1. INTRODUCTION

The subgenus Desmoscolex Claparède, 1863, within the family Desmoscolecidae Shipley, 1896, comprises free-living nematodes inhabiting marine, freshwater, and terrestrial ecosystems, reflecting a broad ecological distribution. This subgenus is frequently documented in diverse marine environments due to its extensive geographic and ecological range (Decraemer and Rho 2014).

In Korean waters, four species of the genus Desmoscolex have been previously recorded. Desmoscolex (Des- molorenzenia) coreensis Rho, Kim, and Chang, 2007, was identified from Geomundo Island, inhabiting sponges and bryozoans on subtidal rocks at a depth of 25 m. Desmoscolex (Desmolorenzenia) pedunculusRho, Kim, and Chang, 2007, was reported from shallow subtidal zones (3-5 m depth) in Geumjin, Youngdeok, associated with invertebrates such as sponges, bryozoans, and polychaetes. Desmoscolex (Desmoscolex) cosmopolitesTimm, 1970, is distributed across various Korean coastal regions, including Samcheok, Pohang, Gyeongju, Masan, Busan, Jindo Island, and Jejudo Island, inhabiting algal beds on rocky shores and sublittoral bottom sands. Desmoscolex (Desmoscolex) maxTimm, 1970, was recorded from Wangdolcho, Uljin, in subtidal coarse sediment at a depth of 68 m (Lim and Chang 2006;Rho et al. 2007;Lee et al. 2022).

As part of a faunistic survey aimed at assessing the biodiversity of free-living marine nematodes in Korean waters, two new species of the subgenus Desmoscolex were discovered: Desmoscolex (Desmoscolex) delticeps sp. nov., collected from subtidal habitats near Ulleungdo Island in the East Sea, and Desmoscolex (Desmoscolex) obtusiseta sp. nov., collected from subtidal zones in distant regions along the western coast of Korea.

The two new species described in this study share a distinct morphological feature: a head bearing robust and blunt cephalic setae. This study provides a detailed taxonomic description of these species, supported by illustrations and micrographs obtained through differential interference contrast (DIC) microscopy and scanning electron microscopy (SEM).

Comparative analyses with previously described taxa are presented, highlighting the unique morphological characteristics of the new species. Furthermore, a comprehensive table summarizing morphological differences within the subgenus Desmoscolex and a pictorial key to aid in the identification of species are provided (Table 1). This study contributes to the taxonomy of marine nematodes and enhances the understanding of nematode biodiversity in Korean waters.

2. MATERIALS AND METHODS

2.1. Sample collection and preparation

Benthic sediment samples with marine nematodes were obtained in 2019 from the areas surrounding Ulleungdo Island and the western coast of Korea.

In the field, meiobenthic organisms were first separated by decantation through a 67 μm mesh sieve, following a quick rinse with freshwater to reduce sediment attachment (Kristensen and Higgins 1989). The samples were subsequently preserved in 5% neutralized formalin prepared with seawater. Further laboratory isolation of meiobenthos was achieved using the Ludox flotation technique with Ludox^® (Grace, Columbia, MD, USA) HS 40 (Burgess 2001). The concentrated samples were preserved again in 5% neutralized formalin for further analysis.

Marine nematode specimens were selected at high magnification using a LEICA M205 C stereomicroscope (LEICA, Wetzlar, Germany). For detailed morphological examination under a light microscope, the specimens were gradually dehydrated using a glycerin series (Seinhorst 1959) before being mounted on HS slides for observation (Shirayama et al. 1993). The nematodes were observed and photographed using a LEICA DM2500 LED microscope fitted with a LEICA K5C color CMOS camera (LEICA, Wetzlar, Germany), which was also used for making sketches and taking measurements.

For scanning electron microscopy (SEM), specimens were preserved in 5% buffered formalin and then washed twice with distilled water to eliminate formalin residues. Subsequently, the specimens were freezedried 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).

