Branchipolynoe pettiboneae

Description. (Based on the largest specimen (MBM 304552 )). Length 34 mm, width 11 mm, excluding chaetae. The mean values of all specimens 23 mm long, 7 mm wide. Body short, spindle-shaped, slightly tapered anteriorly and posteriorly, flattened ventrally and strongly arched dorsally with 21 segments (Fig. 1 a, b). Distinct segmental or nephridial papillae absent, except for 2 pairs of long ventral papillae on segments 11 and 12, projecting posteriorly (Fig. 1 c). With 10 pairs elytra, attached on segments 2, 4, 5, 7, 9, 11, 13, 15, 17 and 19. Elytra large, oval, smooth, elytra do not cover entire body, leaving mid dorsum uncovered. Non-elytra-bearing segments with dorsal cirri, which extend to reach the tips of neurochaetae. Pygidium with one pair of anal cirri. Branchiae well developed, arborescent, with long terminal filaments. Branchiae emerging from body in two short trunks, lower group attached dorsal to the base of notopodia or cirrophores of dorsal cirri and upper one attached to the lateral base of the elytrophores or dorsal tubercles. Branchiae beginning on segment 3 (5 branches on each trunk), becoming larger with more numerous branches in middle (15 branches on each trunk on segment 10), continuing to posterior region (15 branches on upper trunk, 16 branches on lower trunk on segment 18) (Fig. 1 d). Prostomium relatively small, oval and bilobed, with two rounded lateral anterior lobes, lateral antennae and eyes absent. Median antenna inserted anteriorly, distinct, conical, with short tip; ventro lateral palps thick, extending beyond prostomium (Fig. 1 a, b; Fig. 2 a, b). First or tentacular segment fused to prostomium and not distinct dorsally; tentaculophores lateral to prostomium, achaetous, short and bulbous, each with pairs of short cirriform tentacular cirri, shorter and more slender than palps. Notopodia from segment 2, small, papillary (Fig. 2 c, d); segment 2 to 21 with 2 – 3 notochaetae, projecting, stout, with serrated distal margin (Fig. 3 a). Neuropodia larger, rounded, with numerous slender neurochaetae and lower buccal cirri attached basally, similar to tentacular cirri but smaller. Neurochaetae stouter than notochaetae, having two kinds: one with tapered, serrated distal margin (Fig. 3 b, c) and the other with obviously hooked tips (Fig. 3 d).

Remarks. Three species have been reported in the genus Branchipolynoe. The specimens agree with the description of Branchipolynoe pettiboneae with large elytra, sub-biramous parapodia and branchiae beginning on segment 3. The original description did not mention the variation of the number of branches of the arborescent branchiae. The present specimens have large, arborescent branchiae continuing to the posterior region. The type specimens of Branchipolynoe pettiboneae were found from hydrothermal vents in the Kaikata Seamount (580 m) and the Okinawa Trough (1340 m). We also found two specimens in the Okinawa Trough (27 ° 33.06928 ’ N, 126 ° 58.13082 ’ E), very close to the type locality (27 ° 16.0 ′ N, 127 ° 05.0 ′ E). Here is the first time for the species to be found from cold-seep in the South China Sea. A key to all Branchipolynoe species is given.

Distribution. Japan, the South China Sea.

Material examined. 4 specimens (MBM 304552 - MBM 304555), mantle cavity of Bathymodiolus platifrons, coldseep, 22 ˚ 6 ' N, 119 ˚ 17 ' E, 1120 m, northeastern South China Sea, Chinese manned submersible Jiaolong, Dive 54, 18 June 2013, collectors Jianwen Qiu and Xinzheng Li; 10 specimen (MBM 304556 - MBM 304565), ibid, Dive 61, 9 July 2013, 1120 m, collectors Xinzheng Li and Jianwen Qiu. 2 specimens (MBM 304568 - 30456), Hydrothermal vent, Okinawa Trough, MBM 304566, 27 ° 33.06928 ’ N, 126 ° 58.13082 ’ E, 1361 m, coll. Faxian ROV of R / V Kexue, 20 Mar. 2014.