Ischnochiton hakodadensis P. P. Carpenter 1893

Ischnochiton hakodadensis P. P. Carpenter, 1893 Ischnochiton (Ischnoradsia) hakodadensis Carpenter in Pilsbry (1893): 147, pl. 19, figs. 64–66; Pilsbry (1895): 114; Taki (1962): 44; Taki (1964 b): 409; Kaas and Van Belle (1980): 57; Yum (1988): 21, 22, pl. 2, fig. 1, pl. 9, pl. 23, fig. 3, 4, pl. 28, fig. 5. Ischnoradsia hakodadensis: Taki (1938): 373 –375, pl. 15, fig. 8, pl. 26, figs. 1–5, pl. 27, figs. 1–5, pl. 28, figs. 19–20. Ischnochiton (Ischnochiton) hakodadensis: Taki (1962): 43; Kaas and Van Belle (1990): 180 –182, fig. 81, map. 31; Dell'Angelo et al. (1990): 34, 35; Higo et al. (1999): 26; Lee and Min (2002): 94; Min et al. (2004): 70, 71. Ischnochiton (Ischnoradsia) hakodatensis [sic]: Higo (1973): 6; Okada et al. (1981): 8; Higo and Goto (1993): 4. Ischnochiton hakodadensis: Thiele (1909): 111, 112, taf. 8, fig. 44; Jakovleva (1952): 96, 97, fig. 47, pl. 8, fig. 3; Saito (1995): 103; Saito (2000): 12, 13, pl. 6, fig. 17; Saito (2017): 732, pl. 5, fig. 4; Slieker (2000): 98, 99, pl. 37, fig. 12; Qi (2004): 5, pl. 002 E; Sirenko (2013): 148; Owada (2018): 285 –287, figs. 2 G, 3, 7 B. Materials Type status: Other material. Occurrence: individualCount: 6; occurrenceID: D3A30D7C-BDDB-582F-8CA6-CC8D5BE4CF67; Taxon: scientificName: Ischnochiton hakodadensis; 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: 6CB3AB45-7BDB-55F7-9A0E-21E1F02E87C5; Taxon: scientificName: Ischnochiton hakodadensis; Location: country: Korea; locality: Buk-ri, Deokjeok-myeon, Ongjin-gun, Incheon-si; verbatimCoordinates: 37°15'53.09"N, 126°06'00.36"E; Event: eventDate: 18 Apr 2018 Type status: Other material. Occurrence: individualCount: 3; occurrenceID: CBF6FEED-0921-54F9-AA2B-F497D6CA6F3D; Taxon: scientificName: Ischnochiton hakodadensis; Location: country: Korea; locality: Ayajin-ri, Toseong-myeon, Goseong-gun, Gangwon-do; locationRemarks: collected by snorkeling; verbatimCoordinates: 38°16'13.35"N, 128°33'28.75"E; Event: eventDate: 14 May 2018 Type status: Other material. Occurrence: individualCount: 4; occurrenceID: 3E051C4B-8B3C-55FC-B092-5897991E6801; Taxon: scientificName: Ischnochiton hakodadensis; Location: country: Korea; locality: Ayajin-ri, Toseong-myeon, Goseong-gun, Gangwon-do; locationRemarks: collected by snorkeling; verbatimCoordinates: 38°16'13.35"N, 128°33'28.75"E; Event: eventDate: 22 May 2019 Description Body shape oval, rarely over 30 mm in length (Fig. 2 K and M; in examined materials, BL 18.63–25.01 mm, BW 10.92–15.66 mm). Valves mostly dark green or greyish-brown, even reddish-brown in colour, with cream, yellow, blackish-brown stripes or blotches of various size. Girdle rather narrow, with alternating light and dark transverse bands. Gills arrangement holobranchial and adanal in both sides with interspace (Fig. 2 L). Valves: Head valve semicircular, tegmentum sculpted with ca. 65 fine radial riblets, cut into several distinct concentric growth lines; anterior margin round; posterior margin widely V-shaped; (Fig. 5 A). Intermediate valves broadly rectangular in shape, not beaked (Fig. 5 B and C), dorso-ventrally subcarinated, moderately elevated in frontal view, side slope convex (Fig. 5 J; elevation ration of 0.39 in 4 th valve); anterior margin straight, except for 2 nd valve (Fig. 5 B; convex, nearly triangular); lateral margins almost rounded; posterior margin straight; tegmentum in central area (ca) almost smooth with minute, low granules arranged in quincunx or zig-zag patterns (Fig. 5 I); jugal area (ja) somewhat raised, separated from pleural area (pa) by slightly different elevation; lateral area (la) slightly elevated, sculpted with 6–8 fine radial riblets; concentric growth lines