Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region

Lee, Hyo Jin; Lee, Heegab; Rho, Hyun Soo

doi:10.3390/jmse12122310

Open AccessArticle

Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region

by

Hyo Jin Lee

,

Heegab Lee

and

Hyun Soo Rho

^*

East Sea Environment Research Center, Korea Institute of Ocean Science and Technology (KIOST), Uljin 36315, Republic of Korea

^*

Author to whom correspondence should be addressed.

J. Mar. Sci. Eng. 2024, 12(12), 2310; https://doi.org/10.3390/jmse12122310

Submission received: 11 November 2024 / Revised: 11 December 2024 / Accepted: 11 December 2024 / Published: 15 December 2024

(This article belongs to the Special Issue Biodiversity and Population Ecology of Marine Invertebrates)

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Abstract

This study provides a taxonomic analysis of three newly discovered species of free-living marine nematodes in the subgenus Tricoma from the East Sea, Korea. Tricoma (Tricoma) polyringulata sp. nov. is characterized by its relatively small body size, with lengths of 280–370 µm in males and 320–390 µm in females, and 75–89 main rings. Diagnostic features include an uncovered first ring, triangular head shape, amphidial fovea with slight constriction, and a unique pattern of somatic setae, comprising 7–9 subdorsal and 8–12 subventral setae. Tricoma (Tricoma) fortiseta sp. nov. is distinguished by 65–69 main rings, 10–12 subdorsal setae, and 17–20 subventral setae, along with a hexagonal head, a well-defined labial region with six lips, and bifid-tipped cephalic setae located medially on the head. Tricoma (Tricoma) uljinensis sp. nov. exhibits 67–70 main rings, with a thick, sclerotized head that is 1.5 times as wide as its length, gently triangular in shape. Its diagnostic features include a gubernaculum bent downward near its midpoint and a conical terminal ring, with 25–39% of the desmos covered. Detailed morphological descriptions of each species are provided, along with a comparative table of morphological traits for species with 61–75 main rings and an illustrated key for identification.

Keywords:

marine; SEM and DIC imaging; biodiversity; meiobenthos; Northwest Pacific Ocean

1. Introduction

Marine ecosystems are both complex and rich in diversity, providing habitats for countless species, many of which are still not fully explored. Sediment environments, in particular, serve as biodiversity hotspots, essential for supporting marine organisms. Meiofauna, which include nematodes and copepods, play a key role in maintaining sediment stability, nutrient cycling, and organic matter decomposition through bioturbation and feeding, thus indirectly impacting larger organisms. Their high densities, diverse species, and adaptive traits—such as small size, short generation times, and high metabolic rates—enable them to drive essential ecological processes, making them vital to energy flow and nutrient cycling in benthic ecosystems [1]. These functions underscore the significance of meiofauna in sustaining ecosystem resilience, biodiversity, and ecological health in sedimentary habitats, underscoring the need to deepen our understanding of their role within marine systems [2]. Nematodes are the most abundant multicellular organisms in these sediments, comprising 70–90% of meiofauna [3,4].

Within the nematode phylum, the order Desmoscolecida Filipjev, 1929, is notable for its unique morphology, especially the distinctive ring-like structures, or “desmen”, formed by its annulated cuticle and covering material. These rings often embed external particles and secretions, providing both camouflage and structural support [5,6]. The order includes 3 families, 24 genera, and around 280 species, with over 90% inhabiting marine environments, ranging from intertidal zones to depths exceeding 4000 m [7,8,9,10]. This broad ecological range demonstrates Desmoscolecida’s adaptability to diverse marine conditions, from high-energy shorelines to nutrient-poor deep-sea sediments.

The genus Tricoma within this order is remarkable for its extensive species diversity. Originally described in 1894, with Tricoma cincta Cobb, 1894, designated as the type species, the genus has grown to encompass approximately 88 species documented worldwide [11,12]. Species in the subgenus Tricoma are characterized by rounded or triangular desmen, a lack of sudden inversion rings, somewhat triangular heads, and cylindrical terminal rings [9]. However, complex external structures and variability in desmen and somatic setae patterns pose significant taxonomic challenges. The absence of clear morphological synapomorphies further complicates species identification and the discovery of new species within this group [7].

Until the mid-1970s, taxonomic studies on the family Desmoscolecidae primarily focused on external structures [6,7], such as the shape and number of desmen, and the form and arrangement of somatic setae [13]. Over time, research expanded to incorporate both external and internal morphological characteristics. A significant advancement came from Shirayama and Hope (1992), who proposed a method for studying the ultrastructure of the head region using scanning electron microscopy (SEM) [14].

In the late 1990s, two studies investigated the lip region in the genus Desmoscolex [15,16], providing valuable insights. However, for the next two decades, research on this aspect stagnated. Recently, Jung, Khim, and Rho (2024) reignited interest in this topic by reporting on the ultrastructure of the lip region in Desmoscolex, highlighting the need for more detailed and comprehensive morphological data to advance the taxonomic and phylogenetic understanding of the family Desmoscolecidae [17].

This paper aims to build upon these advancements, presenting new data on the ultrastructure of the lip region and the implications for the taxonomy of free-living marine nematodes within the subgenus Tricoma.

The East Sea, located in the Northwestern Pacific, is influenced by the Tsushima Warm Current, a branch of the Kuroshio Current, and the North Korean Cold Current, creating a nutrient-rich and ecologically diverse marine environment [18]. This region is often regarded as a miniature ocean ecosystem due to its high biodiversity [19]. Despite the recognized diversity and ecological importance of nematodes in marine sediments, research on marine nematodes in the East Sea remains relatively limited, with only 43 species reported to date [11,20,21].

In this study, we describe three new species of the subgenus Tricoma found along the eastern coast of Korea and in the waters around Ulleungdo Island, focusing on their detailed morphological characteristics. Using scanning electron microscopy (SEM) and differential interference contrast (DIC) microscopy, we provide comprehensive descriptions of these species, highlighting differences from closely related species. Additionally, we present a comparative table of morphological traits for species with 61 to 75 main rings and a pictorial key to aid in identification within the subgenus.

2. Materials and Methods

Sediment samples were collected from the subtidal zone at Huam, Uljin, using SCUBA diving, and near Ulleungdo Island with a Smith-McIntyre grab (Figure 1). In the field, samples were treated with osmotic shock by rinsing with tap water for 1 min, then filtered through a 63 µm mesh sieve, and preserved in a 5% formaldehyde solution for long-term storage.

In the laboratory, meiobenthic organisms were separated from sediment using Ludox HS40 flotation and sieved through a 63 µm mesh, followed by three centrifugation cycles [22]. Nematodes were carefully isolated from the meiobenthos samples using a Pasteur pipette under high magnification on a LEICA 205 C stereo microscope (Leica, Wetzlar, Germany). Each nematode was transferred to a 3% glycerol solution, allowing for slow evaporation at room temperature over 10 days for preservation.

For morphological observations and measurements, specimens were mounted in glycerin between two coverslips on HS slides using the wax ring method [23]. Observations and measurements were performed with an Olympus BX53 microscope (Olympus, Tokyo, Japan) using cellSens Standard 1.16 software. Photographs were taken with a LEICA DM2500 LED microscope (Leica, Wetzlar, Germany) equipped with a LEICA K5C color CMOS camera, and images were processed in Adobe Photoshop 2022.

Line drawings were created by tracing images captured using a 100× oil-immersion objective lens and Nomarski differential interference contrast (DIC) on a LEICA DM2500 LED microscope (Leica, Wetzlar, Germany) equipped with a drawing tube. The illustrations were further refined on a Wacom Cintiq 22 tablet using Adobe Illustrator.

For scanning electron microscopy (SEM), selected specimens were fixed in 5% buffered formalin, rinsed twice in distilled water for 30 min each to eliminate residual formalin, and freeze-dried on an FDU-1200 cooling stage (EYELA, Tokyo, Japan). The specimens were then mounted on aluminum stubs, sputter-coated with gold/palladium using a high-vacuum evaporator, and examined with an SEC SNE-3200M desktop mini-SEM.

3. Results

Phylum Nematoda Potts, 1932

Class Chromadorea Inglis, 1983

Order Desmoscolecida Filipjev, 1929

Family Desmoscolecidae Shipley, 1896

Subfamily Tricominae Lorenzen, 1969

Genus Tricoma Cobb, 1894

Subgenus Tricoma Cobb, 1894

3.1. Description of Tricoma (Tricoma) polyringulata sp. nov. (Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6; Table 1)

urn:lsid:zoobank.org:act:FED57A5E-DF95–4B2F-B4ED-10E603B57DA1

Table 1. Morphometrics of Tricoma (Tricoma) polyringulata sp. nov. in micrometers (µm). d: dorsal side, v: ventral side.

	Males		Females
	Holotype	Paratypes (n = 5)	Paratypes (n = 2)
Total body length	360	315.2 ± 30.7 (280–370)	375.2 ± 33.3 (320–390)
Body rings	v:83 d:81	v: 81.4 ± 3.4 (77–86) d: 81 ± 3.6 (75–85)	v:81.5 ± 2.5 (79–84) d: 84 ± 5.0 (79–89)
a	16	13.6 ± 1.8 (12.0–17.0)	13.2 ± 1.8 (11.0–15.0)
b	6	6.4 ± 0.2 (6.0–6.5)	6.9 ± 0.5 (6.0–7.0)
c	6	5.2 ± 0.5 (4.5–6.0)	6.2 ± 0 (6.0–6.0)
Head width	13	12.8 ± 0.5 (12.0–13.5)	13.1 ± 0.7 (12.5–14.0)
Head length	12	11.2 ± 0.9 (10.0–13.0)	11.9 ± 0.1 (11.5–12.0)
Body diameter at level of cardia	21	22.5 ± 0.4 (22.0–23.0)	25.4 ± 0.3 (25.0–26.0)
Maximum body diameter	22	23.2 ± 0.8 (22.0–24.0)	27.2 ± 1.2 (26–28.5)
Cephalic setae length	10	10.3 ± 0.7 (9.5–11.0)	10.7 ± 0.6 (10.0–11.5)
Amphideal fovea length	15	12.7 ± 0.9 (11.5–14.0)	13.8 ± 0.2 (13.5–14.0)
Ocelli diameter	8	5.7 ± 1.6 (3.0–8.0)	5.0 ± 1.0 (4.0–6.0)
Ocelli length	8	7.5 ± 2.1 (5.0–10.0)	5.3 ± 1.5 (4.0–6.5)
Anterior end to ocelli	69	51.7 ± 8.3 (37.0–62.5)	55.9 ± 6.1 (50.0–62.0)
Pharynx length	59	49.6 ± 4.5 (43.0–57.0)	51.8 ± 0.7 (51.0–52.5)
Number of subventral setae (Right)	9	10 ± 1.1 (9–12)	10.5 ± 0.5 (10–11)
Number of subventral setae (Left)	9	9.8 ± 0.1 (8–11)	10.0 ± 0 (10–10)
Length of the longest subventral setae at the third position	11	12 ± 0.3 (11.5–12.0)	11.6 ± 0 (11.0–11.5)
Length of the shortest subventral setae in the cloacal region	5	6.4 ± 0.3 (6.0–7.0)	7.9 ± 0.8 (7.0–8.5)
Number of subdorsal setae (Right)	9	8.0 ± 0.9 (7–9)	8.0 ± 0 (8–8)
Number of subdorsal setae (Left)	9	8.2 ± 0.7 (7–9)	8.5 ± 0.5 (8–9)
Length of the longest subdorsal setae	9	11.4 ± 0.4 (11–12)	11.6 ± 0.7 (11.0–12.5)
Length of the shortest subdorsal setae	6	6.5 ± 0.5 (5.5–7.0)	7.4 ± 1.6 (6.0–9.0)
Spicule length	21	22.6 ± 1.2 (21.0–24.0)	-
Gubernaculum length	13	14.8 ± 1.5 (13.5–18.0)	-
Anterior end to vulva	-	-	170.1 ± 18.4 (150–190)
Body diameter at the vulva interzone	-	-	26.4 ± 0.8 (25.5–27.0)
V (%)	-	-	47.6 ± 0.7 (47.0–48.5)
Anal body diameter	18	18.8 ± 0.6 (18.0–19.5)	19.0 ± 0 (19.0–19.0)
Tail length	66	61.2 ± 0.3 (61.0–62.0)	57.6 ± 5.0 (53.0–63.0)
Number of tail’s body rings	v:12 d:11	v: 12.2 ± 1.2 (11–14) d: 12.2 ± 0.7 (11–13)	v: 10.5 ± 0.5 (10–11) d: 11.5 ± 1.5 (10–13)
Terminal ring length	19	20.1 ± 2.4 (16.0–23.0)	19.8 ± 1.3 (18.5–21.0)
Terminal ring width	7	6.5 ± 0.4 (6.0–7.0)	8.4 ± 0.7 (8.0–9.0)
Desmos covering the terminal ring	4	3.3 ± 0.8 (2.0–4.0)	4.7 ± 0.4 (4.5–5.0)
Phasmata	2	2.3 ± 0.3 (2.0–2.5)	2.9 ± 0.1 (2.5–3.0)

