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Article

Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability †

1
East Sea Environment Research Center, Korea Institute of Ocean Science and Technology (KIOST), Uljin 36315, Republic of Korea
2
Ocean Science, University of Science and Technology (UST), Daejeon 34113, Republic of Korea
3
Division of Glacier & Earth Sciences, Korea Polar Research Institute (KOPRI), Yeonsu-gu, Incheon 21990, Republic of Korea
*
Author to whom correspondence should be addressed.
urn:lsid:zoobank.org:pub:55CCFADE-6DF4-4C7A-8958-2DC08E96A484; urn:lsid:zoobank.org:act:B0AD85A5-101F-4CA1-B9FD-80B829B00EF6; urn:lsid:zoobank.org:act:BCEC5738-8875-4ADE-B87E-7EF349517B86.
J. Mar. Sci. Eng. 2024, 12(12), 2168; https://doi.org/10.3390/jmse12122168
Submission received: 22 October 2024 / Revised: 21 November 2024 / Accepted: 25 November 2024 / Published: 27 November 2024
(This article belongs to the Special Issue Biodiversity and Population Ecology of Marine Invertebrates)

Abstract

This study describes two new species of the subgenus Desmoscolex (Nematoda: Desmoscolecidae) from subtidal benthic habitats in Korea, located in the Northwestern Pacific Ocean. Both species are distinguished by the absence of cephalic setae—a rare feature previously documented in only three species within this subgenus: Desmoscolex (Desmoscolex) rostratus Timm, 1970; D. (D.) asetosus Decraemer, 1975; and D. (D.) obscurus Bussau, 1993. The new species exhibit distinct morphological features. Desmoscolex (D.) lanceosetatus sp. nov. is characterized by an asymmetrical, dorsally curved head; large amphidial fovea that extend to the posterior margin of the first main ring; and lance-shaped subdorsal setae. In females, the absence of subventral setae on the 14th ring deviates from the typical arrangement of 17 main rings. Desmoscolex (D.) rotundicephalus sp. nov. is identified by its globular head, oval amphidial fovea, and tapering subdorsal setae with an open tip. In females, the absence of subventral setae on the 14th ring further differentiates it from closely related species. This paper provides detailed morphological descriptions supported by scanning electron microscopy (SEM) and differential interference contrast (DIC) images. Additionally, it includes a comprehensive morphological comparison with previously described species, species diagnoses, and a pictorial key for identification.

1. Introduction

Currently, the genus Desmoscolex Claparède, 1863, within the family Desmoscolecidae Shipley, 1896, is divided into four subgenera: Desmoscolex Claparède, 1863; Pareudesmoscolex Weischer, 1962; Protricomoides Timm, 1970; and Desmolorenzenia Freudenhammer, 1975. Among these, the subgenus Desmoscolex encompasses free-living nematodes that are widely distributed across marine, freshwater, and terrestrial ecosystems [1]. These nematodes are easily recognized by their annulated body cuticle, often accompanied by external rings composed of secreted or foreign materials that can obscure internal morphological structures. Despite the high species diversity in this group, their occurrence in ecological samples tends to be low, and data on intraspecific variability remain scarce, posing challenges for accurate species identification [1,2]. Species identification within Desmoscolex relies on several critical morphological features, including the number and arrangement of main body rings, the configuration of somatic setae, the shape of subdorsal and subventral somatic setae, head morphology, the structure and extent of amphids, and the presence or position of phasmates. Additionally, male-specific traits, such as the shape of spicules and gubernaculum, are crucial diagnostic features. Among these, the presence and characteristics of cephalic setae have traditionally been regarded as vital for taxonomic differentiation. As noted by Timm (1970), “All species of Desmoscolecidae normally have four cephalic setae”, which can vary in length, position, and morphology—from thin to thick, with pointed or knob-like tips—and may exhibit simple or more ornate structures resembling flag-like decorations [1,3]. The subgenus Desmoscolex comprises 97 species [1,3,4,5]; however, the absence of cephalic setae is a highly rare morphological feature observed in only three species within the subgenus Desmoscolex. This characteristic was first documented in Desmoscolex (Desmoscolex) rostratus Timm, 1970 [3]. Since then, only two additional species lacking cephalic setae have been reported: D. (D.) asetosus Decraemer, 1975 [6], and D. (D.) obscurus Bussau, 1993 [7].
In this study, we describe two newly discovered species from subtidal benthic habitats in Korea within the subgenus Desmoscolex, both of which are characterized by the absence of cephalic setae. This paper provides a comprehensive comparative analysis of these new species in relation to previously known taxa. Detailed species descriptions are supplemented with scanning electron microscopy (SEM) images and differential interference contrast (DIC) photomicrographs. Additionally, a pictorial key and comparison table are included to facilitate species identification and highlight key morphological distinctions.

2. Materials and Methods

2.1. Sample Collection and Preparation

Marine nematode samples were collected in November 2015 from benthic sediments in the subtidal zone of the southeastern waters off Jeju Island, Korea, using a box corer. In the field, meiobenthic organisms were initially separated by decantation, employing a 67 µm mesh sieve following a brief rinse with fresh water to minimize sediment adherence [8]. The samples were subsequently preserved in 5% neutralized formalin prepared in seawater. In the laboratory, further isolation of meiobenthos was achieved using the Ludox flotation method with Ludox® (Grace, Columbia, MD, USA) HS 40 [9]. The concentrated samples were then preserved again in 5% neutralized formalin solution for further 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 [10], after which they were mounted on HS slides for observation [11]. 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 utilized for all sketches and measurements. For scanning electron microscopy (SEM), specimens were preserved in 5% buffered formalin and washed twice with distilled water to remove any formalin residues. Subsequently, the specimens were freeze-dried on an FDU-1200 cooling stage (EYELA, Tokyo, Japan). The dehydrated specimens were mounted on aluminum stubs, sputter-coated with gold/palladium using a high-vacuum evaporator, and examined using a JSM-7200F Field Emission Scanning Electron Microscope (JEOL, Tokyo, Japan) [12].

2.2. Terminology and Abbreviations

All measurements were recorded in micrometers along the arc. Ratios a, b, and c were determined following de Man’s method. The abbreviations utilized were as follows: 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 pharynx length; c = body length divided by tail length.

