Sphaerolaimus articulatus Zhai, Liu & Huang, 2025, sp. nov.

Sphaerolaimus articulatus sp. nov. (Figures. 1–3, Table 1) Type material. Five males and four females were found. Holotype on slide 3400-8-2-2. Paratypes: M2 on slide 3400-8-2-2, M3 on slide 3500-4-2-3, M4 and M5 on slide 3400-8-2-1, F1 and F2 on slide 3400-6-2-2, F3 on slide 3400-8-2-2 and F4 on slide 3400-8-2-1. Type locality and habitat. Sea floor silt sediment at Station 3400-8 (34°0′ N, 124°0′ E, water depth 80 m) in the Yellow Sea. Etymology. The species epithet comes from the Latin word “ articulatus ”, referring to the double-jointed spicules. Measurements. All measurement data are given in Table 1. Descriptions Males. Body elongated fusiform. Cuticle with fine transverse striations and inconspicuous lateral alae. The lateral alae is 21 µm wide, extending along the body from about the level of the nerve ring to the cloaca. Anterior end widely conical. Mouth opening surrounded with six lips. Six minute inner labial papillae located at the base of the labial region. Six outer labial and four cephalic sensilla setiform, arranged in a common circle at the level of the anterior edge of the cheilostoma ribs. Subcephalic setae arranged in eight groups, located at the level of the anterior edge of the shagreen band. Each subcephalic group consists of two or three setae of various lengths, 8–20 μm long. Somatic setae arranged in eight longitudinal rows, 10–14 μm long; setae relatively long and dense in the pharyngeal region and tail region, short and sparse posterior to the nerve ring along the body. Amphidial fovea circular, with distinct cuticular rim, located at the level of the gymnostoma. Buccal cavity conspicuous and relatively voluminous. Cheilostoma, major portion of the buccal cavity, consists of three parts. Anterior part cone-shaped and smooth; middle part truncated cone-shaped, with fine longitudinal ribs; posterior part is spacy barrel-shaped with walls forming a sclerotized shagreen band. Anterior and posterior edges of the shagreen band are even, without lobes and inlets. Posterior wide conical part of the buccal cavity presents a pharyngostoma. The anterior part of the pharyngostoma, gymnostoma, is a narrow part just posterior to the shagreen band. The rearmost part of the buccal cavity, stegostoma, is surrounded with muscular pharyngeal tissue. Pharynx strongly muscular along its length, almost cylindrical, slightly broadened gradually to the cardia. Inner cuticular lining of the pharynx strong and wide. Cardia hemispheric, with internal cuticular lining, enveloped by intestinal tissue. Nerve ring difficult to identify, situated about anterior to the middle length of pharynx. Pore and ampulla of the secretory-excretory gland approximately located at the middle of the pharynx. Ventral gland cell body situated posterior to the cardia. Reproductive system diorchic with two outstretched testes. Spicules 1.5–1.8 cloacal body diameter long, arcuated, divided into two parts by a fine suture anterior to the middle, handle-shaped proximally and blunt distally. Gubernaculum with two dorsally hook-shaped apophyses, 40–52 µm long along arc. Precloacal supplements absent. Tail conico-cylindrical with posterior third part cylindrical, and with three terminal setae, 16–18 μm long. Three caudal glands and spinneret present. Females. Similar to males in general characteristics, except smaller amphideal fovea, 17–18% corresponding body diameter in width, located at the posterior edge of the shagreen band, tail with posterior quarter part cylindrical. Reproductive system monodelphic, with single anterior outstretched ovary situated at the right side of intestine. Post-vulvar sac absent. Vagina oblique. Vaginal glands present. Uterus filled with small spermatozoa. Spermatheca absent. Vulva situated far posterior to mid-body, 70–72% of the body length from the anterior end. Dignosis and discussion. Sphaerolaimus articulatus sp. nov. is characterized by circular amphidial fovea 11–12 µm wide in males at the level of the gymnostoma, and 7–8 µm in females at the posterior edge of the shagreen band, the longest subcephalic setae 15–20 µm long, spicules arcuate, divided into two parts by a fine suture anterior to the middle, handle-shaped proximally and blunt distally, gubernaculum with two dorsally hook-shaped apophyses. The genus Sphaerolaimus was divided into two groups of species by Zograf et al. (2020): one group with spicules more than two cloacal body diameter in length and another one with spicules equal or less than two cloacal body diameter in length. Among the second group, a small sub-group of species, characterized by the amphidial fovea situated behind the level of buccal cavity, and the rest of the species of the genus are characterized by amphidial fovea situated at the level of buccal cavity. The described species belongs to the second group which has short spicules and amphidial fovea situated at the level of buccal cavity. The new species is most similar to Sphaerolaimus crenellatus Warwick, 1973 in having double-jointed spicules, but differs from the latter species by shorter body length (1.70–2.06 vs 2.35–2.83 mm), more anterior position of amphidial fovea (at the level of buccal cavity vs behind the buccal cavity base), shorter cervical setae (10–14 μm vs 27.5–37 μm), and gubernaculum with paired strongly curved apophyses (vs with dorsally directed straight apophysis). The new species is also similar to Sphaerolaimus pacificus Allgén, 1947 in having paired strongly upcurved gubernacular apophyses, but differs by longer and double-jointed spicules (110–116 µm vs 72–87 µm long, not jointed). The new species is also similar to Sphaerolaimus limosus Fadeeva, 1983 in having hook-shaped gubernacular apophyses, but differs by double-jointed spicules and anterior position of amphidial fovea (at the level of buccal cavity vs behind the buccal cavity base). In body size, amphidial fovea diameter and position, the new species is close to Sphaerolaimus io Zograf, Pavlyuk, Trebukhova & Nguyen, 2020, but it is distinguished by spicules double-jointed (vs not jointed in the latter species), gubernaculum with paired longer and curved apophyses (vs gubernaculum with shorter and straight apophyses).