Figure 2. Tricoma (Tricoma) polyringulata sp. nov. Males (A–I) and female (J,K). (A) Entire view of male, lateral view (holotype); (B) body showing a pronounced curl at the tail due to fixation (KIOST NEM-1–2732); (C) head region, right side (holotype); (D) head region showing the naked first main ring only on the dorsal side (KIOST NEM-1–2732); (E) spicule and gubernaculum (KIOST NEM-1–2734); (F) spicule and gubernaculum (holotype); (G) posterior region, right side (holotype); (H) terminal ring showing phasmata, left side (holotype); (I) posterior region (KIOST NEM-1–2732); (J) head region of the female, right side; (K) entire view of female. Scale bars: 50 µm (A,B,K); 10 µm (C–J).

Figure 3. Tricoma (Tricoma) polyringulata sp. nov. SEM micrographs of the male, lateral view. (A) Entire view of the body; (B) head region displaying the amphidial fovea, an uncovered first main ring (indicated by an arrow), and a partial first main ring (indicated by an arrowhead); (C) detailed view of the labial region; (D) cephalic setae area; (E) subventral setae with lateral insertion (indicated by an arrow); (F) subdorsal setae; (G) posterior body region; (H) terminal ring. Scale bars: 100 µm (A); 30 µm (G); 10 µm (B,F,H); 5 µm (C–E).

Figure 4. Tricoma (Tricoma) polyringulata sp. nov. DIC photomicrographs of the holotype male (A–C,G) and paratype males, lateral view. (A) Entire view of the body, right side; (B) head region showing the uncovered first main ring (indicated by arrows); (C) amphideal fovea; (D) head region showing the deflated naked first main ring (indicated by arrows) (KIOST NEM-1–2735); (E) amphideal fovea (KIOST NEM-1–2735); (F) subventral setae (KIOST NEM-1–2732); (G) spicules and gubernaculum; (H) spicules and gubernaculum (KIOST NEM-1–2735); (I) posterior body region showing phasmata (indicated by an arrow) (KIOST NEM-1–2733). Scale bars: 50 µm (A); 10 µm (I); 5 µm (B–H).

Figure 5. Tricoma (Tricoma) polyringulata sp. nov. SEM micrographs of the female, lateral view. (A) Entire view of the body; (B) cuticles details in the midbody region; (C) head region showing the amphideal fovea; (D) uncovered first main ring (indicated by an arrow) and very small projections aligned in rows between main rings; (E) cephalic setae completely covered by a thin membrane; (F) subdorsal setae, lateral view; (G) subventral setae; (H) posterior body region. Scale bars: 100 µm (A); 30 µm (H); 20 µm (B); 10 µm (C); 5 µm (D–G).

Figure 6. Tricoma (Tricoma) polyringulata sp. nov. DIC photomicrographs of the females, lateral view. (A) Entire view of the body, right side; (B) head region showing the uncovered first main ring (indicated by arrows); (C) amphideal fovea; (D) vulva region (indicated by an arrow); (E) anal tube region (indicated by an arrow); (F) uncovered vulva region (KIOST NEM-1–2738); (G) posterior body region (KIOST NEM-1–2738). Scale bars: 50 µm (A); 5 µm (B–G).

3.1.1. Type Material

The holotype male (MABIK NA00158738), mounted on an HS slide with glycerin, is preserved at the Marine Biodiversity Institute of Korea (MABIK) in Seocheon, Korea. Additionally, five paratype males (KIOST NEM-1–2732 to KIOST NEM-1–2736) and two paratype females (KIOST NEM-1–2737, KIOST NEM-1–2738) are deposited in the nematode specimen preservation facility at the Bio-Resources Bank of Marine Nematodes (BRBMN) within the East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.1.2. Type Locality

The nematodes were collected on 23 May 2023, by W. I. Jung, H. J. Lee, and H. S. Rho, from benthic sediments in the subtidal zone of Ulleungdo Island, Gyeongsangbuk-do, Korea, East Sea (37°31′39.22″ N, 130°48′15.32″ E), using a Smith-McIntyre grab.

3.1.3. Etymology

The specific name polyringulata is derived from the Greek term poly- (“many”) and the Latin ringulata (“small rings”), reflecting the species’ distinct morphological feature of possessing numerous small rings relative to its compact body size.

3.1.4. Diagnosis

Tricoma (Tricoma) polyringulata sp. nov. is distinguished by the following combination of features: (1) a compact body length of 280–370 µm in males and 320–390 µm in females; (2) cuticles with significant variation in tricomoid main rings, numbering from 75 to 89, with the difference in 1 or 2 main rings between the ventral and dorsal side in the same individual; (3) absence of desmos covering the first main ring; (4) a triangular head in the lateral view, with sclerotized head cuticle, absent in the lip region; (5) vesicular amphidial fovea showing slight posterior constriction, extending to the first exposed main ring; (6) somatic setae comprising 7–9 subdorsal and 8–12 subventral setae; (7) the gubernaculum has a sclerotized corpus and a caudally bent apophysis with curved tip; and (8) tails in males with 11–14 main rings, and in females with 10–13 main rings.

3.1.5. Measurements

All measurement data are provided in Table 1.

3.1.6. Description

Males: The body is relatively small and slender. The holotype male has 81 tricomoid rings on the dorsal side and 83 on the ventral side (Figure 2A and Figure 4A). Paratypes display 75–86 tricomoid rings, with either an equal count or a slight difference (1 or 2 rings) between the ventral and dorsal sides (Figure 2B). The difference in the rings occurs on either the dorsal or ventral side, and this is due to the presence of differentiated partial rings, which are covered with secretions and fine particles. In some individuals, this characteristic is also observed in a partial first main ring (Figure 3B). Each main ring exhibits secondary annulations, covered with secretions and fine particles (Figure 3A). These features are not discernible under an optical microscope; however, the SEM images reveal very small projections irregularly arranged within the interzone between the main rings (Figure 3B). The first main ring lacks desmos, with a relatively wide gap between the head and main rings (Figure 2C, Figure 3B and Figure 4B). In some paratypes, the uncovered first ring appears narrow due to constriction of the body due to fixation (Figure 4D), and in others, it is visible only on the dorsal side (Figure 2D).

The head appears triangular in the lateral view, with a length equal to or slightly greater than its width (Figure 2C and Figure 4B). It tapers from the peduncle of the cephalic setae, ending in a truncated tip measuring 4–5 µm in width. The head cuticle is clearly sclerotized and covered by a thin layer of granular particles, in the anterior head region appearing as prominent granular projections (Figure 3B). The labial region is thin-walled and differentiated from the cephalic region with sclerotized cuticle. The lip region consists of an oral ridge (see Shirayama and Hope) only discernable by SEM and surrounded by a labial ridge; here, with six differentiated lips, and each lip with a labial papilla of the external circle of the anterior sensilla (Figure 3C). The four cephalic setae are slightly shorter than the head width, tapering from a broad cylindrical base to a perforated tip. They are covered by a thin membrane along their entire length and are inserted into low peduncles located toward the back of the head (Figure 3D).

The vesicular amphids show a slight constriction at the posterior part of the head, nearly covering it entirely (Figure 2C and Figure 4E). They extend forward to the posterior labial region and backward to the rear boundary of the uncovered first main ring (Figure 2D, Figure 3B and Figure 4C). The amphidial canal ends in a groove in the sclerotized posterior wall of the head (Figure 2C). The stoma is small, measuring 1–1.8 µm in depth. The pharynx is cylindrical, extending toward the anterior region of the body. It is encircled by the nerve ring at the level between the sixth and seventh rings (Figure 2A,B). In the holotype, the esophageal–intestinal junction is located between the 13th and 14th rings, while in the paratypes, it is positioned between the 12th and 13th or 14th and 15th rings. The cardia is short and cytoplasmic, with the potential to extend into the intestinal lumen (Figure 2A). The glandular and muscular parts of the pharynx are separated posteriorly, and the esophageal glands, especially the dorsal ones, are swollen into rounded, protruding lobes. The intestine is narrow and finely granular at the anterior, gradually expanding into a broad, cylindrical structure with small to large spherical inclusions and fine granules along the lateral walls.

The ocelli are large and dark yellow. In the holotype, they are positioned between the 16th and 17th main rings, while in the paratypes, they are found between the 10th and 18th rings (Figure 2A,B and Figure 4A). Additionally, smaller pigment spots are present along the pharynx.

