Discovery of Two New Deep-Sea Desmoscolex Species (Nematoda: Desmoscolecidae) with Wing-like Cephalic Setae from the Ulleung Basin, the East Sea, Korea ^†

Abstract

This study describes two new species of the subgenus Desmoscolex (Nematoda: Desmoscolecidae) from deep-sea habitats in the Ulleung Basin, the East Sea, Korea, located in the NorthWest Pacific Ocean. Both species exhibit cephalic setae with wing-like appendages—a rare trait documented in only a few species of this subgenus. Desmoscolex (Desmoscolex) globiceps sp. nov. is characterized by a rounded head covered with concretion particles, wing-like cephalic setae equipped with thin, flap-like membranes, and oval amphidial fovea that cover most of the head. The body is composed of 17 main rings, bearing slender somatic setae that taper to an open tip and a conical terminal ring that elongates to a short spinneret. Females of this species lack subventral setae on the 14th main ring. Desmoscolex (Desmoscolex) ovaliceps sp. nov. features an oval head covered with foreign particles, wing-like cephalic setae with thin, flap-like membranes, and oval amphids encompassing much of the head. This species also has a body with 17 main rings, with subdorsal setae gradually tapering toward the tip, which is slightly differentiated from the rest, and shorter subventral setae ending with an open distal tip. The terminal ring is conical, slightly tapered, and terminates in an uncovered spinneret. Detailed morphological descriptions of both species are provided, incorporating scanning electron microscopy (SEM) and differential interference contrast (DIC) images. A comparative analysis with previously described taxa is included, along with a pictorial key to assist in the identification of related species, contributing to a deeper understanding of morphological diversity within the subgenus Desmoscolex.

1. Introduction

In deep-sea environments, meiobenthic community abundance and biomass generally decrease with increasing depth, primarily due to the reduced availability of organic matter and other trophic resources. Despite this overall decline, nematodes exhibit an increasing proportional dominance within the meiobenthic community as depth increases. This trend underscores their remarkable adaptability to the harsh conditions of the deep sea, allowing them to maintain a dominant role even under extreme environmental constraints. The consistently high proportion of nematodes within meiobenthic communities—comprising 83–89% of total abundance across various depth zones—highlights their diverse feeding strategies and exceptional survival capabilities [1].

According to data compiled by Miljutin et al. (2010), based on publications available up to 2008, marine nematodes inhabiting deep-sea environments have been collected globally from 474 deep-sea stations, at depths ranging from 400 to 8380 m. These collections have resulted in the taxonomic description of 548 species across 166 genera [2]. Among these, the subgenus Desmoscolex has consistently been discovered in deep-sea habitats since the identification of Desmoscolex (Desmoscolex) minutus Claparède in 1863. By 1985, Decraemer had provided an overview of Desmoscolex species inhabiting deep-sea environments, documenting 30 species [3]. Subsequent studies by Soetaert (1989) and Bussau (1993) expanded our understanding of Desmoscolex diversity in the deep sea, with estimates suggesting that over 40% of known species within the subgenus are found in deep-sea habitats [2,3,4,5].

The cephalic setae of deep-sea desmoscolecid nematodes may exhibit significant morphological diversity, often featuring distinctive shapes such as knob-like or flag-like termini, as first described by Timm in 1970. These setae are typically positioned submedian (laterodorsal or lateroventral) on the head but can also be located at the extreme anterior [6]. They vary in length, positional arrangement, and structural composition, ranging from delicate to robust forms with terminal modifications that may be pointed, knobbed, or open-ended [7]. Some species within the subgenus Desmoscolex are characterized by cephalic setae with wing- or flag-like appendages, which may be attached either distally or along the entire length of the seta [8].

Among the 99 recognized species within the subgenus Desmoscolex, only 11 are known to have distinctive cephalic setae with wing-like appendages, highlighting the rarity of this feature [6,8,9,10]. The two newly discovered species from the deep-sea region of the East Sea exhibit this unique morphology, enhancing our understanding of cephalic seta diversity within the group.

This study presents detailed taxonomic descriptions of these new species of the subgenus Desmoscolex, highlighting their distinctive cephalic setae. Additionally, it includes a comprehensive comparative analysis with previously described taxa, featuring scanning electron microscopy (SEM) and differential interference contrast (DIC) photomicrographs. A pictorial key and a comparative table are also provided to facilitate accurate species identification and underscore major morphological distinctions.

2. Materials and Methods

2.1. Sample Collection and Preparation

Benthic sediment samples containing marine nematodes were collected from the deep-sea region of the Ulleung Basin, Korea, between 2022 and 2023 using a box corer (Figure 1). In the field, meiobenthic organisms were initially separated through decantation with a 67 µm mesh sieve following a brief rinse with freshwater to minimize sediment adherence [11]. The samples were then preserved in 5% neutralized formalin prepared in seawater. Further isolation of meiobenthos in the laboratory was carried out using the Ludox flotation technique with Ludox^® (Grace, Columbia, MD, USA) HS 40 [12]. The concentrated samples were once 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 analysis under a light microscope, the specimens were gradually dehydrated through a glycerin series [13] before being mounted on HS slides for observation [14]. The nematodes were observed and photographed using a LEICA DM2500 LED microscope equipped with a LEICA K5C color CMOS camera (LEICA, Wetzlar, Germany), which was also used for sketches and measurements.

For scanning electron microscopy (SEM), specimens were preserved in 5% buffered formalin and washed twice with distilled water to remove formalin residues. The specimens were then freeze-dried on an FDU-1200 cooling stage (EYELA, Tokyo, Japan). Dehydrated specimens were mounted on aluminum stubs, sputter-coated with gold/palladium using a high-vacuum evaporator, and examined using an SNE-3200M Desktop Mini SEM (SEC, Suwon-si, Gyeonggi-do, Korea) [15].

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:

L	length of body
hd	maximum head diameter
sdn	length of subdorsal setae on main ring n
svn	length of subventral setae on main ring n
mbd	maximum body diameter
(mbd)	maximum body diameter (foreign material not included)
oes	length of pharynx
spic	length of spicules measured along the median line
gub	length of gubernaculum measured along the median line
abd	anal body diameter
t	tail length
tmr	length of terminal ring
tmrw	maximum width of terminal ring
V	position of the vulva as percentage of the body length
a	body length divided by maximum body diameter
b	body length divided by esophagus length
c	body length divided by tail length

3. Results

3.1. Description of Desmoscolex (Desmoscolex) globiceps sp. nov.

3.1.1. 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.
• Desmoscolex (Desmoscolex) globiceps sp. nov. (Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6)

3.1.2. Diagnosis

Desmoscolex (Desmoscolex) globiceps sp. nov. is characterized by a rounded head covered with concretion particles. The covering material extends further posteriorly on the lateral sides compared to the dorsal and ventral sides, except around the amphid region, the cephalic setae insertion, and the labial region. The relatively short, wing-like cephalic setae are equipped with a thin, flap-like membrane along their entire length, positioned at the anterior margin of the head. The amphids are circular, covering most of the head and restricted to the head region. The body comprises 17 concretion main rings. In females, the arrangement of somatic setae deviates from the typical pattern observed in species with 17 main rings due to the absence of subventral setae on the 14th main ring. This results in sexual dimorphism, where males typically exhibit nine subdorsal and eight subventral setae, whereas females have only seven subventral setae. The somatic setae are slender, tapering to an open tip, with no form differences between the subdorsal and subventral setae. The terminal ring is elongated, conical, and slightly rounded, bending ventrally and tapering towards the tail, with terminal setae situated near the middle of the ring, ending in a short, uncovered spinneret.

