Cryptoplax burrowi (E. A. Smith, 1884 )

Sirenko, Boris I., Saito, Hiroshi & Nguyen, Tai-Tu, 2024, Chitons of the genus Cryptoplax (Mollusca: Polyplacophora) from the Spratly Islands, South China Sea, Zootaxa 5492 (1), pp. 52-70 : 61-64

Cryptoplax burrowi (E. A. Smith, 1884)

( Figures 2B , 9–11 )

Chitonellus larvaeformis ; Reeve 1847: pl. 1, fig. 3, species 3 (not of Chiton larvaeformis de Blainville MS, Burrow 1815 ).

Chiton (Chitonellus) burrowi E. A. Smith 1884: 85 .

Cryptoplax burrowi ; Pilsbry 1893: 54 (in part), pl. 9, fig. 6; Nierstrasz 1905: 71, pl. 5, figs 141–143,?144, 145, pl. 6, 146–150; Iredale & Hull 1925: 106, pl. 12, figs 13, 31; Leloup 1940: 15 (in part), fig. 5, pl. 3, fig. 3; Kaas & Van Belle 1980: 20 (in part); 1998: 36 (in part); Kaas et al. 1998: 189 (in part); Gowlett-Holmes 2001: 46 (in part); Dell’Angelo et al. 2010: 26 , figs 11A–L, 12R–X, 13O–R; Sirenko & Saito 2017: 497 , figs 39–42, 44G; Saito 2023: 54 (in part), fig. 5C, D.

Cryptoplax mjoebergi Bergenhayn 1933: 157 , figs 1–8; Saito 2006: 131; Granpoder et al. 2023: 75.

Type material. Lectotype, NHMUK 1992.053 /1.

Type locality. Northern Queensland, Australia .

Material examined. South China Sea, Spratly Islands, 11°23′45.1″N, 114°35′15.1″E, depth 14–15 m, selected from sand, SCUBA, sample 5, 3 head and 6 intermediate valves, B. Sirenko leg. 25.11.2018; 9°46′53.4″N, 114°22′24.7″E, depth 15.0 m, SCUBA, selected from sand, sample 48, tail valve, B. Sirenko leg. 19.05.2019; 8°49′47.8″N, 113°56′03.2″E, depth 6.0 m, SCUBA, washing off old corals, sample 62, 1 spm, ( ZIN 2451 ) BL 21.0 mm, T. Nguyen leg. 27.05.2019 .

Description. Based on young specimen (BL 21. 0 mm, ZIN 2451): Anterior four valves in contact, remaining valves separated from each other. Color of tegmentum reddish with white portion in valves I–VII, yellowish in valve VIII. Color of girdle consisting of maculation of green, brown and white on dorsum, with brownish orange and white marginal fringe.

Head valve bell-shaped; tegmentum slightly longer than wide, widest of all valves, sculptured with densely packed elongate pustules which are occasionally fused with neighboring ones. Valve II wide oval. Valves III–VIII narrow oval to spindle-shaped, of which valves V–VII are obviously smaller than other valves. Jugum wedge-shaped in valves II and III, narrow wedge-shaped in valves IV, almost parallel sided in valves V–VIII, nearly smooth on dorsal surface and finely denticulate on side margins. Pleurolateral area sculptured with longitudinal series of round to rectangular pustules, which are gradually increasing in size and fused into ribs anteriorly, and arranged in slightly divergent 6 rows in valves II, 3–4 rows in valves III–VIII. Tail valve with subterminal mucro with very short post-mucronal area. Each pustule with 1–3 macraesthetes on top. Micraesthetes sparsely distributed along inner basal portion of riblets and prepustular area of pustules; no micraesthete on top of pustules.

Articulamentum extending outward in anterior 2/5 of tegmentum in valve II, less than 1/ 2 in valves III–VII, almost all around in valve VIII in dorsal view. Insertion plates of head valve rather short, about 1/4 length of tegmentum. Insertion plate of tail valve rather narrow, almost perpendicularly developed ventrally. Slit formula 3/0/0. Color of articulamentum white with rose spot in the middle in head valve, white with rose in the centre in intermediate valves, white in tail valve.

Girdle width rather narrow for genus, covered with minute, strongly ribbed (6–8 double ribs) spicules, up to 100 μm x 20 μm, intermingling with large flattened, round-topped spicules, which are sculptured with 11–19 fine grooves on one side, attaining 350 μm x 100 μm. Sutural tufts of up to 8 slightly curved, smooth, blunt tipped needles, up to 420 μm x 30 μm. Marginal needles numerous, similar to those of sutural tufts, but often with several fine grooves, up to 500 μm x 62 μm. Ventral spicules flat, smooth or weakly grooved at tip, pointed, up to 80 μm x 20 μm.

Gills extending from beginning of valve VII to valve VIII, composed of 13 ctenidia on each side.