The somatic setae include 7–9 subdorsal and 8–12 subventral setae on each side, measuring 6–12 µm and 5–12 µm in length, respectively. The lengths of the subventral setae are generally similar; however, the setae located at the third position from the anterior, inserted at main rings 14–16, are the longest, measuring 11–12 µm, whereas the setae near the cloacal region are the shortest, measuring 5–7 µm. The most anterior subventral setae, located on the fourth, fifth, or sixth rings, are laterally inserted and relatively short compared to the other subventral setae, measuring 3–5 µm (Figure 3E). The subdorsal and subventral setae are similar in length, slender, cylindrical, and tapering toward a perforated distal end (Figure 3E,F and Figure 4F). These setae are set into low peduncles that project from the desmen. Some setae may be damaged or missing, making accurate measurements challenging depending on the specimen’s position or angle. The somatic setae pattern in the males is as follows:

		Subdorsal Setae										Subventral Setae
Holotype	left side	7	12	21	31	41	50	60	67	75	=9	4	7	16		27	36	46		56		66	75	=9
Holotype	right side	6	14	22	32	41	52	61	67	74	=9	4	7	13	24		38	46		57		67	76	=9
Paratype 01	left side	7	14	21	32	40	50	60	68		=8	4	7	14	21	27	31	41	48	55		63	71	=11
Paratype 01	right side	6	14	21	30	40	50	60	69		=8	5	6	13	19	25	32	39	47	54	61	62	71	=12
Paratype 02	left side	8	16	20		37	52	61	70	74	=8	6	8	15	23		31	40	48	59		68	77	=10
Paratype 02	right side	7	15	24	31	41	49	59	70	76	=9	6	8	14	21		30	40	50	60		68	78	=10
Paratype 03	left side	7	15	22	31	41	55	63	70	77	=9			15	21		30	40	50	60		70	77	=8
Paratype 03	right side	6	14	23	31	45	57	63	71	78	=9	4	7	15	24		33	42	51	60		70	77	=10
Paratype 04	left side	7	15	22	32	42	54		71		=7	6	8	13	20	27	34	42	49	57		63		=10
Paratype 04	right side	7	15	23	32	44	52		72		=7	5	7	14	21		28	37	45	53		63		=9
Paratype 05	left side	7	15	23	32	42	52	61	65	73	=9	4	7	13	20		28	37	47	56		65	76	=10
Paratype 05	right side	6	14	23	32	44	52		74		=7	4	7	14	21		30		44	54		65	75	=9

The reproductive system typically consists of two testes. The spicules measure 21–24 µm in length, are arched, and taper distally, with a slightly offset small capitulum at the proximal end (Figure 2E and Figure 4G). The gubernaculum, measuring 13–18 µm in length, has a sclerotized corpus and a caudally bent apophysis with curved tip (Figure 2F and Figure 4H).

The tail consists of 11 rings on the dorsal side and 12 on the ventral side in the holotype (Figure 2G), while the paratypes have 11–14 rings (Figure 2I, Figure 3G and Figure 4I). The terminal ring is cylindrical with a slightly thickened cuticle, except at the terminal spinneret (Figure 2H and Figure 4I). Desmos cover the anterior 12–22% of the terminal ring, leaving the terminal end bare and smooth (Figure 3H). The rounded phasmata, measuring 2–3 µm in diameter, are located at the boundary of the terminal ring desmos, although they were difficult to observe in some paratypes (Figure 4I). Three caudal glands are well developed.

Females: The females resemble the males in morphology, except for sexual characteristics (Figure 2J,K, Figure 5A–G and Figure 6A–C). One paratype female has a cuticle with 84 ventral and 89 dorsal tricomoid rings, while another paratype has 79 rings. These rings are coated with secretions and fine debris (Figure 5B). The somatic setae are arranged as 8–9 subdorsal setae and 10–11 subventral setae on each side. The arrangement of somatic setae in the paratype females is as follows:

		Subdorsal Setae										Subventral Setae
Paratype 06	left side	8	15	24	33	45	53	62	74	82	=9	5	7	13	21	29	41	49	59	68	77		=10
Paratype 06	right side	8	15	24	31	43	51	65	77		=8	6	9	13	21	29	40	49	60	70	77	78	=11
Paratype 07	left side	8	15	22	31	41	51	61	72		=8	6	7	12	19	25	36	45	55	64	73		=10
Paratype 07	right side	8	15	24	31	43	53	61	72		=8	6	7	13	19	26	37	45	55	64	73		=10

The reproductive system is didelphic–amphidelphic. The vulva is situated between the 41st and 42nd main rings (or between the 43rd and 44th rings in the other paratype) and is not covered by desmos (Figure 6D,F). The naked anal tube, measuring 2–4 µm in length, protrudes from the body wall between the 69th and 70th main rings (or between the 72nd and 73rd rings in the other paratype) (Figure 6E). The paratype female’s tail consists of 11 ventral and 13 dorsal rings, while the other has 10 rings (Figure 6G). The cylindrical terminal ring has 20–27% of its anterior portion covered by desmos (Figure 5H). A round phasmata, 3 µm in diameter, is located within the desmos of the terminal ring.

3.1.7. Differential Diagnosis

Tricoma (Tricoma) polyringulata sp. nov. is notably identified by its uncovered first main ring, a characteristic termed the “naked neckring.” This concept, initially introduced by Decraemer (1987) in the description of T. (T.) dimorpha papuensis [24], was further defined by Soetaert and Decraemer (1989) as an extended, cuticle-free region between the head and body, underlining its taxonomic significance [25]. Although not always recognized formally, our review of prior descriptions and illustrations identified 14 species presumed to exhibit this trait: T. (T.) apophysis Soetaert and Decraemer, 1989; T. (T.) breviseta Lee et al., 2023; T. (T.) corsicana Soetaert and Decraemer, 1989; T. (T.) dimorpha Decraemer, 1978; T. (T.) dimorpha papuensis Decraemer, 1987; T. (T.) donghaensis Lee et al., 2023; T. (T.) duopapillata Soetaert and Decraemer, 1989; T. (T.) goldeni Decraemer, 1978; T. (T.) latispicula Soetaert and Decraemer, 1989; T. (T.) megamphida Timm, 1970; T. (T.) rostrata Timm, 1970; T. (T.) lobata Juario, 1974; T. (T.) steineri de Man, 1922; and T. (T.) timmi Decraemer, 1978 [6,11,13,24,25,26,27].

Among these, T. (T.) apophysis, T. (T.) breviseta, T. (T.) corsicana, T. (T.) dimorpha papuensis, T. (T.) donghaensis, T. (T.) duopapillata, T. (T.) latispicula, and T. (T.) megamphida were explicitly noted in original descriptions for having an uncovered first main ring or elongated neck, while the others were inferred based on illustrations. Figure 7 presents a pictorial key to the head morphology of Tricoama (Tricoma) species with presumed uncovered first main rings. Variability in this trait was observed in individuals of T. (T.) dimorpha, T. (T.) dimorpha papuensis, and T. (T.) duopapillata, whereas it consistently appeared across all individuals of other species, including T. (T.) polyringulata sp. nov.

Tricoma (Tricoma) polyringulata sp. nov. is further distinguished by having 75–89 main rings and a body length of under 400 µm. Decraemer’s classification lists only four species—T. (T.) lobata Juario, 1974; T. (T.) meridonalis Kreis, 1934; T. (T.) parabrevirostris Decraemer, 1996; and T. (T.) timmi Decraemer, 1978—with 75–100 main rings and body lengths below 400 µm [5,13,26,29]. With these shared characteristics, T. (T.) polyringulata sp. nov. appears closely related to T. (T.) lobata and T. (T.) timmi, sharing main ring counts, small body size, similar subdorsal and subventral setae patterns, and an uncovered first main ring.

However, T. (T.) lobata is clearly distinguishable by its globular head shape (versus triangular in T. (T.) polyringulata sp. nov.), differing somatic setae arrangement (10–14 subdorsal and 12–16 subventral setae, compared to 7–9 subdorsal and 8–12 subventral setae in T. (T.) polyringulata sp. nov.), and a single curved gubernaculum.

Tricoma (T.) timmi, collected from the Great Barrier Reef and documented from 10 specimens, displays significant variability in main rings and somatic setae, showing superficial similarity to T. (T.) polyringulata sp. nov. However, notable differences persist. Tricoma (T.) timmi consistently exhibits 11 pairs of subdorsal setae (except one paratype with 12 pairs) and 10–18 subventral setae, while T. (T.) polyringulata sp. nov. has 7–9 subdorsal and 8–12 subventral setae. Most T. (T.) timmi specimens contain 13–18 subventral setae (excluding two females), whereas only one paratype male of T. (T.) polyringulata sp. nov. possesses 12 subventral setae.

The two species further differ distinctly in head morphology. Tricoma (T.) polyringulata sp. nov. has a triangular head with cephalic setae peduncles positioned toward the rear, whereas T. (T.) timmi displays setae closer to the head’s center. Moreover, T. (T.) timmi has a non-sclerotized anterior head cuticle, unlike the fully sclerotized head (excluding the labial region) in T. (T.) polyringulata sp. nov., reinforcing its unique taxonomic position.

3.2. Description of Tricoma (Tricoma) fortiseta sp. nov. (Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13; Table 2)

urn:lsid:zoobank.org:act:5617E8A5-EC39–4CFD-B7EA-833E0C295C58

Table 2. Morphometrics of Tricoma (Tricoma) fortiseta sp. nov. in micrometers (µm). d: dorsal side, v: ventral side.

	Males		Females
	Holotype	Paratypes (n = 5)	Paratypes (n = 2)
Total body length	780	753.4 ± 51.5 (700–835)	778.4 ± 38.4 (740–820)
Body rings	v:68 d:69	v: 67.2 ± 1.0 (66–68) d: 67.0 ± 1.3 (65–68)	v: 67.0 ± 2.0 (65–69) d: 67.5 ± 1.5 (66–69)
a	13	11.6 ± 1.3 (9.5–13.0)	10.2 ± 0.7 (9.5–11.0)
b	10	9.4 ± 0.9 (8.5–11.0)	9.4 ± 0.2 (9.0–9.5)
c	7	6.3 ± 0.6 (6.0–7.5)	6.9 ± 0.6 (6.5–7.5)
Head diameter at the level of cephalic setae	23	23.8 ± 0.9 (23.0–25.5)	25.9 ± 1.4 (24.5–27.5)
Head length	17	18.8 ± 0.5 (18.0–20.0)	19.6 ± 0.3 (19.5–20.0)
Body diameter at the level of cardia	51	52.4 ± 5.0 (45.0–61.0)	54.6 ± 2.9 (52.0–57.5)
Maximum body diameter	59	65.8 ± 9.8 (54.5–78.5)	76.6 ± 1.5 (75.0–78.0)
Cephalic setae length	16	18.3 ± 1.3 (17.0–21.0)	21.7 ± 0.9 (21.0–22.5)
Amphideal fovea length	18	17.7 ± 1.3 (16.0–19.5)	17.5 ± 0.2 (17.5–18.0)
Ocelli diameter	22	13.8 ± 2.6 (10.5–17.0)	10.8 ± 0.4 (10.5–11.0)
Ocelli length	29	17.4 ± 2.8 (14.0–22.0)	17.2 ± 0.7 (16.5–18.0)
Anterior end to ocelli	102	101.6 ± 4.7 (96.0–110.0)	91.0 ± 13.3 (80.0–105.0)
Pharynx length	82	80.8 ± 4.4 (75.5–87.5)	82.8 ± 5.9 (77.0–89.0)
Number of subventral setae (Right)	18	18.0 ± 1.1 (17.0–20.0)	18.5 ± 0.5 (18.0–19.0)
Number of subventral setae (Left)	18	17.8 ± 0.7 (17.0–19.0)	18.5 ± 0.5 (18.0–19.0)
Length of the longest subventral setae in the midbody region	19	20.1 ± 1.4 (19.0–22.0)	22.1 ± 1.0 (21.0–23.0)
Length of the shortest subventral setae in the first position	5	6.8 ± 1.6 (5.0–10.0)	4.5 ± 0.3 (4.0–5.0)
Number of subdorsal setae (Right)	10	11.0 ± 0.0 (11.0–11.0)	11.0 ± 0.0 (11.0–11.0)
Number of subdorsal setae (Left)	11	11.0 ± 0.6 (10.0–12.0)	11.0 ± 0.0 (11.0–11.0)
Length of the longest subdorsal setae	18	19.6 ± 1.3 (18.0–22.0)	21.1 ± 1.5 (19.5–22.5)
Length of the shortest subdorsal setae	11	11.9 ± 1.1 (10.5–14.0)	14.5 ± 0.4 (14.0–15.0)
Spicule length	49	47.8 ± 2.9 (44.0–53.0)	-
Gubernaculum length	32	30.7 ± 1.4 (28.5–33.0)	-
Anterior end to vulva	-	-	419.7 ± 35.9 (385–455)
Body diameter at the level of the vulva	-	-	73.4 ± 4.7 (69.0–78.0)
V (%)	-	-	53.8 ± 2.0 (52.0–56.0)
Anal body diameter	40	46.0 ± 2.3 (42.0–48.5)	43.4 ± 1.9 (41.5–45.5)
Tail length	119	119.3 ± 5.1 (110.0–125.0)	112.4 ± 4.0 (110.0–115.0)
Number of tail’s body rings	9	v: 9.8 ± 0.4 (9.0–10.0) d: 10.0 ± 0.6 (9.0–11.0)	8.5 ± 1.5 (7.0–10.0)
Terminal ring length	38	34.6 ± 3.7 (30.0–39.0)	39.4 ± 7.3 (32.0–46.5)
Terminal ring width	11	12.0 ± 1.3 (10.5–14.0)	14.1 ± 1.1 (13.0–15.0)
Desmos covering the terminal ring	16	13.9 ± 1.7 (11.0–15.5)	17.7 ± 5.8 (12.0–23.5)
Phasmata	4	3.0 ± 0.5 (2.5–4.0)	3.6 ± 0.4 (3.0–4.0)

