Two new species of Comesomatidae (Nematoda, Araeolaimida) from the Rizhao coast of the Yellow Sea, China

Huiying Yu; Chunming Wang

doi:10.3897/zookeys.1275.164380

. 2026 Mar 31;1275:259–273. doi: 10.3897/zookeys.1275.164380

Two new species of Comesomatidae (Nematoda, Araeolaimida) from the Rizhao coast of the Yellow Sea, China

Huiying Yu ¹, Chunming Wang ^1,^✉

PMCID: PMC13058601 PMID: 41960289

Abstract

To study the diversity and taxonomy of marine nematodes on the Rizhao coast, undisturbed sediment samples have been systematically collected since 2022. Two new species of the family Comesomatidae, Comesoma macramphidasp. nov. and Paramesonchium tubulosupplementasp. nov. are described. Comesoma macramphidasp. nov. is characterized by having the outer labial sensilla setiform, the buccal cavity cup-shaped with a cuticularized projection, the amphidial fovea wide with 2.5 turns, spicules L-shaped with faint striation, the gubernaculum short with a small caudally directed apophysis, 10 or 11 papilliform precloacal supplements, and a conico-cylindrical tail with two long terminal setae. Paramesonchium tubulosupplementasp. nov. is characterized by having the cuticle ornamented with transverse punctations and lateral pores, four long cephalic setae, a wide buccal cavity with three ridges, a wide amphidial fovea with 2.5–2.75 turns, spicules curved with a cephalate proximal end, the gubernaculum with a dorso-caudal apophysis, 11 or 12 tubular precloacal supplements, and a conico-cylindrical tail with a short, cylindrical portion.

Key words: Comesoma , marine nematode, Paramesonchium

Introduction

Marine nematodes represent the most abundant and taxonomically diverse group within the phylum Nematoda and are a dominant component of meiofaunal communities in marine sediments worldwide (Meldal et al. 2007; Bik et al. 2010). Despite their ecological significance and remarkable species richness, marine nematodes remain one of the most taxonomically challenging groups due to their small size, morphological convergence, and the paucity of molecular data available for phylogenetic analysis (Leduc et al. 2017; Hodda 2022).

The Yellow Sea, situated between China and the Korean Peninsula, represents a biogeographically important region for studies on marine biodiversity. As part of a comprehensive investigation into the meiofaunal diversity of this region, systematic sampling of undisturbed sediments was initiated along the Rizhao coast in May 2022. Preliminary analyses have revealed marine nematodes as the dominant meiobenthic taxon, with several new species already described and subjected to molecular phylogenetic analysis (Guo et al. 2023a, 2023b; Liang et al. 2024a, 2024b). These findings underscore the taxonomic richness of the region and highlight the need for continued systematic research.

The family Comesomatidae Filipjev, 1918, was established with Comesoma Bastian, 1865 as the type genus. The family has long presented taxonomic challenges due to the combined morphological structure. It exhibits a combination of features typically associated with different nematode orders: the punctate cuticle, spiral amphidial fovea, and tubular precloacal supplements are characteristic of Chromadorida, while the outstretched ovaries are more typical of Monhysterida (Jensen 1979; Lorenzen 1994; Fonseca and Bezerra 2014). This morphological complexity has resulted in ongoing debate regarding the phylogenetic placement of Comesomatidae. Currently, Comesomatidae is provisionally assigned to the order Araeolaimida based on morphological criteria (Lorenzen 1994; Fonseca and Bezerra 2014) and comprises three subfamilies: Comesomatinae Filipjev, 1918, Dorylaimopsinae De Coninck, 1965, and Sabatieriinae Filipjev, 1934 (Nemys Eds 2025). Within Comesomatidae, the genus Comesoma was established with the type species C. vulgare Bastian, 1865, currently comprised 13 valid species prior to this study. Similarly, the genus Paramesonchium Hopper, 1967, was established following the transfer of Paramesonchium seriale (Wieser, 1954) Hopper, 1967 from the genus Laimella Cobb, 1920, includes only three valid species before the present study.

