Acanthochitona biformis (Nierstrasz, 1905)

Figs 24–26, 44 B

Acanthochites biformis Nierstrasz, 1905: 55, pl. 1, fig.1, pl. 4, figs 104–108.

Acanthochitona biformis: Kaas & Van Belle, 1980: 16, 1998: 32; Saito, 2006b: 125, fig. 3.

Type material. Syntypes, ZMA Moll 138563.

Type locality. Not designated, original lots were mentioned from near Kupan and Nusa Besi of Timor Island, Sanguisiapo Island and Banda Island, Indonesia.

Material examined. Southern Vietnam, Cau Id., 11°13.227´N, 108°49.786´E, 13–17 m, SCUBA, on old shells, 1 spm, BL 11 mm, 12.05.2013, leg. B. Sirenko.

Description. Animal small, oval, with small, moderately elevated, round backed valves. Color of tegmentum white with brownish flecks; jugum with yellowish tint.

Head valve semicircular with five obsolete radial undulations; posterior margin almost straight; anterior slope convex. Intermediate valves roughly oval to fan-shaped in outline, beaked, round-backed; lateral areas hardly separable; side slopes slightly convex; jugum narrow wedge-shape, smooth. Tail valve small, with central mucro; width of tegmentum slightly wider than half width of head valve.

Tegmentum sculptured with droplet-shaped or oval, weakly concave pustules arranged in quincunx order; prepustular slope very narrow, sunken in slit. Each pustule with one macraesthete pore or occasionally two pores near center. No aesthete pore on tegmental plain. Jugum with fine growth lines, but practically smooth; micraesthete pores sparsely distributed.

Apophyses rounded, widely apart from each other in intermediate valves, truncated in tail valve. Insertion plate rather short, smooth on surface except for tail valve, which has rough surface with eight minute denticles at the edge in studied specimen. Slit formula 5/1/2.

Girdle rather wide, about 2.8 mm near valve V (width 3.3 mm) in studied specimen. Dorsal side of girdle densely covered with minute, pointed spicules, 110–125 µm x 10–16 µm, intermingling with thin needles attaining 218 µm x 9 µm. Sutural tufts prominent, consisting of up to 40 long, slightly curved, smooth or occasionally finely ribbed needles, 770–1200 µm x 40 –42 µm. Marginal needles long, slightly curved, finely ribbed, up to 413 µm x 27 µm. Ventral spicules minute, flat, 66–80 µm x 14–15 µm.

Gills extending from valve VII to valve V, composed of 10 ctenidia on each side in studied specimen. Radula of studied specimen 3.5 mm with 25 rows of mature teeth. Central tooth small, spatulate in outline, thin, sharply keeled in basal portion. First lateral (centro-lateral) tooth with thick, nodulous antero-dorsal corner. Major lateral tooth with tricuspid head; denticles blunt at tip; central one longer than others. Major uncinal tooth with long blade of moderate width.

Distribution. Timor and Banda Islands, Indonesia; southern Sulu Islands and Mindoro Island, Philippines; southern Vietnam, intertidal– 12 m.

Remarks. Nierstrasz (1905) remarked that there are two variations within the present species, which have almost same morphology in the valves and same color (tint) in the sutural tufts, but differences in the color and the width of the valves. One of the two color variations referred by Nierstrasz has white valves with a yellowish jugum and dark reddish flecks as has been reported by Saito (2006b) in a specimen from Mindoro Island, Philippines. This specimen has a little wider valves than the present material but other features match well in both specimens. Another variation reported by Nierstrasz has uniformly orange-yellow valves which are slightly narrower than those of the white variation. Examination of the orange-yellow specimen among the syntypes (from Sanguisiapo Island, Siboga station 93) confirmed the same morphology among those variations. As similar differences are known also from some Acanthochitona species, such as A. crinita (Pennant, 1777) (Kaas 1985), A. fascicularis (Linnaeus, 1767) (Leloup 1941: as A. communis) and A. hirudiniformis (Sowerby, 1832) (Watters 1990), we also regard them as intraspecific variations.

Acanthochitona biformis differs from its Indo-West Pacific congeners by having wide valves with a narrow jugum and densely packed oval pustules. The morphology of the pustules and radula show similarity to those of Acanthochitona intermedia (Nierstrasz, 1905), but A. biformis differs from A. intermedia by having a narrower jugum and more round pustules with a much narrower prepustular slope which is sunken in the slit.

The present species superficially resembles Acanthochitona leopoldi (Leloup, 1933), but differs in shape and in the arrangement of the pustules (oval to droplet-shape and arranged in quincunx order in the former, while narrow droplet-shape and arranged in comarginally in the latter. Also A. leopoldi is more spiculous in the sutures between the valves. This is the first record of this species from Vietnamese waters.