Ilyodrilus mesoprostatus Cui & Wang, 2009, n. sp.

Ilyodrilus mesoprostatus n. sp.

Holotype: IHB YAN 20030405n, whole-mounted specimen.

Type locality: Xingyun Lake (24°18'01΄ N, 102°47'58΄ E), eastern Yunnan Province, China; depth 5.0 m, bottom temperature 16.1°C, dissolved oxygen at bottom 7.6 mg /L, total nitrogen in water 2.960 mg /L, total phosphorus in water 0.129 mg /L, fine silt; 7 Apr 2003, coll. Y. Cui.

Etymology: “ meso ” and “ prostatus ” are Latin for “middle” and “prostate”, respectively. The specific name refers to the prostate glands attaching to the middle part of the atria.

Description: Specimen incomplete, length> 4.4 mm, diameter at XI about 0.7 mm, segments> 22. Clitellum inconspicuous.

Dorsal chaetae 2–5 hairs and 2–4 bifids per bundle, hairs slender and smooth, 250–350 µm long, 2.0 µm thick basally; bifids (Fig. 4 C) 100–125 µm long, 2.5–3.0 µm thick, with upper prong longer and thicker than lower. Dorsal chaetae absent in XI. Ventral chaetae (Fig. 4 A–B) bifid, 2–4 per bundle, 100–120 µm long, 2.5–3.0 µm thick, with upper prong longer and thinner than lower. Spermathecal chaetae unmodified in X. Penial chaetae absent in XI. Male pores paired in line with ventral chaetae in mid-XI. Spermathecal pores paired in line with ventral chaetae in mid-X.

Pharyngeal glands in II–III. Chloragogen cells from VI onwards. No coelomocytes. Male genitalia (Fig. 4 D) paired. Vas deferens (Fig. 4 D, vd) short and broad, 240–360 µm long, 36–46 µm wide, entering atrium apically. Atrial ampulla (Fig. 4 D, aa) somewhat spindle-shaped, 280–320 µm long, 58–108 µm wide. Prostate gland (Fig. 4 D, pr) large, attached to middle portion of atrium by short stalk. Atrial duct (Fig. 4 D, ed) curved, about 54 µm long, 25–36 µm wide. Soft part of penis (Fig. 4 D, pe) cylindrical, about 32 µm long, 20 µm diameter, enclosed in copulatory sac; penis surrounded by thin cuticularized, truncated-cone shaped sheath (Fig. 4 D, ps; Fig. 4 E), 116 µm long, 40–80 µm wide, one side of the ectal opening (Fig. 4 E, eo) curved upwards. Copulatory sac (Fig. 4 D, cs) 64 µm long, 40–50 µm wide.

Spermathecal ampullae (Fig. 4 D, sa) oval to round, 105–125 µm in diameter, with sperm masses (Fig. 4 D, sm) in lumina, ducts (Fig. 4 D, sd) 160–250 µm long, 35–58 µm wide.

Distribution and habitat: Known only from type locality, Yunnan Province, China. Freshwater lake, 5.0 m depth, water temperature 16 ºC, fine silt.

Remarks: The genus Ilyodrilus consists of the type species, I. perrieri Eisen, 1879, together with I. templetoni (Southern, 1909) and the dubious entities I. frantzi Brinkhurst, 1965 and I. fragilis Eisen, 1879. The principal characteristics and the distribution of congeners are shown in Table 3.

Ilyodrilus mesoprostatus n. sp. is distinguishable from congeners mainly in the respect that attachments of prostate glands are situated at middle part of the atria, while those of previously described species are all situated near the ental part of the atria (Table 3). The penial sheaths of different species are dissimilar. Despite the undeveloped type specimens, I. perrieri from California has more or less tubular, cuticular penial sheaths (Holmquist 1985). I. fragilis has thin sheaths (Eisen 1879; Brinkhurst 1978). The cosmopolitan I. templetoni has long conical tapering distally sheath with irregular opening (Brinkhurst 1965; Hrabĕ 1966) and I. frantzi has thin, truncated cone-shaped cuticular sheath (Brinkhurst 1965). The new species has truncated-cone shaped sheath, with ectal opening curved upwards at one side (Table 3).

The presence of spermatozeugmata is regarded a diagnostic character of the genus Ilyodrilus (Brinkhurst & Jamison 1971). Although spermatozeugmata are absent in the present new species, it was assigned to this genus according to the structure of male genitalia. Spermatozeugmata are in fact not always present in Ilyodrilus. For instance, they were not mentioned in previous descriptions of I. perrieri and I. fragilis (Eisen, 1879; Holmquist 1985; Brinkhurst, 1965; Brinkhurst & Jamison 1971). Some specimens of I. templetoni have been described without spermathecae at all (Brinkhurst & Jamison 1971, Wang 2002), while for I. frantzi, the presence of spermatozeugmata was confirmed (Holmquist 1985). In the new species, I. mesoprostatus, the sperm were massed. So, the genus Ilyodrilus Eisen, 1879, needs a revision in the future.