Body firm, muscular, elongated, with constant width in caudal direction, dorsoventrally compressed, BL 23.8–32.5 mm, BW 3.7–4.9 mm (Fig. 2). Caudal sucker elliptic, minor axis 1.0–1.6 mm, major axis 1.9–2.7 mm (Fig. 2b). Somite I completely merged with prostomium. Somites II, III uniannulate. Somites IV, V biannulate. Somites VI, VII triannulate. Somites VIII–XXV quadrannulate (Fig. 3a). Somite XXVI triannulate. Somite XXVII uniannulate; anus behind it. Eyes in 3 pairs, first pair dorsally on posterior margin of II, second and third pairs dorsolaterally on posterior margin of V (a1 + a2). Nephridiopores in 17 pairs, 1 each situated ventrally at posterior margin of a1 of each somites in VIII–XXIV (Fig. 3a).

Pharynx reaching to XIV b5/b6 (Fig. 3b). Crop reaching to XX b5–XX/XXI (Fig. 3b). Gastropore in furrow of XIII a1/a2 (Fig. 3a, c). Gastroporal duct tubular, joining with crop in XIV b5/b6 (Fig. 3b). Intestine reaching to XXV a1/a2–b5/b6. Rectum descending to anus.

Male gonopore at anterior margin of XI b6 (Fig. 3a). Female gonopore in furrow of XIII a1/a2 (Fig. 3c). Gonopores separated by 6 annuli (Fig. 3a). Testisacs undeveloped, undetectable. Paired epididymides in XVI b5/b6–XVII a1/a2, occupying 2 annuli (Fig. 3d). Ejaculatory ducts in XI b5 to XVI b5/b6, nearly straight. Atrial cornua undeveloped. Atrium globular, in XI b6. Paired ovisacs globular, 1 each in XIII a2 and b5. Oviducts short, both oviducts converging into common oviduct in XIII a2. Common oviduct directly descending to female gonopore.

Orobdella kawakatsuorum is distributed in Hokkaido, Japan, and its peripheral islands and inhabited in mountainous regions of these islands (Nakano 2012a). The present specimens have extended the known distributional range of the species north to include the southern tip of the Kuril Islands.

The obtained neighbor-joining tree (Fig. 4) showed that two Orobdella specimens from Kunashir Island (KUZ Z675, Z676) formed a monophyletic lineage with the individual of Orobdella kawakatsuorum from Shiretoko, Hokkaido (KUZ Z152). No difference between the tRNA^Leu–ND1 sequences from the Kunashir specimens. The K2P distance was detected between these two specimens (KUZ Z675, Z676) and that from Shiretoko (KUZ Z152) was 0.5%.

Two specimens of Orobdella from Kunashir Island clearly belong to Orobdella kawakatsuorum based on the following characteristics: male gonopore in the anterior margin of XI b6, female gonopore in the furrow of XIII a1/a2, 6 annuli between gonopores, and epididymides occupying 2 annuli. According to Nakano (2012a), Orobdella kawakatsuorum grows to ca. 10 cm length. However, the body length of the Kunashir specimens is only ca. 3 cm. Since they have undeveloped male atria and undetectable testisacs, they were considered immature individuals. As noted in the Introduction, two quadrannulate species of Orobdella, Orobdella kawakatsuorum and Orobdella koikei, are distributed in Hokkaido. Orobdella koikei is the closest congener of Orobdella kawakatsuorum according to the recent molecular phylogenetic study and the smallest species among the known species of Orobdella (Nakano 2012a). The body length of the known mature leeches of Orobdella koikei is less than 4 cm. Therefore, based only on their body length, a possibility exists that the Kunashir specimens might be misidentified as Orobdella koikei. However, Orobdella kawakatsuorum, as well as the present specimens, are clearly distinguished from Orobdella koikei in the characteristics mentioned above: the latter possesses 1/2 + 4 + 1/2 annuli between gonopores (male gonopore in the middle of XI b6, female gonopore in the middle of XIII a1), and the epididymides occupy 9–12 annuli. Our tRNA^Leu–ND1 data provided additional confirmation that the Orobdella leeches from Kunashir Island were identified correctly as Orobdella kawakatsuorum.

Orobdella kawakatsuorum was collected from Rishirito Island (Nakano 2012a), which is located ca. 20 km away from Hokkaido (Fig. 1). Additionally, Kunashir Island lies offshore of Hokkaido. Thus, anticipating that Orobdella leeches might occur on the island was not difficult. Our findings suggest that Orobdella species may also be present in the southern part of the Kuril Islands, e.g., Iturup Island and Shikotan Island. Further faunal surveys should be conducted not only in the South Kurils, but also in the northern part of the Kuril Islands, as well as the Kamchatka Peninsula, to fully reveal the northern distributional limit of the genus Orobdella.