with foreign particles, approximately 5-6 μm in width. These main rings are separated by narrower intermediate zones consisting of 2-3 secondary rings, each measuring 1-2 μm in width (Figs. 6A, 7A, 8A).

The head is broad and rounded, tapering anteriorly to a truncate end. A non-sclerotized zone is distinctly visible in the anterior third of the head, surrounding the labial region. The remainder of the head is entirely covered with concretion particles, with the exception of the amphidial regions (Figs. 6C, 7B, 8B).

The cephalic setae are slender, fine in structure, and emerge midway along the head (Fig. 8C). The amphidial fovea are longitudinally oval, measuring 6 μm in width and 12 μm in length. They extend from the labial region to the first main ring, with a circular amphidial pore positioned posteriorly on the head.

The pharynx is narrow and cylindrical, extending anteriorly within the body, with the nerve ring encircling it at the level of the second main ring. The pharyngeal- intestinal junction is located at the third main ring, while the intestine, of typical structure, extends posteriorly through the 16th main ring. Paired ocelli are situated between the 4th and 6th main rings.

The somatic setae exhibit the standard arrangement typical of species with 17 main rings, symmetrically paired on both the left and right sides as follows:

Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17=9

Subventral: 2, 4, 6, 8, 10, 12, 14, 15=8

However, deviating from the typical arrangement observed in Desmoscolex species, the 14th subventral seta is shifted laterally, and the 15th subventral seta is displaced subdorsally. SEM observations indicate that the somatic seta on the 15th main ring is morpholog-ically distinct from the other subdorsal setae, more closely resembling the characteristics of subventral setae. Therefore, despite its dorsal displacement, the 15th ring seta is classified as subventral, consistent with the overall somatic setae arrangement.

The subdorsal setae feature a wider basal shaft and a distinct bend marking the transition to the swollen terminal section. Wrinkles and fine striations are evident along the majority of their length. The first to 11th subdorsal setae are uniform in length, while the 13th and subsequent subdorsal setae progressively increase in length, resulting in a distinctly elongated terminal seta (Fig. 8E).

The shorter subventral setae are finely structured and uniform in length. They exhibit wrinkles and fine striations similar to those observed on the subdorsal setae, extending along their surface (Fig. 8F, G).

The male reproductive system is characteristic of the subfamily, comprising a single, outstretched testis that starts with a germinal zone, transitions through a differentiation zone, and leads into a finely granular vas deferens. The spicules are strongly elongated, slender, and strongly arcuate, with a narrow distal portion that terminates in a capitulum. The gubernaculum was not observed. The cloacal tube is positioned centrally within the 15th main ring and terminates in a papilla measuring 3 μm in height (Figs. 6E, 7C, 8D).

The terminal ring is conical, curving ventrally and tapering towards the tail end. The terminal setae are positioned approximately halfway along its length and it ends in a short, uncovered spinneret measuring 3 μm. Phasmata were not observed (Fig. 8H).

Females. The female exhibits most of the morphological features observed in the male, including 17 main rings that taper towards both terminal ends. These main rings are separated by intermediate zones composed of 2-3 secondary rings (Figs. 6B, 9A). The head is broad and rounded, tapering anteriorly to a truncate end. The cephalic setae are slender and delicate, while the amphidial fovea are longitudinally oval with a circular amphidial pore located posteriorly (Figs. 6D, 9B).

The somatic setae exhibit the typical arrangement observed in species with 17 main rings, symmetrically paired on both the left and right sides:

Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17=9

Subventral: 2, 4, 6, 8, 10, 12, 14, 15=8

In females, similar to the male, the 14th subventral seta is laterally shifted, albeit to a lesser extent, while the 15th subventral seta is dorsally displaced, mirroring the arrangement observed in males.

The female reproductive system is didelphic-amphidelphic, consisting of two outstretched ovaries, each containing multiple immature oocytes. The vulva is situated between the 10th and 11th main rings, accompanied by two spermathecae near the vulval region that contain globular sperm. The anal tube is positioned within the 15th main ring.