3.1.3. Type Locality

Marine nematodes were collected at a depth of 2327 m in the deep-sea region of the Ulleung Basin (37°37′42.16″ N, 131°19′39.00″ E), near Ulleungdo Island, Gyeongsangbuk-do, Korea, on 26 April 2023.

3.1.4. Type Material

urn:lsid:zoobank.org:act:D078EE20-7430-4B0E-8861-7F96990A6AEB

The holotype male (MABIK NA00158733) and one paratype female (MABIK NA00158734), mounted in glycerin on HS slides, are preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Korea. Additionally, three paratype males (KIOST NEM-1-2759 to KIOST NEM-1-2761) and one paratype female (KIOST NEM-1-2763), also mounted in glycerin on HS slides, have been deposited in the nematode collection at the specimen conservation facility of the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.1.5. Etymology

The species name Desmoscolex (Desmoscolex) globiceps sp. nov. is derived from the characteristic rounded shape of the head. The term globus means “rounded” or “globe”, and ceps is a Latin suffix meaning “head”. This name succinctly highlights the defining feature of a rounded head.

3.1.6. Measurements

Holotype male: L = 245, hd = 13 × 9, cs = 5, sd₁ = 10, sd₃ = 9, sd₅ = 8, sd₇ = 8, sd₉ = 8, sd₁₁ = 9, sd₁₃ = 8, sd₁₆ = 9, sd₁₇ = 17, sv₂ = 7, sv₄ = 6, sv₆ = 6, sv₈ = 6, sv₁₀ = 6, sv₁₂ = 6, sv₁₄ = 6, sv₁₅ = 6, mbd = 30, (mbd) = 23, oes = 28, spic = 29, gub = 8, abd = 25, t = 51, tmrw = 8, tmr = 21, a = 8.1, b = 8.7, c = 4.7.

Paratype males (n = 3): L = 235–288, hd = 13–15 × 9–11, cs = 6, sd₁ = 9, sd₃ = 7–9, sd₅ = 7–9, sd₇ = 7–8, sd₉ = 7–8, sd₁₁ = 8–9, sd₁₃ = 8–9, sd₁₆ = 8–10, sd₁₇ = 16–18, sv₂ = 7, sv₄ = 7, sv₆ = 7, sv₈ = 7, sv₁₀ = 7, sv₁₂ = 7, sv₁₄ = 6–7, sv₁₅ = 7, mbd = 31–34, (mbd) = 26–27, oes = 28–30, spic = 26–29, gub = 9–10, abd = 26–27, t = 43–55, tmrw = 8–9, tmr = 20–24, a = 7.5–8.4, b = 8.3–9.6, c = 4.9–5.3.

Paratype females (n = 2): L = 263–277, hd = 14 × 9–10, cs = 6–7, sd₁ = 10–12, sd₃ = 9–10, sd₅ = 9–10, sd₇ = 9–10, sd₉ = 9–10, sd₁₁ = 9, sd₁₃ = 9, sd₁₆ = 10–11, sd₁₇ = 16–17, sv₂ = 6–7, sv₄ = 6–7, sv₆ = 6–7, sv₈ = 6–7, sv₁₀ = 6–7, sv₁₂ = 6–7, sv₁₅ = 6, mbd = 32–38, (mbd) = 27–31, oes = 31–33, abd = 21–25, t = 47–49, tmrw = 9, tmr = 20–23, a = 7.3–8.1, b= 8.1–8.4, c = 5.3–5.8, V = 56.5–58.2%.

Figure 2. Desmoscolex (Desmoscolex) globiceps sp. nov. (A) Entire view of the holotype male, lateral view; (B) entire view of the paratype female, lateral view; (C) anterior end of holotype male, surface view; (D) anterior end of paratype female, surface view; (E) posterior end of holotype male, lateral view; (F) posterior end of paratype female, lateral view. Scale bars: (A,B) = 50 μm; (C–F) = 10 μm.

Figure 2. Desmoscolex (Desmoscolex) globiceps sp. nov. (A) Entire view of the holotype male, lateral view; (B) entire view of the paratype female, lateral view; (C) anterior end of holotype male, surface view; (D) anterior end of paratype female, surface view; (E) posterior end of holotype male, lateral view; (F) posterior end of paratype female, lateral view. Scale bars: (A,B) = 50 μm; (C–F) = 10 μm.

Figure 3. Desmoscolex (Desmoscolex) globiceps sp. nov. DIC photomicrographs of holotype male: (A) entire view, lateral view; (B) surface view of the anterior body region, lateral view; (C) tail region, lateral view. Scale bars: (A) = 20 μm; (B,C) = 10 μm.

Figure 3. Desmoscolex (Desmoscolex) globiceps sp. nov. DIC photomicrographs of holotype male: (A) entire view, lateral view; (B) surface view of the anterior body region, lateral view; (C) tail region, lateral view. Scale bars: (A) = 20 μm; (B,C) = 10 μm.

Figure 4. Desmoscolex (Desmoscolex) globiceps sp. nov. SEM micrographs of male: (A) entire view, subventral view; (B) surface view of body cuticle of interzone and of main body rings from the 4th to the 9th main ring, subventral view; (C) detailed view of the interzone between the 7th and 8th main rings, subventral view; (D) anterior end, subventral view; (E) detailed view of the head region, subventral view; (F) posterior body end displaying the terminal main ring and subdorsal somatic setae, subventral view. Scale bars: (A) = 50 μm; (B,F) = 10 μm; (C) = 1 μm; (D,E) = 5 μm.

Figure 4. Desmoscolex (Desmoscolex) globiceps sp. nov. SEM micrographs of male: (A) entire view, subventral view; (B) surface view of body cuticle of interzone and of main body rings from the 4th to the 9th main ring, subventral view; (C) detailed view of the interzone between the 7th and 8th main rings, subventral view; (D) anterior end, subventral view; (E) detailed view of the head region, subventral view; (F) posterior body end displaying the terminal main ring and subdorsal somatic setae, subventral view. Scale bars: (A) = 50 μm; (B,F) = 10 μm; (C) = 1 μm; (D,E) = 5 μm.

Figure 5. Desmoscolex (Desmoscolex) globiceps sp. nov. DIC photomicrographs of paratype female: (A) entire view with white arrow indicating the vulva, lateral view; (B) surface view of the anterior body region, lateral view; (C) posterior body region, lateral view. Scale bars: (A) = 20 μm; (B,C) = 10 μm.

Figure 5. Desmoscolex (Desmoscolex) globiceps sp. nov. DIC photomicrographs of paratype female: (A) entire view with white arrow indicating the vulva, lateral view; (B) surface view of the anterior body region, lateral view; (C) posterior body region, lateral view. Scale bars: (A) = 20 μm; (B,C) = 10 μm.

Figure 6. Desmoscolex (Desmoscolex) globiceps sp. nov. SEM micrographs of female: (A) entire view, sublateral view; (B) body cuticles from the 5th to the 8th main ring, sublateral view; (C) detailed view of the head region, sublateral view; (D) posterior body end displaying the terminal main ring and subdorsal somatic setae, sublateral view; (E) detailed view of cephalic seta, sublateral view; (F) subventral seta on the 4th main ring, sublateral view; (G) subdorsal seta on the 1st main ring, sublateral view; (H) subdorsal seta on the terminal main ring, sublateral view. Scale bars: (A) = 50 μm; (B,D) = 10 μm; (C) = 5 μm; (E–H) = 1 μm.