3. Results

3.1. Description of Desmoscolex (Desmoscolex) lanceosetatus 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) lanceosetatus sp. nov. (Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7 and Figure 8).
urn:lsid:zoobank.org:act:B0AD85A5-101F-4CA1-B9FD-80B829B00EF6.
Figure 1. Desmoscolex (Desmoscolex) lanceosetatus 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) = 20 μm; (CF) = 10 μm).
Figure 1. Desmoscolex (Desmoscolex) lanceosetatus 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) = 20 μm; (CF) = 10 μm).
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Figure 2. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., holotype male, DIC micrographs: (A) entire view of holotype male, 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 2. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., holotype male, DIC micrographs: (A) entire view of holotype male, 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).
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Figure 3. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., male, SEM micrographs: (A) entire view, lateral view; (B) body from the 12th to the 14th main ring, lateral view; (C) detailed view of the interzone between the 12th and 13th main ring, lateral view; (D) head region, enface view; (E) subdorsal setae of the terminal ring, subdorsal view (scale bars: (A) = 20 μm; (B,D,E) = 10 μm; (C) = 1 μm).
Figure 3. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., male, SEM micrographs: (A) entire view, lateral view; (B) body from the 12th to the 14th main ring, lateral view; (C) detailed view of the interzone between the 12th and 13th main ring, lateral view; (D) head region, enface view; (E) subdorsal setae of the terminal ring, subdorsal view (scale bars: (A) = 20 μm; (B,D,E) = 10 μm; (C) = 1 μm).
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Figure 4. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., male, SEM micrographs: (A) entire view, dorsal view; (B) anterior end, lateral view; (C) detailed view of the head region, lateral view; (D) anterior end, dorsal view; (E) detailed view of the head region, enface view; (F) detailed view of the labial region covered in particles, ventral view (scale bars: (A) = 20 μm; (B,D) = 10 μm; (C,E,F) = 1 μm).
Figure 4. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., male, SEM micrographs: (A) entire view, dorsal view; (B) anterior end, lateral view; (C) detailed view of the head region, lateral view; (D) anterior end, dorsal view; (E) detailed view of the head region, enface view; (F) detailed view of the labial region covered in particles, ventral view (scale bars: (A) = 20 μm; (B,D) = 10 μm; (C,E,F) = 1 μm).
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Figure 5. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., male, SEM micrographs: (A) subdorsal seta of the 7th main ring, lateral view; (B) detailed view of the subdorsal seta of the 7th main ring, lateral view; (C) subventral seta of the 4th main ring, lateral view; (D) detailed view of the subventral seta of the 4th main ring; (E) insertion region of the subdorsal seta on the first main ring; (F) cloacal region with cloacal tube and protruding spicule; (G) terminal main ring, lateral view; (H) detailed view of the insertion region of the terminal subdorsal setae on the terminal main ring (scale bars: (A,C,E,F,H) = 1 μm; (B,D) = 0.1 μm; (G) = 10 μm).
Figure 5. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., male, SEM micrographs: (A) subdorsal seta of the 7th main ring, lateral view; (B) detailed view of the subdorsal seta of the 7th main ring, lateral view; (C) subventral seta of the 4th main ring, lateral view; (D) detailed view of the subventral seta of the 4th main ring; (E) insertion region of the subdorsal seta on the first main ring; (F) cloacal region with cloacal tube and protruding spicule; (G) terminal main ring, lateral view; (H) detailed view of the insertion region of the terminal subdorsal setae on the terminal main ring (scale bars: (A,C,E,F,H) = 1 μm; (B,D) = 0.1 μm; (G) = 10 μm).
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Figure 6. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., paratype female, DIC micrographs: (A) entire view of paratype female, 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) lanceosetatus sp. nov., paratype female, DIC micrographs: (A) entire view of paratype female, 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).
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Figure 7. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., female, SEM micrographs: (A) entire view, sublateral view; (B) body from the 5th to the 11th main ring, sublateral view; (C) detailed view of the interzone between the 1st and 2nd main ring, sublateral view; (D) detailed view of the interzone between the 15th and 17th main ring, sublateral view (scale bars: (A) = 20 μm; (B) = 10 μm; (C,D) = 1 μm).
Figure 7. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., female, SEM micrographs: (A) entire view, sublateral view; (B) body from the 5th to the 11th main ring, sublateral view; (C) detailed view of the interzone between the 1st and 2nd main ring, sublateral view; (D) detailed view of the interzone between the 15th and 17th main ring, sublateral view (scale bars: (A) = 20 μm; (B) = 10 μm; (C,D) = 1 μm).
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Figure 8. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., female, SEM micrographs: (A) anterior end, sublateral view; (B) detailed view of the labial region with four protrusions surrounding the oral opening indicated by arrows at the base of the rostrum, sublateral view; (C) anterior end, enface view; (D) detailed view of the labial region with four protrusions surrounding the oral opening indicated by arrows at the base of the rostrum, enface view; (E) subdorsal seta of the first main ring; (F) subventral seta of the 8th main ring; (G) posterior end, lateral view (scale bars: (A) = 5 μm; (B,DF) = 1 μm; (C,G) = 10 μm).
Figure 8. Desmoscolex (Desmoscolex) lanceosetatus sp. nov., female, SEM micrographs: (A) anterior end, sublateral view; (B) detailed view of the labial region with four protrusions surrounding the oral opening indicated by arrows at the base of the rostrum, sublateral view; (C) anterior end, enface view; (D) detailed view of the labial region with four protrusions surrounding the oral opening indicated by arrows at the base of the rostrum, enface view; (E) subdorsal seta of the first main ring; (F) subventral seta of the 8th main ring; (G) posterior end, lateral view (scale bars: (A) = 5 μm; (B,DF) = 1 μm; (C,G) = 10 μm).
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3.1.2. Diagnosis

Desmoscolex (Desmoscolex) lanceosetatus sp. nov. is distinguished by its asymmetrical, dorsally curving head with rounded posterior border anteriorly tapered to a protruding rostrum. The amphidial fovea are large, vesicular structures that nearly cover the entire head laterally and extend to the posterior margin of the first main ring. In females, the somatic setae arrangement deviates from the typical pattern of species with 17 main rings, due to the absence of subventral setae on the 14th main ring, resulting in a sexual dimorphism where males typically exhibit nine subdorsal and eight subventral setae, while females have seven subventral setae. The subdorsal setae have a broad base with a lance-shaped, open tip, and the terminal setae are distinctly elongated and located near the end of the 17th main ring. The terminal ring curves ventrally and ends in a short, uncovered spinneret.

3.1.3. Type Locality

Species was discovered in benthic sediments at a depth of 48 m in the southeastern waters of Jeju Island, Korea (33°00′00.0″ N, 126°30′00.0″ E), on 16 November 2015.

3.1.4. Other Locality

Other specimens were discovered in benthic sediments at a depth of 103 m in the southeastern waters of Jeju Island, Korea (33°00′00.0″ N, 127°00’00.0″ E), on 7 November 2015.

3.1.5. Type Material

The holotype male (MABIK NA00158722) and one paratype female (MABIK NA00158723), mounted in glycerin on HS slides, were preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, one paratype male (KIOST NEM-1-2720) and four paratype females (KIOST NEM-1-2722 to KIOST NEM-1-2725), also mounted in glycerin on HS slides, were deposited in the nematode collection of the specimen conservation room at the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.1.6. Etymology

The species name Desmoscolex (Desmoscolex) lanceosetatus sp. nov. is derived from the characteristic lance-like shape of the subdorsal somatic setae. The prefix “lanceo-“ means “lance”, and “setatus” refers to the presence of bristles (setae), highlighting this distinctive morphological feature.

