From Changdao Island in June 2022, undisturbed samples were collected from rock surfaces with Ulva lactuca and U. prolifera (Fig. 1). Samples used for morphological analysis were fixed with 10% formalin in seawater, and samples used for molecular analysis were fixed with 95% alcohol. In the laboratory, sediment samples fixed with formalin were washed through two sieves (mesh sizes 500 μm and 45 μm) to separate meiofauna from macrofauna larger than 500 μm. Meiofauna were transferred into a grid-lined Petri dish and sorted under a stereoscopic microscope. Nematodes were transferred into a mixture of ethanol (50%) and glycerin (ratio 1:9 by volume), the ethanol was allowed to slowly evaporate (McIntyre and Warwick 1984), and the specimens were mounted in glycerin on permanent slides. Descriptions were made using an Axiscope-5 differential interference contrast microscope (Zeiss, Germany). Line drawings were made with the aid of iPad (Apple, USA), and photographs were taken with the aid of ZEN software (Zeiss).

Figure 1. 

Sampling site (dot) on Changdao Island.

Samples used for molecular analysis were washed and separated as with formalin-fixed samples. Species of C. communis sp. nov. were selected and confirmed on the temporary slides based mainly on buccal cavity, spicules, and numbers of precloacal supplements. Genomic DNA of seven nematode specimens were extracted with DNeasy Blood & Tissue kit (Qiagen, Germany) and used as templates of nearly full-length SSU rRNA gene amplification with two sets of primers, 1096F (5’ – GGT AAT TCT GGA GCT AAT AC – 3’) / 1912R (5’ – TTT ACG GTC AGA ACT AGG G – 3’) and 1813F (5’ – ctg cgt gag agg tga aat – 3’) / 2646R (5’ – gct acc ttg tta cga ctt tt – 3’) (Holterman et al. 2006). PCR was conducted as described by Zhao et al. (2015). The PCR product was sequenced by Genewiz (China). The sequences were assembled in Genious v. 6.1.2. The new SSU sequence was deposited in GenBank under accession number OP680597. Near full-length SSU rRNA sequences of Chromadorida and Desmodorida (genus Prodesmodora Micoletzky, 1923) were retrieved from GenBank with BLAST; other sequences in GenBank from Chromadorida and longer than 600 bp were also used in the phylogenetic analysis.

Sequences were aligned with the Clustal W algorithm, and the final alignment consisted of 76 sequences from 34 genera. Substitution models of (GTR (general time-reversible) + G (gamma distribution) + I (proportion of invariable sites)) were selected as the best-fit model for SSU alignments. A ML analysis was performed with Mega X with 1000 bootstrap replicates. A BI analysis was constructed with CIPRES (http://www.phylo.org/) and MrBayes on XSEDE v. 3.2.7a was used; the trees were run with chain length of 10,000,000, burn-in frac = 0.25, and the analysis was rooted with Enoplolaimus sp. (accession number KR265034). The topology of the resulting tree was viewed in FigTree v. 1.4.3 and edited with PowerPoint.

Order Chromadorida Chitwood, 1933

Family Chromadoridae Filipjev, 1917

Genus Chromadorina Filipjev, 1918

 Chromadorina communis sp. nov.

https://zoobank.org/63F40161-62AA-4836-8F27-9AAF48185A28

Figs 2, 3, Table 1

Diagnosis

Chromadorina communis sp. nov. is characterized by its medium-sized body, finely striate cuticle with punctuations, buccal cavity with three equally sized teeth, absent ocelli, four cephalic setae 6–8 μm long, oval amphidial fovea level with cephalic setae, curved spicules with tapered distal end, boat-shaped gubernaculums, five or six cup-shaped precloacal supplements, and a conical tail with a short spinneret.

Figure 2. 

Chromadorina communis sp. nov. A lateral view of male anterior region showing buccal cavity and pharyngeal region (holotype) B lateral view of male anterior region showing buccal cavity (holotype) C lateral view of male anterior region showing cuticle and amphids (holotype) D lateral view of male whole body (holotype) E lateral view of female anterior body showing buccal cavity and pharyngeal region (22YTCD6-2-1) F lateral view of female entire body showing vulva (22YTCD6-2-1) G lateral view of female posterior body (22YTCD6-2-1) H lateral view of male posterior body, showing precloacal supplements and tail (holotype) I lateral view of spicules and gubernaculums. Scale bars: 20 µm (A–C, E); 50 µm (D, F); 30 µm (G, H).

Material examined

Four males and two females were measured and studied. Holotype: m#1 on slide 22YTCD6-2-17; paratypes: m#2 on 22YTCD6-2-15, m#3 on 22YTCD6-2-18, m#4 on 22YTCD6-2-11, f#1 on 22YTCD6-2-1, and f#2 on 22YTCD6-2-18. Type specimens were deposited in the Institute of Oceanology, Chinese Academy of Sciences, Qingdao.

Figure 3. 

