(Figs 4 A–F; 7B)
Specimens examined. Holotype: SMF 24137, Yonaguni Knoll IV Hydrothermal Vent Field, high CO2 seepage site, Okinawa Trough, 24°50.777’N 122°42.039’E, 1365 m, SO 196, Station 49, TV-MUC, 17 March 2008 (cs). Paratype: SMF 24138, Yonaguni Knoll IV Hydrothermal Vent Field, low CO2 seepage site, Okinawa Trough, 24°50.775’N 122°42.049’E, 1385 m, SO 196, Station 39, TV-MUC, 14 March 2008 (1 cs).
Description. Length 1.8 mm, width 0.38 mm. Prostomium with middle lobe delimited by incision from surrounding lobe, without glandular ridges or eyes. Only tips of buccal tentacles visible. Segment II forming one continuous prominent ventral and ventrolateral lappet (Fig. 4 A). Three pairs of slender, cirriform branchiae, arranged in one transverse row in segment III, not separated by median gap; innermost branchia of right group missing; second outermost branchiae of transverse row originating from segment II, outermost branchiae of transverse row originating from segment III, innermost branchiae of transverse row originating from segment IV (Fig. 7 B). Segment II without chaetae. Notopodia with capillary chaetae from segment III, present in 14 chaetigers; first pair of notopodia much smaller than in following chaetigers; notopodia of thoracic unciniger 8 slightly elevated and connected by prominent dorsal glandular ridge (Fig. 4 B); ventral papilla in notopodia of thoracic unciniger 8, absent in other notopodia; capillary chaetae of elevated notopodia broken and indeterminable. Neuropodial tori with uncini from segment VI, present in 11 thoracic uncinigers; tori without cirri. Continuous ventral shields conspicuous to thoracic unciniger 9. Two intermediate uncinigers (Fig. 4 C). Nine abdominal uncinigers without rudimentary notopodia. Pinnules without dorsal cirrus. Each pair of pinnules connected by dorsal glandular stripe (Fig. 4 C, D). Pygidium with terminal anus and one pair of short, stout, ventrolateral anal cirri (Fig. 4 D). Uncini in thoracic and intermediate uncinigers with 7–8 teeth in 2 vertical rows over rostral tooth and basal prow (Fig. 4 E). Uncini in abdominal uncinigers with 10 teeth in 3 vertical rows over rostral tooth and basal prow (Fig. 4 F).
Remarks. In the paratype specimen the two branchial groups are separated by a small median gap of about half the branchial width and the capillary chaetae of the elevated notopodia are not modified. The paratype specimen is 5.5 mm long and 0.6 mm wide.
Glyphanostomum bilabiatum sp. nov. is unique within the genus because of the modified and elevated notopodia, connected by a dorsal ridge in thoracic unciniger 8. Lateral lappets in segment II have also been described in the congeneric species G. moreirai Aguirrezabalaga & Parapar, 2014 and G. hesslei Reuscher, Fiege & Imajima, 2015. However, in these species the lappets were relatively small and restricted to the sides of segment II. In the new species there is one continuous lappet that stretches from side to side, across the ventrum of segment II, reminiscent of lateral lappets in anterior segments of some terebellid genera. Ventrally the lappet looks like a second lower lip. The modified thoracic unciniger 8 is remarkably similar to the typical modification in the genera Anobothrus and Zatsepinia.
The holotype was collected in the northern area of the Abyss vent. The area was characterized by high CO2 seepage. The sediment had a distinct H2S smell. At the collection site of the paratype gas bubbles were observed, even though it had a lower CO2 seepage rate (Rehder et al. 2008).
Etymology. The species name refers to the ventral lappet in segment II, which looks like a second lower lip. Distribution. Exclusively known from the Yonaguni Knoll IV Hydrothermal Field in the Okinawa Trough off the northeastern coast of Taiwan.