Leptochiton angustidens, Sigwart, Julia D. & Sirenko, Boris I., 2012

Sigwart, Julia D. & Sirenko, Boris I., 2012, Deep-sea chitons from sunken wood in the West Pacific (Mollusca: Polyplacophora: Lepidopleurida): taxonomy, distribution, and seven new species, Zootaxa 3195, pp. 1-38 : 15-17

Leptochiton angustidens n. sp.

Figures 4 B, 7, 8

Leptochiton n. sp. 2 Sigwart 2008b: 2.27–2.29, figs 2.16–2.20; Yearsley & Sigwart 2011: table 1.

Type material. Holotype ( MNHN 23693) disarticulated, consisting of mounts of shell, perinotum and radula, 6 paratypes ( MNHN 23694) and 1 paratype ( ZISP).

Type locality. Vanuatu, Sanma, Espiritu Santo Island, Big Bay, 14°59.7' S 166°55.0' E, 630–705 m; Boa 1 sta. CP2432.

Etymology. From the Latin angustus, narrow, and dens, tooth, referring to the radula in this species, which has narrow central and first lateral teeth.

Material examined. 29 specimens in 4 lots from Vanuatu.

Distribution. From Vanuatu, 630– 705 m. Found living and feeding on sunken terrestrial plant remains.

Diagnosis. Valves fairly flat, slightly beaked. Intermediate valves side margins rounded, anterior and posterior margins straight, lateral areas not raised. Tail valve flat, not inflated, mucro central, postmucronal slope straight. Tegmentum with weakly raised granules arranged in irregular quincunx. Aesthete caps strongly projecting. Girdle wide, dorsally and ventrally covered in flattened conical spicule-scales, dorsally with scattered elongate needles. Gills up to 12–13 per side from valve VI to anus. Radula central and first lateral teeth very narrow and long. Major lateral teeth with tridentate cusps.

Description. Holotype 6.5 × 4.0 mm. Shell semicarinated, with low elevation (elevation ratio 0.29–0.35 in intermediate valves), valves slightly beaked.

Head valve semicircular, of equal width to tail valve. Intermediate valve rectangular, lateral areas not raised, not depressed near apex, anterior and posterior margins nearly straight, except slightly convex anterior margin of valve II. Tail valve with median mucro, anterior margin of jugal area convex, postmucronal slope straight. Whole tail valve is slightly inflated at mucro but, generally flat.

Tegmentum uniformly sculpted with weakly raised, rounded granules (48 µm) arranged in irregular quincunx. Aesthete caps are clearly visible in bundles of eight per granule. Colour of tegmentum cream or pale tan, in some specimens older parts of the valves (near apex) covered with scattered small spots of black deposit ( Fig 4 B).

Each granule with one megalaesthete surrounded by a ring of eight micraesthetes. Central megalaesthete 8.5 µm in diameter.

Articulamentum well developed; apophyses small, widely separated, rounded triangular in valves II–VII, rounded trapezoidal in tail valve. Apophyses on valve II longer than other intermediate valves. Terminal margins on valve I, valve VIII thickened. Aesthete channels are visible in white articulatmentum on terminal margins.

Girdle appears very wide relative to valves, about 0.5 mm wide near valve V in holotype, dorsally covered in elongate, smooth, flattened conical spicule-scales (77 µm long), interspersed with long sharp spicules (170 µm long). Intersegmental areas with larger spicules (up to 230 × 21 µm) of the same type as main dorsal armature.

Marginal spicules up to 115 × 15 µm. Ventrally, the girdle is clothed in smooth, flat narrow scales, very similar to the dorsal armature but slightly smaller.

Radula of holotype 2.6 mm long, with 44 transverse rows of mature central and first lateral teeth very narrow and long. Major lateral teeth with tridentate head; interior denticle very small and indistinct, making cusp sometimes appear to be bidentate.

There are 12 to 13 gills per side, all with laminae on a single side.

Gut contents containing cellulose material of light colour in pellets with a thick mucous coating.

Remarks. This species is unusual for the genus Leptochiton because of its narrow, elongate central and first lateral tooth of radula ( Fig. 8 F), wide, thickened girdle, heavy articulamentum, and enlarged aesthete caps.

Figures

FIGURE 4. Preserved specimens of the seven new species described. In all pictures, anterior is at top and scale bars are 1 mm. A. Leptochiton consimilis n. sp., lateral view of right side, Solomon Islands, Salomon 2 sta. CP 2216. B. Leptochiton angustidens n. sp., lateral view of right side (at left) and dorsal view (at right). Specimen is from Vanuatu, Boa 1 sta. CP 2432. C. Leptochiton dykei n. sp., lateral view of right side (at left) and dorsal view (at right), Vanuatu, Boa 1 sta. CP 2412. D. Leptochiton samadiae n. sp., lateral view of right side (at left) and dorsal view (at right), Solomon Islands, Salomon 2 sta. CP 2285. E. Leptochiton longisetosus n. sp., lateral view of right side (at left), Vanuatu, Boa 1 sta. CP 2422. F. Leptochiton clarki n. sp., paratype, lateral view, Solomon Islands, Salomon 1 sta. CP 1751. G. Leptochiton schwabei n. sp., paratypes, right lateral view (at left), and dorsal view (at right) in different specimens, both from Solomon Islands, Salomon 1 sta. CP 1783.

FIGURE 7 – 8. Leptochiton angustidens n. sp., Boa 1 sta. CP 2432. Figure 7 (at left): SEM images: A. Valve I, dorsal view; B. Valve V, dorsal view; C. Valve VIII, dorsal view; D. Valve IV, rostral view; E. Valve VII, ventral view; F. Valve V, tegmentum detail showing aesthete pores; G. Valve VII, untreated valve surface, showing protruding aesthete caps; H. Valve VIII, left lateral view. Figure 8 (at right). A – D, SEM images: A., B. Central area of radula from middle of mature part of radular ribbon (B. from further anterior); C. Dorsal scales; D. Dorsal scales (at upper right) and ventral scales (at lower left) and marginal spicules, broken and lying on surface; E – L, line drawings: E. Head of major lateral tooth; F. Central and first lateral teeth of radula; G. Aesthete group; H. Intersegmental spicule; I., J. Dorsal scales; K. Marginal spicule; L. Ventral scale.