A New Species of Desmoscolex (Nematoda: Desmoscolecidae) from the Northwestern Pacific and Its Implications for Lip-Region Ultrastructure in Species Delimitation ^†

Abstract

We describe a new species of Desmoscolex collected from subtidal muddy sediments off Jindo Island, on the southern margin of Korea’s west coast. Desmoscolex (Desmoscolex) curuvus sp. nov. is distinguished by 17 main rings, a 9/8 somatic setal arrangement, subdorsal setae with a slightly swollen and hollow distal end, an oval head with laterally extended foreign material, a rounded amphidial fovea confined within the head region, and broad cephalic setae bearing a fine central canal with lateral membranes. The terminal ring is strongly bent ventrally. Females exhibit pronounced sexual dimorphism, including a pair of dorsal setae on the thirteenth main ring and a thicker terminal ring (maximum width 25–26 μm in males and 24–31 μm in females). High-resolution scanning electron microscopy (SEM) observations revealed a distinctive lip-region ultrastructure composed of a tuberculate membrane and six fragment-like membranous elements, representing a rare configuration within the genus. By documenting a new species possessing membranous structures in the cephalic setae and providing detailed ultrastructural characterization of the lip region, this study offers important new evidence for refining species delimitations in Desmoscolex. These findings underscore the diagnostic value of lip-region morphology and highlight the need for targeted sampling and further ultrastructural analyses to better resolve the hidden morphological diversity of the genus, particularly in the underexplored northwestern Pacific.

1. Introduction

The subgenus Desmoscolex (Desmoscolex) is known for its remarkably high species diversity, with approximately 105–112 currently described species and for its wide biogeographic range [1,2,3,4,5,6,7]. Members of this subgenus possess a distinctive morphology characterized by a series of thick main rings encircling the entire body, making them among the most easily recognizable nematodes.

Despite their striking appearance, relatively few external characters are available for taxonomic assessment. The thick, opaque main rings obscure the internal organs and limit access to additional diagnostic features [3]. Thus, species-level identification relies largely on a restricted set of morphological traits, including main-ring number, somatic setal patterns, head shape, amphid morphology, and especially the structure of the cephalic setae [4].

Detailed morphology of the male and female reproductive systems, as well as the anterior sensorial organs such as the lip region, would likely provide more informative characters for resolving taxonomic problems within this group [4,8,9]. However, as noted above, the thick main rings generally obstruct observation of the internal reproductive anatomy, and the anterior sensory structures are extremely fine and complex, making high-resolution SEM observation indispensable for studying them in detail. These challenges have contributed to a prolonged period of stagnation in Desmoscolex research following the active decades of the 1970s and 1980s.

Recent investigations have renewed interest in this group, extending coverage into the historically understudied northwestern Pacific [7,10]. Nonetheless, substantial gaps remain in our understanding of species richness and morphological variability. Notably, subtidal muddy habitats around the Korean Peninsula—particularly the extensive tidal flats of the western coast—are likely to contain considerable, yet still underexplored, meiofaunal diversity.

During subtidal sediment sampling off Jindo Island, on the southern margin of Korea’s west coast, we collected specimens belonging to the subgenus Desmoscolex. Detailed observations using differential interference contrast (DIC) and scanning electron microscopy (SEM) revealed a combination of diagnostic features, most notably thin, delicate membranous extensions flanking the entire length of each cephalic seta and a distinctive lip-region ultrastructure. In combination, these characters clearly distinguish the present material from all known congeners. Here, we describe Desmoscolex (Desmoscolex) curuvus sp. nov., supported by high-resolution SEM imaging of the lip region. This study contributes to a better understanding of the morphological diversity of northwestern Pacific Desmoscolex and underscores the taxonomic importance of cephalic setae and lip-region structures within this morphologically constrained group.

2. Materials and Methods

2.1. Habitat Characteristics of the Study Area

Jindo is a large island located at the southernmost margin of the Korean western coast, characterized by extensive mudflats and a well-developed ria coastline formed by numerous surrounding small islands that provide substantial shelter. These protected conditions have promoted intensive aquaculture, particularly of brown algae and abalone, which in turn increase nutrient and organic-matter inputs into the surrounding waters. In addition, the region experiences a wide tidal range and strong tidal currents, which resuspend fine mud and organic material from the seabed and maintain persistent turbidity in the water column. As a result, nutrient- and organic-rich particles circulate actively, and light penetration remains low, producing a characteristically dim and organically enriched benthic habitat.