Figure 6. Desmoscolex (Desmoscolex) globiceps sp. nov. SEM micrographs of female: (A) entire view, sublateral view; (B) body cuticles from the 5th to the 8th main ring, sublateral view; (C) detailed view of the head region, sublateral view; (D) posterior body end displaying the terminal main ring and subdorsal somatic setae, sublateral view; (E) detailed view of cephalic seta, sublateral view; (F) subventral seta on the 4th main ring, sublateral view; (G) subdorsal seta on the 1st main ring, sublateral view; (H) subdorsal seta on the terminal main ring, sublateral view. Scale bars: (A) = 50 μm; (B,D) = 10 μm; (C) = 5 μm; (E–H) = 1 μm.

3.1.7. Description

Male: Body with 17 main rings gradually tapers toward both extremities (Figure 2A, Figure 3A and Figure 4A). Each main ring consists of numerous concretion particles, with a width ranging from 7 µm to 8 µm. These main rings are separated by narrower intermediate zones, which are made up of three–four secondary rings. The secondary rings within these intermediate zones measure between 2 µm and 3 µm. SEM observations show numerous tubercle- or granular-like materials distributed across the entire surface of the interzones. Additionally, faint junctions between the secondary rings are visible throughout the area (Figure 4B,C).

The head is rounded and covered with concretion particles. The covering material extends further backward on the lateral sides compared to the dorsal and ventral sides, except in the regions surrounding the amphidial pore and the cephalic setae insertion, including the labial area. In observations with a light microscope and scanning electron microscopy, no distinct lips or labial sensory organs are visible in the lateral view (Figure 2C, Figure 3B and Figure 4D,E).

The wing-like cephalic setae, measuring 6 µm in length, are attached to the head cuticle near the anterior margin. These setae have a thin, flap-like membrane that appears all along the cephalic seta. The amphidial fovea are oval, measuring 10 µm in width and 8 µm in length, covering nearly the entire head with a central, circular amphid region. The pharynx is narrow and cylindrical, extending toward the anterior region of the body and encircled by the nerve ring at the level of the second main ring. The pharynx–intestinal junction is located at the third main ring. The intestine, displaying a typical structure, extends into the 16th main ring and includes a postrectal intestinal sac.

The paired ocelli are situated between the third and fifth main rings.

The somatic setae follow the typical arrangement for species with 17 main rings, with each seta symmetrically paired on 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

The somatic setae are slender and taper to an open tip, with fine wrinkles along their length that are visible only under SEM. There is no difference in shape between the subdorsal and subventral setae. The subdorsal setae from the 1st to the 16th main rings are consistent in length, while the terminal setae are distinctly longer. The shorter subventral setae remain relatively uniform in length throughout.

The male reproductive system, characteristic of the subfamily, includes a single, elongated testis that begins with a germinal zone, followed by a vesicula seminalis, and transitions into a finely granular vas deferens. The spicules are almost straight, featuring a slender distal portion that broadens and terminates in a capitulum. A gubernaculum runs parallel to the spicules (Figure 2A,E and Figure 3C). The cloacal tube is positioned at the center of the 15th main ring on a papilliform protrusion of the body cuticle measuring 4–5 µm in height.

The terminal ring is relatively elongated, conical, and slightly rounded, bending ventrally and tapering towards the tail end, with the terminal setae located near the midpoint of the ring. It curves ventrally and concludes in a short, uncovered spinneret measuring 4 µm. Phasmata are present in the posterior region of the terminal ring (Figure 2E and Figure 4F).

Female: The female resembles the male, possessing a similar body structure with 17 main rings, interzones, and slender somatic setae, with no noticeable differences between subdorsal and subventral setae (Figure 2B,D,F, Figure 5A–C and Figure 6A–H). However, in the female, the somatic setae are symmetrically paired on the left and right sides, with a slightly different arrangement as follows:

• Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17 = 9
• Subventral: 2, 4, 6, 8, 10, 12, -, 15 = 7

The reproductive system is didelphic-amphidelphic, with both branches outstretched and containing several immature oocytes. Two spermathecae, each filled with globular sperm, are located near the vulva, which is positioned between the 10th and 11th main rings (Figure 2B).

3.2. Description of Desmoscolex (Desmoscolex) ovaliceps sp. nov.

Desmoscolex (Desmoscolex) ovaliceps sp. nov. (Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11).

3.2.1. Diagnosis

Desmoscolex (Desmoscolex) ovaliceps sp. nov. is defined by its oval-shaped head covered with foreign particles, except for the regions around the amphid region and the insertion points of the cephalic setae. The cephalic setae have thin, flap-like membranes along three-quarters of their length and are inserted subterminally at the base of the lip region. The amphids are oval, covering a large portion of the head. The somatic setae arrangement adheres to a typical 17-ring pattern, with subdorsal setae gradually tapering toward the tip, which is slightly differentiated from the rest. The subventral setae are shorter, tubular, and possess an open distal tip. The terminal ring is elongated and conical, tapering gently, with terminal setae located midway and ending in an uncovered spinneret.

3.2.2. Type Locality

Marine nematodes were collected from benthic sediments at a depth of 2327 m in the deep-sea region of the Ulleung Basin (37°37′42.16″ N, 131°19′39.00″ E), near Ulleungdo Island, Gyeongsangbuk-do, Korea, on 10 November 2022.

3.2.3. Type Material

urn:lsid:zoobank.org:act:B0D88582-39CB-4635-9724-B041CEEDDB4B

The holotype male (MABIK NA00158735) and one paratype female (MABIK NA00158736), mounted in glycerin on HS slides, are preserved in the nematode collection at the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, two paratype males (KIOST NEM-1-2765 to KIOST NEM-1-2766) and three paratype females (KIOST NEM-1-2768 to KIOST NEM-1-2770), also mounted in glycerin on HS slides, have been deposited in the specimen conservation room at the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.2.4. Etymology

The species name Desmoscolex (Desmoscolex) ovaliceps sp. nov. is proposed based on the characteristic oval shape of the head. ‘Oval’ is Latin for “oval”, and ‘ceps’ is a suffix meaning “head”. This name succinctly describes the distinct head shape of the species.

3.2.5. Measurements

Holotype male: L = 371, hd = 24 × 17, cs = 12, sd₁ = 18, sd₃ = 19, sd₅ = 15, sd₇ = 17, sd₉ = 17, sd₁₁ = 17, sd₁₃ = 15, sd₁₆ = 19, sd₁₇ = 27, sv₂ = 16, sv₄ = 17, sv₆ = 17, sv₈ = 17, sv₁₀ = 17, sv₁₂ = 18, sv₁₄ = 13, sv₁₅ = 16, mbd = 57, (mbd) = 50, oes = 37, spic = 45, gub = 13, abd = 46, t = 74, tmrw = 16, tmr = 51, a = 6.4, b = 9.9, c = 4.9.

