Ischnochiton hayamii Owada 2018

Ischnochiton hayamii Owada, 2018 Ischnochiton hayamii Owada (2018): 287, figs 2 A – F, 4, 7 A. Materials Type status: Other material. Occurrence: individualCount: 2; occurrenceID: 5C753217-14FB-5D05-9BCF-8E0933B7989D; Taxon: scientificName: Ischnochiton hayamii; Location: country: Korea; locality: Sinam-ri, Seosaeng-myeon, Ulju-gun, Ulsan-si; verbatimCoordinates: 35°20'16.35"N, 129°19'10.12"E; Event: eventDate: 24 Nov 2016 Description Body oval-shaped, small to medium in size (Fig. 2 N; in examined material, BL 15.28 mm, BW 9.04 mm). Valves dark olive-green in colour, with small spots or large blotches of variable colours; cream, yellow, pink and brown. Girdle rather narrow, alternating with transverse bands of yellowish-brown and light yellow. Gills arrangement adanal and holobranchial on both sides with interspace (Fig. 2 O). Valves: Head valve semicircular in shape, tegmentum almost smooth with faint growth line; anterior margin round; posterior margin widely V-shaped (Fig. 6 A). Intermediate valves broadly rectangular in shape, hardly beaked (Fig. 6 B and C). Intermediate valves dorso-ventrally carinated, moderately elevated in frontal view and side slope nearly straight (Fig. 6 J; elevation ratio of 0.4 in 4 th valve); anterior margin slightly convex, except for 2 nd valve (Fig. 6 B; more convex than other intermediate valves, even appearing triangular); lateral margins somewhat round; posterior margin almost straight; tegmentum of central area (ca) densely arranged minute granules in quincunx or zig-zag patterns (Fig. 6 I); lateral area (la) hardly raised, almost smooth like head valve (Fig. 6 B and C). Tail valve semicircular, as wide as head valve; anterior margin straight (Fig. 6 D); mucro (m) subcentral, slightly elevated; sculpture of antemucronal area (ama) same as central area of intermediate valves; postmucronal area (pma) nearly smooth such as head valve; postmucronal slope (pms) concave (Fig. 6 K). Articulamentum light blue, central part of articulamentum red violet in colour. Apophyses (ap; sutural laminae) rectangular in shape, short, fragile and separated with very short jugal lamina (jl); jugal sinus (js) shallow, about one-third of tegmentum width; insertion plate (ip) teeth (t) uneven in size; slit formula 10 / 1 / 10, slit rays (slr) fine, but relatively distinct (Fig. 6 E – H). Eaves (e) very narrow, solid (Fig. 6 J). Girdle: Perinotum covered with flat, oval, slightly bending, nearly smooth and overlapping scales (Fig. 7 M, N; ps: L 108.2–157.6 μm, W 152.6–184.8 μm), scales arranged parallel to margin; peripheral scales of perinotum (pps: L 18.8–59.8 μm, W 79.5–92.9 μm) much smaller than perinotum scales, with 8–15 fine, flat longitudinal ribs (Fig. 7 M – O); marginal spicules with three types: large one (lmsp: L 77.7–95.6 μm, W 23–27.34 μm) long, flat-ovate, narrower to tip, obtuse, with 5–7 weak longitudinal ribs; middle one (mmsp: L 97.7–100.9 μm, W 8.8–11.7 μm) long, slender, smooth, blunt rod-shaped; small one (smsp: L 18.3–21.6 μm, W 14.7–15.3 μm) very short, obtuse with strong radial ribs (Fig. 7 M and O); hyponotum covered with hyaline, smooth, oblong and overlapping scales (Fig. 7 P; hs: L 70.6–75.6 μm, W 11.3–17.5 μm). Radula: Radula symmetrical rows with 17 teeth per transverse row (Fig. 8 D). Central tooth (c) narrow oblong shape with forward bent blade; each centro-lateral tooth (cl) taller than central tooth, with wide inwardly convex cuspid; head (h) of major lateral teeth (mlt) bicuspid, inner cuspid rounded, outer one shorter and more angled than inner one; petaloid process (pp; wing) near base of major lateral tooth, blunt rod-shape, protruding slightly more than cuspids of major lateral tooth directed towards the centre; major uncinus teeth (mu) tall, slender, curving inwards with round spoon-like tip; marginal teeth (im, mm, om) flat, increasing size from inner to outer teeth. Distribution Japan (Zushi, Shimoda, and Hakodate) and Korea (Ulsan). Type locality Japan: Zushi, Kanagawa Prefecture. Taxon discussion Previously, three Ischnochiton species have been reported in Korea (National list of species of Korea 2022): Ischnochiton boninensis, Ischnochiton comptus and Ischnochiton hakodadensis. I. boninensis and I. comptus are very similar in overall morphology and typically co-occur in sympatric regions in intertidal and subtidal habitats (Taki 1938). Owada (2018) noted that I. hayamii shares morphological similarities with these species in their overall appearance, moderated elevation and sculptures of the central area in the intermediate valves. Nevertheless, I. hayamii is distinguished from the other three Korean Ischnochiton species by their morphological differences in body size, back shape and sculptures of the lateral areas (see Table 2 for detailed comparison). Amongst the four Korean Ischnochiton species, I. hayamii is the smallest in adult body size (<15 mm in length), whereas the other three species typically have larger sizes (I. boninensis and I. comptus range from 15–30 mm and I. hakodadensis is the largest species> 30 mm) (Kaas and Van Belle 1990, Kaas and Van Belle 1994, Owada 2016, Owada 2018). The dorsal back shape of I. hayamii is carinated (Fig. 6 J), while it is rounded in I. boninensis (Fig. 3 J) and subcarinated in both I. comptus and I. hakodadensis (Fig. 4 J and Fig. 5 J) (Pilsbry 1893, Kaas and Van Belle 1990, Kaas and Van Belle 1994, Owada 2018). In microstructures of the valves, the lateral areas are almost smooth with faint radial ribs in I. hayamii (Fig. 6 B and C), whereas the other three species possess 5–8 bifurcating radial ribs (Fig. 3 B, C, Fig. 4 B, C, Fig. 5 B and C) (Taki 1938, Kaas and Van Belle 1990, Kaas and Van Belle 1994, Owada 2016, Owada 2018).