Connexochiton kaasi Saito, 1997

Sirenko, Boris, 2021, Composition of the genus Connexochiton Kaas, 1979 (Mollusca: Polyplacophora Ischnochitonidae), Zootaxa 4950 (1), pp. 83-102 : 87-89

Connexochiton kaasi Saito, 1997

( Figs. 2–4 )

Connexochiton kaasi Saito 1997: 50 , figs. 10–12, pl.2, fig. 5; 2011: 74, figs. 5C, 6.

Ischnochiton crassus ; Sirenko 2008: 42, figs. 1–23, 86, not Ischnochiton crassus Kaas, 1985 .

Leptochiton (L.) belknapi ; Kaas 1991: 10 (in part, non Leptochiton belknapi Dall, 1878 ).

Type material. Holotype ( NSMT –Mo 70684).

Type locality. Japan, Suruga Bay , South Senoumi Bank, 92 m.

Material examined. New Caledonia, SMIB 8 , stn DW167, 23°38’S, 167°43’E, 430–452 m, 1 spm ( ZISP 2379 ) , BL 3.5 mm, 29.01.1993.

Distribution. South Japan, 92–154 m, New Caledonia, 300– 675 m.

Description. This description is a mix of my own and Saito’s (1997) observations. Chitons very small (up to 3.7 mm) elongate oval, highly elevated, dorsal elevation ratio 0.65 (in valve III) and 0.45 (in valve VII); the first intermediate valves are subcarinate, whereas posterior intermediate valves are rounded. Side slope convex, valves slightly beaked. Colour of tegmentum white.

Head valve semicircular, posterior margin widely V-shaped, weakly notched in the middle, noticeably larger than tail valve. Intermediate valves of different length: valve II and valve III are longer than others, valve IV half as wide as long. Lateral areas of valves, very narrow and strongly raised. There is a shallow groove between the anterior and posterior areas of the tail valve; mucro posterior, anterior slope being slightly convex and posterior slope concave.

Valves are delicately sculptured by convex triangular granules slightly overlapping each other, arranged in quincunx, forming a lattice like pattern. Lateral areas with convex granules. Each granule with 9 pores of aesthetes. Pores of megalaesthetes 5 µm in diameter, pores of micraesthetes about 4 µm in width.

Articulamentum thin, apophyses short, triangular, medially connected by a short jugal plate without lateral slits, slit formula 5–10/1/6–7, slits short, slit rays indistinct, teeth blunt.

Girdle moderately narrow, width of each side 5 times less than width of valve IV, dorsally covered with small, thin, short, strongly bent, regularly imbricating scales that are 30–50 μm long, 40–70 μm wide, with 8–11 sharp, narrow riblets on the distal half. Marginal armature consists of three types of calcareous corpuscles: rare elongate scales, each with an abruptly tapering point, sculptured with 7 strong longitudinal ribs on the dorsal side, 100 x 26 μm; very rare needles bearing a single strong rib, 70–80 x 6–7 μm and more numerous small short conical spicules, sometimes truncated at the top, sculptured with 3 coarse ribs, 12–13 x 10–12 μm. Ventral side of girdle covered with radiating rows of rectangular scales up to 45 μm long. Near the margin there are several rows of ventral scales 60 x 30 µm with 2–4 short, obtusely pointed riblets. Near the pallial fold there are several rows of short smooth scales 30–40 x 17 µm.

Radula of a specimen (BL 3.7 mm long) is 0.90 mm long and contains about 52 rows of mature teeth. Central tooth slender, elongate, with a small blade and slightly broadened base. Major lateral tooth with unicuspid and sickle-shaped head, dorsal part of the head with 2 shallow grooves.

Studied specimen (BL 3.7 mm) has 10 gills extending from valve V to valve VII.

Remarks. Connexochiton kaasi is most similar to C. costatus n. sp. and Ischnochiton crassus due to the ribbed dorsal scales and radular teeth. It differs from C. costatus n. sp. by having convex granules on the lateral areas (vs. flattened granules in C. costatus n. sp.) and by the absence of ribs on the head valve (vs. 9 ribs in C. costatus n. sp.). Connexochiton kaasi differs from Ischnochiton crassus by having a jugal plate in all intermediate valves (vs. jugal plate in valve II only in I. crassus ) and a posterior mucro (vs. mucro central in I. crassus ).

Figures

FIGURE 2. Connexochiton kaasi. New Caledonia, SMIB 8, stn DW167, 23°38’S, 167°43’E, 430–452 m, 1 spm (ZISP 2379), BL 3.5 mm: A. Valve I, dorsal view; B. Valve II, dorsal view; C. Valve V, dorsal view; D. Valve VIII, dorsal view; E. Valve IV, ventral view; F. Valve V, detail of tegmentum surface of central area; G. Valve V, rostral view; H. Valve VIII, lateral view.

FIGURE 3. Connexochiton kaasi. New Caledonia, SMIB 8, stn DW167, 23°38’S, 167°43’E, 430–452 m, 1 spm (ZISP 2379), BL 3.5 mm:A. Valve I, detail of tegmentum; B. Dorsal and marginal scales and marginal needle; C. Dorsal, marginal and ventral scales; D. Dorsal scales.

FIGURE 4. Connexochiton kaasi. New Caledonia, SMIB 8, stn DW167, 23°38’S, 167°43’E, 430–452 m, 1 spm (ZISP 2379), BL 3.5 mm: A. Ventral scales; B. Central and first lateral teeth of radula; C. Part of radula; D. Central, first lateral and major lateral teeth of radula.