Two new nematode species of the genus Zalonema (Desmodorida, Desmodoridae, Desmodorinae) from mangrove wetlands, China

195 Two new nematode species of the genus Zalonema (Desmodorida, Desmodoridae, Desmodorinae) from mangrove wetlands, China Yuzhen Chen1, Yuqing Guo1,2 1 Fisheries College, Jimei University, Xiamen 361021, China 2 Fujian Provincal Key Laboratory of Marine Fishery Resources and Eco-Environment, Jimei University, Xiamen 361021, China Corresponding author: Yuqing Guo ([email protected]) Copyright: © Yuzhen Chen & Yuqing Guo. This is an open access article distributed under terms of the Creative Commons Attribution License (Attribution 4.0 International – CC BY 4.0). Research Article Abstract Two new species of the genus Zalonema Cobb, 1920 were identified during a meiofaunal survey of mangrove sediment in China. Zalonema eurysbucca sp. nov. is characterized by body length 1772–2309 µm, cuticle with transverse annuli; cephalic setae measuring 1–2 µm, and eight subcephalic setae (2–3 µm) arranged in a circle between the cephalic setae and the anterior margin of the amphid; amphideal fovea (4–4.5 turns) located laterally on the main posterior portion of the cephalic capsule; buccal cavity large and cup-shaped, with twelve cheilorhabdia in the vestibule, a cuticularized dorsal tooth, and minute ventral tooth; males possess lateral and ventral alae; spicule length (as arc) about 1.13–1.34 times the anal body diameter (abd); a small cup-shaped preanal supplement is present, and the ventral ala exhibit pore-like depressions at the somatic setae positions. Zalonema cylindribucca sp. nov. is distinguished by the following characteristics: body length 1158–1451 μm; cuticle with transverse annuli; four cephalic setae (2–3 µm) and eight subcephalic setae (2–3 µm) arranged in separate circular rows; amphideal fovea with 3–3.5 turns; cylindrical buccal cavity containing 12 cheilorhabdia arranged circularly in the vestibule; a large dorsal tooth located near the cheilorhabdia base and a small ventral tooth at the buccal cavity’s base; males possess both lateral alae and ventral ala, spicules measuring 1.07–1.19 times the anal body diameter (as arc), a small cup-shaped preanal supplement present, and the ventral ala exhibit small pore-like depressions at somatic setae positions. Key words: Free-living marine nematode, Hainan Province, mangrove, meiofauna, new species, taxonomy, Zalonema eurysbucca sp. nov., Zalonema cylindribucca sp. nov. Introduction Meiofaunal sediments were collected during a preliminary survey of the meiofauna in mangrove mudflats adjacent to the cities of Danzhou and Sanya, Hainan Province, China. Nematodes are the dominant group at these two mangrove sampling stations, of which Zalonema Cobb,1920 is dominant genus, accounting for 5.5% and 26.9% of the abundance, respectively. Despite their ecological prevalence, the taxonomic knowledge of free-living marine nematodes in Chinese mangrove wetlands remains limited, with fewer than 50 species recorded to date (Chen et al. 2023; Zhu et al. 2023; Sun and Huang 2024). Academic editor: Steven Nadler Received: 10 July 2025 Accepted: 18 October 2025 Published: 20 November 2025 ZooBank: https://zoobank. org/398AE9DB-879E-4CA2-BEB5D027203C69BE Citation: Chen Y, Guo Y (2025) Two new nematode species of the genus Zalonema (Desmodorida, Desmodoridae, Desmodorinae) from mangrove wetlands, China. ZooKeys 1260: 195–211. https://doi. org/10.3897/zookeys.1260.164776 ZooKeys 1260: 195–211 (2025) DOI: 10.3897/zookeys.1260.164776 196 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China The genus Zalonema Cobb,1920 belongs to the subfamily Desmodorinae within the family Desmodoridae. It was initially established as a family by Cobb based on the species Zalonema nudum Cobb, 1920. Gerlach (1963) considered Zalonema to be one of the subgenera of