Cryptoplax larvaeformis (de Blainville MS, Burrow, 1815 )

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 : 53-57

Cryptoplax larvaeformis (de Blainville MS, Burrow, 1815)

( Figures 1 , 3–5 )

Chiton larvaeformis Blainville MS, Burrow 1815: 191 , pl. 28, figs. 2–4.

Chiton fasciatus Quoy & Gaimard 1835: 408 , pl. 73, figs 21–29.

Chitonellus fasciatus ; Reeve 1847: pl. 1, fig. 2b.

Cryptoplax coronatus Rochebrune 1884: 238 ; Thiele, 1909: 54.

Cryptoplax larvaeformis ; Haddon 1886: 37, pl. 3, fig. 12; Pilsbry 1893: 56, pl. 11, figs. 31–34, 36, 40–43; Nierstrasz 1905: 73, pl. 6, figs. 154–158; Thiele 1909: 54; Ang 1967: 423, pl. 16, figs. 1–4; Slieker 2000: 54, pl. 15, fig. 12; Schwabe 2005: 54, pl. 2, fig. 11; Saito 2006: 131; Schwabe 2007: 149, figs. 3, 4A, 4C, 5A, 5C, 5E, 6A, 6C; Schwabe et al. 2008: 29 , figs 7E, F; Granpoder et al. 2023: 75.

Type material. Holotype, NHMUK 1951.1 .28.1.

Type locality. Unknown.

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, 2 intermediate valves, B. Sirenko leg. 25.11.2018; 11°23′45.1″N, 114°35′15.1″E, depth 0.5–1.0 m, SCUBA, washing off old corals, sample 12, 1 spm, ( ZIN 2449 ) BL 35.0 mm, B. Sirenko leg. 27.11.2018 ; 8°49′42.2″N, 113°57′40.38″E, depth 1.5–4.0 m, SCUBA, washing off old Tridacna shell, sample 60, 1 spm, BL 7.0 mm, B. Sirenko leg. 25.05.2019 ; 8°49′42.2″N, 113°57′40.38″E, depth 1.0 m, SCUBA, washing off old corals, sample 61, 2 spms, ( ZIN 2573 ) BL 62.0–96.0 mm, B. Sirenko leg. 25.05.2019 ; 8°53′15.7″N, 112°14′08.3″E, depth 20.0 m, SCUBA, washing off old corals, sample 76, 1 spm, BL 6.0 mm, B. Sirenko leg. 02.06.2019 ; 8°50′35.2″N, 112°11′26.0″E, depth 10–11.0 m, SCUBA, washing off old corals, sample 78, 1 spm, BL 7.0 mm, B. Sirenko leg. 03.06.2019 .

Description. Based on young specimen (BL 35. 0 mm, ZIN 2449): Anterior five valves in contact, remaining valves separated from each other. Color of tegmentum white or light gray, with brownish tint on pleurolateral area in valves II and III, fine dark brownish streaks on pleurolateral area and dark brownish jugum on valves V and VI. Girdle white in anterior half, yellowish buff in posterior half, with dark brownish transverse bands in the middle part. Ventral side of girdle uniformly light gray.

Head valve elongate semicircular, longer than wide, sculptured with elongate pustules. Pustules are irregularly fused with neighboring ones, which are more frequently fused towards the margin of the tegmentum, forming almost flat surface. Valve II oval, widest among all valves. Valve III–VIII narrow, spindle-shaped, of which valve IV is the longest and valves VII and VIII are the shortest. Jugum smooth, wedge-shaped in valves II and III, very narrow and parallel sided in remaining valves. Pleurolateral area sculptured with 5–6 longitudinal, slightly radiated ribs formed by coalescence of pustules. In larger specimens, outer longitudinal ribs of valves II–VIII extending commarginally into jugal area, which interrupt inner ribs and even the jugum. Those longitudinal and commarginal ribs often obsolete in the anterior portion of valves ( Fig. 1D ). Tail valve with terminal mucro overhanging on very short post-mucronal area where is sculptured with small pustules. Each pustule with 1–2 macraesthetes on top. Micraesthetes sparsely distributed along inner basal portion of ribs and prepustular area of pustules; no micraesthete on top of pustules ( Figure 3I ).

Articulamentum of valves II–VIII extending outward only in anterior half of tegmentum in dorsal view. Insertion plates of head valve long, but slightly shorter than half length of tegmentum. Slit formula 3/0/0. Color of articulamentum white with cherry under jugum.

Girdle rather wide, fleshy. Dorsal side of girdle beset with minute slightly curved, strongly ribbed (8–9 double ribs often on one side) spicules, up to 100 μm x 50 μm, intermingling with large rounded or sometimes weakly flattened, smooth but occasionally striated spicules, attaining 250 μm x 100 μm. Sutural tufts of up to 7 slightly curved, smooth, blunt tipped needles, up to 500 μm x 60 μm. Marginal needles similar to those of sutural tufts, but often with several fine grooves, up to 500 μm x 62 μm. Ventral spicules flat, smooth, up to 100 μm x 24 μm.

Gills extending from beginning of valve VI to valve VIII, composed of 31 ctenidia on each side.

Radula 9.0 mm long with 47 transverse rows of mature teeth. Central tooth small, thick, concave at dorsal two third, keeled at ventral one third, round at top. Centro-lateral (first lateral) tooth with small, cusp at nodulous antero-dorsal corner. Major lateral (second lateral) tooth with large, thick tricuspid head; denticles blunt at tip and about equal length with transverse angular lines on proximal portion. Major uncinal tooth narrow with round tip, weakly keeled on posterior surface.

