Leptochiton tahitiensis, Sirenko, 2020

Leptochiton tahitiensis View in CoL sp. nov.

(Figs 1–4)

Holotype. Adult chiton (No. MNHN-IM-2007- 39265), French Polynesia, Society Islands, Tahiti Island, 17°47′S, 149°23′W, depth 450–720 m (oceanographic cruise TARASOC, station DW 3489, 23 Oct. 2009). GoogleMaps

Note. In the holotype, all valves except seventh and eighth are damaged. The specimen is now disarticulated and consists of the following parts: SEM stubs of the valves I, V, VII, VIII, a part of the perinotum and radula; the slide-mounted parts of the perinotum and radula; and a vial with other valves .

Diagnosis. Chiton of small size; valves low elevated, subcarinated. Intermediate valves broadly rectangular. Tegmentum covered with flattened, oval granules arranged uniformly and quincuncially; granules with fine concentric lines and eleven pores of aesthetes. Dorsal scales obtusely pointed, with eight or nine double ribs. Central tooth of radula very short and strongly narrowed in middle; major lateral tooth with sharply point- ed unicuspid head. Nine gills per side arranged from valve VI to anus.

Description. Holotype small (body length 6.5 mm), valves low elevated (dorsal elevation 0.38 mm), subcarinated, not beaked. Colour of tegmentum and girdle white.

Head valve semicircular, with V-shaped posterior margin. Intermediate valves rectangular, short and wide. Their anterior margin slightly convex; posterior margin nearly straight, not beaked; lateral margins rounded; lateral areas slightly raised. Tail valve roughly triangular in outline, narrower

B.I. Sirenko. Leptochiton tahitiensis sp. nov. from the Society Islands than head valve; anterior mucro not prominent; antemucronal slope slightly convex; postmucronal slope weakly concave.

Tegmentum uniformly sculptured with quincuncially arranged, flattened, oval granules; each granule with fine concentric lines; one megalaesthete and ten micraesthetes around; all aesthete pores of approximately equal size.

Articulamentum weakly developed; apophyses small, widely separated, triangular in valves II– VII, more or less trapezoidal in tail valve.

Girdle moderately wide relative to valves, about 0.5 mm wide near valve V, dorsally densely covered with blunt pointed scales (80 × 46 µm) with eight or nine partly double ribs. Intersegmental areas with long needles (up to 150 × 17 µm) embedded in chitinous cups, in the shape of “Ringschaftnadeln” sensu Thiele (1908). Marginal fringe composed of slender spicules, up to 85 × 15 µm, with four or five longitudinal ribs. Ventrally, girdle covered with elongate, smooth and bluntly pointed scales (up to 78 × 25 µm) being larger than dorsal scales.

Radula of holotype 2.5 mm long, with 43 transverse rows of mature teeth. Central tooth of radula very short, strongly narrowed in middle and with large rhomboidal base; major lateral tooth with sharply pointed unicuspid head.

Nine gills on each side extending from valve VI to anus.

Distribution. The species is known only from the type locality, Tahiti Island, depth 450– 720 m.

Etymology. Named after Tahiti Island.

Comparison. Leptochiton tahitiensis sp. nov. has several unique features that are absent or rare in other species of the genus. The new species has unusual granules with fine concentric lines; the central teeth of the radula are very short, strongly narrowed in the middle and with the large blade and rhomboidal base; the tail valve is triangular in shape. The new species is slightly similar to some species of Leptochiton belknapi Dall, 1878 group ( L. taiwanensis Sirenko, 2018 occurring near Taiwan and L. mutschkeae Sirenko, 2015 occurring near Chile) (Sirenko & Sellanes, 2015), which also have the quincuncially arranged granules on the tegmentum, and the unicuspid head of major lateral teeth of the radula. Leptochiton tahitiensis sp. nov. can be distinguished from the species of