Desmodora and Zalonema nudum to be a synonym of Desmodora megalosoma Steiner, 1918. Later, Verschelde and Vincx (1996) raised Zalonema to genus level. Zalonema is distinguished from other genera of the subfamily Desmodorinae by the following synapomorphies: (1) finely annulate cuticle lacking longitudinal striations or lateral differentiation; (2) well-developed, rounded triangular cephalic capsule; (3) large multispiral amphideal fovea (≥2 turns); and (4) presence of subcephalic setae (Tchesunov 2013; Verschelde and Vincx 1996; Verschelde et al. 1998). While the elongated body form and annulation pattern resemble those of representative genera of the subfamily Spiriniinae Chitwood, 1936, Zalonema is unequivocally differentiated by its distinct cephalic capsule, a structure entirely absent in Spiriniinae (Verschelde and Vincx 1996). Recent taxonomic studies have revealed morphological variability within the genus, particularly in the position of subcephalic setae. The diagnosis of the genus by Verschelde and Vincx (1996) stated that the subcephalic setae are located in front of or at anterior edge of amphids, however seven recently described species, i.e.: Zalonema iranicum Gharahkhani, Pourjam, Leduc & Pedram, 2021, Zalonema supplementorum Gharahkhani, Pourjam, Leduc & Pedram, 2021, Zalonema granda Fadeeva, Mordukhovich & Zograf, 2016, Zalonema kamchatkaensis Fadeeva, Mordukhovich & Zograf, 2016, Zalonema mariae Larrazábal-Filho, Silva & Esteves, 2015, Zalonema sesokoensis Leduc, 2022 and Zalonema vicentei Larrazábal-Filho, Silva & Esteves, 2015, demonstrate subcephalic setae either aligned with, or posterior to, the amphids (Leduc 2022). Furthermore, Gharahkhani et al. (2021) proposed an intrageneric division of Zalonema into two groups based on the presence (Group I) or absence (Group II) of lateral or ventral alae in the posterior body region of males. Group I included species with lateral and/or ventral alae in the posterior body of males, group II species lack lateral or ventral alae in the posterior body region of males. In this study, two new species of the genus Zalonema were found during a preliminary survey of the meiofauna in mangrove mudflats of China and are described. Material and methods Sediment samples were collected from mangrove mudflats adjacent the cities of Danzhou (19.77–19.78°N, 109.26–109.27°E) and Sanya (18.23°N, 109.62°E), Hainan Province, China, and immediately preserved in 5% formaldehyde-seawater solution. Nematodes were extracted from the sediment by decantation and/or Ludox centrifugation in the laboratory, and stained with Rose Bengal for more than 24 h in the laboratory (Warwick et al. 1998). Sediment sampling and nematode extraction were performed as described in our previous studies (Chen et al. 2023). Measurements are in µm. Abbreviations are as follows: a = body length/ maximum body diameter; b = body length/pharynx length; c = body length/tail length; abd = anal body diameter; cbd = corresponding body diameter; vbd = vulval body diameter; c’ = tail length/abd; V = vulva from the anterior end; V(%) = position of vulva as % of body length from anterior end, that is, V/body length. All holotypes and paratypes are deposited in Jimei University, Xiamen, China. 197 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China Results Order Desmodorida De Coninck, 1965 Family Desmodoridae Filipjev, 1922 Subfamily Desmodorinae Filipjev, 1922 Genus Zalonema Cobb, 1920 Generic diagnosis. (modified from Leduc 2022): Body long, cylindrical, goldbrown under light microscope with glass-like or golden cephalic capsule. Cuticular annuli close together, usually without ornamentation. Cephalic capsule rounded, triangular. Labial sensilla inconspicuous, 6+6, cephalic setae four, followed by four to eight subcephalic setae or papillae located anteriorly, posteriorly, or at same