Radula 4.5 mm long with 32 transverse rows of mature teeth. Central tooth small, short, weakly round at top. Centro-lateral tooth thickened at obtusely projected antero-dorsal corner that is shallowly grooved at inside. Head of major lateral tooth with three pointed denticles of nearly equal length, which have weak transverse ridge at the proximal portion of the inner two cusps. Major uncinus tooth with narrow blade.

Distribution. Cryptoplax burrowi was recorded from a vast area of the tropical and subtropical Indo-West Pacific, although some records need confirmation because C. burrowi was commonly confused with C. brunoi and some other species as mentioned above ( Dell’Angelo et al. 2010; Saito 2023). Many of those records are not accompanied by detailed descriptions or illustrations. So far, we think the following records of C. burrowi can be accepted: Northern Australia (type locality); Sulawesi ( Dell’Angelo et al. 2010: 32, Fig. 12T ), and Maratoa (=Maratua) Island, Indonesia ( Bergenhayn 1933, as C. mjoebergi ); Sulu Archipelago, the Philippines ( Leloup 1940); Macclesfield Bank, South China Sea ( Sirenko & Saito 2017); Spratly Islands (present study). Dell’Angelo et al. (2010) tentatively assigned specimens from the Maldive Islands (Indian Ocean) to Cryptoplax burrowi because they were aware of the taxonomic confusion about this species. The morphology of the valves and girdle spicules they showed with SEM images seems to agree with that of C. burrowi . Sykes (1907) recorded it from Wasin Island, Kenya, but Kaas (1979) suspected that this is a misidentification of C. dupuisi . In the Spratly Islands, the living animals were found at a depth of 6.0 m.

Remarks. Cryptoplax burrowi most closely resembles C. brunoi . The distinction between the two species is outlined above (remarks under C. brunoi ). Leloup (1940) regarded C. hartmeyeri Thiele, 1911 from Shark’s Bay, Western Australia and C. mjoebergi Bergenhayn, 1933 from Maratoa (=Malatua) Island, Indonesia as synonyms of C. burrowi . We agree with the assignment of C. mjoebergi , but we think that C. hartmeyeri is a distinct species, because it has thicker and pointed large dorsal girdle spicules. Cryptoplax vittata Ang, 1967 described from Mindoro Island, the Philippines, also resembles C. burrowi by having valves V–VII distinctly smaller than valves I, II, and VIII, and the maculated coloration of the girdle. However, C. burrowi differs from C. vittata by having ventrally extending insertion plates in valve I (in C. vittata , they look obviously longer than those of C. burrowi in dorsal view as they are extending anteriorly in C. vittata ), a smaller valve VI, and wider, large flattened dorsal spicules. Consequently, Saito's (2006) synonymization of C. vittata with C. mjoebergi is rejected. C. dupuisi Ashby, 1931 known from southeastern Africa also has distinctly smaller valves V and VI, but it differs from C. burrowi by the larger valve IV, smaller valve VII, and by having narrower and pointed larger dorsal girdle spicules.

Figures

FIGURE 2. Cryptoplax brunoi, Spratly Islands. A. (ZIN 2450), BL 35.0 mm. Cryptoplax burrowi, Spratly Islands. B. (ZIN 2451), BL 21.0 mm. Cryptoplax acerocostata n. sp., Spratly Islands. C. holotype (ZIN 2453), BL 15.0 mm. D. paratype (ZIN 2452), BL 12.0 mm. Scale: 2 mm for A–D.

FIGURE 9. Cryptoplax burrowi, Spratly Islands. A–K. (ZIN 2451), BL 21.0 mm. A. Valve I, dorsal view. B. Valve II, dorsal view. C. Valve III, dorsal view. D. Valve IV, dorsal view. E. Valve V, dorsal view. F. Valve VI, dorsal view. G. Valve VII, dorsal view. H. Valve VIII, dorsal view. I. Valve V, detail of tegmentum in jugal and pleurolateral areas. J. Valve V, rostral view. K. Valve VIII, lateral view.

FIGURE 10. Cryptoplax burrowi, Spratly Islands. A–D. (ZIN 2451), BL 21.0 mm. A. Dorsal scales and spicules. B. Marginal needles and ventral scales. C. Dorsal spicules and scales and tuft needles. D. Ventral scales.

FIGURE 11. Cryptoplax burrowi, Spratly Islands. A–D. (ZIN 2451), BL 21.0 mm. A. Dorsal scales. B. Radula. C. Marginal needles. D. Central and first lateral teeth of radula.

FIGURE 12. Cryptoplax acerocostata n. sp., Spratly Islands.A–K. holotype (ZIN 2453), BL 15.0 mm. A. Valve I, dorsal view. B. Valve II, dorsal view. C. Valve III, dorsal view. D. Valve IV, dorsal view. E. Valve V, dorsal view. F. Valve VI, dorsal view. G. Valve VII, dorsal view. H. Valve VIII, dorsal view. I. Valve V, detail of tegmentum in jugal and pleurolateral areas. J. Valve V, rostral view. K. Valve VIII, lateral view.