This paper describes two new species of Comesomatidae recovered from the Rizhao coast: Comesoma macramphida sp. nov. and Paramesonchium tubulosupplementa sp. nov. Detailed morphological descriptions and illustrations are provided to improve our understanding of marine nematode diversity in the Yellow Sea, China.

Materials and methods

Undisturbed sediment samples were collected from the intertidal zone of Rizhao coast using a syringe (2.9 cm inner diameter) to a depth of 8 cm. Each core was vertically subdivided into two depth intervals: 0–2 and 2–8 cm. Samples were immediately fixed with 10% formalin in seawater. In the laboratory, nematodes were stained with rose bengal, separated from the sediment samples, and slide-mounted as our previous article (Guo et al. 2023a).

Morphological examination was created using a differential interference contrast microscope (Axiscope-5, Zeiss, Germany). Line drawings were prepared using Procreate software on an iPad (Apple, USA), and photomicrographs were captured using ZEN software (Zeiss Corporation, Germany). Holotype and paratype specimens were deposited at the College of Agriculture and Biology, Liaocheng University.

Results and discussion

Taxonomy

Order Araeolaimida De Coninck & Schuurmans Stekhoven, 1933

Family Comesomatidae Filipjev, 1918

Genus. Comesoma

Bastian, 1865

6EA662EA-5F08-5FC8-87FB-7E22E34B7B23

Diagnosis

(revised based on Fu et al. 2022). Cuticle characterised by transverse punctations without lateral differentiation. Anterior sensilla in three distinctly separated crowns; outer labial lateral setae usually shorter than the cephalic setae. Subcephalic setae present in one or more crowns, just posterior to cephalic setae. Buccal cavity cup-shaped, and posterior portion collapsed, weakly sclerotised and with three blunt or thorn-like projections at the border to anterior portion. Spicules long and slender. Gubernaculum plate-like or indistinct.

Remarks.

The genus Comesoma was first established with C. vulgare and C. profundi Bastian, 1865 from Falmouth, England. The genus diagnostic characteristics include: cup-shaped buccal cavity without teeth, spiral-shaped amphidial fovea, cuticle with transverse rows of fine punctations, lateral differentiation only a bit more coarsely punctuated, spicules usually elongated, and precloacal supplements usually present (Bastian 1865). Later, C. heterura Cobb, 1898 (accepted as Sabatieria heterura (Cobb, 1898) Filipjev 1918), C. jubata Cobb, 1898 (accepted as Setosabatieria hilarula), and C. simile Cobb, 1898 (based on female specimen) were described. Subsequently, C. stenocephalum Filipjev, 1918, C. dubium Filipjev, 1918 (accepted as Paracomesoma dubium (Filipjev, 1918) Schuurmans Stekhoven 1950), C. minimum Chitwood, 1937, C. punctata Schuurmans Stekhoven, 1950, C. arenae Gerlach, 1956, C. sipho Gerlach, 1956 (accepted as Paracomesoma sipho (Gerlach, 1956) Jensen and Gerlach 1977 ), C. bermudense Jensen & Gerlach, 1977, C. solum Pastor de Ward, 1984, and C. hermani Chen & Vincx, 1998 were described. Wieser (1954) synonymised Sabatieria tenuispiculum (Ditlevsen, 1921) with C. tenuispiculum. Comesoma punctata was transferred to Metacomesoma by Wieser (1954), but Jensen (1979) regarded this taxon as a species inquirenda due to the poor description of anterior sensilla arrangement. Recently, with the description of C. sinica Fu, Cai, Leduc & Lin, 2022 and C. quattuordecimsupplementata Xiao & Guo, 2023, 13 species are considered as valid.

According to Jensen (1979), the cephalic organs in the Comesomatidae are usually arranged in three distinct crowns with the inner labial and outer labial sensilla typically papilliform. When the six outer labial sensilla are setiform, the two lateral setae are sometimes longer than the other four setae and situated at the second and third crowns. The four cephalic setae are generally longer than the outer labial setae.

Comesoma macramphida sp. nov.