level as the amphids, amphideal fovea multispiral (two turns or more) located on the cephalic capsule. Short and small somatic setae arranged in several longitudinal rows. Pharynx cylindrical with pyriform basal bulb. Male reproductive system monorchic. Spicules slightly arched, cuticularized, without velum. Gubernaculum short, ventral and/or lateral alae sometimes present in males (ventral ala sometimes difficult to distinguish). Precloacal supplements present or absent. Female reproductive system didelphic, amphidelphic, with reflected ovaries. Tail conical. Caudal gland ducts and spinneret conspicuous. Zalonema eurysbucca sp. nov. https://zoobank.org/054D7601-E14B-4EF1-BDB3-757BEDE2E58F Figs 1, 2; Tables 1, 2 Type material. Six males and five females were collected from Station DZ in December, 2020. Holotype: • ♂1 on slide DZ20201214 2L18. Paratypes: • ♂2 on slide DZ20201214 2L101, • ♂3 and ♀1 on slide DZ20201214 2L111, • ♂4 on slide DZ20201214 2L102, • ♂5 on slide DZ20201214 1L113, • ♂6 on slide DZ20201214 2L117, • ♀2 and ♀3 on slide DZ20201214 2L18, • ♀4 on slide DZ20201214 2L110, • ♀5 on slide DZ20201214 1L101. Type locality and habitat. All specimens were collected from the muddy sediment in the mangrove reserve of Xinying Port in Danzhou City, Hainan Province, China. The main mangrove species here is Bruguiera gymnorrhiza (L.) Lam. Characteristics of surface sediments of sampling stations are shown in Table 1. Etymology. The species’ name is from the Latin (Eurys = broad) and refers to the large buccal cavity. Measurements. Morphological characteristics were observed and measured under a differential contrast microscopy (NIKON 80i) (Table 2). Description. General characteristics: Golden-brown body elongated and cylindrical, with blunt and rounded anterior extremity and conical tail. Cuticle with distinct fine annuli arranged transversely from the posterior margin of the cephalic capsule to the tail, ending in a smooth, non-annulated tail tip approximately 10 µm in length. Somatic setae short, about 1–2 µm in length, arranged in eight longitudinal rows, and each seta is connected to an epidermal gland. Cephalic capsule consists of two parts: an anterior labial region, with a thinner cuticle; and a main posterior portion, with a conspicuously thickened cuticle without annula- 198 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China Figure 1. Drawing of Zalonema eurysbucca sp. nov. (lateral view). A. Male tail region, showing copulatory structure, ventral ala and preanal supplement; B. Male head end; C. Female head end; D. Female tail region; E. Female body part, showing ovaries; F. Female body. Scale bars: 50 μm (A–D); 125 μm (E); 100 μm (F). tions, which is about 15–19 µm wide in males and about 13–16 µm wide in females. Inner labial sensilla not observed; six outer labial sensilla are papilliform, arranged in a circle; four cephalic setae arranged in a circle, about 1–2 µm in length, located slightly to the back of the external labial sensilla; eight subcephalic setae, about 2–3 µm in length, arranged in a circle between the cephalic setae and the anterior margin of the amphid. Large spiral amphideal fovea and aper- 199 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China Figure 2. Microscopic images of Zalonema eurysbucca sp. nov. (lateral view). A. Male head region, showing cheilorhabdia in buccal cavity; B. Male head region, showing buccal cavity, large dorsal tooth and minute ventral tooth; C. Male amphid; D. Male anterior region, showing longitudinal rows of short somatic setae; E. Male pharynx and the pyriform posterior bulb; F. Male posterior region, showing lateral alae, the end of ventral ala and the tail; G. Male posterior region, showing copulatory structure, supplement and ventral ala; H. Female vulva; I. Female middle region, showing longitudinal rows of short somatic