2.2. Terminology and abbreviations

All measurements are recorded in micrometers along the arc. Ratios a, b, c are determined using de Man’s ratio. 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) delticeps sp. nov. (Figs. 1-4, 9; Table 1)

Type locality. Marine nematodes were collected from benthic sediments at depths of 18 to 109 meters near Ulleungdo Island (37°26ʹ50.6ʺN, 130°51ʹ59.0ʺE), Ulleung- gun, Gyeongsangbuk-do, Korea, on February 26, 2019.

Type material. The holotype male (MABIK NA0015 8741), mounted in glycerin on an HS slide, is preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Korea. Additionally, one paratype male (KIOST NEM-1-2772) and two paratype females (KIOST NEM-1-2773 and KIOST NEM-1-2774), also mounted in glycerin on HS slides, are deposited in the nematode collection of the specimen conservation room at the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science and Technology (KIOST), Korea.

Diagnosis.Desmoscolex (Desmoscolex) delticeps sp. nov. is characterized by a broadly triangular head that tapers anteriorly, with robust cephalic setae featuring a broad base that narrows distally. These setae are directly inserted into the head cuticle approximately halfway along the head’s length. The amphidial fovea are oval and centrally positioned on the head, with an amphidial pore located slightly posterior to the midpoint. The somatic setae are arranged in 17 rings, symmetrically paired on both sides, with subdorsal setae tapering to a fine tip and terminal setae distinctly elongated. The terminal ring is conical, curving ventrally, with terminal setae positioned approximately two-thirds of the way from the tail tip, terminating in a relatively long, uncovered spinneret.

Etymology. The species name Desmoscolex (Desmoscolex) delticeps sp. nov. is proposed based on the characteristic broadly triangular head. ‘Delta’ is derived from the Greek letter meaning ‘triangle’, and ‘ceps’ is a suffix meaning ‘head.’ This name succinctly describes the distinct triangular shape of the head in this species.

Measurements.

Holotype male: L=348, hd=11×10, cs=6, sd₁=10, sd₃=10, sd₅=10, sd₇=10, sd₉=10, sd₁₁=10, sd₁₃=10, sd₁₆=16, sd₁₇=30, sv₂=8, sv₄=8, sv₆=9, sv₈=9, sv₁₀= 9, sv₁₂=9, sv₁₄=10, sv₁₅=10, mbd=48, (mbd)=45, oes=26, spic=53, abd=39, t=72, tmrw=17, tmr=38, a=7.1, b=13, c=4.8

Paratype male (n=1): L=309, hd=12×9, cs=6, sd₁= 11, sd₃=11, sd₅=11, sd₇=11, sd₉=12, sd₁₁=12, sd₁₃= 12, sd₁₆=15, sd₁₇=24, sv₂=8, sv₄=8, sv₆=8, sv₈=8, sv₁₀=9, sv₁₂=9, sv₁₄=9, sv₁₅=9, mbd=42, (mbd)=37, oes=25, spic=48, abd=33, t=59, tmrw=16, tmr=33, a=7.3, b=12.2, c=5.2

Paratype females (n=2): L=282-328, hd=10-11×9, cs=6, sd₁=12, sd₃=12, sd₅=12, sd₇=12-13, sd₉=12- 13, sd₁₁=12-13, sd₁₃=12-13, sd₁₆=15-18, sd₁₇=30- 31, sv₂=8-10, sv₄=8-10, sv₆=8-10, sv₈=8-10, sv₁₀= 8-10, sv₁₂=9-10, sv₁₄=11-12, sv₁₅=14-15, mbd=41- 58, (mbd)=39-54, oes=23-29, abd=35-42, t=62- 68, tmrw=15-17, tmr=32-34, a=5.6-6.7, b=11.2- 11.9, c=4.5-4.7, V=50.7-53.3%

Description. Males. The body comprises 17 main rings, gradually tapering towards both terminal ends. Each main ring is covered with numerous fine foreign particles and measures 7-8 μm in width. The main rings are separated by narrower intermediate zones, consisting of 2-4 secondary rings, each measuring 3-4 μm in width (Figs. 1F, 2A).

