Ischnochiton comptus (Gould, 1859)

(Fig. 2C)

Chiton (Leptochiton) comptus Gould 1859: 163 . Ischnochiton thaanumi Dall, 1926: 66 . Ischnochiton (Ischnochiton) comptus; Iw. Taki 1964a: 347, figs 1, 3, 5. Ischnochiton (Simplisschnochiton) comptus; Dell’Angelo et al. 1990: 35. Ischnochiton (Haploplax) comptus; Kaas & Van Belle 1994: 70, fig. 28, map 7 (bibliography and synonymy). Ischnochiton comptus; Is. Taki 1938: 366, pl. 14, figs 5, 10, pl. 15, figs 6, 7, pl. 25, figs 9–16, pl. 27, figs 6, 7, pl. 29, figs

11–16.; Huang & Xu 1964: 8; Kaas & Van Belle 1980: 29; Bullock & Harper 1994: 22, pls. 4A, 6A, B, 7A, B, 8A; Saito

1995: 105; 1998: 150; 2000: 13, pl. 6, fig. 14; 2001: 196, fig. 2; 2006: 212; 2017: 49, 732, pl. 5, fig. 1; Hasegawa et al.

2001: 3; Xu 2008: 457; Zhang et al. 2015: 98, fig. 5; Zhang et al. 2016: 11, fig. 9; Owada 2016: 260, figs 4F–J, 5E–H;

2018: 284, Table 3.

Type material. USNM 1562, holotype by monotypy .

Type locality. Japan, Izu-Oshima Island, south of Sagami Bay .

Material examined. South China Sea, Hainan Island, Sanya, 3 spms, BL 14.0–20.0 mm, 21.03.1958; Sanya, on rocks, intertidal, 1 spm, BL 13.0 mm, 21.03.1958; Sanya, Luhuitou reef, intertidal, 1 spm, BL 16.0 mm, 30.12.1959; stn. 7, 19°24’22’’N, 110°44′03’’E, intertidal, on stones, 11 spms, BL 5.0–12.0 mm, 15.11.2016, leg. B. Sirenko and S. Grebelniy; stn. 27, 18°18′03’’N, 109°13′55’’E, depth 2–5 m, on stones covered with red crustose algae, 1 spm, BL 8.5 mm, 24.11.2016, leg. B. Sirenko; stn. 29, 18°18′03’’N, 109°13′55’’E, depth 1.5–2.0 m, on stones, 21 spms, BL 6.5–12.0 mm, 24.11.2016, leg. B. Sirenko.

Distribution. South China Sea, Beibu Gulf (Gulf of Tongkin) from 17°56’N to 20°47’N, Hainan Island, Hong Kong, Taiwan, Qingdao, south Korea, Japan from Okinawa Island to Hakodate Bay, Philippines, and possibly Indonesia (Nierstrasz 1905) and even Australia (Leloup 1941) from intertidal to 6 m depth.

Remarks. Owada (2016) described the new cryptic species Ischnochiton manazuruensis Owada, 2016, which is very similar to I. comptus . These species are frequently misidentified. The main morphological difference in these species is the structure of tegmentum of the central areas. The central areas of I. comptus are covered with conspicuous quincuncial granules whereas the same areas are smooth or very weakly granulated in I. manazuruensis . Molecular analyses revealed that I. comptus and I. manazuruensis are separated by a relatively big genetic gap, similar to the gap between either of these species and I. boninensis . Ischnochiton comptus has a rather wide distribution, which extends from north Japan and Korea to North Vietnam, whereas I. manazuruensis is only known in the area between Sagami Bay and Kyushu.