Figure 8. Tricoma (Tricoma) fortiseta sp. nov. Males (A–E,G,H) and female (F,I). (A) Entire view of the male body, lateral view (holotype); (B) head region, lateral view (holotype); (C) head region, lateral view (KIOST NEM-1–2744); (D) posterior body region showing the copulatory system, lateral view (holotype); (E) entire view of the male body, ventral view (KIOST NEM-1–2745); (F) head region of the female, lateral view; (G) head region, ventral view (KIOST NEM-1–2745); (H) posterior tail region, lateral view (KIOST NEM-1–2744); (I) entire view of the female body, lateral view. Scale bars: 50 µm (A,E,I); 10 µm (B–D,F–H).

Figure 9. Tricoma (Tricoma) fortiseta sp. nov. SEM micrographs of the males. (A) Entire view of the body, lateral view; (B) anterior region; (C) head region showing a collapsed apparently due to fixation, thin amphidial fovea; (D) labial region, lateral view; (E) head region, oblique enface view; (F) oblique enface view showing detailed labial region; (G) cephalic setae enclosed by a thin membrane; (H) cephalic setae showing V-shaped terminal opening. Scale bars: 300 µm (A); 50 µm (B); 30 µm (E); 20 µm (C); 5 µm (D,F–H).

Figure 10. Tricoma (Tricoma) fortiseta sp. nov. SEM micrographs of the males. (A) Subdorsal setae showing a terminal pore; (B) subdorsal setae detail; (C) subventral setae detail; (D) subventral setae showing V-shaped terminal pore; (E) detailed posterior body region; (F) posterior body region; (G) cloacal/anal tube and protruding spicules; (H) terminal ring region. Scale bars: 100 µm (F); 30 µm (E,G,H); 10 µm (C); 5 µm (A,B,D).

Figure 11. Tricoma (Tricoma) fortiseta sp. nov. DIC photomicrographs of the holotype male (A–C,F,J) and paratype males. (A) Entire view of the body, right side; (B) head region; (C) amphideal fovea; (D) head region showing teeth and sclerotization, with pharynx muscles dissolved by lactic acid (KIOST NEM-1–2744); (E) anterior region showing laterally inserted subventral setae (indicated by an arrow) (KIOST NEM-1–2745); (F) subventral setae, lateral view; (G) subventral setae, ventral view (KIOST NEM-1–2745); (H) anus region, ventral view (KIOST NEM-1–2745); (I) spicules and gubernaculum are clearly visible after the dissolution of muscles with lactic acid (KIOST NEM-1–2744); (J) posterior region showing spicules and gubernaculum; (K) terminal ring with phasmata (indicated by an arrow) (KIOST NEM-1–2742). Scale bars: 50 µm (A); 10 µm (B–I).

Figure 12. Tricoma (Tricoma) fortiseta sp. nov. SEM micrographs of the females. (A) Entire view of the body, lateral view; (B) anterior region; (C) head region showing labial area, lateral view; (D) cephalic setae; (E) subdorsal setae; (F) subventral setae, lateral view; (G) vulva region (indicated by an arrow); (H) posterior region showing anus (indicated by an arrow). Scale bars: 300 µm (A); 50 µm (H); 30 µm (B); 20 µm (E); 10 µm (D,F); 5 µm (C,G).

Figure 13. Tricoma (Tricoma) fortiseta sp. nov. DIC photomicrographs of the females. (A) Entire view of female body, right side (KIOST NEM-1–2747); (B) head region, left side (KIOST NEM-1–2747); (C) head region (KIOST NEM-1–2746); (D) amphideal fovea (KIOST NEM-1–2747); (E) subventral setae (indicated by an arrow) (KIOST NEM-1–2746); (F) vulva region (KIOST NEM-1–2747); (G) lateral view of anterior region, showing subventral somatic setae (indicated by an arrow) (KIOST NEM-1–2749); (H) vulva region, ventral view (indicated by an arrow) (KIOST NEM-1–2748); (I) anal tube, ventral view (indicated by an arrow) (KIOST NEM-1–2748); (J) posterior body region showing anal tube (indicated by an arrow) (KIOST NEM-1–2746). Scale bars: 50 µm (A); 10 µm (B–J).

3.2.1. Type Material

The holotype male (MABIK NA00158739), mounted on an HS slide with glycerin, is archived at the Marine Biodiversity Institute of Korea (MABIK) in Seocheon, Korea. Additionally, six paratype males (KIOST NEM-1–2740 to KIOST NEM-1–2745) and three paratype females (KIOST NEM-1–2746 to KIOST NEM-1–2748) are deposited in the nematode specimen preservation facility at the Bio-Resources Bank of Marine Nematodes (BRBMN) within the East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.2.2. Type Locality

The nematodes were collected on 24 May 2023, by W. I. Jung, H. J. Lee, and H. S. Rho, from benthic sediments in the subtidal zone of Ulleungdo Island, Gyeongsangbuk-do, Korea, East Sea (37°30′27.83″ N, 130°55′47.12″ E), using a Smith-McIntyre grab.

3.2.3. Etymology

The specific name fortiseta is derived from the Latin “forti-” (meaning “strong” or “powerful”) and “seta” (meaning “bristle” or “setae”), indicating a species characterized by robust subventral setae, which serve as a defining feature of this species.

3.2.4. Measurements

All measurement data are presented in Table 2.

3.2.5. Diagnosis

The newly discovered species, Tricoma (Tricoma) fortiseta sp. nov., can be identified by the following characteristics: (1) a relatively long body measuring 700–835 µm in length; (2) a cuticle with 65–69 tricomoid main rings; (3) a hexagonally shaped head with cephalic setae inserted in the middle; (4) a distinctly demarcated labial region with six lips; (5) cylindrical cephalic setae covered by a thin membrane along their entire length, with bifid tips; (6) round amphidial fovea nearly covering the entire head; (7) somatic setae comprising 10–12 subdorsal setae and 17–20 subventral setae, which are short, thick, and have bifurcated ends; (8) the gubernaculum features a sclerotized corpus and a caudally bent apophysis, which exhibits a slight widening in the middle; and (9) a tail composed of 9–11 main rings in males and 7–10 main rings in females.

3.2.6. Description

Males: The body is relatively slender and elongated, tapering more sharply toward the tail (Figure 8A, Figure 9A and Figure 11A). The cuticle of the holotype male consists of 68 tricomoid rings on the ventral side and 69 on the dorsal side (Figure 8A and Figure 11A). The paratypes exhibit 65 to 68 tricomoid rings, with up to a 1-ring difference between the dorsal and ventral surfaces within the same specimen. The cuticle displays a rounded or triangular morphology and is coated with secretions and microscopic debris (Figure 9B).

When observed from a lateral perspective, the head appears slightly hexagonal (Figure 8B,C, Figure 9C and Figure 11B), tapering in both directions from the cephalic setae. It narrows more noticeably toward the distal end, where it forms a truncated shape measuring 7 µm in width. The labial region is distinctly divided into two sections, which are clearly visible in the SEM images (Figure 9C,E). The anterior labial area is separated from the surrounding region and features two concentric circles: an inner circle outlining the closed oral aperture (oral ridge) and an outer circle divided into six slightly swollen lips, each bearing a central labial papilliform. The posterior cephalic region, characterized by thick ridges formed from the collapsed fovea and referred to as the cephalic concretion, is positioned behind the anterior labial section (Figure 9E,F). In this species, the typical labial ridge observed in the genus Tricoma appears spread out and layered. Under light microscopy, the cephalic concretion appears unclear, with the amphidial fovea predominantly covering the lateral head (Figure 8B,C and Figure 11B).

The head cuticle, except for the labial region, is thick and sclerotized, with firm edges that form a distinct boundary (Figure 8C and Figure 11D). The cephalic setae are relatively thick, tapering from a cylindrical base to a perforated tip, and are covered by a thin membrane along the sides (Figure 9G). The distal opening of the cephalic setae features a V-shaped groove (Figure 9H), giving it a split appearance when viewed from the side (Figure 11B). The setae are inserted on relatively low peduncles in the middle of the head (Figure 9C).

The amphidial fovea is round and almost completely covers the lateral head, extending anteriorly to the lip region and posteriorly to the head boundary (Figure 8B). When viewed ventrally, the amphids appear rounded and bulging on either side of the head (Figure 8G and Figure 11E). The amphideal pore is small and located at the lower part of the circular amphid (Figure 11C). The large cheilostome is cylindrical, with a depth of 2 to 3 µm. When treated with lactic acid and observed laterally, three small, fine teeth are visible within the structure (Figure 11D). The pharynx is cylindrical, slightly constricted around the nerve ring, and widens toward the posterior end (Figure 8E), comprising approximately 9–12% of the total body length. The nerve ring encircles the pharynx between the fourth and fifth rings. The junction between the pharynx and intestine is located at the eighth ring or between the eighth and ninth rings. The cardia is large, cellular, and protrudes into the intestinal lumen. The intestine is a broad, cylindrical structure filled with fine granules.

The ocelli are rounded, large, and dark brown, measuring 11–22 µm in width and 14–29 µm in length. In the holotype, they are situated between the 10th and 12th main rings, while in the paratypes, they are positioned between the 8th and 12th rings. Additionally, smaller pigment spots are present near the posterior end of the pharynx (Figure 8A).

