Two new marine nematode species belonging to the order Monhysterida are described from the sea of China. Halomonhystera zhangi sp. nov. is characterized by its relatively small body size; short anterior sensory setae; small, circular amphidial fovea located at the level of buccal cavity base; funnel-shaped buccal cavity; an excretory-secretory system with a large ventral gland and opening as a very narrow canal just posterior to the level of cephalic setae; slightly curved slender spicules with cephalated proximal end and tapered distal end; rod-like gubernaculum without apophysis; two papilliform precloacal supplements just in front of the cloaca; conico-cylindrical tail with two ventral papillae, each with a seta; and distance between the vulva and anus slightly longer than the tail length. This is the first new species of epiphytic nematode reported in China. The second new species, Stylotheristus flagellicaudatus sp. nov., has a relatively shorter body and longer tail; striated cuticle; The anterior sensilla arranged in two circles, the first circle consisting of six inner labial setiform papillae (3–4 µm) and the second circle consisting of 16 long setae (12–19 µm); a transversely oval amphideal fovea; a well-developed muscle around the funnel-shaped buccal cavity; short spicules and a gubernaculum composed of a single piece; and precloacal supplements absent. An updated key to all species of Halomonhystera and pictorial key to all species of Stylotheristus are also given.

Biodiversity, epiphytic, free-living marine nematode, Halomonhystera zhangi sp. nov., Stylotheristus flagellicaudatus sp. nov., taxonomy

To investigate the diversity of epiphytic nematodes growing in seaweed in the intertidal zone along the coast of the Yellow Sea, six species of common seaweeds were collected from 11 sites in 2021. The mean abundance of epiphytic nematodes in the seaweeds (e.g. Ulva lactuca, Gracilaria tenuistipitata, and Sargassum thunbergii) was 3502 ind./g algae (dry weight). Forty species belonging to 29 genera, 16 families, and seven orders were identified. The main species were Neochromadora poecilosomoides (Filipjev, 1918) Micoletzky, 1924, Chromadorina germanica (Bütschli, 1874) Wieser, 1954, Oncholaimus sinensis Zhang & Platt, 1983, and Thalassomonhystera siamensis Kito & Aryuthaka, 1998. Among these materials collected was an unknown species, which is identified as new to science; it is described here as Halomonhystera zhangi sp. nov.

The genus Halomonhystera was proposed by Andrássy (2006) to accommodate marine species previously included within the genus Geomonhystera. Tchesunov et al. (2015) reviewed the genus and gave an annotated list of 21 morphospecies. According to NeMys database (Nemys Eds 2024), 19 Halomonhystera species are accepted as valid. Halomonhystera ambiguoides (Bütschli, 1874) is considered a taxon inquirenda. Halomonhystera paradisjuncta (De Coninck, 1943) is accepted as H. disjuncta (Bastian, 1865) Andrássy, 2006. Halomonhystera zhangi sp. nov. is the first species of the genus found in the Yellow Sea of China.

To research the diversity of free-living marine nematodes in the northern South China Sea, sediment samples were taken at many sites in the intertidal zone. Results showed that the average abundance of free-living nematodes were 1596 ind·10 cm⁻² in the study area. The most dominant genera were Daptonema, Theristus, and Oncholaimus. Among them, an unrecorded species belonging to family Xyalidae was discovered, and it is described here as Stylotheristus flagellicaudatus sp. nov. At present, more than 300 nematode species have been identified in a study of the biodiversity of free-living marine nematodes in the South China Sea (Huang et al. 2021; Lu et al. 2022).

The genus Stylotheristus was established by Lorenzen in 1977 and, until now, included only two species, namely S. mutilus (Lorenzen, 1973) Lorenzen, 1977 and S. multipapillatus Pinto & Neres, 2020. Stylotheristus flagellicaudatus sp. nov. is also the first species recorded within the genus in the South China Sea.

Samples of Sargassum thunbergii (seaweed) containing specimens of Halomonhystera were collected using a shovel from the intertidal rocky reef of Qingdao along the Yellow China sea (36°37.43′N, 120°18.9′E) in May 2021. The whole algal samples were scooped off at the roots, then fixed with equivalent 10% formalin in seawater for long-term preservation. In the laboratory, algal samples were poured into a beaker with filtered water, shaken, and washed. Then washing liquid was poured into two layers of sieves (500 and 42 µm mesh sizes, respectively), and washed with tap water to remove silt and to separate macrofauna from meiofauna. Heavier sediment particles were removed using centrifugation in Ludox-TM (50% colloidal silica, suspension in water; product of Sigma Aldrich Co., USA) with a specific gravity of 1.15 g/ml (de Jonge and Bouwman 1977). Each sample was washed into a Petri dish with distilled water, and the meiofauna was sorted under a stereoscopic microscope. Nematodes were transferred into a cavity block containing a solution of 5% glycerol, 5% pure ethanol, and 90% freshwater by volume (McIntyre and Warwick 1984). After ethanol was slowly evaporated, the specimens were mounted in glycerin on permanent slides. The descriptions were made using a differential interference contrast microscope (Leica DM 2500). Line drawings were made with the aid of a camera lucida. All measurements were obtained using Leica LAS X v. 3.3.3, and all curved structures were measured along the arc or median line.

Sediment samples containing specimens of Stylotheristus were collected at an intertidal muddy beach of Sangengzhi along Hainan Island in the South China Sea (19°26′55″N, 108°37′38″E) in March 2017. The samples were taken from the 0–8 cm sediment layer using a 2.9 cm diameter sawn-off syringe, then fixed with 10% formalin in filtered seawater for long-term preservation. In the laboratory, the samples were stained with 0.1% rose Bengal, poured into two layers of sieves (500 and 42 µm mesh sizes), and washed with tap water to remove silt and separate macrofauna from meiofauna. The following experimental procedure was as mentioned above.

Abbreviations are as follows: a, the ratio of body length to maximum body diameter; abd, body diameter at cloaca or anus; b, ratio of body length to pharynx length; c, ratio of body length to tail length; cbd, corresponding body diameter; c′, ratio of tail length to cloacal or anus body diameter; V%, position of vulva from anterior end expressed as a percentage of total body length.

The characteristics of the new species match well with the main diagnostic of Stylotheristus (Fonseca and Bezerra 2014). The genus only has contained only two species until now, i.e. S. mutilus (Lorenzen, 1973) Lorenzen, 1977 and S. multipapillatus Pinto & Neres, 2020. The new species is distinguished from S. multipapillatus by the absence of precloacal supplements, relatively larger amphidial fovea, and a longer filiform tail. The latter species possess 11–15 papilliform precloacal supplements and a smaller amphidial fovea (32–38% of corresponding body diameter). The new species differs from S. mutilus by absence of inerratic circle of long cervical setae, relatively larger amphidial fovea (vs 37% of corresponding body diameter), shorter spicules not cephalated proximally (vs 18–20 µm with cephalated proximal ends) and the different structure of gubernaculum (only a piece). In S. mutilus, the gubernaculum is formed by two pieces, it has a circle of long cervical setae. Besides that, the body size of new species is smaller than of the two known species but the tail in the new species is relatively longer (Fig. 6).

Figure 6. 

Pictorial key for genus Stylotheristus A S. mutilus (from Lorenzen 1973) B S. multipapillatus (from Pinto and Neres 2020) C S. flagellicaudatus sp. nov. Scale bars: 20 µm (A1, B, C), 50 µm (A2), 10 µm (A3).

We are very thankful to Dr Mengdi Chu for her kind help in samples collection. We are sincerely grateful to two anonymous referees for reviewing and improving on the manuscript.