3.1.7. Measurements

Holotype male: L = 295, hd = 10, sd1 = 14, sd3 = 11, sd5 = 11, sd7 = 11, sd9 = 11, sd11 = 12, sd13 = 9, sd16 = 13, sd17 = 26, sv2 = 9, sv4 = 9, sv6 = 7, sv8 = 9, sv10 = 9, sv12 = 8, sv14 = 9, sv15 = 9, mbd = 36, (mbd) = 29, oes = 36, spic = 22, gub = 7, abd = 34, t = 61, tmrw = 13, tmr = 34, a = 8.2, b = 8.2, c = 4.8
Paratype male (n = 1): L = 440, hd = 14, sd1 = 19, sd3 = 16, sd5 = 16, sd7 = 16, sd9 = 15, sd11 = 15, sd13 = 14, sd16 = 12, sd17 = 37, sv2 = 11, sv4 = 12, sv6 = 10, sv8 = 11, sv10 = 10, sv12 = 9, sv14 = 11, sv15 = 9, mbd = 59, (mbd) = 50, oes = 56, spic = 39, gub = 11, abd = 52, t = 95, tmrw = 23, tmr = 52, a = 7.4, b = 7.9, c = 4.6
Paratype females (n = 5): L = 250–310, hd = 9–11, sd1 = 11–14, sd3 = 9–11, sd5 = 9–11, sd7 = 9–10, sd9 = 10–11, sd11 = 9–11, sd13 = 7–11, sd16 = 9–12, sd17 = 21–23, sv2 = 6–8, sv4 = 7–8, sv6 = 6–10, sv8 = 6–9, sv10 = 8–9, sv12 = 7–9, sv15 = 7–8, mbd = 38–46, (mbd) = 29–37, oes = 30–35, abd = 30–34, t = 43–49, tmrw = 12–15, tmr = 23–33, a = 6.2–7.1, b= 7.1–10.1, c = 5.4–6.7, V = 52.5–56.5%

3.1.8. Description

For males, the body gradually tapers towards both extremities and is composed of 17 main rings (Figure 1A, Figure 2A and Figure 3A). Each main ring is composed of numerous foreign particles. The width of the main rings in the midsection ranges between 10 µm and 11 µm (Figure 3B and Figure 4A). In the anterior part of the body and the tail, the main rings are separated by narrower intermediate zones, which consist of two to three secondary rings. In the rest of the body, the interzone is divided into three to four narrow rings. The secondary rings in these intermediate zones range from 2 µm to 3 µm in width. SEM observations of the interzones reveal numerous tubercle- or granular-like protrusions distributed across the entire surface. The secondary rings of the interzone show multiple transverse ridges crossing them, and these rings are uniformly covered with tubercle- or granular-like protrusions (Figure 3C).
The head appears asymmetrical, curving dorsally. The posterior part is broad and rounded, covered with concretion particles, and tapers anteriorly towards a protruding rostrum (Figure 1C, Figure 2B, Figure 3D and Figure 4B,D–E). In the SEM enface view, the labial region features four protrusions surrounding the oral opening (Figure 8B,D). This feature is visible in females under SEM but cannot be observed in the male specimen due to particle attachment in the labial area (Figure 4F). However, based on the similarity between the male and female labial regions observed in other SEM images, it is concluded that this feature is common to both sexes. Additionally, the labial region is covered with cephalic tubercles that extend towards the rostrum-shaped oral aperture (Figure 4C,F). The oral opening is bordered by a crown-like membrane, which is free of cephalic tubercles. Although this feature is not visible in the male specimen due to particle attachment, it is presumed to be present in both sexes, as observed in the female specimen. Notably, there are no cephalic setae, and no traces of them were found in their usual positions. The amphidial groove begins at the rostrum, narrows in the middle, and then widens again as it extends towards the posterior region of the head (Figure 4C).
The amphidial fovea are large vesicular structures, measuring 19 µm in length and 8 µm in width, nearly covering the entire lateral side of the head region. They narrow at the front and extend towards the distal end, reaching the posterior margin of the first main ring.
The pharynx is narrow and cylindrical, extending toward 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 through the 16th main ring, while the ocelli are positioned between the 3rd and 5th main rings.
The somatic setae follow the typical arrangement for species with 17 main rings:
Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17 = 9
Subventral: 2, 4, 6, 8, 10, 12, 14, 15 = 8
The subdorsal setae have a wider basal shaft with a smooth, swollen, lance-shaped open tip. The broader basal shaft exhibits a distinct pattern of numerous tubercles or wrinkles, which differ from those in the interzone and gradually disappear around two-thirds of the way up, forming the lance-shaped tip (Figure 5A,B). Unlike the other subdorsal setae, the terminal setae display tubercles or wrinkles only along the first quarter of their length. Beyond this point, the wrinkles persist while the tubercles disappear (Figure 5H), eventually forming a lance-shaped open tip. The subdorsal setae on the first main ring are longer than those on the subsequent rings, maintaining nearly equal length until the 11th main ring. The subdorsal setae on the 13th and 16th main rings are relatively shorter, while the terminal setae are significantly elongated.
The shorter subventral setae have a typical bristle structure with a naked open tip and are of similar length. Like the subdorsal setae, the subventral setae also exhibit numerous tubercles or wrinkles, visible along up to four-fifths of their length, after which the setae become smooth, forming an open-tipped distal part (Figure 5C,D). SEM observations show that the insertion points of all body setae are directly embedded into the body (Figure 5E).
The male reproductive system, typical for the subgenus, consists of a single, outstretched testis, starting with a germinal zone followed by a vesicula seminalis that transitions into a finely granular vas deferens. The spicules are nearly straight, with a slender distal portion that broadens toward the base and terminates in a capitulum. A gubernaculum, positioned parallel to the spicules, accompanies them. The naked cloacal tube, 6 µm long, is protruding from the ventrally enlarged 15th main ring (Figure 1A,E and Figure 5F).
The terminal ring tapers slightly towards the tail end. The terminal setae are located near the end of the 17th main ring (Figure 3E and Figure 5G). The terminal ring is slightly rounded and bends ventrally, ending in a very short spinneret (Figure 1E and Figure 2C). Phasmata are not observed.
Female: The female resembles the male in most aspects. Like the male, the body gradually tapers towards both extremities and is comprised of 17 main rings (Figure 1B, Figure 6A and Figure 7A). The interzones between the main rings have two to three secondary rings, displaying tubercles- or granular-like protrusions across the surface (Figure 7B–D).
The subdorsal setae have a broad base with a lance-shaped tip marked by tubercles or wrinkles that fade towards the tip (Figure 8E). The terminal setae differ, showing wrinkles along the first quarter of their length (Figure 8G). The subventral setae are similar but exhibit tubercles along four-fifths of their length (Figure 8F). SEM observations reveal that the setae are directly embedded into the body.
The head is asymmetrical, curving dorsally, with a broad, rounded posterior tapering into a protruding rostrum (Figure 1D, Figure 6B and Figure 8A,C). The labial region shows four protrusions surrounding the oral opening, and cephalic tubercles cover the labial region.
The oral opening is surrounded by a crown-like membrane without tubercles (Figure 8B,D). Notably, there are no cephalic setae, and no traces of them were found in their usual positions. The shape of the amphidial groove and terminal ring is consistent with that observed in the male.
The amphidial fovea are large vesicular structures extending to the posterior margin of the first main ring. However, there is a difference in the arrangement of the somatic setae.
The somatic setae follow this pattern:
Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17 = 9
Subventral: 2, 4, 6, 8, 10, 12, (-), 15 = 7
This arrangement deviates from the typical pattern observed in species with 17 main rings due to the absence of a pair of subventral setae on the 14th main ring. The reproductive system is didelphic-amphidelphic, with both branches outstretched and containing several immature oocytes. Two spermathecae, each containing small globular sperm, are present anterior and posterior to the vulva. The vulva is positioned between the 10th and 11th main rings (Figure 1F and Figure 6C).