2.2. Field Sampling and Sample Processing

Bottom sediments were collected from the subtidal zone near Geumhodo, one of the small islands around Jindo, at a depth of approximately 3.5 m at low tide on 21 May 2025. Bottom-water temperature during sampling was 17.0–17.4 °C, and salinity was 33‰. The sediments were retrieved using a Van Veen grab deployed from a local fishing boat and consisted of dark gray silty mud mixed with patches of coarse sand and abundant shell debris. Samples were washed to extract meiofauna, which included diverse taxa such as Copepoda, Kinorhyncha, and numerous slender nematodes.

In the field, sediments were briefly rinsed with freshwater for less than one minute to induce osmotic shock, then passed through a 67 µm mesh sieve to retain the meiofaunal fraction. The material was fixed immediately in 5% formalin. Coarse debris was removed by gentle decantation, and the remaining meiobenthos was separated using the Ludox HS-40 (DuPont, Wilmington, DE, USA) colloidal-silica flotation method. Specimens of Desmoscolex were hand-sorted from the mixed meiobenthic assemblage under high magnification using a LEICA M205 C stereomicroscope (LEICA, Wetzlar, Germany). For morphological examination, individual nematodes were transferred to anhydrous glycerin following Seinhorst’s slow-dehydration technique, then mounted between two cover slips on HS-slides in pure glycerin and sealed with paraffin to prepare permanent reference slides [11]. All voucher slides were labeled with unique specimen codes and stored under controlled laboratory conditions at the East Sea Research Institute (KIOST) and the Marine Biodiversity Institute of Korea (MABIK). Details of type deposition are provided in Section 3.3 (Material examined).

2.3. DIC and SEM–Based Morphological Examination

Observations and photographs were taken using a LEICA DM2500 LED microscope fitted with a LEICA K5C color CMOS camera (LEICA, Wetzlar, Germany), and image quality was refined in Adobe Photoshop 2023 (Adobe Inc., San Jose, CA, USA). Line drawings were prepared under Nomarski differential interference contrast (DIC) using a 100× oil-immersion objective on the Olympus BX53 microscope (Olympus Corp., Tokyo, Japan), aided by a drawing tube. Final illustrations were produced by digitally tracing the original pencil drawings on a Wacom Cintiq 22 tablet (Wacom Co., Ltd., Saitama, Japan) and completed in Adobe Illustrator 2023 (Adobe Inc., San Jose, CA, USA). Morphometric measurements were carried out using an Olympus BX53 microscope (Olympus Corp., Tokyo, Japan) equipped with cellSens Standard v1.16 software.

For SEM, specimens were rinsed three times in distilled water to remove residual fixative, dehydrated, and subsequently freeze-dried using an Eyela FDU-1200 freeze dryer (EYELA, Tokyo, Japan). Dried individuals were mounted on aluminum stubs with a fine dissecting needle, sputter-coated with gold using a CT-1000 ion sputter coater (ETS Co., Ltd., Hwaseong, Republic of Korea), and examined with an IM-300S scanning electron microscope (ETS Co., Ltd., Hwaseong, Republic of Korea). Special attention was given to the cephalic region to document ultrastructural details relevant for species-level diagnosis.

3. Results

3.1. Systematic Accounts

Phylum Nematoda Cobb, 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) curuvus sp. nov. (

Table 1. Morphometrics of Desmoscolex (Desmoscolex) curuvus sp. nov. Measurements are based only on specimens positioned in lateral view. Dashes ‘–’ indicate that no data are available.