The terminal ring is conical, curving ventrally and tapering towards the tail end, with terminal setae positioned approximately halfway along its length. It concludes in a short, uncovered spinneret, and no phasmata were observed (Figs. 6F, 9C).

Remarks.Desmoscolex (D.) longispiculatus sp. nov. is distinguished by a combination of key morphological features. The new species is characterized by 17 main rings that taper towards both terminal ends, forming a fundamental diagnostic trait. The spicules are highly elongated, narrow, and arcuate, terminating in a well-defined capitulum, which serves as a primary identification feature. The somatic setae arrangement follows the standard pattern observed in species with 17 rings; however, it exhibits unique deviations, with the 14th subventral seta laterally shifted and the 15th subventral seta dorsally displaced while retaining a subventral-like morphology. The subdorsal setae are bent at a swollen terminal section and feature prominent wrinkles and striations along their length, adding to the distinctiveness of the species. The head is broad and rounded, narrowing to a truncate end, with an anterior non-sclerotized region. The cephalic setae are slender and emerge midway along the head. The amphidial fovea are longitudinally oval, extending to the first main ring, with a circular amphidial pore located posteriorly. The terminal ring is conical in shape, curving ventrally and tapering towards the posterior extremity. It terminates in a short, uncovered spinneret measuring approximately 3 μm in length, a feature that provides additional diagnostic value in distinguishing this species within the genus.

Desmoscolex (D.) longispiculatus sp. nov. shares 17 main rings with several congeners but is clearly distinguished by key diagnostic features in somatic setae arrangement, head morphology, and spicule structure.

Desmoscolex (D.) curvespiculatum exhibits a unique somatic setae arrangement influenced by an additional partial ring between the 12th and 13th rings, resulting in deviations in lateral setae placement. In contrast, D. (D.) longispiculatus sp. nov. maintains a typical 17-ring setae pattern but with gradual lateral and subdorsal shifts in the 14th and 15th subventral setae. Subdorsal setae in D. (D.) longispiculatus sp. nov. are broader at the base, with characteristic wrinkles and a bend, whereas those of D. (D.) curvespiculatum are fine and tapered. The head of D. (D.) longispiculatus sp. nov. is broad and rounded, tapering to a truncate end, while D. (D.) curvespiculatum has a longer head with a naked labial region and subterminal cephalic setae. Spicules in D. (D.) longispiculatus sp. nov. are highly elongated and arcuate, distinguishing them from the spicules of D. (D.) curvespiculatum.

Desmoscolex (D.) deconincki differs by lacking the 10th subventral seta, deviating from the typical 17-ring somatic setae arrangement seen in D. (D.) longispiculatus sp. nov. In D. (D.) deconincki, subdorsal setae are longer and thicker, with a curved sheath, while the subventral setae are slender and tipped acutely. Conversely, in D. (D.) longispiculatus sp. nov., the subdorsal setae are characterized by a wider base, a pronounced bend, and distinct wrinkles, while the subventral setae are shorter but similarly wrinkled. The head of D. (D.) deconincki is as long as it is wide, with a sclerotized anterior ending in six minute papillae, whereas D. (D.) longispiculatus sp. nov. has a broad, rounded head tapering to a truncate end. Amphidial fovea in D. (D.) deconincki are sac-like and extend posteriorly between the 1st and 3rd rings, contrasting with the longitudinally oval fovea in D. (D.) longispiculatus sp. nov.

Desmoscolex (D.) decraemerae shares the typical 17- ring somatic setae arrangement but lacks the lateral and subdorsal shifts seen in D. (D.) longispiculatus sp. nov.. Subdorsal setae in D. (D.) decraemerae are spatulate, whereas in D. (D.) longispiculatus sp. nov., they have a wider base with a characteristic bend, wrinkles, and fine striations. The head of D. (D.) decraemerae is slightly wider than long, with a sclerotized cuticle except for the labial region, while D. (D.) longispiculatus sp. nov. features a broad, rounded head with a non-sclerotized zone. Spicules in D. (D.) decraemerae are twisted centrally and surrounded by a velum, distinct from the elongated, arcuate spicules with a capitulum in D. (D.) longispiculatus sp. nov.

