Tubifex laxus n. sp.

(Figs 1–3)

Holotype. IHB XZ20130630 j, mature specimen, stained and whole-mounted in Canada balsam.

Type locality. Wetland in Cuomujiri Mountain (29°47′46″N, 94°24′53″E), southeast Tibet of China, ca. 3,990 m asl. Sediment silt, with abundant hydrophytes. Water depth 15–20 cm, water temperature 11.5 °C, pH 6.2, dissolved oxygen 9.1 mg /L, and conductivity 19.3 µs/cm. Collected on 30 June 2013 by the authors.

Paratype. IHB XZ20130630 k, 1 mature specimen whole mounted in Canada balsam, from the type locality.

Other material. 1 specimen for SEM, from the type locality.

Etymology. The epithet “ laxus ” is Latin for “wide”, and refers to the relatively wide atrium of the species.

Description. Length 6.8–7.2 mm (holotype: 6.8 mm), width at genital segments X–XI 0.4–0.5 mm (holotype: 0.5 mm). Segments 41–46 (holotype: 46). Prostomium obtuse or conical (Figs. 1A, 2A, 3A: pro). Clitellum inconspicuous, in X–XI. No coelomocytes. Dorsal chaetae (1) 2–3 hairs and 2–5 bifids per bundle. Hairs slightly hirsute, 110–300 µm long, 2–4 µm thick (Fig. 1B, C). Dorsal bifids palmate, 40–90 µm long, 2–4 µm thick, upper and lower tooth subequal, 6–8 µm long, with 6–10 fine intermediate teeth, nodulus distal (Figs. 1C, D, 3D). Ventral chaetae bifid, (3) 4–5 per bundle anteriorly, 2–3 per bundle posteriorly, 60–100 µm long, 2–4 µm thick, upper tooth slightly thinner and longer than lower (4–5 µm/3–5 µm), with (0) 1–2 fine intermediate teeth, nodulus distal (Figs. 1E, F, 3B, C). Ventral chaetae in X–XI present, unmodified. Male pores paired in line with ventral chaetae, in middle to posterior of XI.

Pharynx in III–IV, 180–200 µm long, 120–150 µm wide (Figs. 2A, 3A: ph). Chloragogen cells from VI onwards. Male genitalia paired in X–XI. Sperm funnel cup-shaped, 20–30 µm long, 55–75 µm wide (Figs. 2C, 3F: sf). Vas deferens 500–600 µm long, about 3 times as long as atrium, uniformly 15 µm wide, internally ciliated along the whole length and entering atrium apically (Figs. 2C, 3F: vd). Atrium ovoid placed vertically in XI, 140– 160 µm long, maximally 85–100 µm wide; no ejaculatory duct (Figs. 2B, C, 3F: at). Prostate gland solid, attached to ental portion of atrium by short stalk, 25–35 µm long, 70–85 µm wide (Figs. 2C, 3F: pr). Penis inconspicuous, surrounded by cuticular, symmetrical and funnel-shaped penial sheath. Penial sheath 30–40 µm long, 35–40 µm wide at ental end, length to width ratio nearly 1.0 (Fig. 2D, 3E, F: ps). Testes paired in X, small, immediately behind septum 9/10 (Fig. 2A: t). Seminal vesicles in X–XI, with little developing sperm. Ovaries paired in XI, immediately behind septum 10/11 (Fig. 2A: o). Egg present in XI. Spermathecae absent.

Remarks. The new species could be placed in the genera Tasserkidrilus Holmquist, 1985 or Tubifex (sensu stricto) based on: vas deferens at least two times longer than atrium, ejaculatory duct lacking, solid prostate gland attached to ental portion of atrium, and conical penial sheath (Brinkhurst & Jamieson 1971; Holmquist 1983, 1985; Timm 1989). However, in species of Tasserkidrilus the ventral chaetae in XI are lacking in mature specimens, the atrium consists of two or three portions of different thickness, and the penial sheath is asymmetrical (Timm 1989).

These characters are inconsistent with those of the new material. Therefore, we discard Tasserkidrilus as a possible genus for the new species. Although its vasa deferentia are not bipartite, the new material agrees with the generic diagnosis of Tubifex in the following characters: long vas deferens entering atrium apically, ovoid atrium placed vertically in XI with no ejaculatory duct, solid prostate gland attached to ental atrium by a short stalk, and symmetrically funnel-shaped penial sheath (Brinkhurst & Jamieson 1971; He et al. 2012). The new species is easily distinguished from allied species by its relatively wide atrium (Brinkhurst & Jamieson 1971; Pickavance & Cook 1971; Hrabě 1981; He et al. 2012; Peng et al. 2014) (Table 2).

Distribution and habitat. Known only from the type locality, Cuomujiri Mountain, southeast Tibet of China. Freshwater.