Characters (μm)	Males			Females
	Holotype (n = 1)	Paratypes (n = 3)		Paratypes (n = 13)
		Min–Max	Mean ± SD	Min–Max	Mean ± SD
body length	326	398–428	409.4 ± 16.2	341–447	400.3 ± 33.7
hd (width)	22	23–26	23.0 ± 0.5	20–25	23.0 ± 1.1
hd (length)	17	16–18	16.5 ± 1.1	13–17	15.5 ± 1.2
amphid (width)	11	10–14	12.1 ± 2.0	10–14	11.8 ± 1.1
amphid (length)	12	9–12	9.8 ± 1.8	7–10	8.5 ± 1.2
cs (width)	3	2–3	2.4 ± 0.2	2–3	2.7 ± 0.4
cs (length)	12	8–11	10.0 ± 1.5	9–15	11.4 ± 1.4
sd1	16	14–17	15.4 ± 1.1	13–19	16.8 ± 1.9
sd3	12	11–12	11.4 ± 0.5	10–17	12.6 ± 1.9
sd5	10	9–14	11.9 ± 2.4	7–16	12.4 ± 2.4
sd7	10	8–13	11.2 ± 3.1	8–15	12.2 ± 2.2
sd9	12	11–14	12.2 ± 1.6	7–16	12.5 ± 2.6
sd11	12	11–12	11.4 ± 0.7	7–16	12.7 ± 2.4
sd13	12	11–15	12.9 ± 2.2	11–15	12.7 ± 1.2
sd16	17	15–17	15.8 ± 1.4	14–25	16.4 ± 2.8
sd17	24	23–25	24.2 ± 1.3	22–29	25.4 ± 2.3
sv2	7	8–9	8.3 ± 0.5	6–13	9.8 ± 1.8
sv4	6	6–10	8.4 ± 2.1	8–13	10.4 ± 1.4
sv6	6	7–12	9.2 ± 2.2	7–15	11.0 ± 1.9
sv8	9	6–11	9.5 ± 2.8	8–14	10.1 ± 1.8
sv10	10	7–14	11.4 ± 3.7	8–14	10.9 ± 1.9
sv12	11	8–13	11.3 ± 2.8	7–14	10.9 ± 2.0
sv14	11	11–13	11.8 ± 0.9	10–13	11.6 ± 1.1
sv15	12	9–13	11.8 ± 2.1	8–16	12.4 ± 1.8
mbd	64	52–62	57.2 ± 4.9	60–88	73.5 ± 10.9
(mbd)	49	43–51	46.8 ± 4.3	44–75	60.6 ± 12.2
oes	53	51–52	51.2 ± 0.4	34–48	43.5 ± 3.8
spic	33	32–38	34.5 ± 3.2	–	–
V (%)	–	–	–	49.2–53.8	52.5 ± 1.6
abd	52	50–52	50.9 ± 1.1	44–68	57.3 ± 6.7
t	83	79–86	82.4 ± 3.7	69–98	87.3 ± 7.9
tmr	49	44–46	45.5 ± 1.5	36–58	50.2 ± 5.6
tmrw	26	25	25.0 ± 0.0	24–31	28.9 ± 1.9
a	5.1	6.4–7.7	7.2 ± 0.7	4.3–7.3	5.5 ± 0.9
b	6.2	7.8–8.3	8.0 ± 0.3	7.2–12.6	9.3 ± 1.4
c	3.9	4.9–5.1	5.0 ± 0.1	4.0–15.6	5.4 ± 3.1
spinneret	4	4	3.8 ± 0.5	3–6	4.5 ± 1.0

, Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9)

urn:lsid:zoobank.org:act:6F1D26B1-F101-46B6-8EA5-F3A540ACF715

3.2. Type Locality

Subtidal zone near Geumho-do Island, Jindo, Republic of Korea (34°25′28.9″ N, 126°23′7.28″ E), at a depth of approximately 3.5 m at low tide. Collected on 21 May 2025 using a Van Veen grab. Sediment consisted of dark gray silty mud with patches of coarse sand and abundant shell debris.

3.3. Material Examined

One holotype and eighteen paratypes. The holotype, an adult male (MABIK NA00158862), is deposited in the Marine Biodiversity Institute of Korea (MABIK), Seocheon, Republic of Korea. The paratype series consists of three males (KIOST NEM-1-2902 to KIOST NEM-1-2904) and fifteen females (KIOST NEM-1-2905 to KIOST NEM-1-2919), all preserved in the nematode collection of the Bio-Resources Bank of Marine Nematodes (BRBMN) at the East Sea Research Institute, Korea Institute of Ocean Science and Technology (KIOST), Republic of Korea.

3.4. Etymology

The epithet curuvus derives from the Latin curvus (“curved, bent”). The form curuvus is intentionally adapted from the original Latin root to provide a distinctive and euphonic species name while referring to the pronounced ventral curvature of the terminal ring.