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

Desmoscolex (Desmoscolex) rotundicephalus sp. nov. (Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16 and Figure 17).
urn:lsid:zoobank.org:act:BCEC5738-8875-4ADE-B87E-7EF349517B86.
Figure 9. Desmoscolex (Desmoscolex) rotundicephalus 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) = 20 μm; (CF) = 10 μm).
Figure 9. Desmoscolex (Desmoscolex) rotundicephalus 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) = 20 μm; (CF) = 10 μm).
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Figure 10. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., holotype male, DIC micrographs: (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) rotundicephalus sp. nov., holotype male, DIC micrographs: (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).
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Figure 11. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., male, SEM micrographs: (A) entire view, lateral view; (B) entire view, ventral view; (C) body from the 1st to the 5th main ring, lateral view; (D) detailed view of the interzone between the 3rd and 4th main ring, lateral view; (E) subdorsal seta of the 11th main ring, lateral view; (F) subdorsal seta of the 13th main ring, lateral view; (G) subventral seta of the 2nd main ring, lateral view; (H) subventral seta of the 10th main ring, lateral view; (I) insertion region of the subdorsal seta on the 1st main ring (scale bars: (A,B) = 20 μm; (C) = 10 μm; (DI) = 1 μm).
Figure 11. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., male, SEM micrographs: (A) entire view, lateral view; (B) entire view, ventral view; (C) body from the 1st to the 5th main ring, lateral view; (D) detailed view of the interzone between the 3rd and 4th main ring, lateral view; (E) subdorsal seta of the 11th main ring, lateral view; (F) subdorsal seta of the 13th main ring, lateral view; (G) subventral seta of the 2nd main ring, lateral view; (H) subventral seta of the 10th main ring, lateral view; (I) insertion region of the subdorsal seta on the 1st main ring (scale bars: (A,B) = 20 μm; (C) = 10 μm; (DI) = 1 μm).
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Figure 12. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., male, SEM micrographs: (A) anterior end, lateral view; (B) anterior end, ventral view; (C) entire view, enface view; (D) anterior end, enface view; (E) detailed view of the labial region, enface view (scale bars: (A,D) = 5 μm; (B,C) = 10 μm; (E) = 1 μm).
Figure 12. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., male, SEM micrographs: (A) anterior end, lateral view; (B) anterior end, ventral view; (C) entire view, enface view; (D) anterior end, enface view; (E) detailed view of the labial region, enface view (scale bars: (A,D) = 5 μm; (B,C) = 10 μm; (E) = 1 μm).
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Figure 13. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., male, SEM micrographs: (A) body from the 14th to the 17th main ring, lateral view; (B) cloacal region with cloacal tube, lateral view; (C) body from the 14th to the 17th main ring, ventral view; (D) posterior end, enface view, (E) posterior end, lateral view; (F) posterior end with spinneret (scale bars: (A,CE) = 10 μm; (B,F) = 1 μm).
Figure 13. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., male, SEM micrographs: (A) body from the 14th to the 17th main ring, lateral view; (B) cloacal region with cloacal tube, lateral view; (C) body from the 14th to the 17th main ring, ventral view; (D) posterior end, enface view, (E) posterior end, lateral view; (F) posterior end with spinneret (scale bars: (A,CE) = 10 μm; (B,F) = 1 μm).
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Figure 14. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., paratype female, DIC micrographs: (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 14. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., paratype female, DIC micrographs: (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).
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Figure 15. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., female, SEM micrographs: (A) entire view, lateral view; (B) entire view, dorsal view; (C) body from the 12th to the 16th main ring, lateral view; (D) detailed view of the interzone between the 13th and 14th main ring, lateral view; (E) subdorsal seta of the 1st main ring; (F) subdorsal seta of the 5th main ring; (G) subventral seta of the 4th main ring; (H) detailed view of the 14th main ring, lateral view (scale bars: (A,B) = 20 μm; (C) = 10 μm; (DH) = 1 μm).
Figure 15. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., female, SEM micrographs: (A) entire view, lateral view; (B) entire view, dorsal view; (C) body from the 12th to the 16th main ring, lateral view; (D) detailed view of the interzone between the 13th and 14th main ring, lateral view; (E) subdorsal seta of the 1st main ring; (F) subdorsal seta of the 5th main ring; (G) subventral seta of the 4th main ring; (H) detailed view of the 14th main ring, lateral view (scale bars: (A,B) = 20 μm; (C) = 10 μm; (DH) = 1 μm).
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Figure 16. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., female, SEM micrographs: (A) anterior end, lateral view; (B) anterior end, dorsal view; (C) anterior end, enface view; (D) detailed view of the labial region, enface view (scale bars: (A) = 5 μm; (B,C) = 10 μm; (D) = 1 μm).
Figure 16. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., female, SEM micrographs: (A) anterior end, lateral view; (B) anterior end, dorsal view; (C) anterior end, enface view; (D) detailed view of the labial region, enface view (scale bars: (A) = 5 μm; (B,C) = 10 μm; (D) = 1 μm).
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Figure 17. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., female, SEM micrographs: (A) entire view, subventral view; (B) body from the 9th to the 12th main ring, highlighting the vulva region with its position marked by a dashed circle, subventral view; (C) posterior end, subventral view; (D) posterior end, lateral view, (E) posterior end, dorsal view; (F) entire view, enface view; (G) posterior end, enface view (scale bars: (A) = 20 μm; (BG) = 10 μm).
Figure 17. Desmoscolex (Desmoscolex) rotundicephalus sp. nov., female, SEM micrographs: (A) entire view, subventral view; (B) body from the 9th to the 12th main ring, highlighting the vulva region with its position marked by a dashed circle, subventral view; (C) posterior end, subventral view; (D) posterior end, lateral view, (E) posterior end, dorsal view; (F) entire view, enface view; (G) posterior end, enface view (scale bars: (A) = 20 μm; (BG) = 10 μm).
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3.2.1. Diagnosis

Desmoscolex (Desmoscolex) rotundicephalus sp. nov. is characterized by its globular head with a prominent labial region, featuring an oral ring with eight distinct projections and four elongated setae-like structures arranged submedially behind the oral ring, adding a unique morphological trait to the labial region. The amphidial fovea are oval, nearly covering the entire lateral sides of the head and featuring a central amphidial pore. The subdorsal setae taper towards the tip, ending in an open tip, while the terminal setae are distinctly elongated and located near the two-thirds of the 17th main ring. The terminal ring curves ventrally and ends in a short, uncovered spinneret, with small, rounded phasmata. In females, the somatic setae arrangement deviates from the typical 17 main ring species pattern due to the absence of subventral setae on the 14th main ring.