Desmoscolex (D.) longisetosus differs by lacking the 10th and 14th subventral setae and showing unique lateral shifts in the 2nd and 15th subventral setae. In contrast, D. (D.) longispiculatus sp. nov. exhibits gradual lateral and subdorsal shifts in the 14th and 15th subventral setae. The head of D. (D.) longisetosus is long and narrow with thin, pointed cephalic setae, unlike the broad, rounded head with slender cephalic setae seen in D. (D.) longispiculatus sp. nov. Additional features in D. (D.) longisetosus, such as transverse rows of spines on secondary rings and jointed subventral setae, are absent in D. (D.) longispiculatus sp. nov. Spicules in D. (D.) longispiculatus sp. nov. are elongated, narrow, and arcuate, distinguishing them from those of D. (D.) longisetosus.

Desmoscolex (D.) sieverti differs in its somatic setae arrangement, with males lacking subventral setae on the 10th and 14th rings and females lacking them on the 4th, 8th, 10th, and 14th rings. Both sexes of D. (D.) sieverti possess nine subdorsal setae, unlike D. (D.) longispiculatus sp. nov., which retains the typical 17-ring somatic setae pattern with gradual shifts in the 14th and 15th subventral setae. The head of D. (D.) sieverti features a lip region forming a narrow rim around the mouth opening, contrasting with the broad, rounded head of D. (D.) longispiculatus sp. nov. Spicules in D. (D.) longispiculatus sp. nov. are elongated, arcuate, and terminate in a capitulum, distinguishing them from those of D. (D.) sieverti.

Desmoscolex (D.) vanoyei possesses 18 main rings, differing from the 17-ring pattern in D. (D.) longispiculatus sp. nov. The subdorsal setae of D. (D.) vanoyei are hollow with lance-shaped tips, whereas those in D. (D.) longispiculatus sp. nov. have a wider base, a characteristic bend, and wrinkles. The head of D. (D.) vanoyei is rounded with a small, naked labial region, while D. (D.) longispiculatus sp. nov. features a broad, rounded head with a non-sclerotized zone. Additionally, spicules in D. (D.) longispiculatus sp. nov. are highly elongated, narrow, and arcuate, contrasting with those of D. (D.) vanoyei.

These morphological distinctions highlight the unique combination of diagnostic features that establish D. (D.) longispiculatus sp. nov. as a distinct species within the genus.

4. DISCUSSION

The present study describes two new nematode species, Desmoscolex (Desmoscolex) jejuensis sp. nov. and Desmoscolex (Desmoscolex) longispiculatus sp. nov., from intertidal zones in Jejudo Island, Korea. Both species belong to the subgenus Desmoscolex, which is characterized by a body cuticle comprising large main rings separated by interzones, as well as remarkable variation in the morphology of cephalic setae and somatic setae arrangements. These two newly described species demonstrate unique morphological adaptations, adding to the known diversity of the subgenus and contributing to a broader understanding of Desmoscolex diversity in the Pacific region.

Among the approximately 100 described species within the subgenus Desmoscolex, cephalic setae exhibit remarkable diversity, ranging from typical radially symmetrical structures to more specialized forms, including absent, wing-shaped, club-shaped, jointed, or longitudinally grooved setae. Despite this diversity, over half of these species retain the typical cephalic setae morphology, characterized by a gradual tapering from the base to the tip, often accompanied by species- specific features that facilitate taxonomic differentiation (Timm 1970;Gerlach and Riemann 1973;Freudenhammer 1975). The newly discovered species, D. (D.) jejuensis sp. nov. and D. (D.) longispiculatus sp. nov., also exhibit this typical cephalic setae structure but display unique diagnostic traits that distinguish them from morphologically similar species, such as D. (D.) curvespiculatum, D. (D.) deconincki, D. (D.) decraemerae, D. (D.) longisetosus, D. (D.) sieverti, and D. (D.) vanoyei.

