Eleutherolaimus conoicaudatus Huang, Sun and Bai, 2025

Eleutherolaimus conoicaudatus Huang, Sun and Bai sp. n.

( Figures 1 View Figure 1 , 2 View Figure 2 and Table 1)

Holotype and paratype material

Two males and one female were obtained and measured. Holotype (male 1), paratype (male 2) and paratype (female 1) are all on slide BHH3200-1-0-1.

Type locality and habitat

Holotype and paratypes were all collected from sea floor muddy sediment at Station 3200–1 ( 32°0′N, 122°0′E, water depth 30 m) in the Yellow Sea in 2008.

Etymology

The species epithet is composed of the Latin words conoidalis (conical) and caudatus (tail), referring to the conical tail of this species.

Measurements

All measurement data are given in Table 1.

Description

Males. Body slender, cylindrical, tapering slightly towards both extremities. Cuticle finely striated, no lateral differentiation. Short somatic setae largely absent except in pharyngeal and cloacal regions, sparsely distributed, 3 µm long. Cephalic region round, not set off from the body. Six minute labial papillae barely discernible around the mouth opening. A circle of six outer labial setae, 3–4 µm long, located at the level of the anterior gymnostom margin; a circle of four lateroventrally and laterodorsally cephalic setae, ca. 3 µm long, located posterior to the circle of outer labial setae. Another circle of six subcephalic setae situated posterior to amphidial fovea, ca. 4 µm long. Amphidial fovea circular, outline slightly cuticularised, 45–60% corresponding body diameter, located at the level of posterior half of the buccal cavity. Buccal cavity cylindrical, 4 µm wide, 10–11 µm deep, with cuticularised walls. Pharynx muscular, widening slightly posteriorly but not forming true bulb. Nerve ring located slightly posterior to mid-pharynx. Secretory-excretory system with small ventral gland situated immediately posterior to cardia; excretory pore situated at the level of nerve ring or just below the nerve ring. Cardia long, ca. 14 µm, partially surrounded by intestine.

Reproductive system with two opposed testes; position of testes relative to intestine unclear. Spicules ventrally arcuate, slightly swollen proximal portion, unequal in length, right spicule (ca. 35 µm long) slightly longer than left one (30–32 µm long). Gubernaculum with paired strongly cuticularised dorso-caudal apophyses, slightly bent. Precloacal supplements not observed. Tail conical with rounded tip and two 5–6 µm terminal setae. Two rows of subventral caudal setae 2–3 µm long. Three caudal gland cells and spinneret present.

Female. Similar to males but amphidial fovea slightly smaller and tail shorter. Reproductive system with two opposed outstretched ovaries; anterior ovary to the left of intestine, posterior ovary to the right of intestine. Spermatheca not observed. Vulva slightly posterior to mid-body. Vaginal gland present.

Species diagnosis. Eleutherolaimus conoicaudatus sp. n. is characterised by six outer labial setae 3–4 µm long and four cephalic setae 3 µm long arranged in two separate circles. A circle of six subcephalic setae, 4 µm long, located just posterior to the amphidial fovea. Buccal cavity cylindrical, 10–11 µm deep, secretory-excretory pore situated at the level of nerve ring or just below the nerve ring, spicules unequal in length, right spicule 36 µm long (1.1–1.2 cloacal body diameter), left one 30–32 µm long (ca. 1 cloacal body diameter), tail conical, relatively short in the genus, 3.1–4.2 cloacal body diameter with two terminal setae.

secretory-excretory system; (b) view of entire female, showing reproductive system; (c) posterior portion, showing spicules, gubernaculum and tail; (d) anterior end, showing cephalic sensilla, buccal cavity, amphidial fovea and striated cuticle; (e) high-magnification drawing of spicules and gubernaculum; (f) high-magnification drawing of female reproductive system, showing the vagina and vaginal glands. Scale bars: a, c–f, h–j = 20 µm; b, g = 100 µm; f = 30 µm.

Remarks. The new species is most similar to Eleutherolaimus inglisi Timm, 1967 in the body size, number and arrangement of six outer labial setae and four cephalic setae, size of the buccal cavity, size and placement of amphidial fovea, but differs from E. inglisi in the shape and length of spicules (swollen proximal portion not projected dorsally, longer and unequal in length, 30–35 µm vs capitulum projected dorsally, equal in length, 22–25 µm), and the tail shape (both male and female tail conical vs male tail conical and female tail conico-cylindrical with swollen tip). The new species is also like E. chesapeakensis ( Timm 1954) Tchesunov and Miljutin 2006 in the length and arrangement of outer labial setae and cephalic setae, size of the buccal cavity, size and placement of amphidial fovea, but differs by shorter body length and lower values of a (1770–1900 µm vs 3000–3700 µm, a = 41.3–46.6 vs 91.2–100 in E. chesapeakensis ), lower values of c of males (14.2–19.6 vs 24.1–30), and unequal spicules without velum vs equal spicules with velum in E. chesapeakensis . The new species also resembles E. nutus Gerlach, 1964 in the position of amphidial fovea and excretory pore, but differs from the latter species by shorter body length and lower values of a (1770–1900 µm vs 2370–3160 µm and a = 41.3–46.6 vs 61–65), shorter cephalic setae (3 µm vs 11–13 µm), and shorter tail (c′ = 3.1–4.2 vs 5.9). Eleutherolaimus conoicaudatus sp. n. can be distinguished from other known species by the following combination of characters: the number, arrangement and length of cephalic setae, size of the buccal cavity, size and placement of amphidial fovea, position of the secretory-excretory pore, unequal length of spicules and relatively short conical tails.