setae; J. Female tail; K. Male total body; L. Female total body. Scale bars: 10 μm (A–D, G–I); 50 μm (E); 100 μm (F, J–L) 200 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China Table 1. Characteristics of surface sediments of sampling stations. Station Temperature (°C) pH Salinity (‰) DZ20201214 1L 23.5 6.0 28 DZ20201214 2L 26 6.5 30 Table 2. Individual measurements of Zalonema eurysbucca sp. nov. (in µm except for ratios). Specimen Holotype Paratypes average (minimum-maximum) Characters ♂1 5 ♂♂ 5 ♀♀ Body length 1926 2029 (1898–2309) 1969 (1772–2161) Subcephalic setae cbd 23 26 (23–29) 23 (21–25) Width of cephalic capsule 17 17 (16–19) 15 (13–17) Length of cephalic setae 21.4 (1–2) 1.5 (2–2) Length of subcephalic setae 3 2 (2–2) 2.5 (2–3) Buccal cavity depth 34 38 (35–39) 37 (36–39) Buccal cavity diameter 15 16 (13–19) 13 (12–15) Amphids from anterior end 7 9 (7–10) 7 (6–8) Amphid diameter 17 17 (15–18) 16 (15–17) Amphid cbd 33 37 (30–41) 31 (27–37) Amphid diameter/Amphid cbd (%) 52 46 (38–57) 52 (44–58) Amphid turns 4.00 4.00 (4.00–4.00) 4.12 (4.00–4.50) Nerve ring from anterior end 113 118 (106–134) 121 (110–132) Nerve ring cbd 88 86 (75–96) 80 (73–88) Pharynx length 224 218 (195–235) 205 (196–214) Pharynx bulb diameter 76 76 (64–83) 66 (63–70) Pharynx cbd 99 98 (83–107) 93 (88–98) Max. body diameter 110 109 (94–122) 101 (91–108) abd 64 59 (48–68) 43 (38–48) Tail length 100 95 (87–99) 125 (116–136) c’ 1.57 1.61 (1.43–1.82) 2.95 (2.39–3.41) Spicule length as chord 67 65 (56–69) - Spicule length as arc 76 73 (63–81) - Spicule length as arc/abd 1.19 1.25 (1.13–1.34) - Length of lateral alae 566 553 (505–611) - Length of gubernaculum 34 31 (28–34) - V- - 986 (903–1100) vbd - - 94 (86–104) V(%) - - 50 (48–51) a17.51 19.11 (16.19–20.63) 19.59 (16.41–21.96) b8.60 9.54 (9.30–10.64) 9.61 (8.31–10.87) c19.30 21.85 (21.07–23.65) 15.71 (14.55–16.67) ture, 4–4.5 turns, located laterally on the main posterior portion of the cephalic capsule, with a diameter of about 38–58% of cbd. Buccal cavity large, with a maximum diameter of about 12–19 µm and a depth of about 34–39 µm. There are 12 cheilorhabdia arranged in a circle in the buccal cavity vestibule; the inner wall of the buccal cavity is sclerotized, with two teeth: one large dorsal tooth located at about one-third to half the depth of the buccal cavity, and one small ventral tooth at the bottom of the buccal cavity. Cylindrical pharynx with cuticularized lumen and pyriform posterior bulb; inner wall of the pharynx is thickened or sclerotized, and the pharyngeal bulb is broken into two parts. Nerve ring located at 50–63% of the length of the pharynx. Secretory-excretory system not observed. 201 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China Males: lateral alae present on both sides of posterior body part, extending from 505–566 µm in front of anus to a position on either side of the body at the anus. Ventral ala present, extending from 250–356 µm in front of anus to the anus. Reproductive system monorchic with single anterior outstretched testis located to the right of intestine. Spicules paired, equal, arcuate, length 1.13– 1.34 times abd as arc, with proximal capitulum, without velum. Gubernaculum simple, without apophyses. A small cup-shaped preanal supplement located about 13 µm anterior to the anus. The surface of the ventral ala is marked by small pore-like depressions located at the positions corresponding to somatic setae. However, no distinct setal structures were observed within these depressions. These structures appear to correspond to the “small, pore-like, not cuticularized” features reported by Leduc (2022). In the present study, these pore-like depressions were clearly observed to be arranged regularly along the ventral ala, extending anteriorly along the body, and aligning with the pattern of somatic setae in regions