3.5. Diagnosis

Desmoscolex (Desmoscolex) curuvus sp. nov. is distinguished from its congeners by the following combination of characters: a body composed of 17 main rings; a typical 9/8 arrangement of somatic setae; subdorsal setae with a slightly swollen, hollow distal end; an oval head in which foreign material extends farther laterally than dorsally or ventrally; rounded, swollen amphidial fovea confined within the head region; a rounded lip-region formed by tuberculate membranes and six fragment-like membranous structures; broad cephalic setae containing a fine central canal with lateral membranes extensions; and a terminal ring that is strongly bent ventrally. Females are further characterized by pronounced sexual dimorphism, including a pair of dorsal—rather than subdorsal—setae on the thirteenth main ring and a thicker terminal ring (tmrw 25–26 μm in males and 24–31 μm in females).

3.6. Measurements

All morphometric data are provided in Table 1.

3.7. Description

Males. The body is stout, slightly tapering towards both ends, and composed of 17 well-defined main rings (Figure 1A, Figure 2A and Figure 4A). Each main ring consists of coarse foreign particles and finely granular material. The main rings are separated by interzones composed of 2–4 secondary rings (Figure 2E,F), which appear faintly expressed or weakly developed laterally (Figure 2G,H).

Somatic setae follow the typical arrangement of 17-ring Desmoscolex species:

• Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17 = 9
• Subventral: 2, 4, 6, 8, 10, 12, 14, 15 = 8

Subdorsal and subventral setae are similar in overall shape but differ in length. Subdorsal setae have a broad basal portion that gradually tapers toward the distal end. Under DIC, the distal end is very slightly swollen and appears hollow (Figure 1B,E and Figure 4B), a feature that is not visible under SEM (Figure 3C,D). All subdorsal setae are of similar length, except for the terminal pair, which is slightly elongated. In contrast, the subventral setae lack any distal swelling or hollow appearance. They are shorter than the subdorsal setae and remain relatively uniform in length along the body.

The head is oval and largely covered with fine to coarse foreign particles, except for the amphidial area and the anterior lip region (Figure 1B, Figure 2B and Figure 4B). The amphidial fovea are rounded and confined within the head region. At the anterior extremity, the lip region forms a rounded elevation (Figure 1B and Figure 4B). Under SEM, the six thin tuberculate membranes are partially open (rolled outward) to form an outer ridge that delineates six sectors (two dorsal, two subventral, and two lateral). The interior of the outer ridge is depressed, and each sector bears a single irregular, fragment-like membranous structure. These six fragment-like structures converge toward the oral opening, collectively forming a distinct hexagram-shaped configuration (Figure 3A,B).

Four cephalic setae are inserted at the base of the lip region (Figure 3A). The setae are relatively broad and contain a fine central canal (8–11 µm in length). Along both sides of the canal, a thin membrane (about 2 µm wide) extends the entire length of the seta, giving it a slightly flattened, blade-like appearance (Figure 2C).

The pharynx is cylindrical and extends to the level of the second main ring, where the pharyngo-intestinal junction is situated. The intestine continues posteriorly to the level of the fifteenth main ring. A single pair of ocelli is present at the level of the third main ring.

The single, outstretched testis begins with a germinal zone, continues into a vesicula seminalis containing small spermatocytes, and then passes into a finely granular vas deferens. The spicule measures 32–38 µm in length, tapering distally to a sharp tip and proximally to a slightly broadened capitulum (Figure 1C and Figure 4D). A gubernaculum was not observed. The cloacal tube protrudes posteriorly at the level of the fifteenth main ring (Figure 3E).

The tail consists of two main rings, the sixteenth and seventeenth, with the terminal ring forming a conical structure that is strongly bent ventrally (Figure 1C, Figure 2D and Figure 4C). At its distal end, a distinct spinneret, about 4 µm long, is present and appears beak-shaped (Figure 3F). Phasmata were not observed.

Females. Females resemble males in most external characters but are distinctly more swollen in the midbody region (Figure 1D, Figure 5A, Figure 6A and Figure 8A). This difference is reflected in the body ratio a, defined as body length divided by maximum body diameter: males have an a-value of 7.2, whereas females exhibit a markedly lower value of 5.5.