several, distinct across entire tegmentum (Fig. 5 B and C). Tail valve almost semicircular, slightly narrower than head valve; anterior margin nearly straight in centre, convex on the both side (Fig. 5 D); mucro (m) antemedian, not protruding; sculpture of antemucronal area (ama) like central area of intermediate valves; postmucronal area (pma) sculpted similarly to head valve; postmucronal slope (pms) steep, straight (Fig. 5 K). Articulamentum light blue. Apophyses (ap; sutural laminae) short, rounded triangle or subtrapezoid in shape; jugal lamina (jl) very short; jugal sinus (js) shallow, about one-fourth width of valves, straight; insertion plate (ip) short, teeth (t) uneven in size, slightly roughened; slit formula 15 / 2–3 / 17, slit rays (slr) clearly marked (Fig. 5 E – H). Eaves (e) very narrow, solid (Fig. 5 J). Girdle: Perinotum scales (ps: L 64–122 μm, W 124.9–203.5 μm) small, slightly convex, imbricating, nearly smooth or with faint longitudinal ribs, arranged diagonally to outer margin (Fig. 7 I – K); marginal spicules with three types: large one (lmsp: L 100.5–116.5 μm, W 32.9–40.3 μm) flat-triangular, blunt tip, with fine feather-like grooves, middle one (mmsp: L 63.4–77.4 μm, W 10.5–11.4 μm) slender, smooth, slightly curved rod-shaped and small one (smsp: L 37.2–49.6 μm, W 11.7–14.8 μm) very short, obese rod-shaped with radial grooves on tip (Fig. 7 I, K); hyponotum scales (hs: L 80.7–89.5 μm, W 17–23.2 μm) hyaline, smooth, elongated, rounded-edged rectangular, overlapped, arranged radially (Fig. 7 L). Radula: Radula symmetrical rows (Fig. 8 C). Central tooth (c) small, narrow, blade slightly bent forward; centro-lateral teeth (cl) taller than central tooth, widening towards tip; head (h) of major lateral teeth (mlt) bicuspid, cuspid short, blunted, outer one shorter, pointed than inner one; petaloid process (pp; wing) short, rectangular with inwardly concave tip; major uncinus teeth (mu) slender with round, spoon-like head; marginal teeth (im, mm, om) flat, widening towards outer. Distribution China, Japan, Russia (Vladivostok) and Korea. Type locality Japan: Hakodadi (Hakodate, Hokkaido). Taxon discussion This species is relatively large in body size (> 30 mm) and easily distinguished from other congeneric species by the following characteristics. I. hakodadensis has 2–3 slits in the intermediate valves (Fig. 5 F and G), whereas other Ischnochiton species have only one slit (Fig. 3 F, G, Fig. 4 F, G, Fig. 6 F and G; Owada (2018)). Additionally, the perinotum scales are arranged diagonally to the outer margin in I. hakodadensis (Fig. 7 I), while they are arranged parallel in other Ischnochiton species (Fig. 7 A, E and M). It is noteworthy that these two morphological features distinguishing I. hakodadensis from other congeneric species are also present in many other Lepidozona species as well. Moreover, previous molecular phylogenetic analysis using four gene regions (COI, 16 S, 18 S and 28 S) revealed that I. hakodadensis was not monophyletic, with some other Ischnochiton species grouped in a separate branch (Owada 2018). In contrast to the very low individual sequence variation (a maximum of 0.90 %), this species shows very high genetic divergence from other congeneric species, ranging from 15.80 % to 19.93 % (Table 3). These findings, based on both morphological and molecular evidence, highlight the need for further studies to clarify the taxonomic position of this species within the family Ischnochitonidae. Furthermore, Owada (2018) described the accessory process of major lateral teeth in the radula, but these characters were not found in Korean I. hakodadensis specimens (Fig. 8 C), requiring further study to confirm this discrepancy.