Chromadorina communis sp. nov. A lateral view of male anterior region showing buccal cavity and teeth (arrow) (holotype) B lateral view of male anterior region showing cuticle (holotype) C lateral view of female anterior region showing amphids (arrow) and cephalic setae (22YTCD6-2-1) D–F lateral view of male posterior body showing spicules, precloacal supplements, and gubernaculum (holotype). Scale bars: 20 µm.

Measurements

Detailed measurements information of individual specimens are shown in the Table 1.

Table 1.

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Measurements of Chromadorina communis sp. nov. (in µm except for ratios).

Characters	Holotype	Paratypes	Paratypes
	male	males (n = 3)	females (n = 2)
Total body length	711	741.0 ± 73.6 (696–851)	753.5 ± 47.4 (720, 787)
Maximum body diameter	32	27.8 ± 2.9 (26–32)	31.5 ± 0.7 (31, 32)
Head diameter	11	10.5 ± 0.6 (10–11)	10.5 ± 0.7 (10, 11)
Length of cephalic setae	8	6.5 ± 1.0 (6–8)	6.5 ± 0.7 (6, 7)
Diameter of amphidial fovea	5	4.5 ± 0.6 (4–5)	5.0 ± 0.0 (5, 5)
Amphidial fovea*	2	2.5 ± 0.6 (2–3)	2.0 ± 0.0 (2, 2)
Nerve ring*	71	71.5 ± 4.7 (67–78)	72.0 ± 0.0 (72, 72)
Body diameter at nerve ring	24	21.8 ± 1.5 (21–24)	21.5 ± 0.7 (21, 22)
Pharynx length	119	122.5 ± 5.1 (118–129)	125.5 ± 4.9 (122, 129)
Pharynx bulb length	26	24.8 ± 1.5 (23–26)	27.5 ± 0.7 (27, 28)
Body diameter at the base of pharynx	28	24.5 ± 2.4 (23–28)	25.5 ± 0.7 (25, 26)
Anal body diameter	22	22.0 ± 1.4 (21–24)	17.5 ± 2.1 (16, 19)
Spicules length along arc	28	27.5 ± 0.6 (27–28)	-
Gubernaculum length	17	17.5 ± 1.3 (16–19)	-
Vulva*	-	-	360.0 ± 11.3 (352, 368)
Body diameter at vulva	-	-	31.5 ± 0.7 (31, 32)
V%	-	-	47.9 ± 1.5 (46.8, 48.9)
Precloacal supplements	5	5.5 ± 0.6 (5–6)	-
Tail length	89	92.0 ± 5.7 (87–100)	115.5 ± 12.0 (107, 124)
a	22.2	26.9 ± 3.8 (22.2–27.1)	23.9 ± 1.0 (23.2, 24.6)
b	6.0	6.1 ± 0.4 (5.6–6.6)	6.0 ± 0.1 (5.9, 6.1)
c	8.0	8.1 ± 0.3 (7.7–8.5)	6.5 ± 0.3 (6.3, 6.7)
c’	4.0	4.2 ± 0.2 (4.0–4.4)	6.6 ± 0.1 (6.5, 6.7)

Abbreviations are as follows: a = body length/maximum body diameter; b = body length/pharynx length; c = body length/tail length; c’ = tail length/anal body diameter; V% = position of vulva from anterior body end expressed as a percentage of total body length. *Distance from anterior body end.

Description

Males. Body cylindrical and medium-sized. Inner and outer cephalic setae inconspicuous. Four cephalic setae 6–8 μm in length (0.55–0.73 head diameter long), two pairs of sublateral cervical setae (4–5 μm long) present. Somatic setae (3–4 μm long) scarcely present in pharynx and on caudal region. Cuticle faintly striate, with homogeneous punctuations without longitudinal differentiation. Ocelli absent. Amphidial fovea oval, level with cephalic setae, in some specimens difficult to observe. Buccal cavity slightly cuticularized and funnel-shaped, with three equally sized, solid teeth. Pharynx cylindrical, with posterior end widened into obvious bulb (20.1–21.8% of pharynx length). Nerve ring slightly posterior to middle pharynx region (54–63% of pharynx length). Secretory–excretory system pore on anterior end of body (3–4 μm from anterior end).

Reproductive system monorchic, with anterior testis outstretched, located to right of intestine. Spicules paired, curved, 1.2–1.3 cloacal body diameters long; proximal end slightly cephalate and distal end tapered. Gubernaculum simple, boat-shaped, parallel to the distal end of spicules. Five or six cup-shaped precloacal supplements, 12 μm from cloacal opening and 12–16 μm apart. Tail conical, with short spinneret, with three caudal glands in line.

Females. Similar to males in most characteristics. Reproductive system didelphic and amphidelphic, with reflexed ovaries. Anterior ovary to right of intestine and posterior ovary to left of intestine. Vulva slightly anterior to mid-body. Vagina short and sclerotized.

Type locality and habitat

Changdao Island, Shandong Province, China, 37°57'N, 120°43'E, at the confluence of the Yellow and the Bohai seas. Salinity 28.1‰ ± 0.36.

Distribution

Occurred on rock surfaces with Ulva lactuca and U. prolifera.

Etymology

Latin, communis, “common”.

