Isochaetides palmatus, 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 : 162-164

Isochaetides palmatus n. sp.

( Fig. 2, Table 2)

Holotype. Mature, whole-mounted specimen, IHB ZYH20060409 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 ZYH20100624 b-e, 4 whole-mounted specimens; 40 specimens in 10% formalin, 24 June 2010, coll. by X.B. He and Y.D. Cui .

Etymology. The specific name “ palmatus ” is Latin for “palmate”, and refers to palmate dorsal chaetae of this worm.

Description. Length 7–20 mm (Holotype: 20 mm); segments 49–110 (Holotype: 110). Clitellum inconspicuous. Prostomium conical.

Dorsal chaetae ( Fig. 2A) all palmate, 6–15 per bundle anteriorly, 2–6 (8) per bundle posteriorly; 90–120 µm long, about 2.5–3.3 µm thick, with 5–8 intermediate teeth, rod-like shaped, 0.58–0.88 µm thick. Most ventral chaetae ( Fig. 2B,C) pectinate, 5–10 per bundle anteriorly, 2–5 per bundle posteriorly; 82–118 µm long, 2.5–3.8 µm thin, with distal tooth longer than proximal and 1–3(0) intermediate fine teeth, nodulus distal. Spermathecal chaetae unmodified, 8 per bundle. Penial chaetae absent. Male pores paired in line with ventral chaetae, anterior to mid-XI. Spermathecal pores paired in line with lateral line, posterior to mid-X.

Chloragogen cells from VI onwards. No coelomocytes. Male genitalia ( Fig. 2D) paired in X–XI. Vas deferens ( Fig. 2D, vd) about 1500 µm long, 13–18 µm wide, ciliated throughout, entering atrium apically. Atrium ( Fig. 2D, at) tubular, histologically homogeneous, 215–240 µm long, 35–50 µm wide. Prostate gland ( Fig. 2D, pr) small, ovoid and solid, 85–95 µm long, 45–70 µm wide, attached to subapical portion of atrium. Penis ( Fig. 2D, pe) cylindrical, about 80 µm long, 70 µm wide. No penis sheath.

Spermathecae ( Fig. 2D, sa) pear-shaped. Ampulla 155–185 µm long, 150–175 µm wide; duct short, 30–35 µm wide; spermatozeugmata present, worm like.

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

Remarks. The long vas deferens attaching to the atrium apically, the cylindrical atrium being shorter than the vas deferens, and the small prostate gland attached to the atrium subapically all indicate that the new species is most closely related to one of the genera Tasserkidrilus Holmquist, 1985 , Troglodrilus Juget, des Châtelliers & Rodriguez, 2006 and Isochaetides Hrabĕ, 1966 ( Brinkhurst 1981; Timm 1989; Juget et al. 2006). Troglodrilus has a large prostate gland and an atrium with two kinds of epithelial cells ( Juget et al. 2006), but the new species differs from this genus by having a small prostate and a histologically homogeneous atrium; Tasserkidrilus has an asymmetrically conical penial sheath ( Timm 1989), but a penis sheath is absent in the new species. According to the structure of the male genitalia, i.e. long vas deferens, histologically homogeneous and cylindrical atrium, small prostate gland and absence of penis sheaths, the new species fits closely to the definition of Isochaetides , although its spermathecal chaetae are unmodified ( Brinkhurst 1981).

I. palmatus n. sp. is distinguishable from its congeners mainly in the absence of a penial sheath, the small prostate gland, the long straight atrium, the unmodified spermathecal chaetae and the palmate dorsal chaetae (Table 2). Among the known members of Isochaetides , only one species, Isochaetides gianii Rodriguez & Achurra, 2010 , recorded only from a groundwater spring of northern Spain, has pectinate dorsal chaetae ( Rodriguez & Achurra, 2010). The new species differs from it in palmate dorsal chaetae and pectinate ventral chaetae.

This is the first species of Isochaetides recorded from China. Nineteen species were previously known in the genus, among which three species are distributed in the Palearctic region ( Brinkhurst & Jamieson 1971; Timm & Veldhuijzen van Zanten 2002; Rodriguez & Achurra 2010), four in the Nearctic region ( Brinkhurst 1965; Brinkhurst & Cook 1966; Brinkhurst & Jamieson 1971; Brinkhurst & Diaz 1985), eleven in Lake Baikal ( Semernoy 2004), and only one in the neotropical region ( Brinkhurst & Jamieson 1971).