The somatic setae consist of 10–12 subdorsal setae and 17–20 subventral setae, measuring 11 to 22 µm and 5 to 22 µm in length, respectively. The first pair of subventral setae (located on the second, third, or fourth ring) is inserted laterally (Figure 11E). The somatic setae are relatively short and thick, attached to peduncles that slightly protrude from the desmos. They are generally cylindrical, tapering to a perforated tip (Figure 10B,C). When viewed laterally under a light microscope, the setae exhibit grooves along their length, indicating the presence of a lumen inside the setae that opens at the tip (Figure 11F,G). The fine distal opening forms a V-shaped groove (Figure 10A,D), giving the setae a split appearance when viewed from the side (Figure 11F,G). The dorsal and ventral setae are similar in length, with a pair of anterior setae inserted sublaterally that are relatively short. Due to the positioning and angle of the specimens, accurately measuring the setae length can be challenging, and some setae may be broken or damaged. The pattern of somatic setae in the males is described as follows:

		Subdorsal Setae														Subventral Setae
Holotype	left side		7	12	16	21		33	40	46		51	58	64	=10	3	6	9	11	15	18	21		25	29	32	35	39	43	46		51	55	59		65	=18
Holotype	right side		5	11	17	22	28	33	39	45		51	58	64	=11	3	6		12		16	19	22	25	29	32	36	39	43	48		52	56	60		65	=18
Paratype 01	left side		5	9	15	19	24	30	38	44		52	58	62	=11	2	3	7	10	13	16	19	22		26	29	33	37	41	45		49	53	57		62	=18
Paratype 01	right side		4	10	15	21	24	30	38	44		54	58	63	=11	2	4	8	11	14	17	20	23		27	30	34	38	41	46		50	54	57		62	=18
Paratype 02	left side		5	11	15	21	27	34	40	43	46	50	56	62	=12	3	5	8	11	15	18	21	24		28	31	35	39	43	46		50	55	58		63	=18
Paratype 02	right side		5	11	15	21	26	32	38	44		51	57	63	=11	3	4	7	11	14	18	21	24		28	32	35	39	43	47		51	55	58		63	=18
Paratype 03	left side		5	10	16	20	26	32	38	43		49	54	60	=11	2	4	8	12	14	18	21	24		27	30	34	36	40	44		47	51	56		62	=18
Paratype 03	right side		4	11	15	21	28	32	38	46		50	54	61	=11	2	4	8	12	15	18	22	25		28	31	34	37	41	45		49	52	56		61	=18
Paratype 04	left side		5	11	15	20	26	32	39	44		50	56	61	=11	2	4	7	10	13	16	19	22	25	28	31	34	39	42	46		50	54	58		63	=19
Paratype 04	right side		5	11	16	21	26	32	38	42		49	55	62	=11	2	5	9	12		16	19	22	25	28	31	35	38	42	46		49	53	57		63	=18
Paratype 05	left side	3	5			20	24	31	38	44		52	57	63	=10	2	4	8	11	14	17	20	23		26	30	34	37	41	45		50	54	59		64	=18
Paratype 05	right side		5	10	15	21	27	32	39	44		52	57	63	=11	2	4	8	10	14	17	20	23		26	30	33	36	40	44	46	50	54	57	60	64	=20

The reproductive system follows the typical structure observed in the genus Tricoma. The seminal vesicle contains small granules. In the holotype, the spicules protrude from the body wall between the 59th and 60th rings on the ventral side (Figure 8D). In the paratypes, they are observed protruding between the 58th and 60th rings (Figure 10F), with only one individual showing protrusion between the 56th and 57th rings. The anal tube is relatively prominent (Figure 10G) and appears as a broad oval when viewed from the ventral side (Figure 11H). The spicules are strongly sclerotized and arch-shaped, measuring 44–53 µm in length (Figure 8H). They taper distally (Figure 10E) and have a proximally offset capitulum (Figure 8D and Figure 11J). The gubernaculum features a thin, double-walled, sclerotized distal section running parallel to the spicules, while the apophysis is weakly sclerotized. The dorsocaudally bent apophysis exhibits a slight widening in the middle (Figure 11I).

The tail consists of 9 rings in the holotype and 9–11 rings in the paratypes. The terminal ring is cylindrical, with 34–50% of its anterior portion covered by desmos (Figure 10H). The posterior part of the terminal ring appears smooth under DIC; however, SEM observations reveal that nearly half of the area not covered by desmos is instead adorned with prominent granular projections (Figure 10H). A round phasmata, 2–4 µm in diameter, is situated within the desmos of the terminal ring (Figure 11K), although it is difficult to observe in some specimens. Three well-developed caudal glands are present.

Females: Most characteristics are similar to those observed in the males (Figure 8F,H, Figure 12A–F and Figure 13A–E,G). The cuticle of the paratype female consists of 69 tricomoid rings, while another paratype female has 66 dorsal and 65 ventral tricomoid rings. The somatic setae are arranged in a pattern similar to that of the males, with the anterior pair of setae inserted sublaterally. There are 11 subdorsal setae and 18–19 subventral setae on each side. The arrangement of the somatic setae in the paratype females is as follows:

		Subdorsal Setae												Subventral Setae
Paratype 06	left side	6	10	14	20	26	32	37	46	50	55	61	=11	3	4	8	12	15	18		22	25	29	32	36	40	42	46	50	53	57	63	=18
Paratype 06	right side	5	10	15	22	28	32	38	44	50	56	62	=11	2	4	8	11	13	17	19	23	25	29	33	36	39	42	46	50	53	56	63	=19
Paratype 07	left side	6	10	15	19	25	29	37	45	49	56	61	=11	2	4	8	12	15	19	22	26	29	32	5	37	40	43	48	52	55	59	64	=19
Paratype 07	right side	6	11	16	22	27	33	39	45	51	56	62	=11	3	4	8	12		17	20	25	28	31	34	37	41	44	47	51	55	58	64	=18

The reproductive system is typical, with both branches outstretched and opposed. The vulva is situated on a slightly protruding, tube-like cuticular structure. It is positioned along the midline of the body wall, located between rings 39 and 40 in one female (Figure 13F) and between rings 37 and 38 in the other. When viewed ventrally, the vulva appears as an oval-shaped, wrinkled surface (Figure 12G and Figure 13H). The spermathecae contains a mix of large and small amorphous inclusions on both sides. The anal tube is a short structure, measuring 3–4 µm in length, and protrudes from the body wall between rings 58 and 60 (Figure 13I,J). The tail of one paratype female consists of 10 rings, while the other has 7 rings. The terminal ring is cylindrical, with 37–50% of its anterior portion covered by desmos (Figure 12H and Figure 13J). The round phasmata, measuring 3–4 µm in diameter, is situated within the desmos of the terminal ring.

3.2.7. Differential Diagnosis

Tricoma (Tricoma) fortiseta sp. nov. has 65–69 main rings, placing it within the range of species that have 61–75 main rings, as proposed by Decraemer (1978) [13]. Based on the number of main rings, 24 species fall into this group, and a list of these species is presented in Table 3. To aid in species classification and morphological comparison, this study provides a comparative table and an illustrated identification key that highlights the distinguishing features of species with 61–75 main rings (Figure 14, Table 3). The data presented in the comparative table compile all available information from the literature published to date. Among these species, T. (T.) septuaginta Schuurmans Stekhoven, 1942, represents a unique case [30]. This species was initially described based on two female specimens collected from the Bay of Majorca. Later, Timm (1970) identified a single male specimen from Isla Santa Cruz in the Galapagos Islands as T. (T.) septuaginta [6]. However, Decraemer (1978) raised doubts about this identification, questioning whether Schuurmans Stekhoven’s female specimens and Timm’s male specimen truly belonged to the same species [13]. Decraemer (1978) subsequently recorded a male and a female specimen from Yonge Reef and Nymph Island in Australia, which she identified as T. (T.) septuaginta Schuurmans Stekhoven sensu Timm (1970) due to their strong resemblance to Timm’s male specimen [13]. Although we recognize these unresolved taxonomic ambiguities, this paper provisionally provides comprehensive information on all these specimens.

Tricoma (Tricoma) fortiseta sp. nov. is characterized by a hexagonal head in the lateral view, with cephalic setae inserted in the middle of the head. Species sharing similar features within the 61–75 main ring group include T. (T.) cylindricauda Chitwood, 1936; T. (T.) dimorpha Decraemer, 1978; T. (T.) parvaspiculata Decraemer, 1987; T. (T.) spinosa Chitwood, 1951; T. (T.) spinosoides Chitwood, 1951; and T. (T.) steineri de Man, 1922 [13,24,27,33,34]. A detailed comparison of various taxonomic features shows that T. (T.) cylindricauda and T. (T.) dimorpha are the most morphologically similar to T. (T.) fortiseta sp. nov. in terms of general body shape, number of main rings, somatic setae patterns, and the shape of the spicules and gubernaculum [13,33].

However, T. (T.) fortiseta sp. nov. can be distinguished from T. (T.) cylindricauda by its longer body length (700–835 µm vs. 476–500 µm in T. (T.) cylindricauda), differences in de Man’s indices (with b being 9–11 compared to 5.4–6.3, and c being 6–8 compared to 5 in T. (T.) cylindricauda). Additionally, T. (T.) fortiseta exhibits cephalic and somatic setae with bifurcated tips and a distinctly demarcated labial region, features not observed in T. (T.) cylindricauda (as redescribed by Timm [6]).

Tricoma (T.) fortiseta sp. nov. closely resembles T. (T.) dimorpha in head shape, particularly in the distinctly demarcated labial region with six lips. However, it can be differentiated by its longer body length (700–835 µm vs. 360–600 µm), a higher number of main rings (65–69 vs. 62–65), fewer tail rings in males (9–11 vs. 12), and the presence of relatively short, bifurcated somatic setae compared to the body diameter. Additionally, T. (T.) dimorpha exhibits sexual dimorphism in the length and insertion point of the third and fourth pairs of ventral setae, a feature absent in T. (T.) fortiseta sp. nov. These morphological distinctions clearly separate T. (T.) fortiseta sp. nov. from T. (T.) dimorpha and other related species, confirming its status as a distinct new species.

3.3. Description of Tricoma (Tricoma) uljinensis sp. nov. (Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19; Table 4)

urn:lsid:zoobank.org:act:677E54E0–1C0F-40F9–8522-99F702A07BCD

Table 4. Morphometrics of Tricoma (Tricoma) uljinensis sp. nov. in micrometers (µm). d: dorsal side, v: ventral side.