The two new species exhibit distinct morphological features that differentiate them from previously described congeners. Desmoscolex (D.) jejuensis sp. nov. is distinguished by the presence of 18 main rings, an atypical somatic setae arrangement, and characteristic head morphology. In males, the absence of the 8th, 10th, and 14th subventral setae, coupled with the lateral shift of the 2nd and 15th subventral setae, is a key diagnostic trait. Females display an additional absence of the 6th subventral seta. The species also features vesicular amphidial fovea and subdorsal setae enclosed within a wrinkled sheath. In contrast, D. (D.) longispiculatus sp. nov. is characterized by its 17 main rings and notably elongated, arcuate spicules with a well-defined capitulum. While the somatic setae arrangement follows the typical 17-ring pattern, unique deviations include the lateral shift of the 14th subventral seta and the dorsal displacement of the 15th subventral seta. The subdorsal setae are wider at the base, with a characteristic bend and prominent wrinkles and striations along their length. The head of D. (D.) longispiculatus sp. nov. is broad and rounded, tapering to a truncate end, and features a non-sclerotized zone with slender cephalic setae emerging at the boundary. These traits clearly differentiate it from morphologically similar species.

The comparative analysis highlights the importance of diagnostic morphological features, including somatic setae arrangement, cephalic setae morphology, amphidial fovea structure, and spicule shape, in delineating species boundaries within the subgenus. For example, while D. (D.) curvespiculatum is characterized by an additional partial ring between the 12th and 13th main rings affecting somatic setae placement, D. (D.) jejuensis sp. nov. and D. (D.) longispiculatus sp. nov. exhibit deviations in the somatic setae arrangement without additional rings, instead showing shifts in individual setae positions. Similarly, the spicules of D. (D.) longispiculatus sp. nov. are strongly elongated and arcuate, contrasting with the more typical spicule shapes observed in related taxa. The head morphology also provides key insights into species differentiation. While D. (D.) jejuensis sp. nov. has a rounded head covered with concretion particles and vesicular amphidial fovea, D. (D.) longispiculatus sp. nov. has a broader, truncate head with longitudinally oval amphidial fovea. These differences underscore the utility of detailed morphological observations, particularly through advanced imaging techniques such as differential interference contrast (DIC) microscopy and scanning electron microscopy (SEM).

The discovery of D. (D.) jejuensis sp. nov. and D. (D.) longispiculatus sp. nov. enriches the known diversity of Desmoscolex in Korean waters, a region previously understudied in terms of nematode biodiversity. Only four species of the subgenus Desmoscolex were documented in Korea prior to this study, highlighting the need for continued exploration (Lim and Chang 2006;Lee et al. 2022;Jung et al. 2024). Moreover, this study contributes to the broader understanding of Desmoscolex diversity in the Pacific region, where 33 of the 99 known species of the subgenus have been recorded (Timm 1970;Gerlach and Riemann 1973;Freudenhammer 1975;Decraemer and Rho 2014;Jung et al. 2024;Nemys eds. 2024). The traits described here highlight the ecological and evolutionary significance of morphological adaptations in marine nematodes and underscore the value of detailed taxonomic studies in revealing biodiversity.

5. CONCLUSIONS

This study has significantly expanded the known diversity of the subgenus Desmoscolex by documenting two new species, D. (D.) jejuensis sp. nov. and D. (D.) longispiculatus sp. nov., from intertidal areas in Korea. Both species exhibit unique morphological features that distinguish them from related taxa, highlighting the importance of detailed morphological analyses in nematode taxonomy. The use of advanced imaging techniques, such as SEM and DIC microscopy, was instrumental in identifying subtle yet taxonomically significant features, enabling a precise characterization of these species. These findings not only contribute to the global database of marine nematodes but also emphasize the ecological and evolutionary significance of documenting nematode diversity in underexplored habitats. The study underscores the rich biodiversity found in Korean marine ecosystems and the need for continued taxonomic and ecological investigations.