where ventral ala are absent. In addition, each porelike depression extends subcutaneously and connects to the epidermal gland, similar to the association seen in somatic setae. Therefore, we interpret these pore-like structures on the ventral ala not as preanal supplements, but likely as pore-like sensilla that may be homologous to somatic setae. Preanal setae not observed. Conical tail without terminal setae, length about 1.43–1.82 times abd. Spinneret with single opening present. Females: Most characteristics are similar to those of the males, but the tail is longer than that of the males, with a length of about 2.4–3.4 times abd. Without lateral alae or ventral ala. Reproductive system didelphic, reflexed and present on the right side of the intestine. Vulva sclerotized, located at 48–51% of the body length. Diagnosis. Zalonema eurysbucca sp. nov. is distinguished by the following combination of characters: golden-brown body (length 1772–2309 µm) with a finely annulate cuticle; short somatic setae arranged in eight longitudinal rows; eight subcephalic setae (2–3 µm) arranged in a circle between the cephalic setae (1–2 µm) and the amphid’s anterior margin; amphideal fovea (4–4.5 turns) located laterally on the cephalic capsule’s posterior portion (diameter 38–58% of cbd); a large, cup-shaped buccal cavity with twelve cheilorhabdia, a cuticularized dorsal tooth, and a minute ventral tooth; and a conical tail. Males possess lateral and ventral alae; the arcuate spicules measure 1.13–1.34 times the anal body diameter (abd); gubernaculum simple without apophyses; a small cupshaped preanal supplement, and the surface of the ventral ala at the position of the somatic setae sunken into small pore-like depressions. In accordance with the classification proposed by Gharahkhani et al. (2021), the new species Zalonema eurysbucca sp. nov. belongs to Group I, which is characterized by the presence of lateral alae and/or ventral ala on the posterior part of the male body. Zalonema eurysbucca sp. nov. is distinguished by the combined presence of both lateral and ventral alae, contrasting with Zalonema ditlevseni (Micoletzky, 1922) Gerlach, 1963 and Zalonema megalosoma (Steiner, 1918) Gerlach, 1963, which possess only lateral alae and lack a ventral ala (in contrast to Gharahkani’s classification that places Zalonema megalosoma in Group II, the redescription of this species by Gerlach (1963) reports the presence of lateral alae in males); and from Zalonema myrianae Verschelde & Vincx, 1996, which lacks lateral alae but possesses ventral ala. Zalonema eurysbucca sp. nov. can also be 202 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China differentiated from other species in Group I. Zalonema eurysbucca sp. nov. differs from Zalonema mariae Larrazábal-Filho, Silva & Esteves, 2015 in having eight subcephalic setae (vs four), a cup-shaped buccal cavity with a single large dorsal (at 30–50% depth) and small ventral teeth (vs. elongate buccal cavity with two dorsal teeth, one after the other), and spicules lacking a velum (Larrazábal-Filho et al. 2015); from Zalonema vicentei Larrazábal-Filho, Silva & Esteves, 2015 in the subcephalic setae position (between cephalic setae and the anterior margin of the amphid vs. at amphid posterior margin) and number (8 vs. 4) (Larrazábal-Filho et al. 2015); from Zalonema iranicum Gharahkhani, Pourjam, Leduc & Pedram, 2021 in the dental formula (one dorsal and one ventral tooth vs 3 teeth, a large anterior dorsal tooth, a small posterior dorsal tooth, and a medium-sized ventral-subventral tooth) (Gharahkhani et al. 2021); from Zalonema supplementorum Gharahkhani, Pourjam, Leduc & Pedram, 2021 in the subcephalic setae count (8 vs 4) and amphid coils (4–4.5 vs 3 turns) (Gharahkhani et al. 2021); and from Zalonema sesokoensis Leduc, 2022 by lacking amphid sexual dimorphism and