Benthoscolex seisuiae

Description. Body flat, tapered in anterior and posterior regions, whitish both in life and after fixation; pair of brown longitudinal lines on ventral middle line; no pigmentation on dorsal surface (Fig. 1 A). Body surface smooth. Prostomium triangular; eyes absent. Pairs of lateral antennae and palps present, conical, smooth; palps 1.8 times as long as lateral antennae. Median antenna present, conical, as long as lateral antennae (Figs 1 B, 2 A). Caruncle consists of three longitudinal ridges, without ornamentation, extends to chaetigers 1 - 2 (depending on fixation), unattached in posterior part (Fig. 2 A). Mouth composed of chaetigers 1 - 2. Pharynx eversible with black pigmentation. Parapodia biramous, notopodia and neuropodia clearly separated (Fig. 2 B). Dorsal and ventral cirri occur singly on notopodium and neuropodium, conical, whitish, arising from body wall, present in all chaetigers. Branchiae present on chaetiger 6 or 7 and succeeding posterior chaetigers: anterior ones simple, conical lobes; gradually increasing in number and size posteriorly (Fig. 1 B, C), branched from base; filaments digitiform, 8 - 10 filaments per branchia in middle body chaetigers, 15 - 18 filaments per branchia in posterior chaetigers; branchiae in posterior chaetigers differ in size between specimens, but never reaching to tip of notochaetae (Fig. 1 D). Notochaetae contain three types: i) harpoon chaetae, with serrations limited to one side (Fig. 2 C); ii) bifurcate chaetae with weakly serrated or non-serrated short tip (Fig. 2 D); iii) bifurcate chaetae, with long serrated tip (Fig. 2 E). Neurochaetae contain two types: i) bifurcate chaetae, with weakly serrated or non-serrated short tip (Fig. 2 F); ii) bifurcate chaetae, with long serrated tip (Fig. 2 G). Neurochaetae longer than notochaetae. Anus opening dorsally on terminal chaetiger; anal papilla absent (Fig. 1 D).

Confirmed distribution. Only known from the type locality, the Sea of Kumano, Japan, 487 - 596 m depth.

Etymology. The species is named after the TRV Seisui-maru. The type specimens from the Sea of Kumano were collected by beam trawl gear of the ship. The specific name is a noun in the genitive case.

Material examined. Holotype: NSMT-Pol H- 676, 21 mm long, 5 mm wide (without chaetae, at widest chaetiger), 29 chaetigers, female, the Sea of Kumano, 487 - 596 m depth, 8 November 2017, collected by NJ (left parapodium of chaetiger 15 was dissected for DNA extraction). Paratypes: NSMT-Pol P- 677, two specimens, 19 - 28 mm long, 4 - 7 mm wide (without chaetae, at widest chaetiger), 29 chaetigers, female, collection data is the same as that of the holotype.