Ischnochiton comptus

Ischnochiton comptus (Gould, 1859) Chiton (Leptochiton) comptus Gould (1859): 163–164; Gould (1862): 117 (partim). Ischnochiton comptus: Pilsbry (1892): 117 (partim); 1895: 114; Thiele (1909): 111, 114; Taki (1938): 366 –371, 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 (partim, =? boninensis); Lee (1956): 11; Kang (1971): 52; Yoo (1976): 146, pl. 35, fig. 9; Kaas and Van Belle (1980): 29; Kaas and Van Belle (1998): 49; Kira (1982): 183, pl. 67, fig. 1; Saito (1995): 105; Saito (2000): 12, 13, pl. 6, fig. 14; Saito (2017): 732, pl. 5, fig. 1; Slieker (2000): 98, 99, pl. 37, fig. 11; Owada (2016): 6, figs. 4 F – J, 5 E – H; Sirenko and Zhang (2019): 6, 7, fig. 2 C. Ischnochiton thaanumi Dall (1926): 66 (partim). Ischnochiton (Ischnochiton) comptus: Taki (1962): 43; Higo (1973): 6; Yum (1988): 18 –20, pl. 1, fig. 3, pl. 7, pl. 22, fig. 5, 6, pl. 28, fig. 3. Ischnochiton (Ischnochiton) comptus (Gould) forma comptus: Taki (1964 a): 347, text-figs. 1, 3, 5; 1964 b: 408; Okada et al. (1981): 8; Higo and Goto (1993): 3. Ischnochiton (Haploplax) comptus: Kaas and Van Belle (1994): 70 –72, fig. 28, map. 7; Higo et al. (1999): 27; Lee and Min (2002): 94; Min et al. (2004): 71 –73, fig. 9. Materials Type status: Other material. Occurrence: individualCount: 4; occurrenceID: C27ECD33-0416-5309-AEAF-F17C6710DA7D; Taxon: scientificName: Ischnochiton comptus; Location: country: Korea; locality: Gamsan-ri, Andeok-myeon, Seogwipo-si, Jeju-do; verbatimCoordinates: 33°14'02.18"N, 126°21'45.60"E; Event: eventDate: 26 Jan 2015 Type status: Other material. Occurrence: individualCount: 3; occurrenceID: 1B2B3AA3-4F11-5273-AA4E-08B84F18B4E2; Taxon: scientificName: Ischnochiton comptus; Location: country: Korea; locality: Gamsan-ri, Andeok-myeon, Seogwipo-si, Jeju-do; verbatimCoordinates: 33°14'06.31"N, 126°21'34.21"E; Event: eventDate: 22 Dec 2015 Type status: Other material. Occurrence: individualCount: 2; occurrenceID: CDA06FBD-6774-5AB8-867C-BA22C65B8F5E; Taxon: scientificName: Ischnochiton comptus; Location: country: Korea; locality: Dokgot-ri, Daesan-eup, Seosan-si, Chungcheongnam-do; verbatimCoordinates: 36°59'05.79"N, 126°19'50.10"E; Event: eventDate: 22 Mar 2016 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 50DCB9E7-A1D5-5F61-B1D1-5AF5C4A05326; Taxon: scientificName: Ischnochiton comptus; Location: country: Korea; locality: Jukbyeon-ri, Jukbyeon-myeon, Uljin-gun, Gyeongsangbuk-do; locationRemarks: collected by snorkeling; verbatimCoordinates: 37°03'33.85"N, 129°25'42.04"E; Event: eventDate: 08 Sep 2016 Type status: Other material. Occurrence: individualCount: 2; occurrenceID: 1A911F9E-3306-522B-AF9A-26E6BF151A75; Taxon: scientificName: Ischnochiton comptus; Location: country: Korea; locality: Sagye-ri, Andeok-myeon, Seogwipo-si, Jeju-do; verbatimCoordinates: 33°13'37.13"N, 126°18'30.92"E; Event: eventDate: 23 Apr 2017 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 1D4D279E-F639-5747-9CE0-B5B98C3A7CCB; Taxon: scientificName: Ischnochiton comptus; Location: country: Korea; locality: Hangaechang, Is. Mun, Seogwi-dong, Seogwipo-si, Jeju-do; locationRemarks: collected by SCUBA-diving; verbatimCoordinates: 33°13'31.06"N, 126°33'54.57"E; Event: eventDate: 26 Mar 2019 Description Body oval-shaped, less than 25 mm in length (Fig. 2 F and H – J; in examined materials, BL 15.69–19.38 mm, BW 9.9–11.68 mm). Valves overall light green to greyish-green in colour, partially white, cream, pink blotches or dots of various size. Girdle rather narrow, cream in colour with dark brown transverse bands. Gills arrangement holobranchial and adanal on both sides with interspace (Fig. 2 G). Valves: Head valve semicircular, tegmentum sculpted with weak microgranules; ca. 50 radial riblets gradually fainter towards apex; growth lines rather distinct; anterior margin round; posterior margin widely V-shaped (Fig. 4 A). Intermediate valves broadly rectangular in shape, hardly beaked (Fig. 4 B and C), dorso-ventrally subcarinated, moderately elevated in frontal view, side slope slightly convex (Fig. 4 J; elevation ratio of 0.43 in 4 th valve); anterior margin nearly straight to slightly convex, except for 2 nd valve (Fig. 4 B; triangular with rounded angle); lateral margins rounded; posterior margin straight; tegmentum in central area (ca) with low oval granules, arranged in quincunx or zig-zag patterns, converging and weaker towards apex (Fig. 4 I); lateral area (la) somewhat raised, sculpted with 4–5 faint radial riblets (Fig. 4 B and C). Tail valve almost semicircular, as wide as head valve; anterior margin convex (Fig. 4 D); mucro (m) subcentral, not projecting; sculpture of antemucronal area (ama) like central area of intermediate valves; postmucronal area (pma) sculpted similarly to head valve, with more pronounced radial ribs; postmucronal slope (pms) straight (Fig. 4 K). Articulamentum light greenish-blue, generally red violet in centre. Apophyses (ap; sutural laminae) short, rounded-angled subtrapezoid in shape, except for that of 2 nd valve, which is triangular; jugal lamina (jl) vert short; jugal sinus (js) shallow, about one-fourth width of valves, almost straight; insertion plate (ip) short, teeth (t) relatively smooth; slit formula 10 / 1 / 10, slit rays (slr) faint (Fig. 4 E – H). Eaves (e) narrow, solid (Fig. 4 J). Girdle: Perinotum scales (ps: L 114.8–192.5 μm, W 146.7–232.7 μm) large, oval, slightly curved, imbricating, with faint concentric lines and indistinct scratches towards apex, parallel to outer margin (Fig. 7 E and F); peripheral scales of perinotum (pps: L 59.6–73.7 μm, W 64.7–97.2 μm) smaller and narrower than perinotum scales (Fig. 7 E, G); marginal spicules with three types: large one (lmsp: L 93.6 μm, W 20.7 μm) flat-ovate, narrowing towards tip, with fine feather-like grooves, middle one (mmsp: L 50.1 μm, W 11 μm) long, smooth, slightly curved rod-shaped and small one (smsp: L 17.8–26.4 μm, W 6.7–11.1 μm) very short, obese spheroidal (Fig. 7 E, G); hyponotum scales (hs: L 71.2 μm, W 18.5 μm) hyaline, smooth, narrow, oblong, overlapped (Fig. 7 H). Radula: Radula symmetrical rows (Fig. 8 B). Central tooth (c) narrow, blade wide, bent forward; centro-lateral teeth (cl) wing-like shape, each blade inwardly convex; head (h) of major lateral teeth (mlt) bicuspid, cuspid short, rounded-angled, inner one longer than outer one; petaloid process (pp; wing) small rectangular; major uncinus teeth (mu) tall, curving inwards with broad spatula-like tip; marginal teeth (im, mm, om) flat, with outer one largest. Distribution Australia, China, Hong Kong, Indonesia, Japan, Philippines, Taiwan, Vietnam and Korea. Type locality Japan: Oushima (Amami Oshima, Kagoshima Prefecture). Taxon discussion This species is considered taxonomically challenging due to its extremely high morphological variation, as well as recently discovered cryptic species. Recent studies, based on a combination of morphological and molecular analyses, have uncovered two cryptic species (Owada 2016, Owada 2018): I. manazuruensis and I. hayamii. They show very similar morphology to I. comptus, but are distinguished by the microstructure of the central area in the intermediate valves and the size of the girdle scales (Owada 2016, Owada 2018). In I. comptus and I. hayamii, the central area of the intermediate valves is covered with granules arranged in quincunx or zig-zag patterns, whereas I. manazuruensis shows an almost smooth surface in this area. Furthermore, the perinotum scales of I. comptus are similar in size to I. manazuruensis (300–400 μm), but larger than I. hayamii (150–200 μm). This species shows a moderate within-species sequence divergence of the mtDNA cox 1, with a maximum divergence of 1.28 %, but relatively high sequence divergence from their congeneric species compared in this study, ranging from 11.31 % (I. poppei) to 16.70 % (I. hakodadensis) (Table 3).