Ischnochiton boninensis Bergenhayn 1933

Ischnochiton boninensis Bergenhayn, 1933 Ischnochiton boninensis Bergenhayn (1933): 10–13, taf. 1, figs. 2, 3, taf. 2, figs. 24–29, 32, textfig. 3; Taki (1938): 369, 373, 375; Kang (1971): 52; Kaas and Van Belle (1980): 18; Kaas and Van Belle (1998): 33; Kira (1982): 183, pl. 67, fig. 2; Saito (1995): 105; Saito (2000): 12, 13, pl. 6, fig. 15; Saito (2017): 732, pl. 4, fig. 11; Slieker (2000): 98, 99, pl. 37, fig. 10; Owada (2016): 5, 6, figs. 4 A – E, 5 A – E; Sirenko and Zhang (2019): 6, fig. 2 B. Ischnochiton zebrinus Bergenhayn (1933): 13–15, taf. 1, fig. 4, taf. 2, figs. 30, 31, 33–39, textfig. 4; Taki (1938): 375; Taki (1964 a): 342. Ischnochiton (Ischnochiton) boninensis: Taki (1962): 43; Higo (1973): 6; Yum (1988): 20, 21, pl. 1, fig. 4, pl. 8, pl. 23, figs. 1, 2, pl. 28, fig. 4; Kaas and Van Belle (1990): 184 –186, fig. 83, map 39; Higo et al. (1999): 27; Lee and Min (2002): 94; Min et al. (2004): 70, 71, fig. 8. Ischnochiton (Ischnochiton) comptus (Gould) forma isaoi: Taki (1964 a): 348, text-figs. 2, 4, 6; Taki (1964 b): 408; Higo (1973): 6; Higo and Goto (1993): 3. ? Ischnochiton (Ischnochiton) zebrinus: Taki (1962): 43; Taki (1964 b): 408; Higo (1973): 6; Higo and Goto (1993): 3; Higo et al. (1999): 27. Ischnochiton (Simplischnochiton) boninensis: Van Belle (1982): 470; Dell'Angelo et al. (1990): 36, 37. Materials Type status: Other material. Occurrence: individualCount: 3; occurrenceID: B04F80DF-C510-566A-9E5A-CEEA8BBADF80; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Jongdal-ri, Gujwa-eup, Jeju-si, Jeju-do; verbatimCoordinates: 33°29'16.84"N, 126°54'41.37"E; Event: eventDate: 26 Jan 2015 Type status: Other material. Occurrence: individualCount: 5; occurrenceID: C8673915-9FF2-5CC2-AC00-144DD7F27CAD; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Jongdal-ri, Gujwa-eup, Jeju-si, Jeju-do; verbatimCoordinates: 33°29'16.84"N, 126°54'41.37"E; Event: eventDate: 21 Dec 2015 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 283FB9B8-108F-585F-B330-D280BD77DF60; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Daejeong-eup, Seogwipo-si, Jeju-do; verbatimCoordinates: 33°14'27.09"N, 126°13'21.24"E; Event: eventDate: 02 Jul 2016 Type status: Other material. Occurrence: individualCount: 2; occurrenceID: 7BB82550-EA20-5A66-931F-2117EB63AA04; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: 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 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 8B88C27E-086E-5F4E-B663-D333BF8D776F; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Is. Mun, Seogwi-dong, Seogwipo-si, Jeju-do; locationRemarks: collected by SCUBA-diving; verbatimCoordinates: 33°13'35.78"N, 126°34'07.27"E; Event: eventDate: 27 Mar 2019 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 6A0AE6BA-B86B-5D25-A78E-A730101EC7B8; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Geumgye-ri, Gogun-myeon, Jindo-gun, Jeollanam-do; verbatimCoordinates: 34°25'21.32"N, 126°20'56.94"E; Event: eventDate: 14 Dec 2016 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 5DF8577C-F423-5E58-AF2D-9FB504D98202; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Gujora Beach, Gujora-ri, Irun-myeon, Geoje-si, Gyeongsangnam-do; verbatimCoordinates: 34°48'05.58"N, 128°41'28.40"E; Event: eventDate: 12 Jun 2018 Type status: Other material. Occurrence: individualCount: 1; occurrenceID: 1DB2075D-449C-5995-BCB5-B9A6A2EF4C92; Taxon: scientificName: Ischnochiton boninensis; Location: country: Korea; locality: Haengnam Lighthouse, Dodong-ri, Ulleung-eup, Ulleung-gun, Gyeongsangbuk-do; locationRemarks: collected by SCUBA-diving; verbatimCoordinates: 37°29'13.48"N, 130°55'00.18"E; Event: eventDate: 30 Oct 2018 Description Body elongate-oval shaped, medium, less than 30 mm in length (Fig. 2 A and C – E; in examined materials, body length [BL] 14.6–21.76 mm, body width [BW] 7.21–12.23 mm). Valves generally yellowish-green to dark green or wholly whitish or pinkish in colour, brown streaks or cream, yellow, bright pink blotches. Girdle rather narrower in width than valves, dark brown, often with yellow-brown blotches or transverse bands. Gills arrangement