3.2.2. Type Locality

Species found in benthic sediments at a depth of 103 m in the southeastern waters of Jeju Island, Korea (33°00′00.0″ N, 127°00′00.0″ E), on 7 November 2015.

3.2.3. Type Material

The holotype male (MABIK NA00158724) and one paratype female (MABIK NA00158725), mounted in glycerin on HS slides, are preserved in the nematode collection of the Marine Biodiversity Institute of Korea (MABIK), Seochun, Korea. Additionally, three paratype females (KIOST NEM-1-2728 to KIOST NEM-1-2730), also mounted in glycerin on HS slides, have been deposited in the nematode collection at the specimen conservation room of the Bio-Resources Bank of Marine Nematodes (BRBMN), East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.2.4. Etymology

The species name Desmoscolex (Desmoscolex) rotundicephalus sp. nov. is derived from the distinctive round shape of the head. The prefix ‘rotundi-’ means ‘round,’ and the suffix ‘-cephalus’ refers to the head, emphasizing this characteristic feature of the species.

3.2.5. Measurements

Holotype male: L = 275, hd = 14, sd1 = 11, sd3 = 8, sd5 = 8, sd7 = 7, sd9 = 8, sd11 = 8, sd13 = 6, sd16 = 7, sd17 = 21, sv2 = 7, sv4 = 6, sv6 = 6, sv8 = 6, sv10 = 7, sv12 = 7, sv14 = 7, sv15 = 7, mbd = 35, (mbd) = 26, oes = 38, spic = 30, gub = 7, abd = 32, t = 54, tmrw = 12, tmr = 30, a = 7.8, b =7.27, c =5.0
Paratype females (n = 4): L = 255–330, hd = 13–16, sd1 = 12–13, sd3 = 10–11, sd5 = 9–10, sd7 = 9–10, sd9 = 8–10, sd11 = 9–10, sd13 = 8–9, sd16 = 10–12, sd17 = 20–24, sv2 = 7–8, sv4 = 8–9, sv6 = 7–9, sv8 = 8, sv10 = 8–9, sv12 = 8–9, sv15 = 8–10, mbd = 39–52, (mbd) = 32–42, oes = 33–39, abd = 30–36, t = 52–62, tmrw = 12–15, tmr = 33–39, a = 6.3–7.8, b = 6.9–9.9, c = 4.6–5.3, V = 53.4–56.1%

3.2.6. Description

In males, the body gradually tapers towards both extremities and is composed of 17 main rings (Figure 9A, Figure 10A, Figure 11A,B and Figure 12C). Each main ring is composed of numerous large foreign particles. The width of the main rings in the midsection of the body ranges from 9 µm to 11 µm. In the anterior part of the body and the tail, the main rings are separated by narrower intermediate zones consisting of two secondary rings. In the rest of the body, the interzone is divided into three secondary rings. The secondary rings forming the interzones range from 1 µm to 2 µm in width. SEM observations reveal numerous tubercle- or granular-like protrusions distributed across the entire interzone surface. Additionally, faint junctions between the secondary rings are visible throughout the area (Figure 11C,D).
When observed from a lateral view, the head appears globular, while in the dorsal or ventral view, it has a rounded triangular shape with blunt edges, except at the amphid regions (Figure 9C, Figure 10B and Figure 12A,B). The head features a narrow, protruding labial region. The oral opening is encircled by an oral ring bearing eight projections, accompanied by four longer setae-like structures positioned submedially just behind the ring. When the labial region is not swollen, as depicted in Figure 12E, these projections appear distinctly separate, giving the impression of individual structures. No labial papillae or tubercle-like projections were observed in this externally protruding labial area. Notably, there were no cephalic setae, and no traces of them were found in their usual positions. The oval amphidial fovea nearly cover the entire head laterally, with the circular amphidial pore located centrally.
The pharynx is narrow and cylindrical, extending toward 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 situated at the third main ring. The intestine, exhibiting a typical structure, extends through the 16th main ring. The ocelli are positioned between the fourth and fifth main rings.
The somatic setae follow the typical arrangement for species with 17 main rings:
Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17 = 9.
Subventral: 2, 4, 6, 8, 10, 12, 14, 15 = 8.
The subdorsal setae taper towards the tip and have an open terminal end. Faint wrinkles are present along most of the subdorsal setae, except for the distal part, which leads to a smooth open tip (Figure 11E,F). Similarly, the terminal setae show wrinkles along most of their length, ending in a naked open tip. The subdorsal setae on the first main ring are longer than those on the subsequent rings, maintaining nearly equal length until the 16th ring, except on the 13th ring, where the subdorsal setae are relatively shorter. However, the terminal setae are distinctly elongated (Figure 13C).
The shorter subventral setae exhibit a fine structure and are similar in length. Like the subdorsal setae, wrinkles are present along their length, except for the distal part, which is naked and ends in an open tip (Figure 11G,H). SEM observations reveal that the insertion points of all body setae are directly embedded into the body (Figure 11I).
The male reproductive system, characteristic of the subgenus, consists of a single, outstretched testis, starting with a germinal zone followed by a vesicula seminalis that transitions into a finely granular vas deferens. The spicules are slightly bent, with a slender distal portion that widens proximally and terminates in a capitulum. A gubernaculum, positioned parallel to the spicules, accompanies them (Figure 9E and Figure 10C). The cloacal tube protrudes from the ventrally enlarged 15th main ring (Figure 13A). This papilla, devoid of concretion particles, exhibits numerous tubercle- or granular-like protrusions (Figure 13B)
The terminal ring tapers towards the tail. The subdorsal setae of the terminal ring are inserted at approximately two-thirds of the 17th main ring. Beyond this point, the ring curves ventrally and terminates in a short, uncovered spinneret (Figure 13D–F). Small, rounded phasmata are located midway along the terminal ring.
The female is similar to the male in most respects. Like the male, the body gradually tapers towards both extremities and is composed of 17 main rings. The main rings are composed of large concretion particles (Figure 9B, Figure 14A and Figure 15A,B). SEM observations reveal numerous tubercle-like protrusions across the interzone (Figure 15C,D).
The setae taper towards an open tip, with faint wrinkles along most of their length (Figure 15E–G). The terminal setae are distinctly elongated (Figure 17A,F,G). The head appears globular in the lateral view but has a rounded triangular shape in the dorsal or ventral views. The oral ring appears more swollen and is viewed at a slightly oblique angle. In the SEM image of the swollen labial region (Figure 16D), the eight projections and four longer setae-like structures blend together, forming a unified, crown-like arrangement (Figure 9D, Figure 14B and Figure 16A–D). Notably, there are no cephalic setae, and no traces of them were found in their usual positions. The amphidial fovea are oval, nearly cover the entire head laterally, with a central amphidial pore.
The terminal ring bends ventrally and terminates in a short, uncovered spinneret with small, rounded phasmata (Figure 9F, Figure 14C and Figure 17C–E).
There is a difference in the arrangement of the somatic setae. In a paratype female, the somatic setae are arranged as follows:
Subdorsal: 1, 3, 5, 7, 9, 11, 13, 16, 17 = 9.
Subventral: 2, 4, 6, 8, 10, 12, (-), 15 = 7.
In all paratype females, the arrangement deviates from the usual 17 main ring species pattern due to the absence of a pair of subventral setae on the 14th main ring (Figure 15H).
The reproductive system is didelphic-amphidelphic, with both branches outstretched, each containing several immature oocytes. The two spermathecae, although not always distinctly differentiated, contain small, globular sperm. While the exact location of the vulva is difficult to determine under light microscopy, SEM observations revealed that the vulva is positioned between the 10th and 11th main rings (Figure 17B).