Paratype males (n = 2): L = 332–416, hd = 19–22 × 15–16, cs = 10, sd₁ = 18, sd₃ = 17, sd₅ = 16–17, sd₇ = 16–18, sd₉ = 17–18, sd₁₁ = 16–18, sd₁₃ = 16–18, sd₁₆ = 19, sd₁₇ = 27, sv₂ = 15–16, sv₄ = 16–17, sv₆ = 17, sv₈ = 17, sv₁₀ = 17, sv₁₂ = 16–18, sv₁₄ = 15, sv₁₅ = 16, mbd = 54–70, (mbd) = 42–56, oes = 34–38, spic = 40–43, gub = 14–15, abd = 41, t = 74–82, tmrw = 14–16, tmr = 43–55, a = 4.7–6.3, b = 8.7–12.2, c = 4.4–5.

Paratype females (n = 4): L = 390–486, hd = 20–24 × 15–19, cs = 10–11, sd₁ = 17–20, sd₃ = 17–19, sd₅ = 16–19, sd₇ = 16–19, sd₉ = 17–19, sd₁₁ = 17–18, sd₁₃ = 17, sd₁₆ = 19–23, sd₁₇ = 28–33, sv₂ = 15–16, sv₄ = 15–16, sv₆ = 15–16, sv₈ = 15–16, sv₁₀ = 15–17, sv₁₂ = 15–16, sv₁₅ = 15–16, mbd = 61–79, (mbd) = 49–68, oes = 38–41, abd = 41–43, t = 81–92, tmrw = 15–16, tmr = 43–54, a = 4.8–7.5, b= 10–11.8, c = 4.5–5.4, V = 53.8–57.6%.

Figure 7. Desmoscolex (Desmoscolex) ovaliceps sp. nov. (A) Entire view of the holotype male, lateral view; (B) entire view of the paratype female, lateral view; (C) anterior end of holotype male, surface view; (D) anterior end of paratype female, surface view; (E) posterior body end with copulatory apparatus of holotype male, lateral view; (F) posterior body end of paratype female, lateral view. Scale bars: (A,B) = 50 μm; (C–F) = 10 μm.

Figure 7. Desmoscolex (Desmoscolex) ovaliceps sp. nov. (A) Entire view of the holotype male, lateral view; (B) entire view of the paratype female, lateral view; (C) anterior end of holotype male, surface view; (D) anterior end of paratype female, surface view; (E) posterior body end with copulatory apparatus of holotype male, lateral view; (F) posterior body end of paratype female, lateral view. Scale bars: (A,B) = 50 μm; (C–F) = 10 μm.

Figure 8. Desmoscolex (Desmoscolex) ovaliceps sp. nov. DIC photomicrographs of holotype male: (A) surface view of the anterior body region, lateral view; (B) posterior body region, lateral view; (C) entire view, lateral view. Scale bars: (A,B) = 10 μm; (C) = 20 μm.

Figure 8. Desmoscolex (Desmoscolex) ovaliceps sp. nov. DIC photomicrographs of holotype male: (A) surface view of the anterior body region, lateral view; (B) posterior body region, lateral view; (C) entire view, lateral view. Scale bars: (A,B) = 10 μm; (C) = 20 μm.

Figure 9. Desmoscolex (Desmoscolex) ovaliceps sp. nov. SEM micrographs of male: (A) entire view, lateral view; (B) body cuticles from the 3rd to the 11th main ring, lateral view; (C) detailed view of the interzone between the 7th and 8th main rings, lateral view; (D) anterior body region with collapsed amphidial fovea, lateral view; (E) posterior body region, lateral view; (F) detailed view of the head region featuring wing-like cephalic setae, lateral view; (G) subventral seta on the 4th main ring, lateral view; (H) detailed view of the subventral seta on the 4th main ring showing an open tip, lateral view; (I) subdorsal seta on the 3rd main ring, lateral view. Scale bars: (A,E) = 50 μm; (B,D) = 30 μm; (C,F) = 5 μm; (G–I) = 1 μm.

Figure 9. Desmoscolex (Desmoscolex) ovaliceps sp. nov. SEM micrographs of male: (A) entire view, lateral view; (B) body cuticles from the 3rd to the 11th main ring, lateral view; (C) detailed view of the interzone between the 7th and 8th main rings, lateral view; (D) anterior body region with collapsed amphidial fovea, lateral view; (E) posterior body region, lateral view; (F) detailed view of the head region featuring wing-like cephalic setae, lateral view; (G) subventral seta on the 4th main ring, lateral view; (H) detailed view of the subventral seta on the 4th main ring showing an open tip, lateral view; (I) subdorsal seta on the 3rd main ring, lateral view. Scale bars: (A,E) = 50 μm; (B,D) = 30 μm; (C,F) = 5 μm; (G–I) = 1 μm.

Figure 10. Desmoscolex (Desmoscolex) ovaliceps sp. nov. DIC photomicrographs of paratype female: (A) entire view, lateral view; (B) surface view of the anterior body region, lateral view; (C), posterior body region, lateral view. Scale bars: (A) = 20 μm; (B,C) = 10 μm.

Figure 10. Desmoscolex (Desmoscolex) ovaliceps sp. nov. DIC photomicrographs of paratype female: (A) entire view, lateral view; (B) surface view of the anterior body region, lateral view; (C), posterior body region, lateral view. Scale bars: (A) = 20 μm; (B,C) = 10 μm.

Figure 11. Desmoscolex (Desmoscolex) ovaliceps sp. nov. SEM micrographs of female: (A) entire view, lateral view; (B) anterior body region, lateral view; (C) detailed view of the head region showing the wing-like cephalic setae, lateral view; (D) detailed view of the 15th main ring showing the anal tube, lateral view; (E) posterior body end, lateral view. Scale bars: (A) = 50 μm; (B,E) = 20 μm; (C) = 5 μm; (D) = 1 μm.

Figure 11. Desmoscolex (Desmoscolex) ovaliceps sp. nov. SEM micrographs of female: (A) entire view, lateral view; (B) anterior body region, lateral view; (C) detailed view of the head region showing the wing-like cephalic setae, lateral view; (D) detailed view of the 15th main ring showing the anal tube, lateral view; (E) posterior body end, lateral view. Scale bars: (A) = 50 μm; (B,E) = 20 μm; (C) = 5 μm; (D) = 1 μm.

3.2.6. Description

Male: The body features 17 main concretion rings that gradually taper toward both ends (Figure 7A, Figure 8C and Figure 9A). Each main ring consists of numerous particles, with a width ranging from 9 µm to 10 µm. In the anterior and posterior regions, the main rings are separated by narrower intermediate zones, which are made up of two–three secondary rings. Along the rest of the body, these interzones are divided into three–four thin rings. The secondary rings in these zones measure between 3 µm and 4 µm. SEM observations reveal numerous tubercle- or granular-like materials scattered across the entire surface of the interzones, with junctions between the secondary rings clearly visible throughout (Figure 9B,C).

The head is oval in shape, with the cuticle covered in foreign material except around the amphidial pore and the areas where the cephalic setae are inserted, including the labial region. In observations with a light microscope and scanning electron microscopy, no distinct lips or labial sensory organs are visible in the lateral view (Figure 7C, Figure 8A and Figure 9D).

The wing-like cephalic setae, measuring 10–12 µm in length, are attached to the head cuticle near the anterior margin. These setae have a thin, flap-like membrane that is attached along three-quarters of the cephalic seta (Figure 9F).

The amphids are oval, measuring 17 µm in width and 9 µm in length, covering most of the head, with an oval amphidial pore extending from the anterior part to the middle of the head. The pharynx is narrow and cylindrical, extending towards the anterior region of the body. It is encircled by the nerve ring at the level of the second main ring, with the pharynx–intestinal junction occurring at the third main ring. The intestine, exhibiting a typical structure, extends into the 16th main ring and includes a postrectal intestinal sac.

