Acanthochitona ostreaphila, 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 : 491-495

Acanthochitona ostreaphila n. sp.

Figs 36–38 , 44 E, F

Type material. Holotype, ZISP 2231, now disarticulated consisting of mounts of shell, perinotum and radula and 4 paratypes: 3 spms, ZISP 2232; 1 spm, NSMT-Mo 78925.

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

Etymology. Named ostrea = oyster, phil = like, because most of the specimens were found on the shells of oysters.

Material examined. Northern Vietnam, Gulf of Tongking, Cat Ba Id., 20°43.379´N, 107°06.216´E, 3–5 m, SCUBA, sand, stones, on shell of Isognomon sp., 1 spm, BL 6 mm, 0 4.05.2012, leg. B. Sirenko ; 20°46.310´N, 107°07.700´E, 0.5 m, on oysters, 1 paratype (NSMT) and 3 spms, BL 3–8 mm, 0 4.05.2012, leg. B. Sirenko; 20°43.675´N, 107°03.373´E, intertidal, on stones, holotype, BL 5 mm, 0 8.05.2012, leg. B. Sirenko; Van Don , 21°02´09´´N, 107°33´35.4´´E, intertidal, on shells of oysters, 3 spms, BL 4–5.5 mm, 0 3.04.2014, leg. B. Sirenko ; 20°52´3.9´´N, 107°19´16.4´´E, intertidal, on shells of oysters, 2 spms, BL 4–4.5 mm, 0 4.04.2014, leg. B. Sirenko; 20°54´N, 107°22´E, intertidal, on shells of oysters, 5 spms, BL 3–7 mm, 0 5.04.2014, leg. B. Sirenko; 21°04´46.3´´N, 107°34´18´´E, intertidal, on shells of oysters, 2 spms, BL 6–7 mm, 0 6.04.2014, leg. B. Sirenko; 20°53´58.6´´N, 107°19´39.4´´E, intertidal, on shells of oysters, 2 spms, BL 3–4 mm, 0 7.04.2014, leg. B. Sirenko. Southern Vietnam, Phu Vin , 13°24.009´N, 109°13.136´E, 0 m, under stones, 7 spms, BL 5–8 mm, 0 1.05.2013, leg. B. Sirenko ; Nha Trang Bay , Tre Id., Dambay Inlet, 12°12.098´N, 109°13.178´E, intertidal, on oysters that inhabited mangrove Rhizophora sp., 3 paratypes, BL 8–12 mm, 0 5.06.2012, leg. B. Sirenko ; Gom Id. , 12°40.268´N, 109°24.514´E, 0–0.5 m, on bivalve shells inhabited stones, 8 spms, BL 3–5 mm, 0 2.05.2013, leg. B. Sirenko.

Diagnosis. Small chitons with wide, low intermediate valves. Jugum granulose, wide wedge-shaped. Tail valve oval with central, low mucro. Pustules on tegmentum sparsely arranged; top of pustule slightly convex, with single macraesthete pore and 6–10 micraesthete pores. Micraesthete pores present also on tegmental plain. Dorsal side of girdle uniformly beset with minute, short, smooth, blunt tip spicules. Sutural tufts small, of very fine fiburous or “asbestoid” spicules, intermingling with some thicker needles.

Description. Animal small, elongate oval, low in profile. Color of tegmentum light brown with patches of brown, dark brown and greenish brown on lateral and pleural areas, rose pink on jugum. Girdle light brown with regular bands of dark olive green; base of sutural tufts yellow.

Head valve semicircular, posterior margin widely V-shaped; anterior slope nearly straight; without radial ribs. Intermediate valves wide, roughly rectangular, low, subcarinate, beaked; front margin slightly sinuate between apophyses; hind margin slightly concave at both sides of beak; jugum wide wedge-shaped, which comprises of fused pustules; lateral areas slightly raised. Tail valve oval, with central, low mucro; width of tegmentum ca. 75% of that of head valve; posterior slope nearly straight.

Pustules on tegmentum sparsely distributed, arranged in quincunx order on head valve, lateral areas of intermediate valves and posterior area of tail valve, in weakly curved, converging rows in pleural areas. Each pustule slightly convex at top with single macraesthete pore and 6–10 micraesthete pores. Micraesthete pores present also on tegmental plain.

Articulamentum moderately developed, translucent, with transverse callus in middle of valves, porous under jugum. Apophyses well projecting forward, triangular, widely separated from each other in intermediate valves, truncate in tail valve. Insertion plates short with short, narrow slits; insertion plate of tail valve between slits very short. Slit formula 5/1/2.

Girdle rather wide, ca. 0.9 mm near valve V. Dorsal side of girdle beset with minute spicules, 43–48 µm x 12– 13 µm, smooth, blunt at tip. Sutural tufts of up to ten thick needles, up to 248 µm x 12 µm, surrounded by very finefibrous spicules, up to 215 µm x 3 µm. Marginal needles lanceolate, flat, smooth, up to 250 µm x 11 µm. Ventral spicules similar to marginal spicules but shorter, occasionally wider, 65–88 µm x 16 µm.

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

Radula 1.7 mm long, with 29 transverse rows of mature teeth. Central tooth elongate tulip-shaped, with thick semicircular blade and keel at basal half. First lateral (centro-lateral) tooth with thickened antero-dorsal corner. Major lateral tooth with tricuspid head; cusps pointed, somewhat flattened near tips; central cusp longer than others. Major uncinal tooth with comb-like blade.

Distribution. Northern to southern Vietnam from 21°02.15´N to 12°12.098´N, intertidal– 3 m.

Remarks. The present species has a comb-like blade in the major uncinal tooth of the radula. Together with the similar tooth of Leptoplax tongkingi n. sp. described above, this is the first report of this feature in the family Acanthochitonidae (see Remarks to L. tongkingi ).

This small species resembles Acanthochitona saitoi Sirenko, 2012 and Acanthochitona savinkini Sirenko, 2012 described from Vietnamese waters by having very fine fibrous or “asbestoid” spicules on the sutural tufts and minute dorsal girdle spicules which gives smooth appearance to the girdle, but the present species differs from those two species by the wide, rectangular intermediate valves with sparsely arranged pustules on the tegmentum, by having fewer micraesthete pores on the pustules, a narrower girdle, and a comb-like blade of the major uncinus tooth.

Figures

FIGURE 36. Acanthochitona ostreaphila 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. Valve V, detail of tegmentum in jugal and lateropleural areas; F. Valve V, rostral view; G. Valve VIII, lateral view.

FIGURE 37. Acanthochitona ostreaphila n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 5 mm: A. Tuft and dorsal spicules; B. Valve VIII, detail of tegmentum of antemucronal area; C. Dorsal spicules, marginal needles and ventral spicules; D. Radula.

FIGURE 38. Acanthochitona ostreaphila n. sp., Northern Vietnam, Gulf of Tongking, Cat Ba Id., holotype, BL 5 mm: A. Needles of tufts; B. Dorsal 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.