4. Discussion

The subgenus Desmoscolex (Claparède, 1863) includes a small number of species characterized by unique morphological traits, particularly related to the presence or absence of cephalic setae. Since the first discovery of Desmoscolex (Desmoscolex) rostratus Timm, 1970, a species lacking cephalic setae, only five species with this feature have been reported, including D. (D.) asetosus Decraemer, 1975; D. (D.) obscurus Bussau, 1993; and the newly described species in this study, D. (D.) lanceosetatus sp. nov. and D. (D.) rotundicephalus sp. nov. This study provides a comprehensive comparative analysis of the morphological characteristics of these species, along with discussions of their habitats and distributions, thereby contributing to a more refined understanding of the species within the subgenus Desmoscolex. To support this, we present a table comparing the morphological characteristics of Desmoscolex species lacking cephalic setae (Table 1) and a pictorial key (Figure 18) for species sharing common features within the subgenus.

4.1. Comparative Morphology of Desmoscolex Species Lacking Cephalic Setae

Desmoscolex (Desmoscolex) rostratus Timm, 1970. The first species in the subgenus Desmoscolex without cephalic setae, D. (D.) rostratus was described based on a single holotype female found at a depth of 73 m in the Indian Ocean. Morphologically, this species is characterized by its distinct elongated snout (or rostrum), which is asymmetrical and extends anteriorly as a long projection. The amphidial fovea are ovate and extend to the first main ring, a feature that differentiates it from the newly described species. Notably, D. (D.) rostratus has 31 body rings, a significant difference from the newly described species D. (D.) lanceosetatus sp. nov. and D. (D.) rotundicephalus sp. nov., both of which have 17 main rings. Additionally, D. (D.) rostratus possesses a unique morphological trait not observed in the newly described species: a single terminal subdorsal seta. This feature, along with the greater number of main body rings, highlights the distinct taxonomic characteristics that clearly distinguish D. (D.) rostratus from other species lacking cephalic setae, including the two newly described species within the subgenus Desmoscolex.
Desmoscolex (Desmoscolex) asetosus Decraemer, 1975. Desmoscolex (D.) asetosus, discovered in sandy bottoms around Lizard Island, Great Barrier Reef, Australia, at depths of 20 to 21.5 m, exhibits a number of distinctive features. It has an asymmetrical head, dorsally bent, similar to D. (D.) lanceosetatus sp. nov. However, it differs from D. (D.) lanceosetatus sp. nov. in several aspects, including its amphid structures, the morphology of subdorsal setae, and the arrangement of somatic setae.
The amphidial fovea in D. (D.) asetosus are bipartite, tapering anteriorly and ending in a posterior sac that extends to the second main ring. This differs from D. (D.) lanceosetatus sp. nov., where the amphidial fovea extend only to the posterior margin of the first main ring.
The subdorsal setae in D. (D.) asetosus consist of a broader basal shaft with a fine, slightly bent border at the junction with the narrower lance-shaped tip. This description may give the impression of a common lance-shaped tip similar to other species, particularly those addressed in this study. However, as illustrated in Figure 18A, the junction in D. (D.) asetosus is accentuated before sharply narrowing, clearly differing from the subdorsal setae of D. (D.) obscurus and D. (D.) lanceosetatus sp. nov., as shown in Figure 18B and 18D. This differentiation is further evident when considering the description of the subdorsal setae of D. (D.) spec. apud D. (D.) asetosus, which are described as having a large spatulate tip. This highlights how the fine, slightly bent border at the junction with the narrower lance-shaped tip in D. (D.) asetosus is distinct.
Moreover, the arrangement of somatic setae in D. (D.) asetosus is consistent in both males and females, showing the typical pattern for Desmoscolex species. This arrangement differentiates it from D. (D.) obscurus and D. (D.) lanceosetatus sp. nov., where the 14th subventral seta is absent in females, making it an important distinguishing feature.
In terms of habitat, D. (D.) asetosus inhabits sandy bottoms at relatively shallow depths (21.5 m), indicating a preference for shallower marine environments. This contrasts with the deeper silty sand habitats of the newly described Korean species, D. (D.) lanceosetatus sp. nov. (48,103 m) and D. (D.) rotundicephalus sp. nov. (103 m). These habitat differences may reflect divergent ecological adaptations within this group.
Desmoscolex (Desmoscolex) obscurus Bussau, 1993. Desmocolex (D.) obscurus, described from deep-sea environments in the Peru Basin at depths of 4154 and 4174 m, is notable for its adaptation to extreme deep-sea conditions. This species possesses 17 main rings composed of coarse foreign particles and features lance-shaped subdorsal setae. In females, the somatic setae arrangement differs from the typical 17 ring pattern due to the absence of subventral setae on the 14th ring, resulting in males having nine subdorsal and eight subventral setae, while females have seven subventral setae. These characteristics are similar to those found in D. (D.) lanceosetatus sp. nov.
However, D. (D.) obscurus is distinguished by its relatively broad body diameter compared to body length, as evident from Bussau’s illustrations. Specifically, the measurements for the holotype male indicate a body length of 365 µm, a midbody diameter of 78 µm, and a cloacal region body diameter of 57 µm. The ‘a’ ratio, defined as body length divided by maximum body diameter, reveals a significantly wider body relative to its length.
Moreover, the holotype male shows a terminal main ring length of 65 µm, while the terminal main ring width, estimated from the figure’s scalebar, is 15 µm. This relatively narrow width compared to the length of the terminal main ring is a distinct feature that differentiates D. (D.) obscurus from other species, such as D. (D.) asetosus and D. (D.) lanceosetatus sp. nov.
The amphidial fovea in D. (D.) obscurus are vesicular and extend slightly beyond the first main ring, which is also a distinctive feature of this species.
The habitat of D. (D.) obscurus in the deep-sea Peru Basin contrasts significantly with the shallow subtidal and silty sand environments where the two new Korean species were discovered.