The paired ocelli are positioned between the third and fifth main rings.

The somatic setae follow the typical arrangement for species with 17 rings, with each seta symmetrically paired on 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

The subdorsal setae gradually taper to a slightly differentiated tip, with wrinkles along most of their length that are visible only under SEM (Figure 9I). All subdorsal setae from the 1st to the 16th main rings are of similar length, while the terminal setae are significantly longer. The shorter subventral setae possess a fine structure, ending in a narrow, tubular distal tip that is open (Figure 9G,H). The subventral setae are relatively uniform in length, except for the 14th subventral seta, which is distinctly shorter.

The male reproductive system, characteristic of the subfamily, comprises a single, outstretched testis that begins with a germinal zone, transitions through a differentiation zone, and leads into a finely granular vas deferens. The spicules are nearly straight, featuring a slender distal section that broadens and terminates in a small capitulum. A gubernaculum runs parallel to the spicules. The cloacal tube is a small papilliform protrusion of the body cuticle that protrudes from the center of the 15th main ring and measures 3–4 µm in height (Figure 7A,E, Figure 8B and Figure 9E).

The terminal ring tapers gently toward the tail, with the terminal setae positioned approximately midway along its length. It is relatively elongated and conical, ending in an uncovered spinneret measuring 4–5 µm. Phasmata are absent.

Female: The female is similar to the male in most morphological features, sharing the same overall body structure, including 17 main concretion rings, interzones, and somatic setae arrangement and structure, as well as the overall head structure, including head morphology, cephalic setae, and amphids, with no significant differences observed (Figure 7B,D,F, Figure 10A–C and Figure 11A–E).

The reproductive system is didelphic-amphidelphic, consisting of two outstretched ovaries. The vulva is positioned between the 10th and 11th main rings, with two spermathecae containing globular sperm located near the vulval region. The anal tube is located at the 15th main ring (Figure 7B and Figure 11D).

4. Discussion

The subgenus Desmoscolex Claparède, 1863, encompasses numerous species distinguished by unique morphological traits, especially the presence of wing- or flag-shaped appendages on their cephalic setae. Since the initial descriptions of Desmoscolex (D.) remifer Timm, 1970, and D. (D.) velifer Timm, 1970, additional species exhibiting cephalic setae with wing-like appendages have been documented [6]. These appendages can be located on one or both sides of the seta and may attach either at the distal end—as in D. (D.) remifer—or extend along the entire length of the bristle, as seen in D. (D.) chaetalatus Freudenhammer, 1975 [8]. Currently, eleven species displaying this characteristic have been identified: D. (D.) remifer Timm, 1970, D. (D.) velifer Timm, 1970, D. (D.) petalodes Lorenzen, 1972, D. (D.) chaetalatus Freudenhammer, 1975, D. (D.) spinosus Decraemer, 1976, D. (D.) roscoffiensis Decraemer, 1979, D. (D.) complexus Decraemer, 1984, D. (D.) membranifer Soetaert, 1989, D. (D.) brachyrhynchus Bussau, 1993, D. (D.) noctuabundus Bussau, 1993, and D. (D.) opacus Bussau, 1993 [4,5,6,8,16,17,18,19]. This study provides a thorough comparative analysis of the morphological characteristics of Desmoscolex species with wing-like appendages on cephalic setae and includes a pictorial key for valid species within the subgenus, as shown in

Table 1. Comparative characteristics of Desmoscolex (Desmoscolex) species possessing cephalic setae with wing-like appendages (M: male; F: female; -: unknown; *: species known only from a dorso-ventral view are marked separately with *).

Taxa		Characters
	Body Length (µm)	Number of Main Rings	Seta Arrangement	Head	Cephalic Seta	Amphid	Subdorsal Setae	Subventral Setae	Spicule Length (µm)	Habitat	Distribution
* D. (D.) remifer Timm, 1970	M (n = 2): 570, 700 F (n = 2): 580, 650	17	$\mathrm{♂},\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8}}$	Head with reticulate pattern, visible in dorso-ventral view	Short, thick, oar-like tips, positioned at extreme anterior	Raised, extending to the posterior of first main ring	Sword-like, with a blunt tip	Sword-like, with a blunt tip	47	Deep-sea, 3994–4364 m; Deep-sea, 1360 m, 1860 m	Peru; Pacific Ocean
* D. (D.) velifer Timm, 1970	M (n = 2): 625, 635 F (n = 2): 500, 595	♂43–44 ♀31–38	${\displaystyle \frac{\begin{array}{c}♂:\mathrm{1,3},\mathrm{6,12,17,24,30}\left(29\right),\mathrm{38,44}=9\\ ♀:\mathrm{1,3},\mathrm{6,10,15,17,21,27,31}=9\end{array}}{\begin{array}{c}♂ :\mathrm{2,4},\mathrm{7,12}\left(11\right),\mathrm{18,24,30,34}=8\\ ♀ :\mathrm{2,4},\mathrm{7,11,15,19,23,27}=8\end{array}}}$	Thickly cuticularized anterior head	Thin flap-like membrane, positioned at extreme anterior	Darkly granular, raised at the posterior, restricted to head region	Thick, with blunt tip	Thick, with blunt tip	60	Deep-sea, 3100–6200 m	Peru; Pacific Ocean
D. (D.) petalodes Lorenzen, 1972	M (n = 2): 280 F (n = 3): 285–351	17	$\mathrm{♂},\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8}}$	Rounded, triangular head	Sail-like appendage along the entire length	Rounded vesicular, restricted to the head region	Wide base with a curved, lancet-shaped tip	Shorter, slender, laterally shifted on main ring 10	29	Sandy silt, 38 m; fine sand, 10 m	North Sea; White Sea, Kandalaksha Bay
* D. (D.) chaetalatus Freudenhammer, 1975	F (n = 1): 415	17	$\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15},16=9}}$	Head covered with secretions and foreign particles	Outward flange along seta, inserted near labial region	Spherical, double-contoured, restricted to head position	Winged, except first and terminal setae	Winged	-	Deep-sea, 1174 m	Iberian Deep Sea; Atlantic Ocean
D. (D.) spinosus Decraemer, 1976	M (n = 1): 285 F (n = 1): 290	17	$\mathrm{♂},\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8}}$	Head with short, naked anterior part provided with a crown of three rows of fine spines	Flanked by a membrane along the entire length, with a fine central canal	Circular, extending from just in front of the cephalic setae to the head margin	Broader basal part, tapering distally to a narrow, slightly offset spatulate tip	Shorter, slender subventral setae	35	Deep-sea, 535 m	Ross Island; Atlantic Ocean
D. (D.) roscoffiensis Decraemer, 1979	M (n = 1): 165	17	$\mathrm{♂}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14},-=7}}$	Rounded head, tapered to a truncated end	Long, fine, with thin, flag-like membrane, positioned at extreme anterior	Large vesicular, covering most of the head, extending to the first main ring	Large base, tapered to a hooked lance-shaped tip	Shorter, fine, tapered to pointed tip	21	Fine sand, 18 m	Bay of Morlaix, France; Atlantic Ocean
D. (D.) complexus Decraemer, 1984	M (n = 1): 240 F (n = 2): 250, 260	17	$\mathrm{♂},\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14},-=7}}$	Bipartite head, rounded posteriorly and tapered anteriorly, with six-lipped labial region	Complex, 8 feather-like parts with central canal	Large vesicular, extends from stomatal region to the end of the first main ring	Stout, spatulate tip, inserted on peduncles with granular gland cells	Short, slender, pointed tip, on small peduncles	27	Deep-sea, 330 m, 550 m	Mozambique Channel, north of île du Lys; Indian Ocean
D. (D.) membranifer Soetaert, 1989	F (n = 10): 145–185	17	$\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},-,12,-,15=6}}$	Oval head with sclerotized ring and protruding hyaline labial region	Thin, flap-like membrane, positioned at extreme anterior	Oval vesicular, restricted to the head region	Spatulate tip on small peduncles, slightly dorsal displacement	Shorter subventral setae	-	Deep-sea, 530 m	Calvi, Corsica; Mediterranean Sea
D. (D.) brachyrhynchus Bussau, 1993	M (n = 1): 295 F (n = 1): 460	17	$\mathrm{♂},\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8}}$	Protruding rostrum with terminal labial region	Sail-like appendages, positioned at one-third from the anterior end	Balloon shaped, extends to the anterior edge of the first main ring	Feather-like, surrounded by a transparent sheath	Normal bristle structure, with transparent sheath	38	Deep-sea, 4147 m, 4175 m	Peru Basin; Pacific Ocean
D. (D.) noctuabundus Bussau, 1993	M (n = 2): 320, 405 F (n = 1): 375	Only a few, and partially incomplete, main rings are present	$\mathrm{♂},\mathrm{♀}(9/7)$	Slit-shaped mouth, visible in front view only	Cauliflower-like, positioned at one-third from the anterior end	Vesicular	Blunt, notched tips on peduncles covered with secretions	Blunt, notched tips on peduncles covered with secretions	51	Deep-sea, 4154 m, 4175 m	Peru Basin; Pacific Ocean
D. (D.) opacus Bussau, 1993	M (n = 2): 280, 320	17	$\mathrm{♂}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8}}$	Oval head, covered with foreign particles	Cauliflower-like	Vesicular, extending to the middle of the first main ring	Slender, gradually tapering to tip	Slender, gradually tapering to tip	30	Deep-sea, 4154 m, 4178 m	Peru Basin; Pacific Ocean
D. (D.) globiceps sp. nov.	M (n = 4): 235–288 F (n = 2): 263–277	17	${\displaystyle \frac{\mathrm{♂},\mathrm{♀}:\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\begin{array}{c}♂:\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8\\ ♀:\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14},-=7\end{array}}}$	Rounded, covered with foreign deposits, with lateral concretions extending further posteriorly	Thin, flap-like membrane extends along the entire length, positioned at anterior	Oval, centrally located, covering most of head, restricted to head	Slender, gradually tapering to tip	Shorter, slender, gradually tapering to tip	26–29	Deep-sea, 2327 m	Ulleung Basin, Korea; Pacific Ocean
D. (D.) ovaliceps sp. nov.	M (n = 3): 332–416 F (n = 4): 390–486	17	$\mathrm{♂},\mathrm{♀}{\displaystyle \frac{\mathrm{1,3},\mathrm{5,7},\mathrm{9,11,13,16,17}=9}{\mathrm{2,4},\mathrm{6,8},\mathrm{10,12,14,15}=8}}$	Oval head, covered with foreign particles	Thin, flap-like membrane attached along three-quarters of the cephalic seta, positioned at anterior	Oval, centrally located, covering most of head, restricted to head	Gradually tapering to a slightly differentiated tip	Shorter, narrow, tubular distal end	40–45	Deep-sea, 2327 m	Ulleung Basin, Korea; Pacific Ocean