The head is broadly triangular, tapering anteriorly, and covered with fine foreign particles (Fig. 1A). Observations using DIC microscopy reveal that the cuticle extends across the head except in the labial regions (Fig. 2B). The cephalic setae are robust, with a broad base that tapers distally, and are directly inserted into the head cuticle approximately halfway along its length.

The amphidial fovea are oval, measuring 7 μm in width and 8 μm in length, centrally positioned on the head, with an oval amphidial pore located just posterior to the midpoint of 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 situated at the third main ring, and the intestine, with a typical structure, extends through to the 16th main ring. Paired ocelli are located between the 4th and 5th main rings.

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

The subdorsal setae gradually taper to a fine tip, with subtle striations visible along most of their length. All subdorsal setae are uniform in length, except for those at the 16th ring, which are relatively longer, and the terminal setae, which are distinctly elongated. The shorter subventral setae have a fine structure and remain consistent in length throughout. Subtle striations observed on the subdorsal setae are also present on the subventral setae.

The male reproductive system, typical of the subgenus, comprises a single, outstretched testis that begins with a germinal zone, transitions into a vesicula seminalis, and extends into a finely granular vas deferens. The spicules are slender and arcuate, with a narrow distal portion terminating in a small capitulum. No gubernaculum structures were observed. The cloacal tube is centrally located on the 15th main ring and is positioned on a papilla measuring 3 μm in height (Figs. 1C, 2C).

The terminal ring is conical, curving ventrally and tapering toward the tail end, with terminal setae positioned approximately two-thirds along its length. It terminates in a relatively long, uncovered spinneret measuring 7 μm in length. Phasmata were not observed.

Females. The female exhibits most of the same morphological features as the male, including 17 main rings that taper toward both terminal ends. These main rings are separated by interzones composed of 2-4 secondary rings (Figs. 1E, 3A, 4A).

The head is broadly triangular, similar to that of the male. DIC microscopy reveals that the cuticle extends across all regions except the labial areas. However, SEM observations were partially obscured by external particles resembling dust (Figs. 1B, 3B, 4B).

The cephalic setae are robust, featuring a broad base that tapers distally. The amphidial fovea are oval and centrally positioned on the head, with an amphidial pore located just posterior to the midpoint of the head (Fig. 4C).

The somatic setae follow the typical arrangement for species with 17 rings, symmetrically paired on both the left and right sides:

Both the subdorsal and subventral setae taper gradually to a tip, with fine striations visible along most of their length (Fig. 4D, E).

The female reproductive system is didelphic-amphidelphic, comprising two outstretched ovaries, each containing several immature oocytes. The vulva is located between the 10th and 11th main rings, with two spermathecae near the vulval region containing globular sperm. The anal tube is positioned on the 15th main ring and measures 5 μm in length.

The terminal ring is conical, curving ventrally and tapering toward the tail, with terminal setae located approximately two-thirds along its length. The ring terminates in a spinneret. Phasmata were not observed (Figs. 1D, 3C, 4F).

Remarks.Desmoscolex (D.) delticeps sp. nov. differs from D. (D.) bathybius by its shorter body length (309- 348 μm in males, 282-328 μm in females versus 790 μm in female for D. (D.) bathybius). It has fewer main rings (17 compared to 28) and a head that is broadly triangular and tapers anteriorly, whereas D. (D.) bathybius has a broad, flattened head. The cephalic setae in D. (D.) delticeps sp. nov. are robust with a broad base that narrows distally, unlike the thick, twisted, and knobbed setae in D. (D.) bathybius. The oval amphid in D. (D.) delticeps sp. nov. is positioned posterior to the midpoint of the head, differing from the oval amphid with small internal vesicles in D. (D.) bathybius. Subdorsal and subventral setae in D. (D.) delticeps sp. nov. taper to fine tips, contrasting with the thick, straight setae with blunt open tips in D. (D.) bathybius.