EE40F3F1-1625-5384-B6BD-7EDB3C4BF4F9

https://zoobank.org/26C5F69A-BAC9-44C5-9F96-A4A5BB662282

Figs 1 , 2 , 3 , Table 1

Figure 1. — *Comesoma* *macramphida* sp. nov., holotype. A. Lateral view of male anterior region; B. Lateral view of male anterior region showing buccal cavity; C. Lateral view of male anterior region showing cuticle and amphidial fovea; D. Lateral view of spicules and gubernaculum; E. Lateral view of male posterior body; F. Lateral view of male posterior region showing precloacal supplements. Scale bars: 50 µm (A); 30 µm (**B, C, E, F**).

Figure 2. — *Comesoma* *macramphida* sp. nov. Lateral view of male whole body (holotype). Scale bar: 80 µm.

Figure 3. — *Comesoma* *macramphida* sp. nov., holotype. A. Lateral view of male anterior region showing amphidial fovea and cephalic setae; B. Lateral view of male anterior region showing buccal cavity; C. Lateral view of male posterior region showing spicules; D. Lateral view of male posterior body, showing gubernaculum; E. Lateral view of male anterior region, showing lateral cuticle region; F. Lateral view of male posterior body, showing precloacal supplements (arrows). Scale bars: 20 µm (**A–E**); 10 µm (F).

Table 1.

Individual measurements of Comesoma macramphida sp. nov. (in µm, except ratio).

Characters	Holotype male	Paratype male #1	Paratype male #2
Total body length	2776	2753	2933
Maximum body diameter	30	35	28
Head diameter	11	11	10
Length of outer labial setae	6	6	6
Length of cephalic setae	24	23	30
Buccal cavity depth	4	4	4
Amphidial fovea width	10	12	11
Amphidial fovea turns	2.5	2.5	2.5
Amphidial fovea from anterior	5	5	5
Body diameter at amphidial fovea	13	13	14
Nerve ring from anterior	107	119	115
Body diameter at nerve ring	23	21	22
Pharynx length	219	228	228
Body diameter at pharynx	24	24	22
Anal body diameter	29	25	28
Spicule length along arc	64	59	62
Gubernaculum length	14	14	11
Precloacal supplements	10	11	10
Tail length	157	164	156
a	92.5	78.7	104.8
b	12.7	12.1	12.9
c	17.7	16.8	18.8
c'	5.4	6.6	5.6

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Diagnosis.

Comesoma macramphida sp. nov. is characterised by the following: outer labial sensilla setiform, buccal cavity cup-shaped with a cuticularized projection, amphidial fovea wide (77–92% of the body diameter) and consisting of 2.5 turns, spicules L-shaped and with faint striation; gubernaculum short and boat-shaped, with a small caudally directed apophysis, 10 or 11 papilliform precloacal supplements and tail conico-cylindrical, with two long, terminal setae.

Type material.

Three males were measured and studied. Holotype: • ♂ on slide 22WPK10-3-13; paratype 1: • ♂ on slide 22WPK10-2-15; paratype 2: • ♂ on slide 22WPK10-3-2.

Type locality and habitat.

Rizhao coast, Shandong Province, China (35°26'N, 119°34'E). Specimens collected from sandy sediment at a depth of 0–2 cm; collected May 2022 by Wen Guo.

Measurements.

All morphometric measurement data are given in Table 1.

Description.

Males. Body cylindrical, slender, and long; anterior end truncated, and posterior end tapered. Cuticle punctated; punctations arranged in transverse rows throughout body; lateral differentiation with irregularly spaced larger punctations. Cephalic sensilla arranged in three circles of 6+6+4. Inner labial sensilla 6, papilliform. Outer labial sensilla 6, setiform, 0.55–0.6 head diameters in length. Cephalic setae 4, long, 2.1–3.0 head diameters in length. Cervical setae 8, 11–12 μm in length; somatic setae present in pharynx and caudal regions, 2–4 μm in length. Amphidial fovea spiral, with 2.5 turns, and wide, 77–92% of body diameter, located immediately posterior to cephalic setae. Ocelli absent. Buccal cavity cup-shaped, 4 μm in depth, with subventral cuticularized projection at posterior. Pharynx muscular, cylindrical, and slightly expanded at posterior end but not forming true posterior bulb. Cardia conical, surrounded by intestine, and 7 μm in length. Nerve ring located in approximate middle of pharynx region, 48.9–52.2% of pharynx length. Secretory excretory system present; secretory excretory pore 142–146 μm from anterior end. Renette cell oval, 31–41 μm in length, 14–20 μm in width, and 68–76 μm from pharynx posterior end.

