Leptoplax tongkingi, 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 : 470-472

Leptoplax tongkingi n. sp.

Figs 17–20 , 43 E

Type material. Holotype, ZISP 2235, now disarticulated, consisting of mounts of shells, perinotum and radula.Paratype, ZISP 2236.

Type locality. Cat Ba Island , Gulf of Tongking, northern Vietnam, 20°43.675´N, 107°03.373´E, intertidal.

Etymology. Named after Gulf of Tongking.

Material examined. Northern Vietnam, Gulf of Tongking, Cat Ba Id., 20°43.675´N, 107°03.373´E, intertidal, underside of stone, holotype, BL 7 mm, 10.05.2012, leg. B. Sirenko ; 20°45.957´N, 107°07.722´E, 3–4 m, SCUBA, sand, stones, on shells, paratype, BL 5 mm, 0 6.05.2012, leg. B. Sirenko.

Diagnosis. Small chitons with wide, low intermediate valves. Jugum smooth, wide wedge-shaped, with wavy side margins. Tail valve as wide as head valve, oval, with central, low mucro. Pustules on tegmentum elongate oval, which are fused in the posterior margins of the head and intermediate valves; top of pustule flat, with 2–3 aesthete pores on top and several aesthete pores on the prepustular slope. Aesthete pores present also on tegmental plain. Dorsal side of girdle beset with minute, sharply pointed, strongly ribbed spicules intermingling with sharply pointed, smooth needles.

Description. Animal small, elongate oval, low in profile. Color of tegmentum yellowish white with patches of pink and olive green. Girdle light brown with pinkish maculation.

Head valve semicircular, posterior margin widely V-shaped; anterior slope nearly straight; no radial ribs. Intermediate valves wide, roughly rectangular, low, round backed, beaked; front margin straight between apophyses; hind margin concave at both sides of beak; jugum wide wedge-shaped, smooth, with wavy side margins; lateral areas slightly raised. Tail valve oval, with central, moderately raised mucro; width of tegmentum almost equal to that of head valve; posterior slope concave.

Pustules on tegmentum arranged in quincunx pattern on head valve, lateral areas of intermediate valves and posterior area of tail valve, somewhat irregular in pleural areas. Pustules of posterior margin fused in head and intermediate valves. Each pustule elongate oval, flat at top with two to three aesthete pores and several aesthete pores on prepustular slope. Aesthete pores present also on tegmental plain. All aesthete pores of almost uniform size, 8̄9 µm x 5 ̄8 µm, hardly distinguishable macr- and micraesthete pores.

Articulamentum moderately developed, yellowish white, with callus antero-laterally extending from under beak, porous under jugum. Apophyses wide, gently arched, widely separated each other in intermediate valves, protruded at inner corners in tail valve. Insertion plate short with short, rather wide slits; Slit formula 5/1/9.

Girdle rather wide, ca. 0.8 mm near valve V. Dorsal side of girdle beset with minute, sharply pointed, strongly ribbed spicules, up to 36 µm x 15 µm, intermingling with sharply pointed, smooth needles measuring up to 108 µm x 8 µm. Sutural tufts of up to 12 straight, smooth needles, up to 680 µm x 48 µm. Marginal needles similar to those of sutural tufts, but thinner, up to 250 µm x 11 µm. Ventral spicules similar to ribbed dorsal spicules, but larger, up to 59 µm x 16 µm.

Gills extending from valve VII to valve IV, composed of 11 ctenidia on each side.

Radula 2.1 mm long, with 30 transverse rows of mature teeth. Central tooth asymmetrical, widened at lower half, slender at upper half with small blade, weakly keeled at base. First lateral (centro-lateral) tooth with thin, angulated antero-dorsal corner. Major lateral tooth with tricuspid head; cusps sharply pointed; central cusp longer than others. Major uncinal tooth with finely incised comb-likely edge near tip.

Distribution. Gulf of Tongking, northern Vietnam, intertidal– 3 m.

Remarks. The comb-like blade of the major uncinal tooth is known only in other two distinct families: the genus Ferreiraella of the family Ferreiraellidae in the order Lepidopleurida and the genera Tonicella and Boreochiton in the family Tonicellidae of the order Chitonida and this is the first report of this feature other than those two families.

Leptoplax tongkingi n. sp. most resembles Leptoplax richardi ( Kaas, 1990) by having ribbed dorsal girdle spicules and peculiar shape of the central tooth of the radula, which is asymmetrical, widened at lower half, slender at upper half. However, the spicule of the former has sharp riblets in almost its entire length whereas only in swollen distal part in the latter; the valves of the former are wider than those of the latter; the tail valve is as wide as head valve in the former, whereas much narrower in the latter. The comb-like blade of the major uncinal tooth is not possessed by L. richardi .

Figures

FIGURE 17. Leptoplax tongkingi n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 7 mm: A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve IV, ventral view; E. Valve V, rostral view; F, detail of tegmentum in jugal and lateropleural areas; G. Valve V, detail of tegmentum in lateropleural area; H. Valve VIII, lateral view.

FIGURE 18. Leptoplax tongkingi n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 7 mm: A. Radula; B. Dorsal needles and spicules; C. Tuft, dorsal needles and spicules; D. Ventral spicules.

FIGURE 20. Leptoplax tongkingi n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 7 mm: A. Needles of tufts; B. Dorsal spicules; C. Dorsal needles; D. Marginal needle; E. Ventral spicules; F. Head of major lateral tooth of radula; G. Central and first lateral teeth of radula; H. Pustule of lateropleural areas of tegmentum with aesthete pores. Scale bars: 100 µm.

FIGURE 43. Images of species of genera Notoplax and Leptoplax: A. Notoplax conica, Southern Vietnam, Nha Trang Bay, Tre Id, BL 10 mm; B. Notoplax odysseyi n. sp., Southern Vietnam, Off Phan Rang, 95 m, on old corals, holotype, BL 51 mm; C. Leptoplax nhatrangi n. sp., Nha Trang Bay, Dat Id., 7 – 12 m, on shells of Malleus malleus, holotype, BL 5.2 mm; D. Leptoplax doederleini, Vietnam, Nha Trang Bay, Dat Id., intertidal, under stones, BL 15 mm; E. Leptoplax tongkingi n. sp., Gulf of Tongking, Cat Ba Id., 3 – 4 m, on shells, paratype, BL 5 mm.