Leptoplax nhatrangi, Sirenko & Saito, 2017

Sirenko, Boris I. & Saito, Hiroshi, 2017, New species of chitons of the superfamily Cryptoplacoidea (Mollusca: Polyplacophora) from Vietnamese waters, Zootaxa 4299 (4), pp. 451-506 : 464-469

Leptoplax nhatrangi n. sp.

Figs, 13–15, 43C

Notoplax sp.: Sirenko 2012: fig. 36A–C.

Type material. Holotype, ZISP 2239, now disarticulated consisting of mounts of shell, perinotum and radula, 3 paratypes, ZISP 2258 and 2259, intermediate valves.

Type locality. Tam Island , Nha Trang Bay, southern Vietnam, 12°10´N, 109°14´E, 7–12 m, on shells of Malleus malleus, muddy sand,

Etymology. Named after Nha Trang Bay

Material examined. Southern Vietnam, Nha Trang Bay, Nok Id., 12°11´34.1´´N, 109°20´19.2´´E, 16–22 m, SCUBA, shell grit, paratypes, 2 intermediate valves, 17.05.2009, leg. B. Sirenko ; Tam Id., 12°10´N, 109°14´E, 7– 12 m, SCUBA, on shells of the hammer oyster Malleus malleus, muddy sand, holotype, BL 5.2 mm, 14.05.2011, leg. B. Sirenko; East off Tre Id., 12°11.557´N, 109°20. 323´E, 24 m, SCUBA, sand, paratype, 1 intermediate valve, 27.05.2013, leg. B. Sirenko.

Diagnosis. Small chitons with prominent radial ribs that consist of large, convex pustules on head valve, on diagonal lines of intermediate valves and on posterior area of tail valve and narrow wedge-shaped, raised, smooth jugum. Intermediate valves rhombic, rather elevated and carinated. Tail valve round, apparently narrower than head valve, with central, low mucro. Pustules on tegmentum droplet-shaped or elongate oval; top of pustules almost flat or slightly concave except for those on radial ribs. Each pustule with 1–3 macraesthete pores and 2–12 micraesthete pores. Micraesthete pores sparsely distributed also on tegmental plain. Dorsal side of girdle, beset with minute, short, thick spicules intermingling with sharply pointed, smooth needles.

Description. Animal small, elongate oval, moderately elevated. Color of tegmentum yellowish brown with maculation of white and brown and reddish tint on jugum. Girdle yellowish brown.

Head valve more than semicircular with five radiating rows of large pustules; posterior margin nearly straight. Intermediate valves roughly rhombic, moderately elevated, carinate, beaked; front margin concave between apophyses; hind margin concave at both sides of strong beak; jugum narrow wedge-shaped, prominently raised, smooth on surface, with almost straight side margins; narrow lateral areas separated from pleural areas by radiating rows of large pustules. Tail valve round, narrower than head valve, with central, low mucro; posterior slope concave, with four (five in paratype) radiating rows of large pustules.

Pustules on tegmentum droplet-shaped or elongate oval, arranged in quincunx pattern, which are arranged somewhat irregularly near the jugum; top convex in radial rows, flat or slightly concave in other areas; top of each pustule with 1–3 macraesthete pores and 2–12 micraesthete pores. Micraesthete pores sparsely distributed also on tegmental plain.

Articulamentum moderately developed, white, translucent, with transverse callus in middle of valves, several large pores under anterior margin of jugum, numerous minute pores under remaining part of jugum. Apophyses wide, long, rounded at anterior margin in intermediate valves, truncated in tail valve. Insertion plate short with short, rather wide slits; outer surface rough in tail valve. Slit formula 5/1/4.

Girdle rather wide, ca. 0.8 mm near valve IV (width 1.4 mm). Dorsal side of girdle beset with minute, thick, slightly curved, smooth, bluntly pointed spicules, 20–29 µm x 7–10 µm, intermingling with sharply pointed, smooth needles measuring up to 188 µm x 11 µm. Sutural tufts of up to 12 straight, smooth needles, 210–500 µm x 12–19 µm, surrounded by thick, curved, smooth spicules, 75 µm x 15 µm. Marginal needles similar to those of sutural tufts, but smaller, 92–160 µm x 10 µm. Ventral spicules flat, smooth, blunt tipped or with two riblets, pointed, 65–88 µm x 16 µm.

Gills extending from valve VII to valve VI, composed of five ctenidia on each side.

Radula 1.5 mm long with 24 transverse rows of mature teeth. Central tooth large, assymetrical with weakly bilobed blade, keeled at basal half. First lateral (centro-lateral) tooth with thin antero-dorsal corner Major lateral tooth with tricuspid head; cusps sharply pointed; central cusp slightly longer than others.

Distribution. Only known from the type locality, Nha Trang Bay, Vietnam, 7– 12 m.

Remarks. The present species closely resembles Notoplax holosericea ( Nierstrasz, 1905) in shell morphology, such as outline of the valves, the sculpture including convex pustules on radial ribs and flat ones on other areas and also the yellowish coloration of the shell and girdle. However, the present species differs from N. holosericea by having round and more densely arranged pustules on the tegmentum and minute, thick spicules (lance blade-like in N. holosericea ) on the perinotum. Notoplax holosericea appears to be a member of the genus Leptoplax because it has the characteristic sutural laminae on the tail valve. Examination of the radula of N. holosericea is needed to confirm this generic assignment. The present species resembles Notoplax conica Taki and Taki, 1929 , but differs in having a smooth jugum (granular in N. conica ) and a flat tail valve (raised as cone shape in N. conica ).