and Figure 12. The illustrations in Figure 12 are based on the original descriptions of each species but are not exact copies; they have been newly redrawn as computer-generated images by the authors. Most of the illustrations are based on males, but two species known only from females are also included. Additionally, while the majority of species are presented in lateral view, it should be noted that three species are depicted with their entire body or tail region shown in a dorso-ventral view.

4.1. Comparative Morphological Features of Desmoscolex Species with Wing-like Appendages on Cephalic Setae

The species within the Desmoscolex (Desmoscolex) species group are distinguished by unique morphological features, including the position of the cephalic setae, the presence of a differentiated labial region, head morphology, somatic setal patterns, and the number of main rings. Desmoscolex (D.) ovaliceps sp. nov. and D. (D.) globiceps sp. nov. possess rounded heads, resembling D. (D.) petalodes Lorenzen, 1972, whereas D. (D.) roscoffiensis Decraemer, 1979 exhibits a tapered head. The structure and position of the cephalic setae serve as key diagnostic traits: D. (D.) globiceps sp. nov. and D. (D.) ovaliceps sp. nov. feature thin, flap-like membranes near their insertion points, while D. (D.) opacus Bussau, 1993 is characterized by unique cauliflower-like setae. Additionally, D. (D.) brachyrhynchus Bussau, 1993 is distinguished by a prominent labial region, absent in the newly described species. The arrangement of somatic setae also provides critical taxonomic distinctions. For example, D. (D.) membranifer Soetaert, 1989 lacks subventral setae on the 10th and 14th main rings, a feature that differentiates it from the new species. While D. (D.) petalodes Lorenzen, 1972 shares the setal pattern of species with 17 main rings, subtle variations in the setal arrangement further separate it from the new taxa. Other distinguishing features include the number of main rings and the morphology of the terminal ring, which collectively contribute to differences in body structure among species.

Desmoscolex (Desmoscolex) remifer Timm, 1970 is the first species in the subgenus Desmoscolex to exhibit wing- or flag-shaped appendages on the cephalic setae. It was discovered in deep-sea dredgings at depths ranging from 3994 to 4364 m off the coast of Peru and from 1360 to 1860 m in the Indian Ocean. Morphologically, the setal pattern is typical for the subgenus, but the somatic setae are sword blade-shaped with blunt tips, a unique feature. Notably, the cephalic setae are short and thick, with flattened, oar-like tips positioned at the extreme anterior. Wing-like appendages are present but are restricted to the distal half [6].

Desmoscolex (Desmoscolex) velifer Timm, 1970 was collected from deep-sea dredgings at depths of 3100 to 6200 m in the Peru Basin. As noted by Timm (1970), “Such a great difference in the number of rings between the two sexes is unknown in any other species of the Desmoscolecida”. Males have 43–44 rings, while females have 31–38 rings, marking a significant distinction between the sexes. The interzones between each main ring contain one–two distinct secondary annules, each featuring a central row of small spines or pegs—features not commonly observed in other Desmoscolex species. These spines or pegs also occur beneath the concretions in the middle part of the anterior main rings. The somatic setae follow the typical arrangement for species with 17 main rings and are thick, ending in blunt or open tips. The cephalic setae is located at the extreme anterior, bearing a thin, flap-like membrane along the outer margin [6].

Desmoscolex (Desmoscolex) petalodes Lorenzen, 1972 was first described by Lorenzen in 1972 from sandy silt at a depth of 38 m in the southern North Sea, an area abundant with Ophiura albida and Turritella communis. The somatic setae exhibit a typical arrangement for species with 17 main rings, consisting of a robust basal section and a thin-walled distal section. The cephalic setae are characterized by a sail-like appendage extending along their entire length. In 1996, Decraemer and Tchesunov redescribed two female specimens collected from fine sand at a depth of 10 m in Kandalaksha Bay, White Sea. These specimens closely matched Lorenzen’s original description, including the rounded, triangular head shape and the standard setal arrangement. The somatic setae are robust, slightly curved, and broader at the base, with the apical end of the subdorsal setae modified to a lancet-like tip. The cephalic setae are significantly modified, featuring flag-like extensions originating from the setal bar. These characteristics are consistent with Lorenzen’s (1972) description [16,20].