	Males		Females
	Holotype	Paratypes (n = 5)	Paratypes (n = 2)
Total body length	630	615 ± 24.2 (570–640)	695.6 ± 15.1 (680–710)
Body rings	68	68.4 ± 0.8 (67.0–69.0)	69.5 ± 0.5 (69.0–70.0)
a	13	13.5 ± 0.7 (12.5–14.5)	13.4 ± 0.6 (13.0–14.0)
b	7	8.0 ± 0.4 (7.5–8.5)	8.8 ± 0.5 (8.5–9.5)
c	7	7.1 ± 0.2 (7.0–7.5)	8.1 ± 02 (8.0–8.5)
Head diameter at the level of cephalic setae	26	25.1 ± 1.0 (23.5–26.0)	27.9 ± 1.0 (27.0–29.0)
Head length	16	16.8 ± 0.7 (16.0–18.0)	17.9 ± 0.5 (17.5–18.5)
Body diameter at level of cardia	43	42.1 ± 2.7 (39.0–45.5)	46.3 ± 0.5 (46.0–47.0)
Maximum body diameter	48	45.9 ± 3.2 (41.5–51.0)	52.1 ± 1.0 (51.0–53.0)
Cephalic setae length	18	17.0 ± 1.3 (15.0–19.0)	17.0 ± 0.4 (16.5–17.5)
Amphideal fovea length	13	15.1 ± 0.7 (14.0–16.0)	16.0 ± 0.8 (15.0–17.0)
Ocelli diameter	7	7.3 ± 2.3 (3.5–9.5)	6.4 ± 0.5 (6.0–7.0)
Ocelli length	11	10.0 ± 2.9 (5.5–15.0)	11.6 ± 2.0 (9.5–13.5)
Anterior end to ocelli	97	93.2 ± 5.8 (87.0–104.0)	81.8 ± 17.4 (64.5–99.0)
Pharynx length	92	77.4 ± 6.0 (69.5–85.5)	79.5 ± 6.5 (73.0–86.0)
Number of subventral setae (right)	17	15.6 ± 1.9 (13.0–18.0)	16.0 ± 0.0 (16.0–16.0)
Number of subventral setae (left)	17	16.2 ± 0.7 (15.0–17.0)	16.0 ± 1.0 (15.0–17.0)
Length of the longest subventral setae in the anterior position	20	21.0 ± 1.0 (19.5–22.5)	21.7 ± 0.6 (21.0–22.5)
Length of the shortest subventral setae in the first position	6	6.6 ± 0.8 (5.5–7.5)	7.8 ± 0.6 (7.0–8.5)
Number of subdorsal setae (Right)	11	10.8 ± 0.4 (10.0–11.0)	10.5 ± 0.5 (10.0–11.0)
Number of subdorsal setae (Left)	11	10.2 ± 1.0 (9.0–11.0)	10.5 ± 0.5 (10.0–11.0)
Length of the longest subdorsal setae	22	19.2 ± 1.6 (16.5–21.0)	22.9 ± 1.2 (21.5–24.0)
Length of the shortest subdorsal setae	8	8.7 ± 1.1 (7.0–10.0)	10.5 ± 0.5 (10.0–11.0)
Spicule length	39	39.7 ± 2.4 (36.5–43.5)	-
Gubernaculum length	22	24.0 ± 2.3 (20.0–27.0)	-
Anterior end to vulva	-	-	385.1 ± 11.8 (340–395)
Body diameter at the level of the vulva	-	-	50.3 ± 2.1 (48.0–52.5)
V (%)	-	-	54.8 ± 0.5 (54.5–55.0)
Anal body diameter	41	38.0 ± 2.2 (34.5–41.0)	41.2 ± 0.1 (41.0–41.5)
Tail length	89	87.2 ± 5.2 (78.5–93.5)	85.5 ± 0.1 (85.0–85.5)
Number of tail’s body rings	8	8.0 ± 0.6 (7.0–9.0)	8.1 ± 0.0 (8.0–8.0)
Terminal ring length	27	26.9 ± 1.2 (25.5–28.5)	26.2 ± 0.7 (25.5–27.0)
Terminal ring width	12	12.1 ± 0.4 (11.5–12.5)	11.5 ± 0.6 (11.0–12.0)
Desmos covering the terminal ring	9	8.7 ± 1.4 (6.5–10)	9.0 ± 0.1 (8.5–9.0)
Phasmata	2	2.9 ± 0.7 (2.0–3.5)	2.8 ± 0.1 (2.5–3.0)

3.3.1. Type Material

The holotype male (MABIK NA00158740), mounted on an HS slide with glycerin, is archived at the Marine Biodiversity Institute of Korea (MABIK) in Seocheon, Korea. Additionally, six paratype males (KIOST NEM-1–2750 to KIOST NEM-1–2755) and two paratype females (KIOST NEM-1–2756, KIOST NEM-1–2757) have been deposited in the nematode specimen preservation facility at the Bio-Resources Bank of Marine Nematodes (BRBMN) within the East Sea Research Institute, Korea Institute of Ocean Science & Technology (KIOST), Korea.

3.3.2. Type Locality

The nematodes were collected on 13 October 2023, by W. I. Jung and H. S. Rho, from a coralline algae bed in the subtidal zone of the East Sea, Uljin-gun, Gyeongsangbuk-do, Korea (37°04′19.55″ N, 129°25′00.55″ E), using SCUBA diving techniques.

3.3.3. Etymology

The chosen specific name, ‘uljinensis’, is derived from the type locality, Uljin.

3.3.4. Measurements

All measurement data are presented in Table 4.

3.3.5. Diagnosis

Tricoma (Tricoma) uljinensis sp. nov. is characterized by the following combination of features: (1) body length ranging from 570 to 640 µm in males and 680 to 710 µm in females; (2) cuticle with 67–70 tricomoid main rings; (3) head gently triangular in shape, with a width approximately 1.4–1.6 times greater than the length and a narrow posterior region covered by a thin layer of foreign material; (4) cylindrical cephalic setae covered along their entire length by a relatively broad, thin membrane; (5) round amphidial fovea almost entirely covering the head; (6) somatic setae consisting of 9–11 subdorsal setae and 13–18 subventral setae, which are relatively long and slender; (7) gubernaculum with a thin, hardened double wall at its distal end, dorsocaudally apophysis at the midpoint of its length; (8) tail composed of 7–9 main rings; and (9) terminal ring conical, with the anterior 25–39% covered by relatively thick desmos.

3.3.6. Description

Males: The body is relatively thick and tapers toward the extremities, especially at the tail region (Figure 15A, Figure 16A and Figure 17A). The cuticle of the holotype male consists of 68 tricomoid rings, while the paratypes display 67–69 tricomoid rings, with one individual showing a 1-ring discrepancy between the dorsal and ventral sides. The cuticular structure of each ring is covered by an opaque desmos that contains coarse foreign material (Figure 16B).

The head tapers gently anteriorly from the base of the cephalic setae, ending in a broadly truncated form with a width of 6–8 µm. It is 1.4–1.6 times wider than it is long, and the cuticle, except in the short labial region, consists of distinctly different layers of sclerotized material, with the outer covering layer being very thin (Figure 15B and Figure 17B,E). The labial region is a thin-walled structure, apparently forming a ridge, with the labial sensilla being difficult to discern. The lips appear to be covered by a granular substance (Figure 16D). A narrow posterior zone of the head is covered by a thin layer of foreign material (Figure 15B, Figure 16C and Figure 17C). The cephalic setae are cylindrical at the base, tapering toward the tip, and are covered along their entire length by a relatively broad, thin membrane. They are inserted on peduncles positioned slightly posteriorly on the head. The vesicular amphidial fovea is circular with thickened borders, creating a double-ring appearance when viewed laterally and almost entirely covering the head (Figure 17C). The amphideal pore opens through a small groove in the sclerotized posterior part of the head.

Figure 15. Tricoma (Tricoma) uljinensis sp. nov. Males (A,B,D,E) and female (C,F). (A) Entire view of the male body, lateral view (holotype); (B) detailed head region (holotype); (C) detailed head region; (D) posterior body region showing the copulatory system (holotype); (E) posterior body region (KIOST NEM-1–2754); (F) entire view of the female body. Scale bars: 50 µm (A,F); 10 µm (B–E).

Figure 16. Tricoma (Tricoma) uljinensis sp. nov. SEM micrographs of the male. (A) Entire view of the body, lateral view; (B) cuticle of the midbody region; (C) head region, lateral view; (D) head region showing the labial rim; (E) subdorsal setae; (F) subventral setae; (G) spicules region; (H) posterior body region. Scale bars: 200 µm (A); 50 µm (B); 30 µm (C,H); 10 µm (D–G).

Figure 17. Tricoma (Tricoma) uljinensis sp. nov. DIC photomicrographs of the holotype male (A–C,G,H) and paratype males (D–F,I). (A) Entire view of the body, right side; (B) head region, lateral optical section; (C) amphideal fovea; (D) head region showing sclerotized teeth (KIOST NEM-1–2750); (E) head region showing sclerotized teeth after treatment with lactic acid and pharynx muscle dissolution (indicated by an arrow) (KIOST NEM-1–2755); (F) spicules and gubernaculum are clearly visible following the dissolution of the muscles by lactic acid (KIOST NEM-1–2755); (G) subdorsal setae, lateral view; (H) posterior body region showing spicules and gubernaculum; (I) terminal ring with phasmata (indicated by an arrow) (KIOST NEM-1–2752). Scale bars: 50 µm (A); 10 µm (B–G).

The depth of the stoma is approximately 3–4 µm. In the lateral view, three small, fine teeth are observable within the structure (Figure 17D). The pharynx is cylindrical, with the pharynx–intestinal junction situated at the anterior end of the 11th ring in the holotype and between the 8th and 10th rings in the paratypes. The cardia is small and cellular, protruding into the intestine. The intestine is cylindrical and filled with fine granules. The ocelli are brown, round, and measure 4–12 µm in diameter and 6–15 µm in length. In the holotype, the ocelli are located at the 12th ring, while in the paratypes, they are observed between the 10th and 13th rings.

The somatic setae are relatively long and slender, tapering to a perforated tip (Figure 16E,F). When observed laterally under a light microscope, they appear to have a lumen with an open tip (Figure 17G). The somatic setae consist of 9–11 subdorsal setae and 13–18 subventral setae, measuring 7–22 µm and 5–24 µm in length, respectively. The first pair of setae, situated on rings two, three, or four, show a slight lateral displacement. While the length of the somatic setae is generally consistent, they become relatively shorter toward the posterior region near the cloaca. The anterior pair of subventral setae, inserted laterally, measure approximately 5–7 µm, making them significantly shorter. It should be noted that accurate length measurements may vary due to the position or angle of the body, and some setae may be broken or damaged. The somatic setae pattern of the males is as follows:

		Subdorsal Setae												Subventral Setae
Holotype	left side	7	12	17	22	29	35	43	50	54	59	63	=11	4	5	8	12	16		20	24	28	32	37	41	45	48	52	58	62	65	=17
Holotype	right side	8	12	18	22	29	35	42	48	53	59	64	=11	3	5	8	12	15		20	23	28	33	37	41	46	50	54	59	62	64	=17
Paratype 01	left side		12		22	28	38	43	49	55	60	64	=9	3	4	7		16		20	24	29	35		39	44	50	54	58	63	65	=15
Paratype 01	right side	5	11	18	22		35	43	47	54	61	65	=10					15		19	23	28	33	37	41	47	51	55	59	63	65	=13
Paratype 02	left side	6	10	15	21	27	33	43	51	55	61	64	=11	3	5	8	12	16		20	23	28	33	38	42	47	51	55	59		64	=16
Paratype 02	right side	5	11	17	23	26	36	42	48	53	60	64	=11	3	5	8		16			24	29	34	38	42	46		54	58	61	64	=14
Paratype 03	left side	6	11	16	21	27	36	43	49	53	58	63	=11	4	6	10	14		18	21	24	28	33	38	42	46	50	54	58	62	66	=17
Paratype 03	right side	5	11	17	22	29	33	40	47	51	58	62	=11	4	6	10	13		17	20	24	28	33	38	42	45	49	53	58	63	65	=17
Paratype 04	left side	7	12	16	22	27	35	42	49	54	60	64	=11	3		8	12	15		22	25	30	33	37	42	46	50	54	59	62	65	=16
Paratype 04	right side	5	11	17	24	28	35	42	48	53	60	65	=11	3	4	7	11	15	18	22	25	29	33	37	41	46	49	54	59	63	66	=18
Paratype 05	left side	6	12	18	22	27	35	43	47	53	60	63	=11	2	5		10	13	17	21	25	29	33	38	42	46	50	54	57	62	64	=17
Paratype 05	right side	6	11	17			32	41	47	54	60	63	=9	2	5	10		14	17	21	25	29	34	37	42	46	50	54		61	64	=16

The reproductive system is typical, featuring a large ejaculatory gland. The spicules are slightly arched, measuring 37–44 µm in length, and are characterized by thick sclerotization, tapering toward the distal end (Figure 16G), with an offset capitulum at the proximal end (Figure 15D). The gubernaculum measures 20–27 µm in length, with a double wall running parallel to the spicules at the distal end and weak sclerotization at the proximal end. The apophysis, positioned at the midpoint of the gubernaculum, is dorsocaudally oriented (Figure 17F,H) and, in some individuals, exhibits slightly widened projections on either side of the midsection of its proximal part (Figure 15E).

