Doliodrilus bisaccus, 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 : 274-276

Doliodrilus bisaccus sp. n.

(figure 2)

Holotype. SMNH Type Coll. 5457 , whole-mounted specimen. Type locality. Japan, Honshu, Mie Prefecture, Gokasho Bay , brackish-water; 16 May 1990, coll. K. Ogawa.

Paratypes. SMNH Type Coll. 5458–5460, three specimens from type locality.

Other material. IHB HANA 2000009al, one specimen from Hainan Island, Station SY00-6.

Etymology. The specific name bisaccus is Latin for ‘with two sacs’, and refers to the bilobed atrial ampullae, a feature unique within the genus.

Description of type material. Two complete specimens, holotype 9.7 mm with 43 segments, one paratype 8.4 mm with 41 segments. Diameter at XI 0.3–0.5 mm. Prostomium usually conical. Clitellum well developed over XI–XII. Chaetae bifid, with upper teeth more or less as long as, and thinner than, or sometimes as thick as, lower (figure 2A). Chaetae 45–60 m m long, about 2.5 m m thick; (zero) one to two (three) per bundle anteriorly, zero to two 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 2B). Spermathecal pores paired in line with ventral chaetae in anterior part of X.

Pharyngeal glands well developed in IV–V. Chloragogen cells from VI onwards. Oesophagus in IX barrel-shaped, thick-walled and granulated, without chloragogen cells; semi-embedded blood plexus permeating entire region, with regular transverse vessels and less regular longitudinal ones.

Male genitalia (figure 2C, D) paired. Vas deferens (figure 2C, D: vd) short and wide, about 70 m m long, 19–22 m m wide, entering atrium apically. Ental part of atrial ampulla bilobed, thin-walled and dilated; lobes separated by constriction. First ‘lobe’ of ampulla (figure 2C, D: at1) conical or oval, 35–65 m m long, maximally 39–53 m m wide. Second ‘lobe’ oval (at2), 35–50 m m long, maximally 39–44 m m wide. Ectal part of atrial ampulla more duct-like, thick-walled, with numerous slender nuclei close to (figure 2C, D: ncc) and opposite to (nco) prostatic pad. This part 150–155 m m long, 16–48 m m wide. Prostatic pad (figure 2C, D: ppd) round, 46-48 m m long, ventrally attached to middle of duct-like part of atrial ampulla (aa), bulging out from atrium. Prostate gland (figure 2C, D: pr) large, with small nuclei and large nucleus-like bodies, latter oblong or round, maximally 16 m m long, 15 m m wide. Atrial duct (figure 2C, D: ad) with (1) posterior blind sac (bs), 25–50 m m long, 15–48 m m wide, and (2) efferent duct (ed), 45–55 m m long, 31–48 m m wide, opening directly to exterior through simple pore. Sperm sac in IX–XII, or XI–XII. Egg sac in XII–XIII or absent. Spermathecae (figure 2B) large; ducts 75–105 m m long, 36–48 m m wide, with ectal vestibules; ampullae oblong, thin-walled, 145–195 m m long, 50–75 m m wide, with sperm masses in lumina.

Brief description of specimen from Hainan. Specimen complete, 5 mm long, 39 segments. Chaetae 45–55 m m long, about 2.5 m m thick, bifid, with equal teeth; one to two (three) per bundle anteriorly, one to two per bundle in post-clitellar segments. Male pores posterior to middle of XI. Male ducts somewhat twisted and difficult to make out. Spermathecae with oblong ampullae, but without sperm; specimen appears pre-copulatory.

Remarks. This species is easily distinguished from all its congeners by its bilobed atrial ampullae, and low number of chaetae; latter usually no more than two per bundle or even absent. The atria have dorsal thickenings and clusters of slender nuclei around the prostatic pads, as well as blind sacs, suggesting a close relationship to D. tener . In the new species, however, the vasa deferentia are shorter than those of the latter, and the prostatic pads are situated at mid-point of the atrium, as opposed to the more ental location in D. tener .

Although not all its features were clearly seen, the specimen from Hainan has fewer chaetae than D. tener , and appears to have blind sacs and clusters of slender nuclei in the atria. Therefore, it is identified as D. bisaccus , at least for the time being.

Distribution and habitat. Japan and southern China. Brackish-water, muddy sands.