Desmoscolex (D.) delticeps sp. nov. can be distinguished from D. (D.) galeatus by its shorter body length (309-348 μm in males, 282-328 μm in females compared to 600 μm in female for D. (D.) galeatus). Both species have 17 main rings, but D. (D.) delticeps sp. nov. has a broadly triangular, tapering head, while D. (D.) galeatus has a spherical head with thin ridges and an exposed lip region. The cephalic setae of D. (D.) delticeps sp. nov. are robust and taper distally, whereas those of D. (D.) galeatus are thick and cylindrical with a distal groove. The amphid in D. (D.) delticeps sp. nov. is oval and positioned near the midpoint of the head, while in D. (D.) galeatus, it is bubble-shaped. The subdorsal and subventral setae in D. (D.) delticeps sp. nov. taper to fine tips, compared to the lance-shaped tips in D. (D.) galeatus.

Desmoscolex (D.) delticeps sp. nov. is shorter in body length (309-348 μm in males, 282-328 μm in females) compared to D. (D.) multianulatus (345-415 μm in males, 380-550 μm in females). It has fewer main rings (17 versus 27-34 in D. (D.) multianulatus). The head of D. (D.) delticeps sp. nov. is broadly triangular and tapers anteriorly, unlike the broad head with a rounded posterior and truncated anterior in D. (D.) multianulatus. The cephalic setae in D. (D.) delticeps sp. nov. are robust and taper distally, while D. (D.) multianulatus has stout setae with a central canal. The amphid of D. (D.) delticeps sp. nov. is oval and positioned posteriorly, differing from the large vesicular amphid with an anterior pore in D. (D.) multianulatus. The subdorsal and subventral setae in D. (D.) delticeps sp. nov. taper to fine tips, while in D. (D.) multianulatus, they are inserted on low peduncles with an enlarged distal end and granular gland at the base.

Desmoscolex (D.) delticeps sp. nov. differs from D. (D.) quadricomoides in body length (309-348 μm in males, 282-328 μm in females versus 300 μm in male, 500 μm in female for D. (D.) quadricomoides). Both species have 17 main rings and a broadly triangular, tapering head, as well as robust and distally tapering cephalic setae. However, the amphid in D. (D.) delticeps sp. nov. is oval and positioned near the midpoint of the head, in contrast to the large, raised, pear-shaped amphid in D. (D.) quadricomoides. The subdorsal and subventral setae in D. (D.) delticeps sp. nov. taper to fine tips, while D. (D.) quadricomoides has thick setae with an open tip. Notably, D. (D.) quadricomoides displays a unique main ring morphology in males, characterized by thick cuticularization and a resemblance to that of Quadricoma, with an anterior slope slanting forward and a slight posterior incline. This distinctive feature is absent in D. (D.) delticeps sp. nov., providing a clear distinction between the two species.

Desmoscolex (D.) delticeps sp. nov. and D. (D.) obtusiseta sp. nov. both possess 17 main rings but differ significantly in body length, with D. (D.) delticeps sp. nov. being notably shorter (309-348 μm in males, 282- 328 μm in females) compared D. (D.) obtusiseta sp. nov. (509-540 μm in males, 428-442 μm in females). The two species also differ in head morphology and seta structure. Desmoscolex (D.) delticeps sp. nov. has a broadly triangular head tapering anteriorly, with robust cephalic setae narrowing distally, whereas D. (D.) obtusiseta sp. nov. has a broader, flattened head with blunt cephalic setae inserted on peduncles. The amphidial fovea in both species are oval and positioned posterior to the midpoint of the head, but differences are evident in the seta morphology. In D. (D.) delticeps sp. nov., both the subdorsal and subventral setae taper uniformly to fine tips. Conversely, in D. (D.) obtusiseta sp. nov., the subdorsal setae taper to slightly differentiated tips, while the subventral setae are short and taper to fine tips highlighting distinct morphological differences between the two species.

3.2. Desmoscolex (Desmoscolex) obtusiseta sp. nov. (Figs. 5-9; Table 1)

Type Locality. Marine nematodes were collected from benthic sediments at a depth of 88 meters in distant subtidal zones off the western coast of Korea (34°59ʹ 40.14ʺN, 125°00ʹ2.82ʺE), on April 20, 2019.

