Acanthochitona spratlyenses, Yuen & Chen & Yuen & Liu & Leung & Ko & Chan & Somani & Herrmann & Jung & Tokas & Croghan & Traxer & Chiu & Chew & Teoh & Gauhar & Li & Ng, 2021

Acanthochitona spratlyenses View in CoL sp. nov.

( Figs 1D–F, 10–13)

Holotype. Female (adult specimen); South China Sea, Spratly Is., Truong Sa Dong I., 8°53′15.7″N, 112°14′08.3″E, depth 20 m, on dead corals, SCUBA, 2 June 2019, B. Sirenko leg. ( ZIN, No. 2416). GoogleMaps

Paratypes. South China Sea, Spratly Is .: 1 specimen, same data as for holotype ( ZIN, No. 2419) GoogleMaps ; 1 specimen, Song Tu Tay I., 11°23′45.1″N, 114°35′15.1″E, depth 14–15 m, on dead coral, SCUBA, 25 Nov. 2018, B. Sirenko leg. ( ZIN, No. 2417) GoogleMaps ; 1 specimen, Da Lat I., 8°39′30.0″N, 111°39′36.0″E, depth 23 m, on old tridacna clam, SCUBA, 6 June 2019, B. Sirenko leg. ( ZIN, No. 2418) GoogleMaps .

Note. The holotype and paratypes are now disarticulated and consist of the following parts: SEM stubs of valves I, II, V, IV, VIII, a part of the perinotum and the radula; slide-mounted parts of the perinotum and the radula; and a vial with other valves.

Measurements. BL 4.0 mm ( holotype), 4.9 mm ( paratype; ZIN, No. 2417), 5.0 mm ( paratype; ZIN, No. 2418), 4.0 mm ( paratype; ZIN, No. 2419).

Diagnosis. Small chiton with rounded beak and rather low intermediate valves. Jugum wide, wedge-shaped. Tail valve small, oval, with posteri- or mucro. Pustules on tegmentum droplet-shaped or oval, quincuncially arranged in pleurolateral areas; pustules in jugal area becoming more oblong, merging with each other in centre of area. Top of pustule flat, with single megalaesthete pore and two micraesthete pores in pustules of pleurolateral area and up to six in pustules near jugal area. Tegmental plain without aesthete pores. Girdle dorsally covered with minute, somewhat flattened, sharply pointed spicules and larger, curved in low- er part, smooth, pointed needles. Sutural tufts prominent, consisting of thick, slightly curved, smooth needles. Marginal needles weakly ribbed in upper half.

Description. Small elongate-oval chiton. Body length up to 5.0 mm. Valves rounded with small beak, rather elevated (dorsal elevation of valve V equal to 0.35). Tegmentum white with reddish brown maculation, sometimes completely red.

Head valve more than semicircular, with posterior margin straight and anterior slope slightly convex. Intermediate valves roughly rectangular or widely trapezoidal, low, beaked; front margin concave; hind margins slightly concave at both sides of beak; jugum wide wedge-shaped, tail valve trapezoidal, with posterior mucro; anterior and posterior slopes slightly convex.

Pustules densely covering tegmentum, arranged in quincunx order in all areas except for jugum. In jugal area, pustules becoming more oblong, merging with each other in centre of area. Each pustule flat at top, with a single megalaesthete pore and two micraesthete pores in pleurolateral area and up to six pores near jugal area. Tegmental plain without micraesthete pores.

Articulamentum translucent, with transverse callus in middle of valves and several small pores under anterior margin of jugum. Apophyses projecting forwards, 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 wide, its width near valve V about 360 µm. Girdle dorsally densely covered with small, sharply pointed, flattened spicules 52–53 × 8 µm and rarer but larger, curved in lower part, smooth pointed needles 65–121 × 8–10 µm. Sutural tufts prominent, consisting of up to ten thick, sharply pointed, smooth, slightly curved needles

B.I. Sirenko & T. Nguyen Tai. Acanthochitona from the Spratly Islands up to 1100 × 23 µm, surrounded by much shorter, thin, sharply pointed, smooth needles 120–130 × 8 µm. Marginal spicules of two kinds: sharply pointed, weakly ribbed in upper half spicules up to 60–80 × 10–11 µm, and smooth needles up to 300–350 × 17 µm. Ventral spicules small, smooth, sharply pointed 61 × 10 µm.

Radula of holotype 1.2 mm long, with 18 transverse rows of mature teeth. Central tooth with expanded top and sharply pointed base; first lateral tooth with thick nodulous anterodorsal corner; major lateral tooth with tricuspid head.

Holotype with seven gills on each side extending from valve V to valve VII; paratypes with eight gills.

Holotype is female with about 30 mature eggs (egg diameter 200–230 µm).

Comparison. The new species belongs to a group of acanthochitons which inhabit shoals of mainland Vietnam, the Ryukyu Islands and the Spratly Islands. This group is characterised by the absence of distinct border between the jugal and pleurolateral areas and by merged pustules on the jugal area of intermediate valves. The group consists of five species: A. saitoi , A. savinkini Sirenko, 2012 , A. nigra Sirenko et Saito, 2017 , A. ostreaphila Sirenko et Saito, 2017 and A. spratlyenses sp. nov.

Acanthochitona spratlyenses sp. nov. differs from A. saitoi and A. savinkini in having the sharply pointed dorsal needles ( vs. obtusely point- ed spicules in both species), long (300–350 µm) marginal needles ( vs. short, 60–67 µm, marginal needles in both species), two thick, sharply pointed, stout, smooth needles ( vs. several stout and hundreds of fine needles in A. savinkini ), and pustules on tegmentum with 3–7 pores of aesthetes ( vs. pustules with more than 20 pores in A. saitoi ). The new species differs from A. nigra in having the curved sharply pointed dorsal needles ( vs. lacking dorsal needles in A. nigra ), a posterior mucro ( vs. an anterior mucro in A. nigra ), small (5 µm) pores of megalaesthetes ( vs. large, 10 µm, pores in A. nigra ).

Acanthochitona spratlyenses sp. nov. differs from A. ostreaphila in having the long insertion plates of valve VIII ( vs. very short insertion plates in A. ostreaphila ), pustules arranged close together ( vs. pustules sparsely arranged in A. ostreaphila ), a posterior mucro ( vs. a central mucro in A. ostreaphila ), and sharply pointed dorsal spicules ( vs. curved bluntly pointed spicules in A. ostreaphila ).

Distribution. The species is known only from the type locality in the Spratly Islands, at depths of 15– 28 m.

Etymology. The species is named after the Spratly Islands.