possessing a larger buccal cavity (Leduc 2022). Zalonema cylindribucca sp. nov. https://zoobank.org/D9F11052-B88B-4498-AA92-82C0BBF8BA33 Figs 3, 4; Table 3 Type material. Five males and three females were collected from Station SY in July, 2020. Holotype: • ♂1 on slide SY2020730 2H108. Paratypes: • ♂2 & ♀1 on slide SY2020730 1L126, • ♂3 & ♀2 on slide SY2020730 2H102, • ♂4, • ♂5 & ♀3 on slide SY2020730 2H108. Type locality and habitat. All specimens were collected from the muddy sediment in the mangrove reserve of Yalong Bay Qingmei Port in Sanya City, Hainan Province, China. The primary mangrove species here are Rhizophora stylosa Griff. and Avicennia marina (Forssk.) Vierh. Etymology. The species is name for its cylindrical buccal cavity. Measurements. Morphological characteristics were observed and measured under a differential contrast microscopy (NIKON 80i) (Table 3). Description. General characteristics: Body elongate-cylindrical, golden-brown in color, with a cephalic capsule and a conical tail. Cuticle with annuli arranged transversely from the posterior margin of the cephalic capsule to the tail, with tail tip about 10 µm smooth and without annuli. The annuli are spaced by about 0.6 µm at both the anterior and posterior ends of the body, and by about 0.8 µm in the middle part. Somatic setae short, 1–2 µm in length, and arranged in eight longitudinal rows. Cephalic capsule divided into two parts: an anterior labial region, with a thinner cuticle; and a main posterior portion, with a width of 8–10 µm, and its cuticle distinctly thickened without annuli. No inner labial sensilla were observed; the six outer labial sensilla are papilliform and arranged in a circle; four cephalic setae and eight subcephalic setae are each arranged in a circle, about 2–3 µm in length, with the cephalic setae located slightly posterior to the outer labial sensilla; the subcephalic setae are positioned between the cephalic setae and the anterior margin of the amphid. Large spiral amphideal fovea and aperture, 3–3.5 turns, located laterally on the main posterior portion of the cephalic capsule, with diameter about 35–41% of cbd. Buccal cavity cylin- 203 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China drical, with a maximum diameter of approximately 5–6 µm and a depth of about 28–31 µm. There are 12 cheilorhabdia arranged in a circle in the buccal cavity vestibule. At the anterior part of the buccal cavity, near the base of the cheilorhabdia, there is a large dorsal tooth; and another small ventral tooth is located at the bottom of the buccal cavity, close to the esophagus. Cylindrical pharynx with cuticularized lumen and a pear-shaped posterior bulb; the inner wall of the pharynx is thickened or sclerotized, and the pharyngeal bulb slightly broken. Nerve ring located at 60–65% of the pharynx. The excretory system was not observed. Figure 3. Drawing of Zalonema cylindribucca sp. nov. (lateral view). A. Male head end; B. Male tail region, showing copulatory structure, ventral ala and preanal supplement; C. Female tail region; D. Female head end; E, F. Male copulatory structure; G. Female body; Scale bars: 50 μm (A–F); 100 μm (G). 210 ZooKeys 1260: 195–211 (2025), DOI: 10.3897/zookeys.1260.164776 Yuzhen Chen & Yuqing Guo: New nematode species of Zalonema from mangrove wetland of China Conflict of interest The authors have declared that no competing interests exist. Ethical statement No ethical statement was reported. Use of AI No use of AI was reported. Funding This research was funded by the National Natural Science Foundation of China (grant number 31772416), and the Natural Science Foundation of Fujian Province (grant number 2022J01324) Author contributions Methodology, taxonomy and writing original draft: Y.C.; project management, work program: Y.G. 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