Type Material. The holotype male (MABIK NA001 58742), mounted in glycerin on an HS slide, is preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Korea. Additionally, two paratype males (KIOST NEM-1-2776 to KIOST NEM-1-2777) and two paratype females (KIOST NEM-1-2778 to KIOST NEM-1-2779), also mounted in glycerin on HS slides, are deposited 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 and Technology (KIOST), Korea.

Diagnosis.Desmoscolex (Desmoscolex) obtusiseta sp. nov. is characterized by short, blunt cephalic setae inserted on peduncles protruding from the head cuticle at the anterior margin. The head is broader than long, featuring a broad, flattened anterior region extending to the insertion points of the cephalic setae and a wider, longer posterior part covered with coarse foreign particles, except in the amphidial regions. The amphidial fovea are oval and centrally positioned on the head, with a circular amphidial pore located at the midpoint of the head. The somatic setae are arranged in a typical 17-ring pattern, with subdorsal setae tapering gradually to a slightly differentiated tip and exhibiting wrinkles along most of their length. Subventral setae are shorter and display a fine structure. The terminal ring is conical, tapering ventrally, with terminal setae located near the midpoint and ending in a short, uncovered spinneret.

Etymology. The species name Desmoscolex (Desmoscolex) obtusiseta sp. nov. is proposed to reflect the characteristic blunt morphology of the cephalic setae, which is a key distinguishing feature of this species. The name is derived from the Latin ‘obtusus,’ meaning ‘blunt,’ and ‘seta,’ referring to the setae. This etymology emphasizes the unique blunt shape of the cephalic setae observed in this new species.

Measurements.

Holotype male: L=509, hd=36×23, cs=8, sd₁=24, sd₃=18, sd₅=18, sd₇=19, sd₉=18, sd₁₁=20, sd₁₃=20, sd₁₆=23, sd₁₇=36, sv₂=12, sv₄=12, sv₆=13, sv₈=13, sv₁₀=13, sv₁₂=13, sv₁₄=14, sv₁₅=14, mbd =86, (mbd) =70, oes =63, spic =62, abd =71, t =128, tmrw=39, tmr=67, a=5.9, b=8, c=3.9

Paratype males (n=2): L=510-540, hd=32-35× 18-20, cs=9, sd₁=23-25, sd₃=18-20, sd₅=18-22, sd₇=18-22, sd₉=18-22, sd₁₁=18-22, sd₁₃=18-22, sd₁₆=21-25, sd₁₇=32-36, sv₂=15-16, sv₄=15-16, sv₆=15-16, sv₈=15-16, sv₁₀=15-16, sv₁₂=15-16, sv₁₄=15-18, sv₁₅=13-18, mbd=83-102, (mbd)=72- 90, oes=59-65, spic=61-72, abd=73-86, t=121- 124, tmrw=38, tmr=64-67, a=5.2-6.1, b=8.3-8.6, c=4.1-4.3

Paratype females (n=2): L=428-442, hd=30-35× 20-22, cs=10-11, sd₁=24-25, sd₃=21-22, sd₅=20- 21, sd₇=20-21, sd₉=20-21, sd₁₁=20, sd₁₃=20, sd₁₆=23-24, sd₁₇=33-35, sv₂=12-13, sv₄=12-13, sv₆=12-13, sv₈=12-13, sv₁₀=13, sv₁₂=12-13, sv₁₄= 14, sv₁₅=15-16, mbd=80-95, (mbd)=67-80, oes= 43-50, abd =68-69, t =105-121, tmrw=37-42, tmr=56-64, a=4.6-5.3, b=8.4-10, c=3.6-4, V= 52.3-52.4%

Description. Males. The body comprises 17 main rings that gradually taper toward both extremities. Each main ring is covered with numerous foreign particles and measures 16-18 μm in width. These main rings are separated by narrower intermediate zones composed of 3-4 secondary rings, with individual secondary rings measuring 4-5 μm in width (Figs. 5A, 6A, 7A).

The head is broader than long, with a broad, flattened anterior region extending to the insertion points of the cephalic setae and a broader, elongated posterior region covered with coarse foreign particles, except in the amphidial areas. The cephalic setae are short, broad, and blunt, inserted on peduncles protruding from the head cuticle at the anterior margin (Figs. 5B, 6B, 7B, C).