Desmoscolex (Desmoscolex) chaetalatus Freudenhammer, 1975 was identified from a single holotype male collected at a depth of 1174 m in the Iberian deep-sea sediment, composed of olive-green foraminiferous mud interspersed with heterogeneous fine sand. This species stands out from typical 17-ring Desmoscolex species by having an additional pair of subventral setae on the 16th main ring. Uniquely, all somatic setae, except the first and last pairs of subdorsal setae, are winged and inserted submedially, which is a distinctive trait among Desmoscolex species. The four submedian cephalic setae have outward-facing flanges and are inserted anteriorly, near the mouth region [8].

Desmoscolex (Desmoscolex) spinosus Decraemer, 1976 was identified at a depth of 535 m near Scott Base, Ross Island, in the Atlantic. This species exhibits a somatic setae pattern typical of 17-ring Desmoscolex species. The somatic setae are positioned on low peduncles that do not protrude from the main ring. The subdorsal setae feature a broader basal region that gradually tapers to a narrow, slightly offset, spatulate tip, whereas the subventral setae are more slender and shorter than the subdorsal setae. The head morphology of D. (D.) spinosus is unique within the subgenus Desmoscolex. The anterior region of the head is characterized by a short, naked section that bears a crown of three rows of fine spines. The overall head shape is elongated laterally relative to the dorsal and ventral regions. The cephalic setae are bordered by a membrane along their entire length and contain a fine central canal. These setae are inserted relatively anteriorly, just posterior to the naked anterior region of the head. The amphids are nearly circular and are situated at the level of the posterior covered region of the head, extending anteriorly to just in front of the insertion of the cephalic setae and posteriorly to the margin of the head [17].

Desmoscolex (Desmoscolex) roscoffiensis Decraemer, 1979 was identified from a single holotype male collected from fine sand at a depth of 18 m in the Bay of Morlaix, France. This species is characterized by its relatively small body length and a setal pattern that notably lacks subventral setae on the 15th main ring, setting it apart from typical 17-ring species. The subdorsal setae have a larger basal shaft and taper distally into a lance-shaped end, with the terminal setae pair slightly hooked. In contrast, the subventral setae are fine, distally pointed, and shorter than the subdorsal setae. The head is slightly wider than long, broadly rounded at the posterior, and tapered towards a truncated anterior end. The cephalic setae are long, fine, and equipped with a thin, flag-like membrane, inserted far anteriorly, just behind the extreme anterior margin. The amphids are large and vesicular, covering most of the head, extending from the cephalic setae insertion anteriorly to the first main ring posteriorly. The amphidial canal terminates in a small pore located in the posterior half of the head [18].

Desmoscolex (Desmoscolex) complexus Decraemer, 1984 was discovered north of Île du Lys, at depths ranging from 330 m to 550 m in coral sand. This species is notable for its relatively small body length and the absence of subventral setae on the 15th main ring, deviating from the typical somatic setae pattern. The subdorsal setae are robust, tapering distally to an open spatulate tip and are inserted on peduncles that slightly protrude from the main ring. In contrast, the subventral setae are shorter, slender, and tapered to a pointed tip, inserted on smaller peduncles. At the base of the somatic setae, a finely granular glandular cell can be observed. The head is bipartite, rounded posteriorly, and tapered anteriorly, featuring a labial region with six lips and a crown of very fine labial setae. The cephalic setae are composed of eight feather-like structures, each with a central canal. The amphid is large, vesicular, and extends from the stomatal region to the end of the first main ring. Additionally, a short, naked spinneret and small phasmata are present [19].

Desmoscolex (Desmoscolex) membranifer Soetaert, 1989 was discovered in a transect off Calvi, Corsica, at a depth of 530 m in the deep sea. This species is characterized by its relatively small body length and a somatic setae arrangement that deviates from the typical pattern of 17-ring species, notably lacking a pair of subventral setae on main rings 10 and 14. The subdorsal setae are positioned on small peduncles and have a spatulate tip, while the subventral setae are shorter. The head is oval and strongly sclerotized, featuring a protruding hyaline labial region with an anterior sclerotized ring. The cephalic setae bear a thin, flap-like membrane and are located at the extreme anterior margin of the head. The amphids are vesiculate, transversely oval, and confined to the head region. Additionally, a prominent anal tube protrudes, and the terminal main ring is conical, slightly bent toward the ventral side, with a naked spinneret [5].

Desmoscolex (Desmoscolex) brachyrhynchus Bussau, 1993 was discovered in the Peru Basin at depths of 4147 m and 4175 m in the deep sea. This species exhibits the typical somatic setae arrangement of 17-ring Desmoscolex species, with nine pairs of subdorsal setae and eight pairs of subventral setae. The subdorsal setae are feather-like in structure, whereas the subventral setae are shorter and have a normal seta structure. Both types of setae are enclosed in a transparent sheath. The cephalic setae are equipped with sail-like appendages, and the amphids are balloon-shaped, extending to the anterior edge of the first main ring. The head features a protruding snout with a labial region, and both the spinneret and phasmids are absent [4].

Desmoscolex (Desmoscolex) noctuabundus Bussau, 1993 was discovered in the deep-sea Peru Basin at depths of 4154 m and 4175 m. It exhibits only a few, partially incomplete main rings, with 91 visible cuticular rings that likely represent secondary annules. Unlike the typical 17-ring Desmoscolex species, D. (D.) noctuabundus lacks a subventral seta on the fourth main ring and features 17 medioventral papillae extending along the body from anterior to posterior. The somatic setae have blunt, notched ends and are positioned on pedestals covered with secretions and fine foreign particles. Additionally, the 17 medioventral papillae form a distinct row along the body, with no similar structures present below the subdorsal setae. The cephalic setae are expanded, resembling a cauliflower-like structure. The amphids are vesicular, and the mouth is slit-shaped. Phasmids are present [4].

Desmoscolex (Desmoscolex) opacus Bussau, 1993 was discovered at depths of 4154 m and 4178 m in the Peru Basin. The somatic setae arrangement adheres to the typical pattern for 17-ring species, with both subdorsal and subventral setae exhibiting a normal seta structure. The cephalic setae are broadened, taking the form of a cauliflower leaf-like shape. The amphids are vesicular and extend to the middle of the first main ring. A spinneret is present, but phasmids are absent [4].

4.2. Comparative Analysis of the New Species with Related Desmoscolex Species

Desmoscolex (Desmoscolex) ovaliceps sp. nov. and D. (D.) globiceps sp. nov. are notable for their rounded heads and cephalic setae with thin, flap-like membranes near their insertion points. These features are shared with some other Desmoscolex species possessing 17 main rings, including D. (D.) brachyrhynchus, D. (D.) membranifer, D. (D.) opacus, D. (D.) roscoffiensis, and D. (D.) petalodes.

Desmoscolex (D.) brachyrhynchus can be distinguished from the newly described species by a prominent labial region and amphids that extend to the anterior edge of the first main ring.

