Tubifex conicus, He & Cui & Wang, 2012

He, Xuebao, Cui, Yongde & Wang, Hongzhu, 2012, Two new species of Tubificinae (Annelida: Clitellata: Naididae) from Tibet, China, Zootaxa 3458, pp. 159-165 : 160-162

Tubifex conicus n. sp.

( Fig. 1, Table 1)

Holotype. Mature, whole-mounted specimen, IHB XIZ20060409 a.

Type locality. Lake Yamzho Yumco in Tibet, China; Silt, depth of sample 12–38 m (holotype from 28 m, 29°10.672'N, 90°36.466'E), surface water temperature 12ºC, pH 9.0, DO 7.2 g /L, salinity 1.2‰, surface water conductivity 2.369 mS/cm, 9 April 2006, coll. by X. B. He and Y. D. Cui .

Paratypes. IHB XIZ20060409 b, IHB XIZ20100624 b-e, 5 whole-mounted specimens; 21 specimens in 10% formalin, 9 April 2006 and 24 June 2010, coll. by X.B. He and Y.D. Cui.

Etymology. The specific name “ conicus ” is Latin for “conical”, and refers to the conical penial sheath.

Description. Length 4.6–5.2 mm (Holotype: 5.2 mm), width at X about 0.5 mm; segments 27–32 (Holotype: 32). Clitellum inconspicuous. Prostomium conical.

Dorsal chaetae 1(0) hair and 1–4 bifids per bundle. Hairs smooth, 105–240 µm long, sometimes absent in posterior segments; bifids ( Fig. 1B) pectinate, 3–4 per bundle anteriorly, 62–88 µm long, upper tooth slightly longer than lower, with 2–3 thin intermediate teeth, nodulus distal, 1–3 per bundle in posterior segments, 65–80 µm long, teeth shorter and thicker than those of anterior dorsal bifids, teeth equally long, with 2–3 thin intermediate teeth. Ventral chaetae ( Fig. 1A) bifid, 4–5 per bundle anteriorly, 65–102 µm long, 2.0–2.5 µm thick, with upper tooth longer than the lower; 2–3 per bundle posteriorly, 72–80 µm long, 2.0–2.3 µm thick. Spermathecal chaetae unmodified, 4 per bundle in posterior to mid-X. No penial chaetae. Male pores paired in line with ventral chaetae, anterior to mid-XI. Spermathecal pores paired in line with ventral chaetae, posterior to mid-X.

Chloragogen cells from VI onwards. No coelomocytes. Male genitalia ( Fig. 1D) paired in X–XI. Vas deferens ( Fig. 1D, vd) ciliated throughout, 593–675 µm long, 15–25 µm wide, entering atrium apically. Atrium ( Fig. 1D, at) spindle-shaped, histologically homogeneous, 140 µm long, up to 44 µm wide, thin-walled in holotype (6.8–8.3 µm thick in paratypes). Prostate gland ( Fig. 1D, pr) large, attached to nearly middle portion of atrium by short stalk. Penial sheath ( Fig. 1D, ps; Fig. 1E) symmetrically conical, 48 µm wide at the base, 10 µm in diameter ectally, 45 µm long, wall 3.3–5.2 µm thick.

Spermatheca ( Fig. 1D, sa) long, pear-shaped. Ampulla 118–125 µm long, 59–92 µm wide; duct ( Fig. 1D, sd) short, 44–94 µm long, 22–24 µm wide, spermatozeugmata unknown.

Distribution. Known only from Lake Yamzho Yumco, Tibet, China.

Remarks. Judging from the apical junction between the long vas deferens and the atrium, the conical-shaped penis, the large prostate gland attaching to nearly middle portion of atrium, and the absence of an ejaculatory duct, the new species appears to be most closely related to the genera Christerius Holmquist, 1985 , Tasserkidrilus Holmquist, 1985 and Tubifex Lamarck, 1816 (sensu stricto) ( Brinkhurst 1962, 1984; Holmquist 1983, 1985; Timm 1989).

The systematics of the genus Tubifex is complex. The Tubifex group (sensu lato) was classified by Holmquist (1985) into 12 genera, but this revision has met with different levels of acceptance ( Holmquist 1985; Timm 1989; Juget et al. 2006). The new species has symmetrically conical penial sheaths, and it does not fit the main diagnostic characters of Christerius and Tasserkidrilus , due to the fact that Christerius has no penial sheaths and Tasserkidrilus has asymmetrically conical sheaths ( Holmquist 1985; Timm 1989). The long and winding vas deferens, entering the histologically homogeneous and spindle-shaped atrium apically, large prostate gland attaching to atrium by a short stalk, no ejaculatory duct and symmetrically conical penial sheaths of the new species fit closely the definition of Tubifex , although its vasa deferentia are not bipartite.

There are three species of Tubifex , viz T. newfei Pickavance & Cook, 1971 , T. montanus Kowalewski, 1919 , T. natalensis Brinkhurst, 1966 , that have conical penial sheaths ( Pickavance & Cook 1971; Brinkhurst & Jamieson 1971). With regard to the unmodified genital chaetae and the pectinate needles, T. conicus resembles T. newfei , but the latter has granular penis sheaths and apically curved atria ( Pickavance & Cook 1971). Tubifex montanus is well distinguishable from the new species by its palmate chaetae and larger length/width ratio of the penial sheaths ( Brinkhurst & Jamieson 1971). Moreover, T. natalensis differs from the new species by serrated hair chaetae and broad conical penis sheaths ( Brinkhurst & Jamieson 1971). A comparison of Tubifex species with conical penial sheaths is given in Table 1.