holobranchial and adanal on both sides with interspace (Fig. 2 B). Valves: Head valve semicircular in shape, tegmentum sculpted with about 50 fine radial riblets and a few strong growth lines; radial riblets bifurcate towards margin, but faint towards apex; anterior margin round; posterior margin widely V-shaped (Fig. 3 A). Intermediate valves broadly rectangular in shape, not beaked (Fig. 3 B and C), dorso-ventrally rounded or subcarinated, somewhat high in frontal view, side slope slightly convex (Fig. 3 J; elevation ratio of 0.46 in the 4 th valve); anterior margin nearly straight to slightly convex, except for 2 nd valve (Fig. 3 B; more convex than other intermediate valves, almost triangular); lateral margins somewhat round; posterior margin straight; tegmentum in central area (ca) with rows of densely small, elongate granules in quincunx or zig-zag patterns (Fig. 3 I); lateral area (la) slightly raised, sculpted with 5–7 riblets, dividing to almost double towards margin, similar to head valve (Fig. 3 B and C). Tail valve almost semicircular, as wide as head valve; anterior margin nearly straight (Fig. 3 D); mucro (m) subcentral, not pointed; sculpture of antemucronal area (ama) similar to central area of intermediate valves; postmucronal area (pma) sculpted like head valve; postmucronal slope (pms) weakly concave (Fig. 3 K). Articulamentum light blue, generally red violet in centre, very thin, reflecting colouration and patterns on tegmentum. Apophyses (ap; sutural laminae) short, delicate, triangular in 2 nd valve, but broad rectangular in the other valves; jugal lamina (jl) extremely short; jugal sinus (js) shallow, nearly straight; insertion plate (ip) short, teeth (t) relatively smooth with varying size; slit formula 10–13 / 1 / 9 – 10, slit rays (slr) distinct (Fig. 3 E – H). Eaves (e) narrow, solid (Fig. 3 J). Girdle: Perinotum scales (ps: length [L] 91–110.8 μm, width [W] 120.3–190.7 μm) small, oval, flat, slightly bent, sculpted with 9–16 fine longitudinal ribs converging to distal tip, densely overlapped, parallel to outer margin (Fig. 7 A and B); peripheral scales of perinotum (pps: L 25.8–56.4 μm, W 33.8–86.5 μm) much smaller than perinotum scales, elongate-oval, with 4–7 fine, flat longitudinal ribs (Fig. 7 A – C); marginal spicules of three types: large one (lmsp: L 67.2–87.6 μm, W 16.9 μm) flat-ovate, narrower to tip, blunt, with weakly longitudinal ribs, middle one (mmsp: L 68.2–80.3 μm, W 7.3–10 μm) long, nearly straight, smooth, rod-shaped with blunt tip and small one (smsp: L 22.1–33.5 μm, W 9.9–11.5 μm) very short, stout with strong radial ribs at tip (Fig. 7 A and C); hyponotum scales (hs: L 56.7 μm, W 14.7 μm) hyaline, smooth, oblong, overlapped, radiated to outline (Fig. 7 D). Radula: Radula teeth symmetrical (Fig. 8 A). Central tooth (c) small, narrow, keeled from basal, blade broader than stalk, deeply concaved, bent forward; centro-lateral teeth (cl) slightly taller than central tooth, cuspids thickened, convex; head (h) of major lateral teeth (mlt) bicuspid, cuspids sharp, tapering towards tip, inner cuspid longer than outer one; petaloid process (pp; wing) below at head of major lateral tooth, large, rectangular, inner distal concave; major uncinus teeth (mu) long, spatula-shaped with rounded tip; marginal teeth (im, mm, om) flat, increasing in size from inner to outer teeth. Distribution China, Hong Kong, Japan, Vietnam and Korea. Type locality Japan: Sagami Misaki and Bonin Islands (Ogasawara). Taxon discussion This species is morphologically similar to I. comptus (Gould, 1859), but easily distinguished by the size and microstructure of its perinotum scales (Table 2). I. boninensis has small perinotum scales sculpted with conspicuous fine longitudinal ribs (Fig. 7 A and B), while I. comptus displays relatively large, smooth, glossy scales (Fig. 7 E and F). In contrast to the markedly high individual variations in tegmentum colour and pigmentation patterns, the mtDNA cox 1 sequences of this species showed moderate within-species variations, less than 1.8 % (Fig. 2 A and C – E, Table 3), even amongst individuals sampled from different geographic regions (i. e. Japan and Korea).