Acanthochitona condaoi, 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 : 482-484

Acanthochitona condaoi n. sp.

Figs 27–29 , 44 C

Type material. Holotype, ZISP 2228, now disarticulated consisting of mounts of shell, perinotum and radula and 4 paratypes: 3 spms, ZISP 2229; 1 spm, NSMT-Mo 78923.

Type locality. Con Dao Island , Con Son Archipelago, southern Vietnam, 8°41´N, 106°38´E, 0.5–1.0 m, sand, on shell of Malleus malleus.

Etymology. Named after the type locality.

Material examined. Southern Vietnam, Con Son Archipelago , Con Dao , Id., 8°41´N, 106°38´E, 0.5–1.0 m, sand, on shell of Malleus malleus, 4 spms: holotype, BL 5 mm and 3 paratypes, BL 4–7 mm, 0 9.04.2010, leg. B. Sirenko ; Ba Id. , 8°38´N, 106°33´E, 8–9 m, SCUBA, on old shells with red crustose calcareous algae, 1 spm, BL 4.5 mm, 15.04.2010, leg. B. Sirenko ; Con Dao Id. , 08°40.403´N, 106°33.518´E, 13–15 m, SCUBA, attached substratum unknown, among old shells and corals, 1 spm, BL 2.5 mm, 13.04.2011, leg. B. Sirenko ; Con Dao Id. , 08°40.403´N, 106°37.541´E, 3–5 m, SCUBA, on old corals, 3 spms, BL 4–6 mm, 14.04.2011, leg. B. Sirenko ; Bay Conh Id. , 08°39.653´N, 106°41.557´E, 15–17 m, SCUBA, attached substratum unknown, among old shells and corals, 1 spm, BL 3 mm, 15.04.2011, leg. B. Sirenko ; Con Dao Id. , 08°41´N, 106°38´E, 0.5–1 m, attached substratum unknown, among sand, shells of Malleus malleus and Pinna spp., 1 spm, BL 5 mm, 18.04.2011, leg. B. Sirenko ; Con Dao Id. , 08°41´N, 106°38´E, 0.5–1 m, sand, on shells of Pinna sp., 1 spm, BL 5 mm, 20.04.2011, leg. B. Sirenko ; Con Dao Id. , 08°41´N, 106°38´E, 0.5–1 m, sand, on shells of Malleus malleus, 11 spms (1 paratype in NSMT), BL 3–10 mm, 20.04.2011, leg. B. Sirenko ; Gulf of Thailand, Anthak Ids , Vang, Id., 09°55.201´N, 104°01.201E, 9–11 m, SCUBA, sand, stones, 1 intermediate valve, 0 4.04.2011, leg. B. Sirenko.

Diagnosis. Small chitons with fan-shaped intermediate valves. Jugum very narrow, with wavy side margins. Tail valve small, semicircular, with central, moderately elevated mucro. Pustules on tegmentum droplet-shaped, rather sparsely and randomly arranged; top of pustules flat, with single macraesthete pore and 2–4 micraesthete pores. No micraesthete pores on tegmental plain. Dorsal side of girdle beset with minute, flat, finely ribbed spicules intermingling with much longer, sharply pointed spicules, which are curved near the base, sculptured with fine, sharp ridges on the entire length. Sutural tufts prominent, of thick, curved, smooth needles.

Description. Animal small, oval, moderately elevated. Color of tegmentum creamy white with dark brownish maculation or flecks on lateropleural areas; jugum pale greenish gray. Girdle light brown with translucent prominent sutural tufts which are surrounded by greenish fine spicules.

Head valve semicircular, posterior margin almost straight; anterior slope slightly convex; no radial ribs. Intermediate valves fan-shaped, moderately elevated, beaked; front margin gently rounded; hind margin concave at both sides of beak; jugum narrow wedge-shape, smooth, wavy on both sides; lateral areas hardly separable. Tail valve roughly semicircular, with central, moderately elevated mucro; width of tegmentum slightly wider than half width of valve IV; posterior slope almost straight.

