Cryptoplax acerocostata, Sirenko & Saito & Nguyen, 2024

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 : 64-68

Cryptoplax acerocostata n. sp.

( Figs 2C, 2D , 12 , 13 )

Type material. Holotype, ZIN 2453 and paratype, ZIN 2452 .

Type locality. Spratly Islands, South China Sea, 8°50′35.2″N, 112°11′26.0″E, depth 18.0 m.

Etymology. From the Latin acer, sharp, and costata, ribbed, referring to the ribs on pleurolateral area, which are sharply angulate at outer edge.

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 tail valves and 11 intermediate valves, B. Sirenko leg. 25.11.2018 ; 11°22′33.9″N, 114°18′04.2″E, depth 9.0–15.0 m, SCUBA, under stones, sample 43, 1 spm, BL 18.0 mm, T. Nguyen leg. 16.05.2019 ; 9°46′53.4″N, 114°22′24.7″E, depth 15.0 m, SCUBA, selected from sand, sample 48, intermediate valve, B. Sirenko leg. 19.05.2019 ; 8°50′35.2″N, 112°11′26.0″E, depth 18.0 m, SCUBA, washing off old corals, sample 79, holotype ( ZIN 2453 ) BL 15 mm and paratype ( ZIN 2452 ) BL 12 mm, B. Sirenko leg. 03.06.2019 .

Description. Based on the holotype (BL 15 mm, ZIN 2453): Anterior four valves in contact, remaining valves separated from each other. Color of tegmentum dark red. Color of girdle yellow with white and brownish bands.

Head valve bell-shaped; tegmentum slightly longer than wide, widest of all valves, sculptured with elongate pustules which are fused into longitudinal ribs in lateral portion. Valve II wide oval; valve III rhomboid; valves IV–VIII elongate oval. Among valves II–VIII, tegmentum of valves II and III almost same length, shortest, that of valve IV longest; length of tegmentum gradually decreasing from valve IV toward valve VIII. Jugum wedge-shaped in valves II and III, almost parallel sided in valves IV–VIII, nearly smooth on dorsal surface and on side margins. Pleurolateral area sculptured with longitudinal ribs, which are smooth on surface, sharply angulate on outer edge, and arranged in slightly divergent 5 rows in valve II, 2–3 rows in valves III–VIII. Tail valve with terminal mucro that is overhanging on short post-mucronal area. Each rib with single row of macraesthetes. Many micraesthetes distributed between jugum and rib, and between ribs; no micraesthete on jugum and outside of macraesthetes on ribs.

Articulamentum extending outward in approximately anterior 1/2 of tegmentum in valves II–VIII in dorsal view. Insertion plates of head valve rather long, about 2/5 length of tegmentum. Insertion plate of tail valve rather narrow, almost perpendicularly developed ventrally. Slit formula 3/0/0. Color of articulamentum rose, especially hind part of valves, in which only apophyses are white.

Girdle narrow. Dorsal side of girdle beset with minute slightly curved, strongly ribbed (8–9 double ribs) spicules, up to 80 μm x 25 μm, intermingling with large rounded or occasionally a little flattened, finely grooved spicules, attaining 200 μm x 50 μm; grooves present in entire length or only distal 1/4–1/3 portion. Slender, parallel sided, bluntly tipped needles near sutural tufts, which are weakly grooved only around the tip. Sutural tufts of up to 12–13 slightly curved, smooth, blunt tipped needles, up to 340 μm x 23 μm. Marginal needles, similar to those of sutural tufts, but with several grooves, up to 320 μm x 25 μm. Ventral spicules long, pointed, smooth, up to 110 μm x 21 μm.

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

Radula 3.4 mm long with 36 transverse rows of mature teeth. Central tooth small, short, with weak cusp 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 obtusely pointed, rather flat denticles of nearly equal length; innermost denticle with weak transverse ridgeline at the proximal portion. Major uncinus tooth with narrow blade.

Distribution. Known only from the type locality, Spratly Islands, at a depth of 15– 18 m.

Remarks. Cryptoplax acerocostata n. sp. most closely resembles Cryptoplax vittata Ang, 1967 regrading the banded coloration of the girdle, general shape of the shell valves and narrow, blunt tipped, striated dorsal girdle spicules. However, it differs by having the following features: Length of the tegmentum is gradually decreasing from valve IV toward valve VIII in C. acerocostata , whereas it is longer in valves VII and VIII than in valves V and VI in C. vittata ; the jugum and the longitudinal ridges of the pleurolateral areas of at least valve VIII in C. acerocostata are wider than those in C. vittata (ratio of width of tegmentum of tail valve to width of jugal area is 4.4 in C. acerocostata and 8.5 in C. vittata ); width of tegmentum of the tail valve is equal to that of the valve VII in C. acerocostata , while in C. vittata , the width of tegmentum of the tail valve is greater than that of the valve VII; number of the ridges in the pleurolateral area in the valve VIII is three in C. acerocostata , 4–5 in C. vittata .

Cryptoplax acerocostata n. sp. differs from the other three species reported herein from the Spratly Islands by having longitudinal ribs with sharply angulated outer edge on the tegmentum of the valves II–VIII. Among those three species, the new species most closely resembles C. burrowi by its coloration, but it differs by the relative size of valve VI that is larger than valve VIII (it is obviously smaller than valve VIII in C. burrowi ), and the narrower and thicker dorsal larger spicules. Cryptoplax acerocostata n. sp. resembles young of C. oculata (see Sirenko, 2012: 104, showing features of specimen of BL 13.5 mm from Nha Trang, Vietnam) in valve sculpture and shape of girdle spicules, but it differs from the latter by having a smooth inner surface of the ribs in the pleurolateral area (vs. denticulate inner edge of the ribs in C. oculata ), shorter insertion plates of the head valve and absence of black and white ring on the girdle around anterior valves (the ring is already obvious in young individuals of C. oculata in the specimen of BL 13.5 mm).