The amphidial fovea are oval, measuring 32 μm in width and 23 μm in length, centrally positioned on the head, with a circular amphidial pore located at the midpoint of 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 situated at the third main ring. The intestine, exhibiting a typical structure, extends through to the 16th main ring. Paired ocelli are located between the 3rd and 5th main rings.

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

The subdorsal setae gradually taper to a slightly differentiated tip and exhibit fine wrinkles along most of their length. All subdorsal setae, except those on the 1st and 16th rings, are of similar length, with the terminal setae being distinctly elongated (Fig. 7F). The subventral setae are shorter, finely structured, and consistent in length throughout (Fig. 7E).

The male reproductive system, typical of the subgenus, consists of a single, outstretched testis that begins with a germinal zone, passes through a vesicula seminalis, and leads into a finely granular vas deferens. The spicules are nearly straight, with a slender distal section that broadens and terminates in a small capitulum. No gubernaculum was observed. The cloacal tube is centrally positioned in the 15th main ring on a papilla measuring 9-10 μm in height (Figs. 5D, 6C).

The terminal ring is conical, curving ventrally and tapering toward the tail end, with terminal setae positioned near the midpoint of the ring. It terminates in a short, uncovered spinneret measuring 4 μm in length. Phasmata are situated midway on the terminal ring (Fig. 7D).

Females. The female resembles the male in most morphological features, possessing 17 main rings that taper toward both extremities. Each main ring is separated by interzones composed of 3-4 secondary rings (Figs. 5F, 8A).

The head is broader than long and comprises two distinct regions. The cephalic setae are relatively broad and short, with a blunt structure. The amphidial fovea are oval and centrally positioned on the head, with a circular amphidial pore located at the midpoint of the head (Figs. 5C, 8B).

The somatic setae follow the typical arrangement observed in species with 17 rings, with each seta symmetrically paired on the left and right sides as follows:

Variability was observed among the paratype females. In one paratype, the typical 9-subdorsal seta pattern was present, but the 10th subventral seta was missing, likely broken, while an additional subventral seta was observed on the right side at the 14th ring. In another paratype, the 11th subdorsal seta on the right side was broken; however, the overall subdorsal pattern remained normal. On the left side, the subventral setae followed the typical 8-seta pattern, but on the right side, both the 10th and 12th subventral setae were absent, with a subventral seta appearing instead at the 11th ring.

The subdorsal setae gradually taper to a slightly differentiated tip, with fine wrinkles visible along most of their length. All subdorsal setae, except those on the 1st and 16th rings, are uniform in length, while the terminal setae are distinctly elongated. The shorter subventral setae have a delicate structure and maintain consistent lengths throughout.

The reproductive system is didelphic-amphidelphic, comprising two outstretched ovaries, each containing several immature oocytes. The vulva is situated between the 10th and 11th main rings, with two spermathecae containing globular sperm located near the vulval region. The anal tube, measuring 10 μm in length, is positioned at the 15th main ring.

The terminal ring is conical, tapering toward the tail, with terminal setae located near its midpoint. It terminates in a spinneret, with phasmata positioned midway along the ring (Figs. 5E, 8C).

Remarks.Desmoscolex (D.) obtusiseta sp. nov. differs from D. (D.) bathybius in several morphological characteristics. The body length of D. (D.) obtusiseta sp. nov. is significantly shorter (509-540 μm in males, 428-442 μm in females) compared to the female of D. (D.) bathybius (790 μm). The number of main rings is also distinct, with D. (D.) obtusiseta sp. nov. having 17 main rings, whereas D. (D.) bathybius has 28. The head structure in D. (D.) obtusiseta sp. nov. is broader and flattened, with cephalic setae inserted on peduncles, contrasting with the broad, flattened head of D. (D.) bathybius, which lacks peduncle insertion. The cephalic setae of D. (D.) obtusiseta sp. nov. are blunt, broad, and short, whereas those of D. (D.) bathybius are thick, twisted, and knobbed. The amphid in D. (D.) obtusiseta sp. nov. is oval and positioned posterior to the midpoint of the head, while in D. (D.) bathybius it is oval and contains small internal vesicles. Additionally, the subdorsal and subventral setae in D. (D.) obtusiseta sp. nov. are short and taper to a fine tip, differing from the thick, straight setae with blunt open tips observed in D. (D.) bathybius.