4.2. Comparative Analysis of the New Species

Desmoscolex (Desmoscolex) lanceosetatus sp. nov. The newly described D. (D.) lanceosetatus sp. nov. from Jeju Island, Korea, exhibits unique morphological traits that distinguish it from the known species lacking cephalic setae. Its asymmetrical head, bent dorsally, is comparable to that of D. (D.) asetosus, but the head in D. (D.) lanceosetatus sp. nov. tapers more smoothly, leading to a small, pointed labial region, which is surrounded by four protrusions. This labial region is notably different from the more elongated snout-like rostrum of D. (D.) rostratus.
The amphid structure in D. (D.) lanceosetatus sp. nov. is particularly distinctive. While other species, such as D. (D.) asetosus and D. (D.) obscurus, have amphidial fovea extending to the first and second main rings, the amphidial fovea in D. (D.) lanceosetatus sp. nov. reach only the posterior margin of the first ring. This amphidial groove, which begins at the rostrum, narrows in the middle, and widens toward the posterior head region, is a feature unique to this species.
Additionally, the subdorsal setae in D. (D.) lanceosetatus sp. nov. resemble those in D. (D.) asetosus, with a broader basal shaft and a lance-shaped open tip. However, unlike D. (D.) asetosus, the transition from base to tip is smooth, lacking the “fine slightly bent border” described by Decraemer (1975) [6].
Unlike D. (D.) asetosus, which exhibits the typical setae arrangement in both males and females, D. (D.) lanceosetatus sp. nov. shows a deviation in females, characterized by the absence of subventral setae on the 14th main ring. This results in sexual dimorphism, where males have nine subdorsal and eight subventral setae, while females have only seven subventral setae. The combination of these unique amphid and setal features further highlights the distinct morphology of D. (D.) lanceosetatus sp. nov. within the subgenus.
In these characteristics, D. (D.) lanceosetatus sp. nov. is most similar to D. (D.) obscurus. However, unlike D. (D.) obscurus, D. (D.) lanceosetatus sp. nov. has a relatively slender body diameter, evident from the measurements of the holotype male, which indicate a body length of 295 µm, a midbody diameter of 36 µm, and a cloacal region diameter of 34 µm. The ‘a’ ratio, calculated as body length divided by maximum body diameter, is 8.24 for D. (D.) lanceosetatus sp. nov., compared to 4.7 for D. (D.) obscurus, indicating a notable difference in overall body shape.
For the holotype male of D. (D.) lanceosetatus sp. nov., the terminal main ring length is 34 µm, while the terminal main ring width is 13 µm. The ratio of terminal main ring length to width is 2.6 for D. (D.) lanceosetatus sp. nov., compared to 4.3 for D. (D.) obscurus, further highlighting differences in terminal morphology between the two species.
Additionally, D. (D.) lanceosetatus sp. nov. inhabits silty sand at depths of 48 to 103 m, a habitat similar to that of D. (D.) rotundicephalus sp. nov. However, both species are found in relatively shallower environments compared to the deep-sea habitat of D. (D.) obscurus.
Desmoscolex (Desmoscolex) rotundicephalus sp. nov. Desmoscolex (D.) rotundicephalus sp. nov., also from Jeju Island, is easily distinguished by its more globular head shape, which stands in stark contrast to the asymmetrical heads seen in D. (D.) asetosus, D. (D.) obscurus, D. (D.) rostratus, and D. (D.) lanceosetatus sp. nov. The labial region of D. (D.) rotundicephalus sp. nov. is crowned by eight distinct projections and four elongated setae-like structures forming a unified structure, unlike the four protrusions observed in D. (D.) lanceosetatus sp. nov. Furthermore, this species lacks labial papillae or tubercle-like projections.
The amphidial fovea in D. (D.) rotundicephalus sp. nov. are confined to the head region, unlike in D. (D.) asetosus or D. (D.) obscurus, where the amphidial fovea extend to the first or second main ring. This feature, combined with the globular head shape, suggests significant morphological divergence within the subgenus. Additionally, sexual dimorphism is evident in the arrangement of somatic setae between males and females, further highlighting the species’ distinct characteristics.
Desmoscolex (D.) rotundicephalus sp. nov. inhabits silty sand at a depth of 103 m, further distinguishing it from deep-sea species like D. (D.) obscurus. The habitat of this species, similar to that of D. (D.) lanceosetatus sp. nov., indicates an ecological preference for subtidal benthic environments.

4.3. Insights from Scanning Electron Microscopy (SEM): Shedding Light on Subtle Morphological Features in Desmoscolex Species

The application of scanning electron microscopy (SEM) in this study has significantly enhanced the understanding of the fine morphological details of Desmoscolex species, particularly the newly described D. (D.) lanceosetatus sp. nov. and D. (D.) rotundicephalus sp. nov. SEM analysis revealed numerous tubercle- or granule-like protrusions in the interzone regions of both species, which appeared smooth under light microscopy. These structures, distinctly visible under SEM, provide a deeper understanding of the surface architecture that may be important for species differentiation. Additionally, SEM revealed faint wrinkles on the lower portions of the somatic setae, a feature that was not detectable using light microscopy. This subtle setal morphology further differentiates the two new species from their congeners, indicating that SEM is essential for uncovering such detailed features that could otherwise be overlooked.
The labial region of both new species displayed previously unreported features, particularly in the lip region, which showed intricate patterns not described in earlier studies of Desmoscolex species. These patterns differ from those described by Shirayama and Hope (1992) and Decraemer (1996), suggesting that there is more variation in the labial morphology within the genus than previously recognized [2,13]. The use of SEM allowed for the observation of fine details in the labial region, such as the shape and arrangement of the lips, as well as the presence or absence of tubercles. This level of detail is crucial for understanding the functional morphology and taxonomic relationships within Desmoscolex.
It is also important to note that Decraemer (1996) suggested that the presence of cephalic tubercles is no longer considered a synapomorphy for species within the genus Desmoscolex. However, the detailed morphological analyses facilitated by SEM in this study indicate that the labial region and associated tubercles, while variable, still hold taxonomic significance. Further SEM studies are necessary to clarify the role of these morphological traits in distinguishing species across the subgenus. The findings of this study underscore the importance of SEM in marine nematode taxonomy, not only for uncovering previously unreported features but also for providing a more comprehensive and accurate characterization of morphological variation within the subgenus. This approach enhances the ability to identify species-specific traits and supports a more refined understanding of species relationships in Desmoscolex [2].

4.4. Habitat and Ecological Considerations

The newly described species, D. (D.) lanceosetatus sp. nov. and D. (D.) rotundicephalus sp. nov., were found in silty sand environments at depths of 48 m and 103 m, respectively. This contrasts with the deep-sea habitat of D. (D.) obscurus and the sandy-bottom habitats of D. (D.) asetosus.
Moreover, the discovery of these species in the Northwestern Pacific Ocean expands the known geographical range of Desmoscolex species lacking cephalic setae. The localities of D. (D.) lanceosetatus sp. nov. and D. (D.) rotundicephalus sp. nov. provide valuable insights into the biogeography of the genus, suggesting that more undiscovered species may inhabit similar subtidal environments.