Another species recorded from the Vietnamese coast is Cryptoplax dawydoffi Leloup 1937 . Later, Leloup (1940) regarded this species as a synonym of C. sykesi Thiele, 1909 , which was described from the Red Sea, but we consider the two to be separate species. The sculpture of the pleurolateral areas of C. dawydoffi consists of very narrow longitudinal ribs that have smooth tops, although the edges are undulate, whereas those of C. sykesi consist of longitudinal rows formed by a weak fusion of granules, as described by Thiele (1909) and Strack (1993). We confirmed the presence of this sculpture in the lectotype of C. sykesi : ZMB Moll 102.074. The new species differs from both, C. dawydoffi and C. sykesi by having the posterior valves (V–VIII) clearly separated from each other (all valves in contact, or almost in contact to each other in both, C. dawydoffi and C. sykesi , even in larger animal than the present specimens: BL 16.5 mm in Leloup (1940) for C. dawydoffi (shown as C. “ sykesi ”); BL 22 mm in Strack (1993) for C. sykesi , and the sharply angulate outer edge of the longitudinal ribs in the pleurolateral areas. The new species differs from C. dawydoffi also in the ratio of the length of each shell valve and the morphology of the marginal spicules of the girdle. The marginal spicules of the new species are round in cross section, but those of C. dawydoffi are flattened, and parallel-sided. Based on these examinations and comparisons, we identify Cryptoplax sp. ( Sirenko 2012) from Nha Trang, Vietnam as C. dawydoffi .

To clarify that the present species is distinct from all other species, we should mention here several species described by Rochebrune (1882) from the West Pacific, because their morphology is well less known than that of other species. Those species described by Rochebrune (1882) are Cryptoplax montanoi from Borneo and Luzon Islands, Cryptoplax peroni and Cryptoplax torresiana from Australia, Cryptoplax caledonica , Cryptoplax heurteli , and Cryptoplax uncinifera from New Caledonia. High quality photographs of their type material (mostly one of the syntypes) are shown at the MNHN website (https://science.mnhn.fr/institution/mnhn/collection/im/item/ list?full_ text= Cryptoplax ). Cryptoplax montanoi was regarded conspecific with C. oculata by Thiele (1909: 52), which was followed by Schwabe (2007). The fleshy girdle and the shape of the valves shape, especially the wide valve II, resemble those of C. larvaeformis and C. oculata . But neither the tufts of the spicules nor the black and white rings around the anterior valves can be seen in the syntype material (MNHN-IM-2000-6066). The present species differs from C. montanoi by having a narrower body and smaller valve II. The identity of C. peroni is unclear because valves and girdle spicules of the type material (syntypes: MNHN-IM-2000-5985) are strongly damaged or lost, already when Thiele (1909) examined it, although Thiele (1909: 56) mentioned that valve VIII appears to have a fairly extended posterior margin of the articulamentum, and the calcareous bodies of the girdle are long conical, distally tapering and rounded and longitudinally grooved. Those features are different from those of C. acerocostata n. sp. Cryptoplax torresiana was regarded to be conspecific with C. striata ( Lamarck, 1819) by Thiele (1909), which was followed by Gowlett-Holmes (2001). All shell valves of C. torresiana are in contact with each other and the longitudinal ridges of the pleurolateral area are granular (syntypes: MNHN-IM-2000-6071), which matches the features of C. striata well. C. caledonica , C, heurteli , and C. uncinifera (syntypes: MNHN-IM-2000-5845, MNHN-IM-2000-5905, and MNHN-IM-2000-6026, respectively) share the following features: Valves VII and VIII are clearly longer than valves V and VI; the mucro is located slightly anterior to the posterior end and 5–6 fine, undulating ridges on the pleurolateral area. Those three species may be conspecific, but further studies are needed to clarify their identities. In C. acerocostata n. sp., the length of the tegmentum is gradually decreasing from valve IV toward valve VIII, valve VIII has a terminal mucro that slightly protruding on the post-mucronal area, and the pleurolateral area of valves III–VIII is sculptured with 2–3 longitudinal ribs, which have a smooth surface and a sharply angulated outer edge.

Risbec (1946) described “ Cryptoplax caledonica ” as a new species from New Caledonia, for which Kaas & Van Belle (1998) gave the new name Cryptoplax risbeci . This species also has similar features as the three above mentioned species described by Rochebrune (1882): C. caledonica , C. heurteli and C. uncinifera : valves VII and VIII being clearly longer than valves V and VI; the mucro being located slightly anterior to the posterior end, and having 5–6 fine, undulating ridges on the pleurolateral areas ( Risbec 1946, pl. II, figs 1, 5–8; fig. 7 is erroneously explained as valve VII instead of valve VIII).

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 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.

FIGURE 13. Cryptoplax acerocostata n. sp., Spratly Islands. A–D. holotype (ZIN 2453), BL 15.0 mm. A. Dorsal spicules, marginal needles and ventral spicules. B. Tuft needles and dorsal spicules. C. Dorsal spicules and marginal needle. D. Radula.