Desmoscolex (D.) obtusiseta sp. nov. can be distinguished from D. (D.) galeatus by body length, with D. (D.) obtusiseta sp. nov. measuring 509-540 μm in males and 428-442 μm in females, compared to 600 μm in female for D. (D.) galeatus. Both species share the same number of main rings (17), but they differ significantly in head morphology and setae structure. The head of D. (D.) obtusiseta sp. nov. is broader and flattened, with cephalic setae inserted on peduncles, whereas D. (D.) galeatus has a spherical head with thin ridges and an exposed lip region. The cephalic setae of D. (D.) obtusiseta sp. nov. are broad, short, and blunt, while those of D. (D.) galeatus are thick, cylindrical, and feature a distal groove. The amphidial fovea in D. (D.) obtusiseta sp. nov. are oval and positioned posterior to the midpoint of the head, contrasting with the bubble-shaped amphid of D. (D.) galeatus. The subdorsal setae of D. (D.) obtusiseta sp. nov. taper slightly to a differentiated tip, while the subventral setae are short and taper to fine tips. In contrast, the subdorsal and subventral setae of D. (D.) galeatus are lance-shaped, further emphasizing the morphological differences between the two species.

Desmoscolex (D.) obtusiseta sp. nov. differs from D. (D.) multianulatus in several key morphological traits. In terms of body length, D. (D.) obtusiseta sp. nov. measures 509-540 μm in males and 428-442 μm in females, whereas D. (D.) multianulatus measures 345-415 μm in males and 380-550 μm in females. The number of main rings also varies, with D. (D.) obtusiseta sp. nov. having 17, compared to the 27-34 rings observed in D. (D.) multianulatus. The head structure is another distinguishing feature. D. (D.) obtusiseta sp. nov. has a broader, flattened head with cephalic setae inserted on peduncles, while D. (D.) multianulatus dis- plays a broad head with a rounded posterior and truncated anterior. The cephalic setae in D. (D.) obtusiseta sp. nov. are broad, short, and blunt, contrasting with the stout cephalic setae with a central canal observed in D. (D.) multianulatus. The amphidial fovea in D. (D.) obtusiseta sp. nov. are oval and positioned posterior to the midpoint of the head, whereas in D. (D.) multianulatus, the amphid is large, vesicular, and extends over the sides of the head, with a small anterior pore. The subdorsal setae in D. (D.) obtusiseta sp. nov. taper to slightly differentiated tips, while the subventral setae are short and taper to fine tips. In contrast, the subdorsal and subventral setae in D. (D.) multianulatus are inserted on low peduncles and feature enlarged distal ends with granular glands at their base, providing a clear distinction between the two species.

Desmoscolex (D.) obtusiseta sp. nov. can be distinguished from D. (D.) quadricomoides by body length, with D. (D.) obtusiseta sp. nov. measuring 509-540 μm in males and 428-442 μm in females, compared to 330 μm in male and 500 μm in female for D. (D.) quadricomoides. Both species possess 17 main rings, but D. (D.) obtusiseta sp. nov. has a broader, flattened head with cephalic setae inserted on peduncles, whereas D. (D.) quadricomoides has a broadly triangular head with a truncated anterior region. The cephalic setae in D. (D.) obtusiseta sp. nov. are broad, short, and blunt, contrasting with the thick cephalic setae of D. (D.) quadricomoides. The amphidial fovea in D. (D.) obtusiseta sp. nov. are oval and positioned posterior to the midpoint of the head, while in D. (D.) quadricomoides, the amphid is large, raised, and pear-shaped. The subdorsal setae in D. (D.) obtusiseta sp. nov. taper slightly to a differentiated tip, while the subventral setae are short and taper to fine tips. In contrast, D. (D.) quadricomoides has both subdorsal and subventral setae that are thick and terminate with an open tip, providing a clear distinction between the two species. Notably, D. (D.) quadricomoides displays a unique main ring morphology in males, characterized by thick cuticularization and a resemblance to that of Quadricoma, with an anterior slope inclining forward and a slight posterior slope inclining backward. This distinctive feature is absent in D. (D.) obtusiseta sp. nov., providing a clear distinction between the two species.

