Doliodrilus fibrisaccus sp. n.

(figure 8)

Holotype. IHB HANA2000018 a, whole-mounted specimen.

Type locality. China, southern Hainan, brackish-water pond SE of Teng Qiao Town (Station SY00-10).

Paratypes. IHB HANA2000009 a–d, HANA2000017 a, five specimens: four from SY00-6, one from SY00-9C. SMNH Type Coll. 5475–5479, five specimens: one from type locality, four from SY00-6.

Other material. IHB HANA2000009 e–h, four specimens from SY00-6. SMNH Main Coll. 43682–43685, four specimens from SY00-6. SMNH Main Coll. 43686– 43695, 10 specimens from Fiji, Viti Levu, Lauthala Bay (E of Suva), Rewa River Delta at N end of Lauthala Island, mangrove swamp, mud, intertidal, oligohaline brackish-water; 3 December 1982, coll. C. Erséus (Station FI 82-16).

Etymology. The specific name fibrisaccus, Latin meaning ‘with fibrous sac’, refers to the muscular ectal part of the male duct.

Description of material from Hainan (figure 8A–C). Six complete specimens 3.5–9.7 mm long (holotype: 6.5 mm), 23–57 segments (holotype: 33). Diameter at XI 0.3–0.4 mm. Prostomium more or less conical. Clitellum more or less developed over XI–XII. Chaetae (figure 8A) bifid, with upper teeth thinner, and shorter than, or as long as, lower. Chaetae 45–60 m m long, about 2.5 m m thick, (one) two to four (five) per bundle anteriorly, (one) two (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 8C). Spermathecal pores paired, in line with ventral chaetae in anterior part of X (figure 8B).

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

Male genitalia (figure 8C) paired. Vas deferens (figure 8C: vd) longer than atrium, 155–200 m m long, 4–7 m m wide, straight or irregularly coiled, entering atrium apically. Atrium tubular, totally 150–190 m m long; ental (shorter) part of ampulla (figure 8C: aa) thin-walled and somewhat dilated, oval to round, about 25 m m wide; middle (longer) duct-like part of atrium 14–16 m m wide, with thicker inner epithelium. Prostatic pad (figure 8C: ppd) oval to round, 35–60 m m long, ventrally attached to atrium, ental to middle of duct-like part. Prostate gland (pr) small, posteriorly situated, with oval to round nuclei, latter maximally 7 m m long and 5 m m wide. Ectal part of atrium sacciform (figure 8C: ead), 70–80 m m long, maximally 40–65 m m wide, with 10–12 m m thick muscular layer. Sperm sac, when developed, extending through one of segments IX–XI. Egg sac, when developed, extending through one to two segments within XI–XIII. Spermathecae (figure 8B) slender; ducts 45–80 m m long, 15–30 m m wide, with ectal vestibules; ampullae oval to round, thin-walled, 55–60 m m long, 20–45 m m wide, with scattered spermatozoa in lumina.

Brief description of material from Fiji (figure 8D, E). Four complete specimens 3.0– 8.5 mm long, 17–52 segments. Diameter at XI 0.2–0.3 mm. Chaetae 45–55 m m long, about 2.5 m m thick, with upper teeth thinner and shorter than lower; (two) three to four per bundle anteriorly, usually two per bundle in post-clitellar segments. Vasa deferentia (figure 8E: vd) 160–195 m m long, about 12 m m wide. Atria totally 160–200 m m long, with ental part of ampullae (figure 8E: aa) 19–22 m m wide, middle duct-like part 15–17 m m wide. Prostatic pads (figure 8E: ppd) about 30 m m long. Prostate glands (pr) broadly attached to pads, with nuclei maximally 7 m m long, about 5 m m wide. Ectal sacciform parts of atria (figure 8E: ead) 75–100 m m long, maximally 36–48 m m wide, with 3–10 m m thick muscular layer. Spermathecal ducts about 60 m m long, 24–27 m m wide; ampullae oval, 55–65 m m long, 25–30 m m wide, containing sperm masses (figure 8D).

Remarks. This taxon has a heavily muscular, dilated, ectal part in each male duct, which is not present in any other species of Doliodrilus. It is not conclusive whether this structure is a proper copulatory sac, i.e. a secondary ectodermal invagination associated with the male pore, or whether it is homologous to the ectal part of the atrial duct, but the latter hypothesis is implied in the description above. In several other species of Doliodrilus, such as D. tener and D. diverticulatus, the ectalmost part of the atrium is modified and includes a posterior blind sac. In most of our specimens of D. fibrisaccus, the irregularity in the outline of the muscular sac (i.e. with the posterior face of the sac bulging more than the anterior face; see figure 8C, E: ead) appears to suggest homology with the ‘blind sac’ of other species.

The material from Fiji is similar to that from Hainan. However, in the Fijian worms, the vasa deferentia are thicker, and the ectal dilatations on the atria are somewhat longer and generally thinner, than the corresponding features in the Hainanese material.

Distribution and habitat. Southern China and Fiji. Brackish-water, lower intertidal, muddy sand.