Parabathylaimus gracilis Wang & Guo & Wang 2022, sp. nov.

Parabathylaimus gracilis sp. nov.

(Figures 1–2, Table 1)

Type material. Three males and two females were measured and studied. Holotype: ♂ 1 on slide 19YMD2-2-108; paratypes: ♂ 2 on slide 19YMD2-2-28, ♂ 3 on slide 19YMD2-2-46, ♀ 1 on slide 19YMD2-2-30, and ♀ 2 on slide 19YMD2-2-27.

Type locality and habitat. Specimens were collected from intertidal muddy sediment (0-2 cm) at Yangma Island, Shandong Province. 37º45′N, 121º61′E.

Etymology. Species name gracilis means graceful and slender.

Measurements. All measurement data are given in Table 1.

Description. Males. Body cylindrical, 1882–1975 μm in length. Six short inner labial setae unjointed, 3 μm in length (0.13–0.14 head diameter long). Six outer labial setae three-jointed, 15–19 μm in length (0.7–0.8 head diameter long). Four cephalic setae unjointed, 6–8 μm in length (0.29–0.36 head diameter long), outer labial setae and cephalic setae in a same circle. Somatic setae (4–5 μm in length) scarcely present in pharynx and caudal region. Cuticle faintly striated. Amphidial fovea situated at middle of buccal cavity, unispiral with circular aperture, 6–7 μm in diameter (23%–28% corresponding body diameter), 19–20 μm from anterior body end. Three lips high and deeply incised. Buccal cavity cuticularized, cylindrical in shape and without teeth, 11–14 μm in width and 34–35 μm in length. Pharynx cylindrical, anterior end partly surrounding buccal cavity and posterior end slightly swollen, 343–357 μm in length. Nerve ring situated 143–161 μm from anterior end. Secretory-excretory system and cardia not observed. Reproductive system monorchic with anterior testis outstretched, 276 μm in length (14.2% of total body length), located on the left of the intestine. Spicules paired and slightly curved, 37–41 μm in length (1.1–1.2 cloacal body diameter). Gubernaculum kidney-like, distal end cuticularized with a short projection. Precloacal supplements absent. Tail conical, rounded at posterior end, 130–140 μm (3.6–4.1 anal body diameter) in length, three caudal glands in line. Caudal setae present, long subterminal setae absent.

Females. Similar to males in most characters. Reproductive system didelphic and amphidelphic with reflexed ovaries, 494 μm in length (25% of total body length).Anterior and posterior ovary on the right of the intestine. Vulva located near mid-body, 941–1019 μm from anterior end. Vagina short and sclerotized.

Differential diagnosis and discussion. Parabathylaimus gracilis sp. nov. is characterized by six outer labial setae three-jointed in males and females, cephalic setae and inner labial setae unjointed, amphidial fovea positioned at middle of buccal cavity, spicules slightly curved, gubernaculum kidney-like and distal end cuticularized with a pointed projection, tail conical without long subterminal setae.

Parabathylaimus gracilis sp. nov. is similar to P. jare, P. arthropappus, and P. denticaudatus in tail shape, but differs from P. arthropappus in the shorter body length (1882–1995 μm vs. 3100–3300 μm), inner and outer labial setae shape (inner unjointed, outer three-jointed vs. inner three-jointed, outer four-jointed), shorter outer labial setae and cephalic setae (15–19 μm, 6–8 μm vs. 47–52 μm, 18–19 μm), amphidial fovea position (at middle of buccal cavity vs. posterior to buccal cavity), spicules length and shape (37–41 μm, proximal end not cephalated vs. 50–57 μm, proximal end cephalated), and absence of long subterminal setae (present in P. arthropappus). The new species differs from P. denticaudatus in the shorter outer labial setae (15–19 μm vs. 25–30 μm), amphidial fovea position and diameter (at middle of buccal cavity, 23–28% corresponding body diameter vs. posterior to buccal cavity, 14% corresponding body diameter), outer labial setae shape (three-jointed vs. four-jointed), unjointed inner labial setae and cephalic setae (jointed in P. denticaudatus). The new species differs from P. jare in the shorter body length (1882–1995 μm vs. 2982–3492 μm), outer labial setae shape (three-jointed vs. three-jointed in male and two-jointed in female), amphidial fovea position (at the middle buccal cavity vs. posterior end at level of posterior buccal cavity), spicules shape (slightly curved vs. slightly curved, with heel-shaped projection), and gubernaculum shape (distal end swollen with one projection vs. distal end with two tooth-like projections).