Differential diagnosis

Chromadorina communis sp. nov. is similar to the cultured species C. hirommi Kito & Nakamura, 2001 in body length, length of cephalic setae, and numbers of precloacal supplements, but it differs in tooth shape (three equally sized teeth vs dorsal tooth large and subventral teeth small), absence of ocelli (brownish pigment present in C. hirommi), position of excretory pore (0.3 head diameter from anterior body end vs 1.8 head diameter from anterior body end), spicule length and shape (27–28 μm, distal end tapered vs 22–25 μm, distal end blunt and bifurcate), and gubernaculum shape (slightly cuticularized and boat-shaped vs well cuticularized with wavy dorsal fringe, distal parts of lateral pieces distinct and directed ventrad).

Eleven sequences of Chromadorina are included in the phylogenetic analysis, but only three species (C. bioculata, C. germanica, and C. communis sp. nov.) are identified to species, and C. bioculata was found in freshwater. Chromadorina communis sp. nov. shows a close relationship with Chromadorina sp. (KJ636255), but it differs by 3.1% (52 in 1678 bp, including three gaps). Chromadorina communis sp. nov. is supported as a new species by both phylogenetic and morphological analyses.

The ML topology obtained with the SSU rRNA gene sequence is in accordance with the BI topology, and only the BI tree is shown (Fig. 4). The main Chromadorida clade is split into two major clades: Clade A composed of Selachinematidae and Chromadoridae and Clade B composed of Cyatholaimidae, Achromadoridae, Neotonchidae, and Prodesmodora (100% posterior probability and 74% bootstrap value). Ethmolaimidae (with one species only, Ethmolaimus pratensis) splits early from the main Chromadorida clade. Six Chromadorida families constitute well-supported monophylectic clades.

Figure 4. 

Bayesian inference tree of the order Chromadorida inferred from small subunit (SSU) sequences under the general time-reversible (GTR) + proportion of invariable sites (I) + gamma distribution (G) model. Posterior probability (left) and bootstrap values (right) are given on corresponding clades. The sequence obtained in this study is shown in bold. Subfamilies and families are listed on the right. Main clades are shown by red letters or letters with numbers. The scale stands for substitutions per site.

In clade A1, nine genera of Selachinematidae form a well-supported monophyletic group (100% posterior probability and 83% bootstrap value), but Selachinematinae and Choniolaiminae are not shown as monophyletic clades at the subfamily level. In Clade A2, Chromadoridae forms a well-supported monophyletic group (100% posterior probability and 96% bootstrap value). In the subfamily level, only Chromadorinae constitutes a monophyletic clade based on the BI analysis (93% posterior probability); however, it is weakly supported by the ML analysis (39% bootstrap value). Hypodontolaiminae and the genus Endeolophos (Euchromadorinae) constituted a monophyletic clade (97% posterior probability and 85% bootstrap value). Endeolophos shows a close relationship with the genus Chromadorita, which was highly supported by the BI analysis (100% posterior possibility), and it clustered with Hypodontolaiminae. The morphological characters of Endeolophos–cuticle homogeneous, amphidial fovea at the level of the cephalic setae, and gubernaculum without telamons–conform to the subfamily Hypodontolaiminae, while other characters–outer labial sensillar setiform, cuticular with complex structure, pharyngeal bulb absent, and precloacal supplements absent–conform to the Euchromadorinae (Tchesunov 2014). We prefer to keep Endeolophos in Euchromadorinae despite the molecular evidence.

Chromadora shows a close relationship with Chromadorina in clade A2, and Venekey et al. (2019) also noted this and explained this relationship as a result of a misidentification; there are great morphological similarities between Chromadora and Chromadorina (Venekey et al. 2019).

In clade B, Gomphionema parvum (clade B1) of family Neotonchidae split early, but it is weakly supported (65% posterior probability and 40% bootstrap value). The position of Neotonchidae has been uncertain for a long time. Platt (1982) united Neotonchinae and Ethmolaiminae into Ethmolaimidae with the holapomorphy of “cup-shaped precloacal supplements with an external articulated flange”, but this character was doubted by Lorenzen (1994), as it also occurs in Dichromadora. Tchesunov (2014) also considered the validity of the family Neotonchidae due to its mixed characters of Chromadoridae, Cyatholaimidae, and Microlaimidae. According to our BI and ML analyses, Neotonchinae (clade B1) is a sister taxon to Achromadoridae (clade B2), Prodesmodora (clade B3), and Cyatholaimidae (clade B4), but there is no close relationship with Ethmolaiminae. This is in accordance with the topology tree by Leduc et al. (2017), but it is contrary to Bezerra et al.’s (2013) results based on morphological characters. We prefer to retain Neotonchidae until more molecular evidence is available.

In clade B3, two freshwater genera, Achromadora and Prodesmodora, form a highly supported clade (100% posterior probability and 100% bootstrap value). Lorenzen (1981, 1994) concluded that Achromadora is a holophylectic based on the the position of the ovary and prevailing parthenogenetic reproduction, and holapomorphy was presented in Prodesmodora. Taxonomic position of Prodesmodora should be reconsidered.