Orbinia dicrochaeta Wu 1962

Orbinia dicrochaeta Wu, 1962

Figures 3–4

Orbinia dicrochaeta Wu, 1962: 421, pl. 1, A–G.

Diagnosis. Prostomium conical, pointed. Branchiae from chaetiger 11–13. Thorax with 24–27 chaetigers, posterior thoracic neuropodial postchaetal lobes with 3 papillae. Interramal cirrus present on posterior 3–5 thoracic chaetigers continuing over anterior 7–13 abdominal chaetigers. Thoracic neuropodia with 3–4 long dorsoventral rows of subuluncini, one posterior short row of uncini present from inferior of postchaetal lobes.

Material examined. Paratypes: MBM304674, 1 specimen, West coast of Zhifu Island, Yantai, Shandong Province, 37°37ʹ N, 121°23ʹ E, 30 Jun. 1957; MBM304675, 1 specimen, intertidal zone of Zhanshan, Qingdao, Shandong Province, 36°03ʹ N, 120°23ʹ E, 15 Oct, 1958.

Additional material. MBM009988: SCS, R/V 228, Sta. 6020, 22°30ʹ N, 116°00ʹ E, 37 m, fine sand substrate, coll. Shaozong Wu, 11 Feb. 1960. MBM009989: SCS, R/V 101, Sta. 6152, 20°30ʹ N, 110°45ʹ E, 17 m, sand substrate, coll. Shaozong Wu, 10 Jan. 1960. MBM009992?YS, R/V V, Sta. 4005, 33°30ʹ N, 122°00' E, 16.5 m, hard mud substrate, coll. Yuheng Cui, 12 Jan. 1959. MBM009993: YS, R/V V, Sta. 4025, 32°30ʹ N, 122°45ʹ E, 31 m, brown fine sandy substrate, coll. Yuheng Cui, 23 Jan. 1959. MBM023237: ECS, R/V Handan, Sta. 4033, 32°00ʹ N, 123°30ʹ E, 36.3 m, fine sand substrate, coll. Liren Cheng, 8 Dec. 1959. MBM023238: ECS, R/V Handan, Sta. 4033, 32°00ʹ N, 123°30ʹ E, 36.3 m, fine sand substrate, coll. Liren Cheng, 8 Dec. 1959. MBM023268: YS, R/V V, Sta. 4033, 32°00ʹ N, 123°30ʹ E, 37 m, brown silty sand substrate, coll. Liren Cheng, 24 Oct. 1959. MBM193691: Qingdao, 36°04ʹ N, 120°23ʹ E, sandy beach, coll. Ruiping Sun & Dejian Yang, 4 Dec. 1982. MBM193692: Qingdao, 36°04ʹ N, 120°23ʹ E, sandy beach, coll. Ruiping Sun, 24 May. 1963. MBM023274: ECS, R/V 404, Sta. 4004, 33°30ʹ N, 121°45ʹ E, 33.5 m, black sand substrate, coll. Jinzhao Zhu, 23 Jan. 1959.

Description. Holotype lost; all paratypes posteriorly incomplete, one specimen (MBM304674) 51 mm long, 1.2 mm wide for 131 chaetigers, one specimen (MBM304675) 27 mm long, 1.1 mm wide for 82 chaetigers. Colour in alcohol: light brown.

Thoracic region with 24–27 chaetigers, depressed, abdomen cylindrical. Dorsal organs present on medial anterior edge of intersegmental annulations from posterior thoracic chaetigers, (Fig. 4C). Branchiae from chaetiger 11–13, short and slender at first (Fig. 3A), becoming larger in posterior thoracic chaetigers, with lateral cilia, continuing posteriorly (Figs 3B–E, 4C).

Prostomium conical, acutely pointed on anterior margin; no eyespots; peristomium a single short achaetous ring; nuchal organs as transverse slits at anterior border of peristomium (Figs 3A, 4A–B).

Thoracic notopodia with single finger-like postchaetal lobe from chaetiger 1. Thoracic neuropodia reduced to low thickened ridge from which chaetae emerge, with single postchaetal papilla present from medial posterior edge of first 13–17 chaetigers, gradually increasing to three on posterior thoracic chaetigers (Figs 3A–B, 4F). Abdominal neuropodia bilobed, with inner lobe larger than outer one (Fig. 3A–E). Fleshy subpodial flange conspicuous from anterior to middle abdominal region, present ventral to neuropodium, with one upper and one lower papillae (Fig. 3C–E). Stomach papillae from chaetiger 21–25, beginning as 1–2 papillae, increasing to 11 on subsequent 5–8 chaetigers on each side, encircling ventral side of worm, gradually decreasing in number, 1–3 on subsequent 20–40 chaetigers, then absent (Figs 3B–C, 4E). Interramal cirrus present between noto– and neuropodia of posterior 3–5 thoracic chaetigers, continuing over anterior 7–13 abdominal chaetigers (Fig. 3B–C).

Thoracic notopodia with fascicles of crenulated capillaries. Abdominal notopodia with fascicles crenulated capillaries, three embedded aciculae present (Fig. 3C–E), and 1–3 furcate chaetae (Fig. 4H).

Thoracic neuropodia with 3–4 long dorsoventral rows of subuluncini; one posterior short row of uncini present from inferior of postchaetal lobes, curving ventrally to anterior rows and one last row of crenulated capillaries (Fig. 4F). Subuluncini with distinct transverse rows (about 10 rows) of barbs on broad base, capillary extension weakly notched, with rows of indistinct barbs (Fig. 4D, F). Uncini with blunted tips covered by hood, bearing rows of weakly barbs (Fig. 4F–G). Abdominal neuropodia with only 4–6 fine crenulated capillaries and one thin acicula. Flail chaetae absent.

Distribution: Yellow Sea (Type locality), East China Sea, and South China Sea, intertidal to 37 m.

Remarks: Among species of Orbinia, eight species can be classified into Group B. Orbinia dicrochaeta has branchiae from chaetigers 11–13, posterior thoracic neuropodial postchaetal lobe only with 1–3 papillae, and thus belongs to Group B. In addition, O. dicrochaeta is characterized by having four rows of subuluncini and one short, curved fifth transverse row of uncini on thoracic neuropodia.

In addition, Orbinia dicrochaeta has an interramal cirrus on the posterior thoracic and anterior abdominal chaetigers. In Group B, five additional species possess interramal cirri and should be compared: O. vietnamensis Gallardo, 1968, O. oligopapillata López, Cladera & San Martín, 2006, O. orensanzi Blake, 2017, O. riseri (Pettibone, 1957) and O. dubia Day, 1955. In the first two species, interramal cirri are present only in the abdominal region; in O. orensanzi, interramal cirri are present only on the posterior thoracic chaetigers; whereas in last two species, interramal cirri occur from the posterior thoracic region through most of the abdominal chaetigers.