Pustules on tegmentum rather sparsely distributed, randomly arranged except for jugum. Each pustule flat or slightly concave at top with single macraesthete pore near center, accompanied with 2–4 micraesthete pores. No micraesthete pores on tegmental plain.

Articulamentum rather thin, translucent throughout, with transverse callus in middle of valves, a few pores under anterior margin of jugum. Apophyses well projecting forward, round, widely separated each other in intermediate valves; anterior margins concave in tail valve. Insertion plate short with short slits. Slit formula 5/1/2.

Girdle rather wide, ca. 0.7 mm near valve V (width 2.0 mm) in studied specimen (BL 5 mm). Dorsal side of girdle beset with minute, 30 µm x 10 µm, flat, finely ribbed spicules intermingling with much longer, up to 230 µm x 17 µm sharply pointed needles, which are curved near base, sculptured with fine, sharp ridges throughout entire length. Sutural tufts prominent with about 20 needles, which are thick, curved, sharply pointed, smooth, up to 1000 µm x 40 µm, surrounded by flat, sharply pointed, smooth spicules, up to 220 µm x 15 µm. Marginal needles similar to those of sutural tufts, but smaller, up to 262 µm x 38 µm. Ventral spicules small, flat, blunt at tip, 27–71 µm x 14–15 µm.

Gills extending from valve VII to valve V, composed of 10 ctenidia on each side in studied specimen (BL 5 mm).

Radula 1.6 mm long with 32 transverse rows of mature teeth in studied specimen (BL 5 mm). Central tooth elongate tulip-shaped with weakly bilobed blade, keeled at basal half. First lateral (centro-lateral) tooth with thickened antero-dorsal corner. Major lateral tooth with tricuspid head; cusps rather thin, sharply pointed. Major uncinus tooth with rather wide blade.

Distribution. Con Son Archipelago, Gulf of Thailand, southern Vietnam, 0.5– 15 m.

Remarks. The present species differs from all other Indo-West Pacific Acanthochitona species by having a very narrow jugum and strongly curved spicules on the perinotum.

Figures

FIGURE 27. Acanthochitona condaoi n. sp., Southern Vietnam, Con Dao Id., holotype, BL 5 mm: A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve V, rostral view; E. Valve IV, ventral view; F. Valve V, detail of tegmentum in lateropleural area; G. Valve VIII, detail of tegmentum in antemucronal and postmucronal areas; H. Valve VIII, lateral view.

FIGURE 28. Acanthochitona condaoi n. sp., Southern Vietnam, Con Dao Id., holotype, BL 5 mm: A. Radula; B. Pustules in antemacronal area of valve VIII; C, D. Tufts, dorsal needles and spicules and marginal needles.

FIGURE 29. Acanthochitona condaoi n. sp., Southern Vietnam, Con Dao Id., holotype, BL 5 mm: A. Needles of tufts; B. Dorsal needles and spicules; C. Marginal needle; D. Ventral spicules; E. Head of major lateral tooth of radula; F. Central and first lateral teeth of radula; G. Pustule of lateropleural area of tegmentum with aesthete pores. Scale bars: 100 µm.

FIGURE 44. Images of species of genus Acanthochitona and Cryptoplax: A. A. achates, near Da Nang, intertidal, on barnacles, BL 15 mm; B. A. biformis, Southern Vietnam, Cau Id., 13 – 17 m, on old shells, BL 11 mm; C. A condaoi n. sp., Gulf of Thailand, Vang Id., 9 – 11 m, on old shells, paratype, BL 6 mm; D. A. lanae n. sp., near Da Nang, intertidal, on barnacles, paratype, BL 12 mm; E. A. ostreaphila n. sp., Nha Trang Bay, Tre Island, Dambay Inlet, intertidal, mangrove community, on oysters, paratype, BL 12 mm; F. A. ostreaphila n. sp., Gulf of Tongking, Cat Ba Island, 0.5 m, on oysters, paratype, BL 5.5 mm; G. Cryptoplax burrowi, South China Sea, Macclesfield Bank, 21 m, BL 29 mm.