EOPILIDION MISAKIENSE SP. NOV.

(FIGS 2A, 4A–C)

Zoobank registration: u r n: l s i d: z o o b a n k. org:act: DBA0D4FA-ACAC-4440-9EA3-894F582D62F0.

Material examined: Holotype, ICHUM 6303, extracted total DNA (no morphological voucher remains); 12 February 2015, dredged at station 2 of the 5 th JAMBIO Coastal Organism Joint Survey (Nakano et al., 2015), off Misaki (between 35°08′29′N, 139°32′34″E, 198 m depth, and 35°08′27″N, 139°32′12″E, 274 m depth), Sagami Bay, Kanagawa, Japan, collected by H. Kajihara.

Sequences: From the holotype: LC178608, 18S (1768 bp); LC178641, 28S (2144 bp); LC178690, 16S (506 bp); LC190962, COI (658 bp) .

Etymology: The new specific name is an adjective, derived from the type locality.

Description: Known from anterior fragment, 4 mm long, 1 mm wide, uniformly pale red in colour. Head anteriorly pointed (Fig. 2A), similar to that of capitellid polychaetes. Pre-cerebral region remarkably reduced, about 200 µm long, 320 µm in diameter at base; brain region 500 µm long, 700 µm wide. Transverse cephalic furrow present just behind brain (Fig. 4A, C), encircling head, without secondary furrows. Cerebral organ just behind cephalic furrow, elongated pit, 380 µm long along anterior–posterior axis, 130 µm wide along dorsoventral axis (Fig. 4C). Mouth ventral, small, opening 1.6 mm behind tip of head. Proboscis pore opening subterminal (Fig. 4A–C), immediately anterior to brain.

enlargement of the head (K); L, M, Baseodiscus aff. princeps (Coe, 1901a), ICHUM 6335, entire body (L) and enlargement of the head (M); N, Baseodiscus nipponensis (Hubrecht, 1887), ICHUM 6338; O–Q, Baseodiscus cf. princeps (Coe, 1901a), ICHUM 6339, entire body (O), ventral view of anterior end (P), and lateral view of head (Q). Photos by A. V. Chernyshev (A, B), H. Kajihara (C–F, N–Q), G. Giribet (G, H), T. C. Hiebert (I), and K. Kakui (J, K, L, M).

Distribution: Known only from the type locality, Sagami Bay, Japan.

Remarks: The unusually reduced precerebral region and the wide subventral proboscis pore opening just in front of the brain, as well as phylogenetic position as sister-taxon to all other molecularly known heteronemerteans, justify a new family for E. misakiense . However, information on the body-wall structure is lacking, and it remains to be determined whether Eopilidion misakiense belongs to the Heteronemertea (with cutis layer) or not (without cutis layer) (see Discussion below).