The main rings and secondary rings, as well as the head shape, amphidial fovea, and cephalic setae, are similar to those of the male (Figure 1E, Figure 5B, Figure 6B and Figure 8B). A ventral view of the head shows that the foreign material extends farther laterally than on the dorsal or ventral sides (Figure 8D). A transverse row of peg-like structures is exposed beneath the area where the main ring is partially fractured (Figure 5C,E).

The lip region is composed of tuberculate membranes, which are divided into six sectors (one dorsal, four sublateral, and one ventral) by a fragment-like membranous structure (Figure 7A–H).

The arrangement and morphology of the somatic setae are similar to those of the male (Figure 5G). However, on the thirteenth main ring, a pair of distinct dorsal setae is present rather than subdorsal setae (Figure 5F and Figure 8G), indicating sexual dimorphism.

The reproductive system is didelphic–amphidelphic, with two outstretched ovaries, each containing several branches of immature oocytes. The vulva is situated in the middle of the interzone between main rings 10 and 11 (Figure 1D, Figure 6C–F and Figure 8F), at approximately 49–54% of the body length from the anterior end. The anal tube protrudes posteriorly at the level of the fifteenth main ring, as in the male (Figure 6G,H and Figure 8E).

The terminal ring is strongly bent ventrally and bears a distinct spinneret at its distal end, as in the male, but is noticeably thicker and more robust in the female (Figure 1F, Figure 5D,H and Figure 8C).

3.8. Differential Diagnosis

Desmoscolex (Desmoscolex) curuvus sp. nov. is most similar to D. (D.) spinosus Decraemer, 1976 [12], sharing several key characters: a stout body composed of 17 main rings; a typical 9/8 somatic setal arrangement; an oval head in which foreign material extends farther posteriorly on the lateral sides; rounded, swollen amphidial fovea confined within the head region; a rounded lip-region elevation; cephalic setae bearing lateral membranes and containing a fine central canal; and a conical terminal ring distinctly bent ventrally. Despite these similarities, D. (D.) spinosus is readily distinguished from D. (D.) curuvus sp. nov. by the presence of a short, narrow, and slightly offset naked anterior region that extends to the level of the cephalic setae and bears three transverse rows of fine, short spines, as well as six fine setiform papillae (2 µm long). In contrast, D. (D.) curuvus sp. nov. entirely lacks both the transverse rows of spines and the setiform papillae, a condition confirmed by multiple individuals examined under both DIC and SEM. In addition, D. (D.) spinosus shows no marked sexual dimorphism, whereas D. (D.) curuvus sp. nov. exhibits clear sexual dimorphism, with females possessing a pair of dorsal setae on the thirteenth main ring and a noticeably thicker terminal ring (tmrw 25–26 μm in males and 24–31 μm in females). Additional differences include the shape of the subdorsal setae (tapering with a spatulate tip in D. (D.) spinosus vs. a slightly swollen, hollow distal end in D. (D.) curuvus sp. nov.), the position of the ocelli (fourth main ring in D. (D.) spinosus vs. third in D. (D.) curuvus sp. nov.), and the presence of a gubernaculum in D. (D.) spinosus, which is absent in D. (D.) curuvus sp. nov. Taken together, these characters provide a robust and consistent basis for separating the two species.

Desmoscolex (Desmoscolex) curuvus sp. nov. is also similar to the two Korean congeners D. (D.) globiceps Jung & Rho, 2024 and D. (D.) ovaliceps Jung & Rho, 2024 [6]. However, D. (D.) curuvus sp. nov. is readily distinguished from D. (D.) globiceps by the following characters: a 9/7 somatic setal pattern in females with sexual dimorphism in D. (D.) globiceps, whereas D. (D.) curuvus sp. nov. exhibits a normal 9/8 pattern in both sexes; a round head in D. (D.) globiceps vs. an oval head in the new species; cephalic setae with a membrane running along one side of the seta in D. (D.) globiceps vs. a narrow membrane present on both sides of the seta in D. (D.) curuvus sp. nov.; the absence of distinct lips or labial sensory organs in lateral view in D. (D.) globiceps vs. a distinctly rounded lip-region elevation in D. (D.) curuvus sp. nov.; and a narrow terminal ring (tmrw 8–9 μm) that is only slightly bent ventrally in D. (D.) globiceps, whereas D. (D.) curuvus sp. nov. possesses a thick terminal ring (tmrw 25–26 μm) that is distinctly bent ventrally.

