Acanthochitona leopoldi ( Leloup, 1933 )

Acanthochitona leopoldi ( Leloup, 1933) View in CoL

( Figs 1–5)

Acanthochites leopoldi Leloup, 1933: 27 View in CoL , pl. II, figs 1–9. Acanthochitona leopoldi View in CoL : Burghardt et al., 2006: 43;

Sirenko & Schwabe, 2011: 119.

Holotype (No. IRSN I.G. 9223/MT. 3805) [not examined]. Type locality: Mansfield Island , Indonesia.

Material examined. South China Sea, Spratly Islands: 1 specimen ( ZIN, No. 2368), 10°10′17.0″N, 114°21′59.2″E, depth 31–42 m, on dead corals, SCUBA, 4 Dec. 2018, B. Sirenko leg. GoogleMaps ; two intermediate valves ( ZIN, No. 2371), 11°23′45.1″N, 114°35′15.1″E, depth 14–15 m, in sand, SCUBA, 25 Nov. 2018, B. Sirenko leg. GoogleMaps ; 1 specimen ( ZIN, No. 2369), 11°23′05.7″N, 114°18′30.6″E, depth 10–15 m, on dead corals, SCUBA, 15 May 2019, T. Nguyen Tai leg. GoogleMaps ; 1 specimen ( ZIN, No. 2370), 08°48′47.8″N, 113°56′03.2″E, 2–4 m, on old shells, SCUBA, 15 May 2019, B. Sirenko leg. GoogleMaps

Redescription. Chiton of small size, largest body length 15 mm. In our material, BL 3 mm (No. 2370), 4.5 mm (No. 2368) and 8 mm (No. 2369). Body oval; valves hardly beaked, low elevated (dorsal elevation 0.23–0.24 mm), back evenly rounded. Colour of tegmentum as various combinations of white, yellow and brown.

Head valve semicircular, front slope convex, posterior margin almost straight with projecting apex. Intermediate valves rather wide, trapezoidal, low; front margin slightly sinuate between apophyses and splayed laterally; hind margin slightly concave at both sides of beak; side margins rounded. Tail valve oval, with central, low mucro; width of tegmentum ca. 62% of that of head valve.

Tegmentum sculptured with droplet-shaped, concave, chequered pustules. Pustules forming rows; transverse rows especially noticeable on pleurolateral areas. Each pustule with single macraesthete pore located approximately in centre. No pores on tegmental plain. Jugal area smooth, with growth lines only and with sparsely distributed aesthete pores.

Articulamentum well-developed, white; apophyses rounded, widely apart from each other in intermediate valves, truncated in tail valve. Slit formula 5/1/2.

Girdle rather wide, ca. 0.8 mm from valve V (width 1.5 mm in examined specimen; its BL 4.5 mm), white with large light brown patches and with violet spots at base of each tuft. Dorsal side of girdle surrounded with minute, slightly bent, sharply pointed, smooth needles 60–100 × 6 µm; some needles violet, but most of them white. Same small needles located in a narrow strip between valves ( Fig. 3A). Sutural tufts of up to 20 weakly curved, smooth needles 500–1000 × 17 µm. Marginal needles similar to those of sutural tufts but smaller, up to 500 × 16 µm. Ventral spicules flattened, bluntly pointed, 50 × 14 µm.

All three available whole specimens with eight gills on either side, extending from valve IV to valve VII.

Radula 1.2 mm long, with 22 transverse rows of mature teeth in specimen with BL 4.5 mm. Central tooth small, spatulate in outline, thin. First lateral tooth with thick, nodulous anterodorsal angle. Major lateral tooth with tricuspid head; denticles blunt at tip; central one a little longer than others.

Notes. We would like to note the differences between the Spratly specimens and the type specimen. All available specimens from the Spratly Islands have small needles located in a narrow strip between the valves ( Figs 1A, 1C, 3A). Leloup (1933) did not mention this feature in the holotype , possibly because he did not see it. All our specimens ( BL 3.0, 4.5 and 8.0 mm) have eight gills on either side, whereas the holotype ( BL 15.0 mm) has 13 gills, perhaps also because the latter is twice as large as our largest specimen. Leloup (1933) provided excellent drawings of the valves and the girdle armature of the holotype. All other features of our specimens conform to those of the holotype .

During our examination of the Spratly specimens with SEM, we found the age variability in some of the features. With age, the width of the pleurolateral area increases as compared with the jugal area ( Figs 2C, 5B, 5C), the notch on top of the central tooth of radula increases ( Fig. 4B–D) and the shape of the dorsal spicules changes from bottle-shaped to needle-shaped ( Figs 5G, 3B).

Acanthochitona leopoldi View in CoL resembles A. penicillatus (Deshayes, 1863) View in CoL from the Indian Ocean and the Philippines, A. jugotenuis Kaas, 1979 View in CoL from the Indian Ocean, A. intermedia (Nierstrasz, 1905) View in CoL from Indonesia, and A. biformis (Nierstrasz, 1905) View in CoL from Indonesia, the Philippines and Vietnam.

From A. penicillatus View in CoL , A. leopoldi View in CoL differs in having the central mucro ( vs posterior mucro), one macraesthete and no micraesthete pores in pustule ( vs one macraesthete and one to three micraesthete pores in pustule) ( Dell’Angelo et al., 2010).

From A. jugotenuis , A. leopoldi differs in having the smooth jugal area ( vs finally longitudinally ribbed), droplet-shaped pustules ( vs rounded pustules), central mucro ( vs posterior mucro), and smooth marginal needles ( vs ribbed marginal needles) ( Kaas, 1979).

From A. intermedia , A. leopoldi differs in having the wider valves, central mucro ( vs posterior mucro), chequered pustules which at the same time form rows ( vs pustules do not form rows), and each tuft with ca. 20 needles ( vs each tuft with more than 50 needles).

From A. biformis , A. leopoldi differs in having the wider valves with a wider jugum and droplet-shaped pustules in pleurolateral areas ( vs oval pustules in pleurolateral areas), and smooth marginal needles ( vs ribbed marginal needles).

Of all the related species mentioned above, A. penicillatus is the closest to A. leopoldi . It should also be mentioned that both these species have been recorded in Sri Lanka ( Sirenko & Schwabe, 2011).

Distribution. Indonesia, Sri Lanka. The new records expand the known distribution up to the Spratly Islands in the north. The species occurs at the depths from 4 to 31 m.