Doliodrilus adiacens, Wang & Erséus, 2004

Wang, Hongzhu & Erséus, Christer, 2004, New species of Doliodrilus and other Limnodriloidinae (Oligochaeta, Tubificidae) from Hainan and other parts of the north-west Pacific Ocean, Journal of Natural History 38 (3), pp. 269-299 : 281-283

Doliodrilus adiacens sp. n.

(figure 6)

Holotype. IHB HANA2000017 b, whole-mounted specimen.

Type locality. China, southern Hainan, lower end of estuary SE of Teng Qiao Town (Station SY00-9C).

Paratype. IHB HANA2000017 c–d, two specimens from type locality . SMNH Type Coll. 5472–5474, three specimens from type locality .

Etymology. The specific name ‘ adiacens ’ is Latin for ‘adjacent’, and alludes to the position of the spermathecal pores, which are located immediately behind the anterior septum of segment X.

Description. Two complete specimens, holotype 4.6 mm with 43 segments, one paratype 7.0 mm with 53 segments. Diameter at XI 0.3–0.4 mm. Prostomium blunt. Clitellum well developed over XI–XII. Chaetae bifid, with upper teeth variable in length (ranging from longer to shorter than lower teeth), and thinner than, or sometimes as thick as, lower (figure 6A). Chaetae 40–60 m m long, about 2.5 m m thick; (one) two to four (five) per bundle anteriorly, (one) two to three per bundle in post-clitellar segments. Ventral chaetae absent in XI. Male pores paired, in line with ventral chaetae in posterior part of XI (figure 6C, D). Spermathecal pores paired in line with ventral chaetae, immediately behind septum 9/10 (figure 6B).

Pharyngeal glands in IV– V; those in V sometimes indistinct. Chloragogen cells from VI onwards. Oesophagus in IX barrel-shaped, thick-walled and granulated, with chloragogen cells; semi-embedded blood plexus permeating dorsal region, with regular transverse vessels and less regular longitudinal ones.

Male genitalia (figure 6C, D) paired. Vas deferens (figure 6C, D: vd) 120–155 m m long, about 18 m m wide, entering atrium subapically. Atrium tubular, totally 150– 180 m m long, 16–24 m m wide; ental ampulla dilated but not thin-walled; dorsal wall opposite prostatic pad thin, and clusters of slender nuclei around pad absent (figure 6C, D: aa). Prostatic pad (ppd) small, oval to round, 16–24 m m long, situated at middle or ectal to middle of atrium. Prostate gland (pr) medium-sized, with oblong to round nuclei, latter maximally 8 m m long, 5 m m wide. Atrial duct (figure 6C, D: ad) with (1) posterior blind sac (bs), varying in size mainly with regard to extent of evagination of male pore, 15–50 m m long, 10–20 m m wide, and (2) efferent duct (ed), also variable, 10–40 m m long, 12–17 m m wide. Spermathecal ducts (figure 6B) about 30 m m long, about 23 m m wide, with ectal vestibules; ampullae cylindrical, 70–100 m m long, 25–35 m m wide, with more or less sigmoid spermatozeugmata (figure 6B: sz) in lumina.

Remarks. This species has two distinguishing characters. First, its spermathecal pores are adjacent to the anterior septum of segment X, whereas those of all the congeners are well separated from the septum. Second, in D. adiacens sp. n., the vasa deferentia enter the atria at a considerable distance from the apical ends of the latter, while the vasa join the atria apically in most of the congeners (vasa slightly subapical in D. brachyductus sp. n. and, probably, also in D. ciliatus sp. n.). With regard to the male ducts, it seems likely that D. adiacens is closely related to D. tener and D. longidentatus sp. n. In addition to the above-mentioned features, D. adiacens differs from D. tener by its lack of clusters of slender atrial nuclei and the more ectal location of its prostatic pads, and from D. longidentatus sp. n. by the shorter upper teeth of its chaetae, and the thicker walls of the ental part of the atria.

Distribution and habitat. Known only from type locality, Hainan, China. Brackish-water, lower intertidal, muddy sand.