Body fusiform, flat. Eyes absent. Caruncle consisting of three ridges without ornamentation. Branchiae absent at least in first five chaetigers, dendritically branched. Dorsal and ventral cirri occur singly on the notopodium and neuropodium.

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.

LC360809, COI gene, 507 bp, determined from holotype.

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. 1A). 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 1B, 2A). Caruncle consists of three longitudinal ridges, without ornamentation, extends to chaetigers 1–2 (depending on fixation), unattached in posterior part (Fig. 2A). Mouth composed of chaetigers 1–2. Pharynx eversible with black pigmentation.

Parapodia biramous, notopodia and neuropodia clearly separated (Fig. 2B). 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. 1B, 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. 1D).

Notochaetae contain three types: i) harpoon chaetae, with serrations limited to one side (Fig. 2C); ii) bifurcate chaetae with weakly serrated or non-serrated short tip (Fig. 2D); iii) bifurcate chaetae, with long serrated tip (Fig. 2E). Neurochaetae contain two types: i) bifurcate chaetae, with weakly serrated or non-serrated short tip (Fig. 2F); ii) bifurcate chaetae, with long serrated tip (Fig. 2G). Neurochaetae longer than notochaetae.

Anus opening dorsally on terminal chaetiger; anal papilla absent (Fig. 1D).

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.

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

Benthoscolex seisuiae sp. n. can be discriminated from B. coecus and B. cubanus by the following features: i) palps 1.8 times as long as lateral antennae (vs. same length as lateral antennae in B. cubanus; 2.0 times as long as lateral antennae in B. coecus), and ii) branchiae do not reach to tip of notochaetae (vs. extending beyond tip of notochaetae in B. coecus; they also do not reach to tip of notochaetae in B. cubanus). In addition, the tip of the bifurcate neurochaetae is reportedly serrated in B. cubanus, whereas it is only weakly serrated, or not serrated at all, in B. seisuiae sp. n., although chaetal serration is known to be variable in Eurythoe (Barroso and Paiva 2007).

Benthoscolex cubanus is reported to be endocommensal in the body cavity of the bathyal irregular sea urchin Heterobrissus hystrix (A. Agassiz, 1880) (Hartman 1942; Emson et al. 1993). Benthoscolex seisuiae sp. n. was collected by a beam trawl and found free living. In the same haul, 49 specimens representing five species of irregular sea urchins [Brisaster latifrons (A. Agassiz, 1898) (n = 13, NSMT E-10723–10724), Brissopsis luzonica (Gray, 1851) (n = 6, NSMT E-10721–10722), Brissopsis sp. (n = 1, NSMT E-10727), Lovenia gregalis Alcock, 1893 (n = 22, NSMT E-10719–10720), Schizaster sp. (n = 7, NSMT E-10725–10726)] were present and some were broken in the net. However, examination of body cavity in all but one specimen (used for species identification and photography, Fig. 3) for each species revealed no commensal Benthoscolex worms (A. Ogawa pers. obs.); Brissopsis sp. was not examined because it was represented by only one specimen. Therefore, whether the new species is also endocommensal in sea urchins or not cannot be ascertained at the moment. Future studies are required to confirm the present observations of a free-living lifestyle in the new species.