Distribution. C. larvaeformis has been recorded from a wide area in the tropical West Pacific, but we have not verified many of those records. At least the following records can be accepted: Tongatapu (Tongatabu), Tonga Islands ( Rochebrune 1884, as C. coronatus ); Bulia, Cicia, Kandav, Tuvutha, Viti, and Yaukuve Leve Islands, Fiji ( Haddon, 1886; Pilsbry 1893; Schwabe et al. 2008); Pombo Island, Ambon, Indonesia ( Slieker 2000); Woka Island, Indonesia ( Schwabe 2007); Banacon Island, Bohol, the Philippines ( Granpoder et al. 2023); “the Philippines ” ( Ang 1967); Spratly Islands, South China Sea (present study). Phi Phi Don Island, Phuket, Thailand ( Granpoder et al. 2023). Record from Okinawa, Japan ( Saito 2000) is an error caused by misidentifying C. oculata as C. larvaeformis (see below in remarks). In the Spratly Islands, the living animals were found at a depth of 1– 20 m.

Remarks. Cryptoplax larvaeformis resembles C. oculata ( Quoy & Gaimard 1835) most in its large body size (attaining BL 140 mm) and in having a fleshy girdle with dark brownish transverse bands. These two species were treated as distinct species by Haddon (1886), Pilsbry (1893), Nierstrasz (1905), and Thiele (1909), but Iredale & Hull (1925) synonymized them. Ang (1967) separated them, but Kaas & Van Belle (1980), again, synonymized them. Since then, subsequent authors (e.g., Slieker 2000; Saito 2000; Gowlett-Holmes 2001) followed this synonymization. However, Schwabe (2007) clearly demonstrated that they represent two different species. Accordingly, C. larvaeformis differs from C. oculata mainly by having spicule tufts around the anterior valves. Burrow (1815) referred to them as “pores” (of the tufts) in the original description of C. larvaeform is. They are lacking in C. oculata . Instead, C. oculata has “a black (inner ring) and a white spicule ring around the first three valves” ( Schwabe 2007: 150). Schwabe (2007) depicted shell valves of C. larvaeformis of the type specimen (NHMUK 1951.1.28.1) and a specimen from Woka Island, Indonesia (ZSM Mol 20052064) as well as shell valves of C. oculata from Ceram Island, Indonesia (ZSM Mol 20052088). However, valve features of both species have never been compared yet. The shell morphology of these two species is very similar. We regard the following as an important taxonomic diagnostic feature to distinguish them: Valve III has a narrowly extending pleurolateral area toward the anterior end in C. larvaeformis , whereas almost only the jugum is extending at the anterior portion in C. oculata . Thus the anterolateral margin of the tegmentum is almost straight and wider in C. larvaeformis and concave and narrower in C. oculata . The similar shape of the valve III of C. larvaeformis was illustrated by Haddon (1886), Pilsbry (1893) and is also shown in the present study ( Figs 1D , 3C ). In addition to the features mentioned above, C. larvaeformis differs from C. oculata by having a finer sculpture on the pleurolateral area.

C. dimidiata Ang 1967 also closely resembls C. larvaeformis . They share even the feature of the valve III mentioned above, but Ang (1967) listed as many as 16 differences between the two species. Further studies will be needed to clarify the identity of C. dimidiata .

The type specimen of Chitonellus laevis Lamarck, 1819 is probably conspecific with C. larvaeformis . One of the syntypes of Chitonellus laevis (MNHN-IM-2000-6017) shown on the MNHN site (https://science.mnhn. fr/institution/ mnhn/collection/im/item/ list?full_text= Cryptoplax ) has a dark reddish coloration at least in valves II–V, small valves V and VI, large valve II that has its widest point at anterior to the center of the tegmentum, and tegmental sculpture that lost jugum at anterior portion in valves VII and VIII. Moreover, Blainville (1825) illustrated Lamarck’s type specimen and described it as follows: “always provided with bundles of setae or hairs arranged in pairs”( Blainville, 1825: 600), all of which match the features of C. larvaeformis ( Figs 1A, B, D ). A formal synonymization should await close examination of the type material of Chitonellus laevis , especially of its girdle elements.

There are many other species that have been synonymized with Cryptoplax larvaeformis (see Iredale & Hull 1925; Kaas & Van Belle 1980, 1998), but a review of all of their identities needs extensive studies, which is beyond the scope of the present work. Only obvious synonyms are treated herein. Some of the species which were synonymized with Cryptoplax larvaeformis by Iredale & Hull (1925) and Kaas & Van Belle (1980, 1998) but are regarded to be distinct herein are discussed below in the remarks about Cryptoplax acerocostata n. sp.

Figures

FIGURE 1. Cryptoplax larvaeformis, Spratly Islands. A, B. (ZIN 2449), BL 35.0 mm. C, D. (ZIN 2573), BL ca. 96 mm (ca. 140 mm when alive). A, C. Whole animal dorsal view. B. Close up of anterior body. D. Valves I–VIII. Arrow heads indicates sutural tufts. Scale: 5 mm for A, C, D.

FIGURE 3. Cryptoplax larvaeformis, Spratly Islands. A–K. (ZIN 2449), BL 35.0 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 4. Cryptoplax larvaeformis, Spratly Islands. A–D. (ZIN 2449), BL 35.0 mm.A. Dorsal spicules, marginal needles and ventral spicules. B, C. Dorsal spicules. D. Tuft needles and dorsal spicules.

FIGURE 5. Cryptoplax larvaeformis, Spratly Islands. A, B. (ZIN 2449), BL 35.0 mm. A. Radula. B. Central, first lateral teeth and head of major lateral teeth of radula.