The tail consists of 8 rings in the holotype and 7–9 rings in the paratypes. The terminal ring is conical, tapering toward the terminal spinneret. The anterior 25–39% of the terminal ring’s cuticle is covered with thick desmos (Figure 16H). The phasmata are round and located on the desmos of the terminal ring (Figure 17I). Three well-developed caudal glands are present.

Females: Apart from sexual characteristics, most features are identical to those of the males (Figure 15C,F, Figure 18A–D and Figure 19A–C). The cuticle of the paratype females consists of 69–70 tricomoid rings. The cuticular zone of each main ring features secondary annulation and is covered with secretions and coarse foreign material. The somatic setae are arranged similarly to those of the males, with 10–11 subdorsal setae and 15–17 subventral setae on both sides (Figure 18E,F). The arrangement of somatic setae in the paratype females is as follows:

		Subdorsal Setae												Subventral Setae
Paratype 06	left side	7	11	16	21	28	35	43	50	54	61	64	=11	4	5		11	15	19	23	26	31	36	40	45	49	54	58	62	65	=16
Paratype 06	right side	6	12	15	24	29	33	41		54	60	64	=10	3	4	8	12	16	19	22	26	31	36	40	45	49	53	58	62	65	=17
Paratype 07	left side	7	10	15	21	26	35	42		54	61	65	=10	4	5	7	12	15	19	24	28	32	37	42	47	51	56	61	66		=16
Paratype 07	right side	6	11	16	23	28	34	42	47	54	61	65	=11	3		8	13	17	21	25	28	33	37	42	46	51	56	60	66		=15

The reproductive system is typical, with both branches extending in opposite directions. The uterus contains both large and small amorphous inclusions. The vulva is located on the midline of the ventral body wall at the 39th ring (Figure 19D) and appears as a wrinkled surface (Figure 18G). The anal tube is 5 µm in length and is positioned between the 61st and 62nd rings (Figure 19E). The tail consists of eight rings. The terminal ring is conical, with the anterior 33–36% covered by desmos (Figure 18H). The phasmata are circular, 3 µm in width, and are situated on the desmos of the terminal ring. Three well-developed caudal glands are present.

Figure 18. Tricoma (Tricoma) uljinensis sp. nov. SEM micrographs of the female. (A) Entire view of the body, lateral view; (B) anterior region, lateral view; (C) head region showing labial area; (D) cephalic setae covered by a thin membrane; (E) subdorsal setae; (F) subventral setae; (G) vulva region (indicated by an arrow); (H) posterior body region. Scale bars: 200 µm (A); 30 µm (B,H); 20 µm (C,F); 10 µm (D,E,G).

Figure 19. Tricoma (Tricoma) uljinensis sp. nov. DIC photomicrographs of the paratype female (KIOST NEM-1–2756). (A) Entire view female body, right side; (B) head region, optical section; (C) amphidial fovea, surface view; (D) naked vulva region (indicated by an arrow); (E) posterior body region showing anal tube (indicated by an arrow). Scale bars: 50 µm (A); 10 µm (B–E).

3.3.7. Differential Diagnosis

Tricoma (Tricoma) uljinensis sp. nov. is characterized by having 65–69 main rings and a strongly sclerotized head, with a width approximately 1.5 times greater than the length. Among species with 61–75 main rings, as outlined by Decraemer (1978) [13], those with a similar head shape include T. (T.) amydramphida Timm, 1970; T. (T.) brevispicula Decraemer, 1978; T. (T.) denticulata Timm, 1970; T. (T.) oblita Blome, 1982; T. (T.) platapophysa Decraemer, 1978; T. (T.) platycephala Filipjev, 1922; T. (T.) reimani Decraemer, 1978; and T. (T.) spuria Inglis, 1967 [6,13,33,34,35,36,37,38].

Among these species, T. (T.) amydramphida, T. (T.) brevispicula, T. (T.) denticulata, T. (T.) platapophysa, and T. (T.) platycephala can be clearly distinguished from T. (T.) uljinensis sp. nov. by the structure of the male copulatory system. Tricoma (T.) uljinensis sp. nov. is characterized by a strongly downward-curved apophysis of the gubernaculum near its midpoint, whereas these species have a shorter or upward-curved gubernaculum near the midpoint. Additionally, T. (T.) oblita and T. (T.) spuria possess 60–63 main rings and a typical triangular head, while T. (T.) uljinensis sp. nov. is distinguished by having 67–70 main rings and a slightly gentler sloped triangular head.

Tricoma (T.) uljinensis sp. nov. shows similarities with T. (T.) riemanni in body length, number of main rings, arrangement of somatic setae, and the structure of the male copulatory apparatus, as revealed through comprehensive taxonomic character analysis. However, several distinguishing features set them apart. Tricoma (T.) riemanni has a relatively thinner body (with a de Man’s a index of 17–22, compared to 12–13 in T. (T.) uljinensis sp. nov.), and the number of tail rings is also different: T. (T.) riemanni has 9–10 rings in both males and females, while T. (T.) uljinensis sp. nov. has 7–9 rings in males and 8 rings in females. Additionally, T. (T.) riemanni has a cylindrical terminal ring, whereas T. (T.) uljinensis sp. nov. possesses a slightly conical terminal ring. Notably, T. (T.) riemanni displays a lateral displacement of the eighth pair of subdorsal setae, a feature absent in T. (T.) uljinensis sp. nov. Lastly, T. (T.) riemanni presents a distinctly triangular head, in contrast to the more gently sloping triangular head shape of T. (T.) uljinensis sp. nov., providing a clear basis for distinguishing between the two species.

4. Discussion

The genus Tricoma was first established by Cobb in 1894 and has undergone revisions by several researchers over the years [6,7,13,39]. Decraemer (1978) notably classified Tricoma species into six groups based on the number of main rings, a criterion that has proven to be highly effective for species differentiation based on similarity. While the number of main rings remains a key diagnostic characteristic for identifying Tricoma species, some taxa show variability in this feature, making it an important factor to consider when evaluating intraspecific variation [11].

In particular, species within the genus Tricoma exhibit high species diversity but relatively low individual abundance. As a result, many taxa have been described based on limited sample sizes, which has constrained the collection of morphological and morphometric data [11]. To address this bottleneck in classification, a more comprehensive approach is needed. Studies should include a broader range of samples and geographical regions to better account for intraspecific variation. Diagnostic features such as the number of main rings, tail rings, and somatic setae patterns exhibit significant variability within species, and this variation must be considered for accurate species differentiation. For instance, T. (T.) polyringulata sp. nov. shows considerable variation in the number of main rings, ranging from 75 to 89, which serves as a key indicator for assessing intraspecific variability. The presence of the uncovered first main ring is another critical feature for species identification in Tricoma, first highlighted by Decraemer (1987) and Soetaert and Decraemer (1989), but not adequately emphasized in previous studies. In this study, we propose a list of 14 species that may have an uncovered first main ring, contributing to new species classification and identification efforts (Figure 7). This discovery also suggests the possibility of identifying new diagnostic features within this genus. In addition to previously recognized features, characteristics such as the presence of the uncovered first main ring, the curvature of the gubernaculum, head shape, position of the cephalic setae, form of the amphidial fovea, presence of ejaculatory glands in males, and the presence of teeth in the buccal cavity should also be considered important for differentiating Tricoma species.

Moreover, many species within the genus Tricoma have often been reported based solely on a single specimen or illustrations of the holotype. As a result, subtle variations in morphological characters may be overlooked, whether due to differences among individuals within a species or slight alterations caused by the angle of observation under a microscope after specimen preparation. For instance, in the case of T. (T.) fortiseta sp. nov., as shown in Figure 8D,H, the degree of curvature of the gubernaculum, a male reproductive structure, can vary depending on the individual or the angle of observation, potentially affecting accurate species recognition. Similarly, for T. (T.) polyringulata sp. nov., as described in Figure 3B,D, contraction during specimen preparation may cause the first main ring to appear uncovered. To address these issues, this study provides illustrations and DIC photographs not only of the holotype but also of several paratype specimens.

Despite various attempts to identify morphological variation, a significant issue in species differentiation remains the lack of molecular data due to insufficient sample sizes. Molecular data play a vital role in clearly defining species boundaries, and this is an area that needs to be addressed in future studies. Relying solely on morphological characteristics has often proven insufficient for reliable species identification, and resolving this issue will require combining morphological data with molecular evidence to define species more accurately.

The use of scanning electron microscopy (SEM) in this study significantly enhanced our ability to observe fine morphological details in Tricoma species. For example, while Tricoma nematodes possess important features such as labial sensilla, these characteristics are often difficult to observe under light microscopy due to the thick cuticular rings in Desmoscolecidae. SEM allowed us to closely examine the ultrastructure of the lip region, revealing previously overlooked morphological features. Specifically, T. (T.) polyringulata exhibited distinctive granular projections in the lip region, T. (T.) fortiseta displayed a bilayered lip region, and T. (T.) uljinensis showed a single lip margin. These findings suggest considerable morphological variation in the lip region within the genus Tricoma, which could be assessed more precisely using SEM to observe the structure, arrangement, and presence of projections. Additionally, the impact of specimen fixation on the integrity of morphological features should not be underestimated. For example, SEM images of thin-walled structures like the amphidial fovea can be influenced by the fixation process. This can introduce structural distortions or masking effects that may affect species identification. To address this, future studies should focus on collecting more samples and refining fixation techniques to improve our understanding of the morphological and taxonomic relationships within Tricoma.

In conclusion, this study emphasizes the need for careful consideration of variability in the diagnostic features of Tricoma species and demonstrates that reliable species identification is achievable through sufficient specimen collection. It also emphasizes the need to consider both morphological variation, such as the number of main rings, and molecular evidence in the identification of Tricoma species. By combining morphological analysis with molecular data, we can improve the accuracy of species identification within the genus, which will contribute significantly to future taxonomic research.