5. Conclusions

This study describes two new species of the subgenus Desmoscolex from Korea, Desmoscolex (Desmoscolex) lanceosetatus sp. nov. and D. (D.) rotundicephalus sp. nov., both of which share the rare trait of lacking cephalic setae. These species contribute to our understanding of morphological diversity within Desmoscolex, a group that has traditionally relied on the presence of cephalic setae for taxonomic differentiation. Detailed morphological analyses using SEM and DIC have revealed key diagnostic features that distinguish these new species from previously known taxa within the subgenus. Desmoscolex (D.) lanceosetatus sp. nov. is characterized by its asymmetrical, dorsally curving head, large amphids, and lance-shaped subdorsal setae. The absence of subventral setae on the 14th ring in females further differentiates this species from other 17-ring Desmoscolex species. On the other hand, Desmoscolex (D.) rotundicephalus sp. nov. displays a globular head with a unique arrangement of labial projections forming a crown-like structure. Its amphidial fovea is confined to the head, and the absence of subventral setae on the 14th ring in females highlights its distinction from other species. The discovery of these species not only expands the known diversity of the subgenus Desmoscolex but also provides new insights into the variability of key morphological features such as the shape and arrangement of amphids, somatic setae, and the structure of the labial region. This study underscores the importance of advanced microscopy techniques, such as SEM, in revealing morphological details that are essential for accurate species identification and comparative taxonomy. Further research on the labial region and other morphological characteristics, using both traditional and modern methods, is necessary to enhance our understanding of Desmoscolex species and their evolutionary relationships. These findings contribute valuable knowledge to the taxonomy of marine nematodes and offer a foundation for future studies on species identification and classification within Desmoscolex.

Author Contributions

Data curation and writing—Original draft preparation, W.J.; SEM observation and imaging, J.-H.K.; Writing—Reviewing, editing, and funding acquisition, H.S.R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the management of Marine Fishery Bio-resources Center (2024) funded by the National Marine Biodiversity Institute of Korea (MABIK) (PG54100), and by the research projects “Development of original technology to verify factors influencing barren ground on the East Sea coast according to climate change (PEA0205)” of the Korea Institute of Ocean Science & Technology (KIOST).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank Heegab Lee for assistance with sample collection and express their gratitude to Hyo Jin Lee for offering valuable insights during the morphological analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Figure 18. Pictorial key of species group in subgenus Desmoscolex lacking cephalic setae. (A) Desmoscolex (Desmoscolex) asetosus Decraemer, 1975; (B) Desmoscolex (Desmoscolex) obscurus Bussau, 1993; (C) Desmoscolex (Desmoscolex) rostratus Timm, 1970; (D) Desmoscolex (Desmoscolex) lanceosetatus sp. nov.; (E) Desmoscolex (Desmoscolex) rotundicephalus sp. nov.
Figure 18. Pictorial key of species group in subgenus Desmoscolex lacking cephalic setae. (A) Desmoscolex (Desmoscolex) asetosus Decraemer, 1975; (B) Desmoscolex (Desmoscolex) obscurus Bussau, 1993; (C) Desmoscolex (Desmoscolex) rostratus Timm, 1970; (D) Desmoscolex (Desmoscolex) lanceosetatus sp. nov.; (E) Desmoscolex (Desmoscolex) rotundicephalus sp. nov.
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Table 1. Comparative characteristics of Desmoscolex (Desmoscolex) species lacking cephalic setae (M: male; F: female; -: unknown).
Table 1. Comparative characteristics of Desmoscolex (Desmoscolex) species lacking cephalic setae (M: male; F: female; -: unknown).
TaxaCharacters
Body Length
(µm)
Number of Main RingSeta ArrangementHeadAmphidSubdorsal
Setae
Subventral SetaeHabitatLocality
D. (D.) asetosus
Decraemer, 1975
M: 225–250
F: 275, 320
17 , 1 , 3 , 5 , 7 , 9 , 11 , 13 , 16 , 17 = 9 2 , 4 , 6 , 8 , 10 , 12 , 14 , 15 = 8 Asymmetrical, bent to the dorsal sideBipartite, extends to the second main ringWide base with a bent lance-shaped tipWide base with a sharp, short tipSandy bottom with a layer of silt and rich in Foraminifera,
21.5 m
Lizard Island, Great Barrier Reef, Australia
D. (D.) lanceosetatus sp. nov.M: 295–440
F: 250–310
17 , : 1,3 , 5,7 , 9,11,13,16,17 = 9 : 2,4 , 6,8 , 10,12,14,15 = 8 : 2,4 , 6,8 , 10,12 , - , 15 = 7 Asymmetrical, bent to the dorsal sideVesicular, extends to the posterior margin of the first main ringWide base with a lance-shaped tipNormal bristle structure with an open tipSilty sand,
48 m, 103 m
Jeju Island, Korea
D. (D.) obscurus
Bussau, 1993
M: 365
F: 445
17 , : 1,3 , 5,7 , 9,11,13,16,17 = 9 : 2,4 , 6,8 , 10,12,14,15 = 8 : 2,4 , 6,8 , 10,12 , - , 15 = 7 -Vesicular, slightly beyond the first main ringLance-shaped tipNormal bristle structureDeep-sea,
4154 m, 4174 m
Peru Basin
D. (D.) rostratus
Timm, 1970
F: 33031 1 , 4 , 7 , 11 , 15 , 19 , 23 , 29 , 31 = 9 2 , 5 , 9 , 13 , 17 , 21 , 27 = 7 Asymmetrical with a long snoutOvate, extending to first main ringInconspicuous lancet tip, single terminal subdorsal seta-Rock dredge,
73 m
Indian Ocean
D. (D.) rotundicephalus sp. nov.M: 275
F: 255–320
17 , : 1,3 , 5,7 , 9,11,13,16,17 = 9 : 2,4 , 6,8 , 10,12,14,15 = 8 : 2,4 , 6,8 , 10,12 , - , 15 = 7 Globular head with a protruding labial regionOval, covering most of the head with central amphidial poreTapering to an open tip without lance-shaped tipNormal fine structureSilty sand, 103 mJeju Island, Korea
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Jung, W.; Kihm, J.-H.; Rho, H.S. Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability. J. Mar. Sci. Eng. 2024, 12, 2168. https://doi.org/10.3390/jmse12122168

AMA Style

Jung W, Kihm J-H, Rho HS. Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability. Journal of Marine Science and Engineering. 2024; 12(12):2168. https://doi.org/10.3390/jmse12122168

Chicago/Turabian Style

Jung, Wooin, Ji-Hoon Kihm, and Hyun Soo Rho. 2024. "Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability" Journal of Marine Science and Engineering 12, no. 12: 2168. https://doi.org/10.3390/jmse12122168

APA Style

Jung, W., Kihm, J.-H., & Rho, H. S. (2024). Two New Species of Desmoscolex (Nematoda: Desmoscolecidae) from Subtidal Benthic Habitats in Korea, with a Comparative Analysis of Cephalic Setae Variability. Journal of Marine Science and Engineering, 12(12), 2168. https://doi.org/10.3390/jmse12122168

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