Linhystera filiformis Yanwei, Wen & Mengna, 2024, sp. nov.

Linhystera filiformis sp. nov. (Figures 3–4, Table 1) Type material. Four males and three females were measured and studied. Holotype: ♂ 1 on slide 19YMD2–1–47, paratypes: ♂ 2 on slide 22HI7–2–3, ♂ 3 on slide 22HI7–3–5, and ♂ 4 on slide 22HI7–3–20, ♀ 1 on slide 22HI7–3–10, ♀ 2 on slide 22HI7–3–9, ♀ 3 on slide 22HI7–2–4. Type locality and habitat. Specimens were obtained from intertidal sandy sediment at Yangma Island and Rizhao coast, Shandong Province, China. 35º14´N, 121º38΄E (YMD); 37º28´N, 119º24΄E (HI), respectively. Etymology. Species epithet filiformis refers to the filiform body shape. Measurements. All measurement data are given in Table 1. Description. Males. Body cylindrical, filiform, tapering towards both extremities. Cuticle striated. Six outer labial setae and four cephalic setae in the same circle and with the same length, 4–5 μm (0.6–0.8 head diameter). Amphidial fovea circular, 4–5 μm in diameter (0.4–0.6 c.b.d.), located at 7–8 μm (1.0–1.2 head diameter) from anterior body end. Buccal cavity minute, slit-like. Pharynx muscular and cylindrical, posterior end not widened and without posterior bulb. Cardia triangular. Nerve ring positioned slightly posterior to half of pharynx length (0.5–0.7 pharynx length). Excretory pore and ventral gland not observed. Tail filiform, 14–16 c.b.d. Three terminal setae, 5 μm in length. Three caudal glands in tail region. The reproductive system with one testis outstretched to the left of the intestine. Spicules paired and equal in length (1.4–1.6 cloacal body diameter), sickle-shaped, with a small capitulum in the proximal end, middle part enlarged and distal end tapered. Gubernaculum boat-shaped, 0.4–0.5 cloacal body diameter. Precloacal supplements absent. Females. Similar to males in most characters. Single anterior outstretched ovary to the left of the intestine.Vulva located at mid-body, 460–489 μm (49.8–54.6 % body length) to anterior body end. Vagina short and sclerotized. Differentiation diagnosis and discussion. Linhystera filiformis sp. nov. is characterized by filiform body, six outer labial setae and four cephalic setae in one circle with the same length, amphidial fovea circular, buccal cavity minute, slit-like, spicules sickle-shaped, gubernaculum boat-shaped and tail filiform. According to the diagnostic character of “presence and absence of gubernacular apophysis” (Figure 5), genus Linhystera can be divided in two groups, and morphometric characters of valid species of Linhystera are given in Table 2. L. breviapophysis and L. longiapophysis are characterized with the presence of gubernaculum with apophysis, while L. filiformis sp. nov., L. longa and L. problematica with gubernaculum without apophysis. Linhystera filiformis sp. nov. is differentiated from L. problematica in the body length (829–1048 μm vs. 720–833 μm), amphidial fovea size (3–5 μm vs. 5–8 μm in diameter), amphidial fovea location (6–8 μm vs. 8–11 μm from anterior end) and the tail length (154–233 μm vs. 95–106 μm); differs from L. longa in shorter body (829–1048 μm vs. 1250–1320 μm), shorter tail (154–233 μm vs. 270–322 μm) and shorter spicules (19–21 μm vs. 51–53 μm). Furthermore, differences between L. filiformis sp. nov. and other congeners are listed in the key below.