5. Conclusions

This study provides a detailed taxonomic analysis of three new Tricoma species discovered along the coast of Uljin and around Ulleungdo Island in the East Sea, Korea. It emphasizes the importance of considering variations in diagnostic features, such as the number of main rings and the pattern of somatic setae. Tricoma (T.) polyringulata sp. nov. is characterized by its small body size and uncovered first main ring, T. (T.) fortiseta sp. nov. features a hexagonal head with six lips and thick, bifid cephalic setae, and T. (T.) uljinensis sp. nov. is distinguished by a gently sloping triangular head that is 1.5 times wider than its length and a gubernaculum that curves downward near its midpoint. Our findings underscore the importance of examining multiple specimens to account for intraspecific variability and avoid misinterpretations arising from limited sample sizes. The integration of illustrations, DIC, and SEM images for both the holotype and paratype specimens provides a more comprehensive approach, addressing the shortcomings of prior studies that often relied on single or insufficient specimens. Particularly noteworthy is the emphasis on the diagnostic value of the uncovered first main ring, a feature that has been underappreciated in previous taxonomic efforts. By proposing a list of 14 species presumed to exhibit this characteristic, this study paves the way for refined identification criteria and future taxonomic research. Moreover, this research highlights the necessity of revisiting overlooked diagnostic features, including head morphology, amphidial fovea structure, somatic setae arrangement, and gubernaculum details, to establish more reliable species distinctions. The variability observed in key traits, such as the number of main rings and head features, further underscores the importance of comprehensive morphological analyses and the inclusion of sufficient specimens. In addition, SEM analysis has revealed fine-scale structural details, such as lip morphology and cephalic sensilla arrangements, that were previously unobservable with conventional optical methods. In conclusion, this study enhances our understanding of Tricoma species diversity in the Northwest Pacific and contributes to the broader taxonomic framework of the genus. It calls for future research to combine robust morphological approaches with molecular data, ensuring accurate delineation of species boundaries. Such efforts are vital for capturing the full extent of biodiversity within Tricoma and advancing marine nematode taxonomy.

Author Contributions

Data curation and writing—original draft preparation., H.J.L.; investigation, H.L.; 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 Marine Fishery Bio-resources Center Management (2024), funded by the National Marine Biodiversity Institute of Korea (MABIK) (PG54100). Additional support was provided by the research projects “Development of Original Technology to Identify Factors Affecting Barren Grounds on the East Sea Coast under Climate Change (PEA0205)” and “Enhancing the Capacity for Analyzing and Assessing Marine Environment and Ecosystem Variability in the Waters Surrounding the Korean Peninsula (PEA0201)” at the Korea Institute of Ocean Science & Technology (KIOST).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

All data are available in the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Map of sampling locations: Yellow triangle markers indicate the sampling sites (ST. 01: Ulleungdo Island, 37°31′39.22″ N, 130°48′15.32″ E; ST. 02: Ulleungdo Island, 37°30′27.83″ N, 130°55′47.12″ E; ST. 03: Uljin, 37°04′19.55″ N, 129°25′00.55″ E).

Figure 7. Pictorial key depicting the head morphology of species presumed to have an uncovered first main ring. Source for figures: (A) Soetaert and Decraemer (1989); (B) Lee, Lee, and Rho (2023); (C) Soetaert and Decraemer (1989); (D) Decraemer (1978); (E) Decraemer (1987); (F) Lee, Lee, and Rho (2023); (G) Soetaert and Decraemer (1989); (H) Decraemer (1978); (I) Soetaert and Decraemer (1989); (J) Timm (1970); (K) Timm (1970); (L) Juario (1974); (M) Decraemer (1979); (N) Decraemer (1978) [6,11,13,24,25,26,28].

Figure 14. Pictorial key to the species group with 61–75 main rings in the subgenus Tricoma. Source for figures: (A) Timm (1970); (B) Timm (1970); (C) Timm (1970); (D) Decraemer (1978); (E) Decraemer (1983); (F) Chitwood (1936); (G) Timm (1970); (H) Decraemer (1978); (I) this study; (J) Steiner (1916); (K) Freudenhammer (1975); (L) Decraemer (1983); (M) Timm (1970); (N) Blome (1982); (O) Decraemer (1987); (P) Timm (1970); (Q) Decraemer (1978); (R) Filipjev (1922); (S) Decraemer (1978); (T) Timm (1978); (U) Timm (1970); (V) Chitwood (1951); (W) Chitwood (1951); (X) Decraemer (1986); (Y) Decraemer (1979); (Z) this study [6,7,13,24,28,31,32,33,34,35,36,37].

Table 3. Comparison of mainly morphometric data among species groups with 61–75 main rings in the subgenus Tricoma. Morphometric values are rounded, and dashes (-) indicate unknown measurements.

Species	Characters
	Males			Females			Head Diameter	Head Length	Cephalic Setae Length	Spicules Length	Gubernaculum Length	Vulva (Ring)	Number of Tail Rings (Males)	Number of Tail Rings (Females)
	Body Length	Body Rings	Setae Pattern (sd/sv)	Body Length	Body Rings	Setae Pattern (sd/sv)	Head Diameter	Head Length	Cephalic Setae Length	Spicules Length	Gubernaculum Length	Vulva (Ring)	Number of Tail Rings (Males)	Number of Tail Rings (Females)
T. (T.) adelpha (Greeff, 1869)	-	-	-	200–325	70–72	9/10	16	16	16	-	-	38	-	12
T. (T.) amydramphida Timm, 1970	600–860	70–74	7–8/16–17	490	75	8/12–13	21–33	19–21	16	59–65	-	40	10	10
T. (T.) antarctica Timm, 1970	-	-	-	1330	72	4–8/5–9	55	30	15	-	-	-	-	-
T. (T.) brevispicula Decraemer, 1978	290	68	10/11–12	-	-	-	15	11	16	16	10	-	9	-
T. (T.) bullapophysa Decraemer, 1983	395	66–68	8/11	345–500	61–68	8/12–13	13–14	13–15	15–20	27	18	34–35, 39	9	9
T. (T.) cylindricauda Chitwood, 1936	500	69–70	11–13/20–21	476	72	13/15–17	16	16	16	35	8	-	12	-
T. (T.) denticulata Timm, 1970	645–800	63–64	10/15–17	760	65–66	-	32–39	19–26	20–22	96–104	50–56	43	11–12	9
T. (T.) dimorpha Decraemer, 1978	305–600	52–65	12–13/16–17	400	62–67	13/17	15–20	11–16	15–21	16–27	16–19	27–29	10–12	12–13
T. (T.) disparseta Lee et al., 2024	527–613	59–61	9–10/14–18	490–575	60–62	9/16–18	19–22	15–18	22–28	29–32	16–19	38–40	9–10	8–9
T. (T.) fortiseta sp. nov.	700–836	65–69	10–12/17–20	740–817	65–69	11/18–19	23–27	17–20	16–23	44–53	29–33	37–40	9–11	7–10
T. (T.) gracilis Steiner, 1916	178	67	-	-	-	-	-	-	-	-	-	-	12	-
T. (T.) incomposita Fredenhammer, 1975	420	72	7/7–9	-	-	-	16	18	9	29	-	-	8	-
T. (T.) longirostris (Southern, 1914)	250–900	63–78	8–9/12–15	700–1000	-	-	15–36	12–31	21–32	19–35	14–16	-	10–12	-
T. (T.) megamphida Timm, 1970	370–400	72–74	11–12/17–18	380–435	-	12/12–13	15–16	11–14	15–17	24–30	-	-	9–10.5	9–9.5
T. (T.) oblita Blome, 1982	477–506	60–63	8–9/15–16	513	63	10/18	25–27	-	21–22	27	11–13	37–38	9–10	-
T. (T.) parvaspiculata Decraemer, 1987	240	69–70	9/13	195	69–70	9/12–13	8–9.5	10–11	10–12	17	10	37	10–11	11
T. (T.) perparvula Timm, 1970	275	61–62	7–9/13	-	-	-	14	11	11	24	9	-	9	-
T. (T.) platapophysa Decraemer, 1978	435–460	66–67	10/20	-	-	-	23–28	16–18	19–23	29–33	16–17	-	8	-
T. (T.) platycephala Filipjev, 1922	460	71	9/14	-	-	-	22	11	14	32	20	-	9	-
T. (T.) riemanni Decraemer, 1978	510–620	66–67	11/17–19	605–700	65–70	11/16–22	20–24	15–21	20–29	28–38	20–27	36–42	9–10	8–10
T. (T.) septuaginta Schuurmans Stekhove, 1942	425–440	68–71	9/12–13	345–680	68–72	9–13/11–19	12–22	9.5–15	13–22	30–56	13–16	42	9–10	9
T. (T.) septentrionalis Timm, 1978	-	-	-	1260–1360	72–76	-	36	25	28	-	-	-	-	9
T. (T.) spinosa Chitwood, 1951	512	66	12/14	-	-	-	-	-	-	32	14	-	11	-
T. (T.) spinosoides Chitwood, 1951	400	61	10/17	380	-	10/14	-	-	-	26	13	26	-	12
T. (T.) spuria Inglis, 1967	710	62	11/21	735	62	10–11/18–19	27–28	18–21	29	46	31	33	10	9
T. (T.) steineri de Man, 1922	310–408	63–55	11–13/13–16	310–460	63–64	12/15–16	13–15	12–13	12–17	24–27	17	28–30	12	11
T. (T.) uljinensis sp. nov.	571–639	67–69	9–11/13–18	681–711	69–70	10–11/15–17	23–29	16–18	15–19	37–44	20–27	39	7–9	8

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Lee, H.J.; Lee, H.; Rho, H.S. Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region. J. Mar. Sci. Eng. 2024, 12, 2310. https://doi.org/10.3390/jmse12122310

AMA Style

Lee HJ, Lee H, Rho HS. Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region. Journal of Marine Science and Engineering. 2024; 12(12):2310. https://doi.org/10.3390/jmse12122310

Chicago/Turabian Style

Lee, Hyo Jin, Heegab Lee, and Hyun Soo Rho. 2024. "Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region" Journal of Marine Science and Engineering 12, no. 12: 2310. https://doi.org/10.3390/jmse12122310

APA Style

Lee, H. J., Lee, H., & Rho, H. S. (2024). Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region. Journal of Marine Science and Engineering, 12(12), 2310. https://doi.org/10.3390/jmse12122310

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Taxonomic Study of Free-Living Marine Nematodes in the Subgenus Tricoma (Desmoscolecida: Desmoscolecidae) from the Subtidal Zone of the East Sea, Korea, with Insights into the Ultrastructure of the Lip Region

Abstract

1. Introduction

2. Materials and Methods

3. Results

3.1. Description of Tricoma (Tricoma) polyringulata sp. nov. (Figure 2, Figure 3, Figure 4, Figure 5 and Figure 6; Table 1)

3.1.1. Type Material

3.1.2. Type Locality

3.1.3. Etymology

3.1.4. Diagnosis

3.1.5. Measurements

3.1.6. Description

3.1.7. Differential Diagnosis

3.2. Description of Tricoma (Tricoma) fortiseta sp. nov. (Figure 8, Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13; Table 2)

3.2.1. Type Material

3.2.2. Type Locality

3.2.3. Etymology

3.2.4. Measurements

3.2.5. Diagnosis

3.2.6. Description

3.2.7. Differential Diagnosis

3.3. Description of Tricoma (Tricoma) uljinensis sp. nov. (Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19; Table 4)

3.3.1. Type Material

3.3.2. Type Locality

3.3.3. Etymology

3.3.4. Measurements

3.3.5. Diagnosis

3.3.6. Description

3.3.7. Differential Diagnosis

4. Discussion

5. Conclusions

Author Contributions

Funding

Institutional Review Board Statement

Informed Consent Statement

Data Availability Statement

Conflicts of Interest

References

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