Desmoscolex (Desmoscolex) curuvus sp. nov. also differs from D. (D.) ovaliceps in several key characters: the membrane of the cephalic setae is broad and located on one side of the canal, extending three-quarters of the setal length in D. (D.) ovaliceps, whereas it is narrow and present on both sides of the canal, extending the entire setal length in D. (D.) curuvus sp. nov.; the absence of distinct lips or labial sensory organs in lateral view in D. (D.) ovaliceps vs. a distinctly rounded lip-region elevation in D. (D.) curuvus sp. nov.; and a narrow terminal ring (tmrw 14–16 μm) that is only slightly bent ventrally in D. (D.) ovaliceps, in contrast to the thick (tmrw 25–26 μm) and strongly ventrally bent terminal ring of D. (D.) curuvus sp. nov.

3.9. Intraspecific Variation

One male paratype of D. (D.) curuvus sp. nov. exhibited notable deviations in the pattern of main rings and somatic setae, including two ventral partial rings between main rings 2–3 and 11–12 (designated as rings 2.5 and 11.5). Additional setae were also observed: an extra subdorsal seta on the right side of ring 10; extra subventral setae on both sides of rings 2 and 2.5; two subventral setae on each side of ring 8; and single extra subventral setae on ring 11 (right side) and ring 11.5 (left side).

In female paratypes, occasional absence of individual setae was documented, although this is interpreted as post-mortem seta loss rather than genuine morphological variation. No major deviations (extra setae, partial rings, or anomalous structures) were observed in females. Overall, intraspecific variation in this species appears limited and is expressed mainly in subtle deviations of the male somatic setal pattern and main ring configuration. Although based on a small number of specimens, documenting such deviations provides objective baseline information that may prove useful for future assessments of character stability and intraspecific variation within Desmoscolex.

4. Discussion

4.1. Limitations of Cephalic Setae as Diagnostic Characters

The morphology of the cephalic setae is widely recognized as one of the most informative characters for distinguishing species within the subgenus Desmoscolex [2,3,4,6]. However, in the present study, the cephalic setae of D. (D.) curuvus sp. nov. are nearly indistinguishable from those of D. (D.) spinosus, making it difficult to separate the two species based on this character alone. This limitation emphasized the need to examine additional, finer-scale diagnostic structures—particularly those of the lip region.

The diagnostic features characteristic of D. (D.) spinosus—three transverse rows of fine spines and six setiform papillae on the naked anterior region—were entirely absent in D. (D.) curuvus sp. nov. Instead, the new species exhibits a unique lip-region configuration not previously documented in the genus. To clarify the structural details of this configuration, we examined the lip regions of one male and six females using SEM, allowing ultrastructural components to be visualized at a level of resolution far beyond what is possible with light microscopy.

4.2. Interpretation of Lip-Region Ultrastructure

A total of one male and six females were examined under SEM to clarify the lip-region morphology. However, the male was observed with the lip region in an open state, whereas all females were examined in a closed state. Because both conditions (open vs. closed) were not represented in both sexes, direct comparison between males and females cannot be used to infer sexual dimorphism. In addition, the two states themselves cannot be confidently attributed to sex. Nevertheless, the tuberculate membrane exhibited a consistent fundamental configuration across sexes, allowing a meaningful comparison of its open and closed conditions.

Figure 9 clearly illustrates the contrast between these two states. In females (Figure 9B), the tuberculate membranes remain closed, forming a rounded elevation at the anterior tip. In contrast, the male specimen (Figure 9A) displays the membranes in an open condition, rolled outward to delineate six distinct sectors, each bearing a single fragment-like membranous structure.

Interpretation of these fragment-like elements is complicated by the coexistence of two morphological conditions and two sexes. In the male, the structures lie at approximately the same level as—or slightly below—the tuberculate membrane (Figure 9A). In females, however, the same structures occur above the membrane, forming a more distinctly elevated superficial layer (Figure 9B). Morphologically, the male structures appear as irregular membranous fragments that converge toward the oral opening, together forming a hexagram-like configuration (Figure 3A,B). In females, by contrast, the elements consist of six triangular differentiations—one dorsal, four sublateral, and one ventral—that also converge toward the oral opening but produce either a hexagonal outline (Figure 7B) or an elongated, six-pointed, star-like form (Figure 7H).