The reproductive system diorchic, with opposed and outstretched testes. Anterior testis to left of intestine and posterior testis to right. Spicules paired, L-shaped, with faint striation, 2.21–2.36 cloacal body diameters long. Gubernaculum boat-shaped, short, and parallel to distal end of spicules, and with a very small caudally directed apophysis. Precloacal supplements 10 or 11, papilliform; anteriormost precloacal supplement 133–171 μm from cloacal opening and posteriormost 10–14 μm from cloacal opening, respectively; distance slightly widening from posterior to anterior. Tail conico-cylindrical, 5.4–6.6 anal body diameters long, with cylindrical part about 1/3 of total length. Two terminal setae present, 16–17 μm in length. Four caudal glands in line with separate outlet.

Female not observed.

Etymology.

The species epithet macramphida is derived from Latin macro (large) and amphida (amphidial) and refers to the large size of amphidial fovea.

Differential diagnosis and discussion.

Comesoma macramphida sp. nov. can be differentiated from other species by the combination of having spicules shorter than 100 μm and six outer labial sensilla setiform. The new species is similar to C. bermudense, C. hermani, C. minimum, C. quattuordecimsupplementata, C. sinica, C. stenocephalum, and C. tenuispiculum in amphidial fovea turns. This new species differs from C. bermudense in the length of the outer labial setae (6 μm vs 2 μm), ornamentation and length of spicules (59–64 μm, with faint striation vs 150–194 μm, without striation), gubernaculum length (11–14 μm vs 25 μm), and number of precloacal supplements (10–11 vs 18–22). Comesoma macramphida sp. nov. differs from C. hermani in the length of the body (2753–2933 μm vs 1694–2108 μm), length of the cephalic setae (23–30 μm vs 13 μm), spicule length (59–64 μm vs 169–184 μm), and length of the gubernaculum (11–14 μm vs 20–25 um). Comesoma macramphida sp. nov. differs from C. minimum in the length of the body (2753–2933 μm vs 1370–1520 μm), length of the cephalic setae (2.1–3.0 head diameters vs 1.1 head diameters) and spicule length (2.2–2.4 anal body diameters vs 6.3 anal body diameters) (Xiao and Guo 2023). Comesoma macramphida sp. nov. differs from C. quattuordecimsupplementata in the shape of the buccal cavity (cup-shaped, wide and with cuticularized projection vs cup-shaped and small, without a projection), length of cephalic setae (23–30 μm vs 9–10 μm), spicule length (59–64 μm vs 120–128 μm), and number of precloacal supplements (10–11 vs 14). Comesoma macramphida sp. nov. differs from C. sinica in spicule length (59–64 μm vs 166–179 μm) and number of precloacal supplements (10–11 vs >20); the new species differs from C. stenocephalum in the length of the body (2753–2933 μm vs 4600 μm), depth of the buccal cavity (4 μm vs 8 μm), spicule length (59–64 μm vs 165 μm), and length of the gubernaculum (11–14 μm vs 40 μm). Comesoma macramphida sp. nov. differs from C. tenuispiculum in the length of the body (2753–2933 μm vs 1600 μm), the length of the cephalic setae (2.1–3.0 head diameters vs 1.5 head diameters), spicule length (59–64 μm vs 112 μm), and tail length (5.4–6.6 anal body diameters vs 3.5 anal body diameters, measured according to Filipjev 1918).

Genus. Paramesonchium

Hopper, 1967

683425B9-6236-5767-BC23-5FB15734370D

Revised diagnosis

(based on Fonseca and Bezerra 2014). Anterior sensilla in three circles of 6+6+4. Cuticle laterally differentiated, with transverse or longitudinal rows of punctations; lateral pore complex either absent or present. Posterior portion of buccal cavity conical, with three ridges; each ridge ending in an acute projection at border to anterior portion. Spicules curved. Gubernaculum with dorso-caudally directed apophysis. Precloacal supplements present, setiform or tubular.