Desmoscolex (D.) membranifer, described only from females, has a smaller body length (145–185 µm) and a distinctive somatic seta pattern, lacking subventral setae on the 10th and 14th main rings. These differences set it apart from both new species from Korea.

Desmoscolex (D.) opacus, although known only from males, has a comparable body length and somatic seta arrangement. However, it features unique cauliflower-like cephalic setae and amphids that extend only to the middle of the first main ring, differentiating it from the new species.

Desmoscolex (D.) roscoffiensis, described from a single male specimen, shares head morphology similar to the newly identified Korean species, particularly resembling D. (D.) globiceps sp. nov. in the shape of the terminal ring. Nevertheless, it differs in having a smaller body length (165 µm) and the absence of subventral setae on the 15th main ring. The head is also more tapered anteriorly with a truncated end and a non-sclerotized region. The cephalic setae of D. (D.) roscoffiensis are long, fine, and bear a thin, flag-like membrane, unlike the short, flap-like membranes of the Korean species. The presence of a small amphidial pore, not seen in the Korean species, further sets D. (D.) roscoffiensis apart. Additionally, D. (D.) roscoffiensis has subdorsal setae with a prominent basal shaft that tapers to a lance-shaped tip, a feature absent in the newly described species.

Desmoscolex (D.) petalodes is morphologically closest to the two new species. It shares characteristics like a rounded, triangular head, vesicular amphids, and flag-like cephalic setae. However, D. (D.) petalodes was found in shallow waters at depths of 10–38 m, compared to the deep-sea habitat (2327 m) of the new species. This significant difference in habitat depth is likely to be accompanied by pronounced differences in morphological characteristics.

For D. (D.) globiceps sp. nov., similarities with D. (D.) petalodes include body length and a slightly rounded terminal ring tapering toward the tail. However, D. (D.) globiceps sp. nov. differs in the absence of subventral setae on the 15th main ring in females, while D. (D.) petalodes retains a typical setal arrangement for 17-main-ring species in both sexes. Furthermore, the somatic setae of D. (D.) petalodes, as detailed by Lorenzen (1972), have a robust basal segment and a thin, membranous terminal part, features not seen in D. (D.) globiceps sp. nov. Additionally, the broad, slightly curved subdorsal setae with a lancet-like tip described by Decraemer and Tchesunov (1996) are also absent in D. (D.) globiceps sp. nov. The presence of phasmata in D. (D.) globiceps sp. nov. further distinguishes it from D. (D.) petalodes [16,20].

For D. (D.) ovaliceps sp. nov., D. (D.) petalodes exhibits a shorter body length (280 µm for males and 285–351 µm for females). As with D. (D.) globiceps sp. nov., the lancet-like tips of D. (D.) petalodes are not found in either Korean species. Lastly, D. (D.) ovaliceps sp. nov. has a relatively elongated, conical terminal ring, setting it apart from D. (D.) petalodes.

4.3. Habitat Preferences, Distribution Patterns, and Ecological and Biogeographic Insights of Desmoscolex (Desmoscolex) Species with Wing-like Appendages on Cephalic Setae

Species within the Desmoscolex (Desmoscolex) genus that possess cephalic setae with wing-like appendages exhibit diverse habitat preferences and distribution patterns, ranging from shallow coastal environments to extreme deep-sea habitats (Table 1).

Habitat preferences. The data indicate that these species predominantly inhabit deep-sea environments, often exceeding depths of 500 m. For instance, D. (D.) renifer and D. (D.) velifer are recorded at depths ranging from 1360 to 6200 m in the Pacific Ocean, highlighting their adaptation to extreme deep-sea conditions. Similarly, D. (D.) brachyrhynchus, D. (D.) noctuabundus, and D. (D.) opacus are exclusively found in the Peru Basin at depths exceeding 4000 m, reflecting their specialization for abyssal habitats.

However, certain species inhabit shallower waters, indicating ecological flexibility. For example, D. (D.) petalodes is found in sandy silt and fine sand at depths of 10–38 m in the North and White Seas, while D. (D.) roscoffiensis is recorded in fine sand at 18 m in the Bay of Morlaix, Atlantic Ocean. These records demonstrate that some Desmoscolex species are capable of thriving in both shallow and deep marine environments.

Distribution patterns. The geographic distribution of Desmoscolex (Desmoscolex) species with wing-like cephalic appendages spans multiple oceanic regions, showcasing their adaptability to diverse marine environments. These species are recorded in the Pacific, Atlantic, Indian, and Mediterranean Seas, displaying notable patterns of habitat specialization and biodiversity.

In the Pacific Ocean, the Peru Basin serves as a key biodiversity hotspot, hosting several species, including D. (D.) renifer, D. (D.) velifer, D. (D.) brachyrhynchus, D. (D.) noctuabundus, and D. (D.) opacus. These species are found at significant depths, emphasizing the region’s importance for deep-sea nematode diversity. Similarly, the Ulleung Basin, Korea, in the northwestern Pacific, represents a recent addition to the subgenus’s known range. The discovery of D. (D.) globiceps sp. nov. and D. (D.) ovaliceps sp. nov. highlights the unexplored biodiversity of East Asian seas and underscores the need for further research in these under-sampled regions.

In the Atlantic Ocean, D. (D.) spinous has been recorded near Ross Island, while D. (D.) roscoffiensis is documented in the Bay of Morlaix. These findings indicate the subgenus’s presence in temperate and polar regions, suggesting its ability to adapt to a variety of environmental conditions.

The Indian Ocean hosts D. (D.) complexus, which is found in the tropical Mozambique Channel, reflecting the subgenus’s adaptability to warm, nutrient-rich waters. Meanwhile, the Mediterranean Sea harbors D. (D.) membranifer in Calvi, Corsica, demonstrating the subgenus’s ability to thrive in temperate deep-sea environments.

Ecological and biogeographic insights. The broad bathymetric range and diverse geographic distribution of Desmoscolex (Desmoscolex) species suggest remarkable ecological versatility. These nematodes have adapted to a wide array of environmental conditions, from nutrient-rich shallow sands to oligotrophic deep-sea sediments. Their specialized morphological features, such as wing-like cephalic appendages and sensory structures, likely enable them to exploit different ecological niches effectively.

These findings emphasize the importance of further sampling and research in both well-studied and less-explored regions to better understand the biogeographic and ecological roles of Desmoscolex species in marine ecosystems.

5. Conclusions

This study describes two new deep-sea nematode species, Desmoscolex (Desmoscolex) globiceps sp. nov. and Desmoscolex (D.) ovaliceps sp. nov., discovered in the Ulleung Basin of Korea in the Northwest Pacific Ocean. Both species exhibit cephalic setae with wing-like appendages, a rare trait within the subgenus Desmoscolex, contributing to our understanding of cephalic seta diversity. Desmoscolex (D.) globiceps sp. nov. is characterized by a rounded head with concretion particles, short wing-like cephalic setae with flap-like membranes, and a unique somatic seta arrangement. Desmoscolex (D.) ovaliceps sp. nov. is characterized by an oval head, wing-like cephalic setae with thin membranes, and subdorsal setae that taper to a slightly differentiated tip. Our comprehensive morphological analysis, using SEM and DIC techniques, has allowed for detailed comparisons with similar species, highlighting key diagnostic features and emphasizing the significant role of habitat differences in morphological variability. These findings enhance our understanding of the morphological diversity and adaptive traits within the subgenus Desmoscolex, and the inclusion of a pictorial key provides an essential tool for future taxonomic studies.