Acanthochitona nigra, 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 : 489-491

Acanthochitona nigra n. sp.

Figs 33–35

Type material. Holotype, ZISP 2230, now disarticulated consisting of mounts of shells, perinotum and radula.

Type locality. Cat Ba Island , Gulf of Tongking, northern Vietnam, 20°47´09´´N, 107°06´10´´E, 1–1.5 m.

Etymology. The species name refers to the dark tegmentum.

Material examined. Northern Vietnam, Gulf of Tongking, Cat Ba Id., 20°47´09´´N, 107°06´10´´E, 1–1.5 m, SCUBA, sand, stones, on bivalve shell, holotype, BL 5 mm, 0 1.05.2012, leg B. Sirenko.

Diagnosis. Small chitons with beaked, low intermediate valves. Jugum wide wedge-shaped, smooth on surface. Tail valve small, oval, with central, low mucro. Pustules on tegmentum droplet-shaped or round, quincuncially arranged; top of pustule flat, with single macraesthete pore and 7–8 micraesthete pores. No micraesthete pores on tegmental plain. Dorsal side of girdle beset with minute, somewhat flattened spicules which are sculptured with fine riblets near pointed tip. Sutural tufts prominent, of thick, slightly curved, smooth needles.

Description. Animal small, elongate oval, low in profile. Color of tegmentum dark brown with brownish maculation. Girdle dark brown or almost black, with white sutural tufts.

Head valve more than semicircular, posterior margin almost straight; anterior slope slightly convex; without radial ribs. Intermediate valves rather wide, roughly rectangular or wide trapezoidal, low, beaked; front margin concave; hind margin slightly concave at both sides of beak; jugum wide wedge-shaped, smooth; lateral areas hardly separable from pleural areas. Tail valve oval, with central, low mucro; width of tegmentum more than half width of valve IV; posterior slope slightly concave.

Pustules on tegmentum densely distributed, arranged in quincunx order on all areas except for jugum. Each pustule flat at top with single macraesthete pore and 7–8 micraesthete pores. No micraesthete pores on tegmental plain.

Articulamentum translucent, with transverse callus in middle of valves, several small pores under anterior margin of jugum. Apophyses well projecting forward, triangular, widely separated from each other in intermediate valves, truncated in tail valve. Insertion plate short with short, narrow slits. Slit formula 5/1/2.

Girdle rather narrow. Dorsal side of girdle covered with minute, 22–33 µm x 8–10 µm, somewhat flattened, finely ribbed near pointed tip. Sutural tufts prominent, of up to 10 thick, sharply pointed, smooth needles, measuring up to 500 µm x 20 µm, surrounded by much shorter, thin, sharply pointed, smooth needles. Girdle margin of smooth needles similar to those of sutural tufts, up to 100 µm x 18 µm and lanceolate, flat finely ribbed spicules, up to 100 µm x 27 µm. Ventral spicules small, 22–51 µm x 11–18 µm, flat, blunt at tip.

Gills extending from valve VII to valve IV, composed of 10 ctenidia on each side.

Radula 1.6 mm long, with 31 transverse rows of mature teeth. Central tooth tulip-shaped, with almost straight blade at top and keel at basal half. First lateral (centro-lateral) tooth with thick nodulous antero-dorsal corner. Major lateral tooth with tricuspid head; cusps of almost equal length, weakly pointed, somewhat flattened near tips. Distribution. Known only from the type locality

Remarks. This species resembles Acanthochitona savinkini Sirenko, 2012 by the sculpture of the shells and the dorsal side of the girdle which is uniformly covered with minute short spicules, but differs by having fewer micraesthete pores (7–8 in the former, more than 20 in the latter) and the lack of very fine fibrous or 'asbestoid' ( Pilsbry, 1893: 18; Iredale & Hull, 1927: 71) silvery sutural tufts.

Figures

FIGURE 33. Acanthochitona nigra n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 5 mm: A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve IV, ventral view; E, F. Valve V, detail of tegmentum in lateropleural area; G. Valve V, rostral view; H. Valve VIII, lateral view.

FIGURE 34. Acanthochitona nigra n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 5 mm: A, B. Tufts, dorsal spicules, marginal needles and spicules and ventral spicules; C. Tufts, dorsal spicules and marginal needles and spicules; D. Radula.

FIGURE 35. Acanthochitona nigra n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 5 mm: A. Needles of tufts; B. Dorsal spicules; C. Marginal needle and spicules; D. Ventral spicules; E. Head of major lateral tooth of radula; F. Central and first lateral teeth of radula; G. Pustule of lateropleural areas of tegmentum with aesthete pores. Scale bars: 100 µm.