4. DISCUSSION

The subgenus Desmoscolex is characterized by significant diversity in cephalic setae morphology, a key feature that aids in the identification and classification of its species. Among the 100 described species within the subgenus, cephalic setae exhibit a wide range of forms, from typical radially symmetrical structures to more specialized morphologies, including robust and blunt setae (Timm 1970; Gerlach and Riemann 1973;Freudenhammer 1975;Decraemer 1983). In this study, two new species, D. (D.) delticeps sp. nov. and D. (D.) obtusiseta sp. nov., are described from subtidal zones in Korea, and both exhibit robust and blunt cephalic setae-a morphological characteristic that links them to several previously described species, including D. (D.) bathybiusTimm, 1970, D. (D.) quadricomoidesTimm, 1970, D. (D.) galeatusFreudenhammer, 1975, and D. (D.) multiannulatus Decraemer, 1983 (Fig. 9).

Despite sharing similar cephalic setae characteristics, the two new species exhibit distinct morphological differences that justify their taxonomic status as separate species. For example, D. (D.) delticeps sp. nov. is characterized by a broadly triangular head with cephalic setae directly inserted into the head cuticle, while D. (D.) obtusiseta sp. nov. has a broader, flattened head with cephalic setae inserted on peduncles. These differences in head morphology and the insertion of cephalic setae are consistent with interspecific variation observed within Desmoscolex. Additionally, differences in amphidial fovea structure, somatic setae morphology, and body size further distinguish the two species.

The establishment of an artificial species group based on robust and blunt cephalic setae provides a useful framework for understanding the morphological diversity within the subgenus Desmoscolex. This grouping includes species that inhabit diverse marine environments, from shallow subtidal zones to deep-sea habitats, indicating that cephalic setae morphology may not necessarily correlate with habitat specificity but instead reflect functional adaptations.

The discovery of D. (D.) delticeps sp. nov. and D. (D.) obtusiseta sp. nov. expands the known diversity of Desmoscolex in Korean waters, increasing the number of recorded species to six. These findings highlight the importance of detailed morphological studies, particularly using advanced techniques such as differential interference contrast (DIC) microscopy and scanning electron microscopy (SEM), in uncovering subtle but taxonomically significant features. Moreover, the comparative analysis of these two new species with related taxa underscores the utility of morphological features such as cephalic setae, amphidial fovea, and somatic setae in delineating species boundaries.

5. CONCLUSIONS

This study describes two new species of the subgenus Desmoscolex, D. (D.) delticeps sp. nov. and D. (D.) obtusiseta sp. nov., from subtidal zones in Korea. Both species exhibit robust and blunt cephalic setae, a distinctive morphological trait that links them to an artificial species group within the subgenus. Detailed morphological analyses revealed clear differences between the two species in head shape, cephalic setae insertion, amphidial fovea structure, and somatic setae morphology, supporting their recognition as distinct taxa.

The findings contribute to the growing understanding of the biodiversity of free-living marine nematodes in Korean waters and emphasize the importance of morphological characteristics in nematode taxonomy. Furthermore, the establishment of an artificial species group based on cephalic setae morphology provides a valuable framework for future studies on the systematics and functional morphology of Desmoscolex.

These results underscore the need for continued exploration and documentation of nematode diversity in underexplored regions, particularly through the integration of advanced imaging techniques and detailed comparative analyses. The addition of these two new species to the global inventory of Desmoscolex highlights the rich biodiversity of marine nematodes in Korea and contributes to the broader understanding of nematode taxonomy and biogeography.