To describe this complex region, we apply the same term—fragment-like membranous structures—to both sexes. However, the subtle yet consistent morphological differences between males and females introduce uncertainty regarding the homology of these elements. The male lip region exhibits a degree of resemblance to that of Desmoscolex (Desmolorenzenia) camerunensis Decraemer & Sturhan, 1997, in which each of the six sectors bears a labial seta, and all labial setae curve inward toward the oral opening [13]. Although speculative, this similarity may suggest that the fragment-like membranous structures in the male represent a modified expression of the labial setae.

Labial setae in Desmoscolex exhibit considerable morphological diversity, ranging from the prominent papilla-like projections of D. (D.) parvospiculatus Decraemer, 1996 to the more concealed forms located within the six cephalic-tubercle–bounded sectors of D. (D.) brevisetosus Decraemer, 1974 [9,14]. To date, however, no instance of sexual dimorphism in labial setae has been reported within the genus. Importantly, in our material, the male lip region is documented in an open condition, whereas that of the females remains closed, complicating direct comparison and making the assessment of homology between these structures particularly challenging.

4.3. Current Knowledge of Lip-Region Diversity in Desmoscolex

At present, detailed SEM-based descriptions of the lip region are available for only nine species within the subgenus Desmoscolex: D. (D.) brevisetosus, D. (D.) laevis Kreis, 1926, D. (D.) membranosus Decraemer, 1974, D. (D.) granulatus Decraemer, 1975, D. (D.) parvospiculatus, D. (D.) cosmopolites Timm, 1970, D. (D.) lanceosetatus Jung et al., 2024, D. (D.) rotundicephalus Jung et al., 2024, and D. (D.) curuvus sp. nov. [9,10,15,16,17,18]. When species of the subgenus Desmolorenzenia are included, two additional taxa—D. (Desmolorenzenia) camerunensis and D. (Desmolorenzenia) pedunculus Rho et al., 2007—bring the total number of species with documented lip-region morphology to eleven [13,19].

The very small number of species for which lip-region ultrastructure has been described highlights the substantial gaps that remain in our understanding of this character complex. Nevertheless, the morphologies that have been documented to date exhibit remarkably divergent structural patterns, revealing a level of variation far greater than historically recognized for this morphologically conservative genus. The configuration observed in D. (D.) curuvus sp. nov. represents an additional, clearly differentiated pattern and therefore contributes important new comparative data to this still-limited but expanding body of knowledge.

4.4. Taxonomic Implications of Lip-Region Ultrastructure

Although the currently available material is insufficient to determine whether lip-region characters can function as stable, species-level diagnostic features, the substantial diversity documented thus far indicates that these structures may hold significant taxonomic potential. The newly discovered lip-region morphology of D. (D.) curuvus sp. nov., which is clearly distinct from all previously described configurations, further expands the known range of variation within the genus.

Taken together, these findings underscore the need to re-evaluate the taxonomic relevance of lip-region morphology within the subgenus Desmoscolex. The ultrastructure of the lip region—an aspect of morphology that has been largely overlooked—likely represents a concealed but informative source of variation, one that may ultimately enhance the resolution of species boundaries and provide insights into evolutionary patterns within this morphologically constrained lineage. Future research incorporating broader species sampling and systematic high-resolution SEM analyses will be essential for fully assessing the phylogenetic, diagnostic, and evolutionary significance of these structures.

5. Conclusions

Desmoscolex (Desmoscolex) curuvus sp. nov. is established as a new species based on a unique combination of somatic, cephalic, and lip-region characters. Although it is superficially similar to D. (D.) spinosus, the new species is clearly distinguished by the complete absence of transverse rows of fine spines and setiform papillae, as well as by distinct female sexual dimorphism in the position of dorsal setae on the 13th main ring and the terminal ring thickness. High-resolution SEM observations revealed a previously undocumented lip-region configuration, thereby expanding the currently limited but highly diverse set of lip-region morphologies known within the subgenus. These ultrastructural characters provide valuable diagnostic information and indicate that key species-level differences in Desmoscolex may be concentrated in fine-scale lip-region architecture rather than in traditional external traits such as somatic or cephalic setae. Overall, this study broadens the known morphological diversity of northwestern Pacific Desmoscolex and underscores the importance of continued ultrastructural analyses for refining species boundaries and uncovering hidden morphological complexity in this taxonomically challenging group.