Eubranchus rupium (Møller, 1842)

(Fig. 8B, C, I)

Material studied: MIMB48068 – MIMB48070, 3 specimens, all dissected, Sea of Okhotsk, Aniva Bay, Moguchi River, Hirano ridge, 46°05’06.5”N, 142°12’39.4”E, 0.1–1 m in depth, on Zostera sp. with Obelia longissima colonies, 7 August 2023, coll. I. Ekimova, D. Grishina.

Diagnosis: Body elongate, narrow, up to 5 mm in length, yellowish semitransparent with small reddish-brown spots. Rhinophores and oral tentacles elongated, smooth. Cerata finger-shaped, with pointed tips, bearing small thickening in upper part, arranged in 5–7 groups with 1–2 cerata per group. Jaws triangular-shaped with masticatory border bearing one row of small conical denticles. Radular formula 18–22 × 1.1.1. Triangular rachidian tooth with depressed central cusp and 6-7 lateral denticles on each side. Lateral teeth with rectangular base and triangular cusp. Ampulla small, bean-shaped. Vas deferens without prostatic part. Penis conical. Receptaculum seminis bent in middle part, expanded.

Molecular data: A BLAST-n search of COI sequences resulted in 99.57% identity to sequences of E. rupium from GenBank thus confirming distinctness of the studied specimens.

Distribution: This species has a wide distribution, having been recorded from Greenland [type locality, see Møller, 1842], Norway, Sweden, the Barents Sea, and the White Sea [Martynov, 1998; Cella et al., 2016], the Sea of Japan [Martynov, 1998], Maine [Cella et al., 2016]. This is the first record of this species from Sakhalin Is. (Aniva Bay), although it has been found from adjacent areas [Martynov, 2013].