Branchinotogluma nikkoensis Jimi & Chen & Fujiwara 2022, sp. nov.

Branchinotogluma nikkoensis sp. nov.

[Japanese name: Nikko-tsunodashi-urokomushi]

Zoobank LSID: urn:lsid:zoobank.org:act: 380201B0-0231-4D7B-9AE8-1334B54A1FB0

(Figures 1–3)

Type locality: Nikko Seamount hydrothermal vent field (23°04.855' N, 142°19.506' E), Japan, 458 m in depth.

Material examined: Holotype (NSMT-Pol H-880): male, Nikko Seamount, 458 m depth, ROV KM-ROV, R / V Kaimei, December 8, 2020, collected by NJ. Paratypes (NSMT-Pol P-881–884): 4 specimens, male and female, collected with the holotype.

Description: Male 7 mm long, 2 mm wide (with parapodia, without chaetae), with 21 segments, flat. Living colour purple-red (Fig. 1). Body without dorsal tubercles. Ten pairs of elytra on segments 2, 4, 5, 7, and alternating segments up to 19. Elytra transparent, without fringing papillae at outer margins, without microtubercles and macrotubercles. Bilobed prostomium pinkish in live (whitish in ethanol), with distinct anterior lobes; anterior lobes triangular, with conical frontal filaments. Median antenna with short cylindrical ceratophore (Figs 1C, 3A), inserted at anterior notch of prostomium; style 0.5 mm, not papillate, thin and tapered. Lateral antennae absent. Eyes absent. Palps as long as ventral tentacular cirri (Figs 1C, 3A), whitish, smooth. Dorsal and ventral tentacular cirri with tentaculophores lateral to prostomium. Dorsal tentacular cirri longer than ventral tentacular cirri. Dorsal and ventral tentacular cirri whitish, not papillated. One pair of long, thick ventral papillae present on segment 12. Ventral segmental lamellae near ventral bases of neuropodia present on segments 13–17. Pygidium with a pair of pygidial cirri (Fig. 3B).

Parapodia biramous (Figs 2A, 3C). Notopodia shorter than neuropodia. Notopodia with short conical acicular lobes, on elytrigerous segments, with prominent bracts enclosing notochaetae dorsally and posteriorly (Figs 2B, 3C). About 50 notochaetae in each notopodium. Notochaetae short to long, with distinct rows of spines along one side, tapering to blunt tips, unidentate, stouter than neurochaetae (Figs 2C, 3D). Neuropodia with subconical prechaetal lobes tapering to sharp tips; post chaetal lobes shorter, rounded. Neurochaetae of one type, numerous, short to long, with borders finely spinous, tips slightly hooked and finely spinous, spines long and widely spaced in sub-distal region, without semi-lunar pockets (Figs 2D, 3E). Robust acicula in each rami. Ventral cirri tapered, not papillated, attached on middle part of parapodia. On segment 2, ventral cirri elongated and projecting from half of neuropodia. Branchiae present from chaetigers 3 to 16, short, digitate, with 2–3 filaments, two pouches per parapodium.

Pharynx with six pairs of papillae terminally and two pairs of jaws. Jaws growing inwardly, with minute denticles.

Variation: Paratypes 7–10 mm long, 2 mm wide without chaetae. Ventral papillae absent in female specimens.

Etymology: Named after the type locality (Nikko Seamount). The specific name is an adjective.

Distribution: Hydrothermal vent, 458 m, Nikko Seamount, Japan, Northwest Pacific Ocean.

Remarks: This new species resembles Branchinotogluma hessleri Pettibone, 1985 in having a body with 21 segments, long and thick ventral papillae, bilobed prostomium, dentated jaws. The new species can be distinguished from B. hessleri by the following features: i) ventral segmental lamellae near ventral bases of neuropodia present on segments 13–17; ii) dorsal tentacular cirri longer than ventral tentacular cirri; iii) dorsal tubercles absent. Branchinotogluma japonicus (Miura & Hashimoto, 1991) is known from the Kaikata Seamount on the Izu-Ogasawara Arc and vents in the Okinawa Trough (Zhang et al. 2018) but has not been found in Nikko Seamount which is much further south to the Kaikata Seamount. Branchinotogluma elytropapillata Zhang, Chen & Qiu, 2018, was described from the Okinawa Trough and also has not been found at Nikko Seamount. The new species can be differentiated from these two other known Japanese species by having elytra with smooth edges and jaws with denticles.