Chiton (Rhyssoplax), Thiele, 1893

Schwabe, Enrico & Lozouet, Pierre, 2006, Chitons (Mollusca, Polyplacophora) from Rapa, the southernmost island of Polynesia, Zoosystema 28 (3), pp. 617-633 : 623-624

Subgenus Rhyssoplax Thiele, 1893

TYPE SPECIES. — Chiton affinis Issel, 1869 , by subsequent designation ( Thiele 1909). Members of the subgenus are distributed in the Indo-Pacific (except of the East Pacific) and East Atlantic, Oligocene-Recent.

Chiton (Rhyssoplax) rapaitiensis n. sp. ( Figs 5-7 ; 10C, D)

TYPE MATERIAL. — Holotype: type locality, pd ( MNHN). Paratypes: type locality and stns 25 and 30, 47 paratypes ( MNHN). — Stns 25 and 30, 2 paratypes ( ZSM).

Schwabe E. & Lozouet P.

TYPE LOCALITY. — French Polynesia, archipel des Australes, Rapa, Pointe Komiré, RAPA 2002, stn 10, 27°34.8’S, 144°22.8’W, 16- 18 m.

ETYMOLOGY. — From Rapa Iti (little Rapa), one of the Polynesian names of Rapa, as opposed to Rapa Nui (big Rapa) applied to Easter Island.

MATERIAL EXAMINED. — A total of 228 specimens (16 lots).

French Polynesia. Rapa Island , Baie de Hiri , 27°37.3’S, 144°22.2’W, RAPA 2002 , stn 9, 3- 24 m, corals, 44 spms. — Pointe Komiré, 27°34.8’S, 144°22.8’W, RAPA 2002 , stn 10, 16- 18 m, boulders covered by brown algae, 1 spm ( MNHN) ( Figs 5 ; 6 ). — W of Pointe Aukura, 27°38.1’S, 144°21.1’W, RAPA 2002 , stn 15, 20 m, boulders on sand bottom, 1 spm. — W of Tauna islet, 27°36.3’S, 144°18.4’W, RAPA 2002 , stn 16, 5 m, dead corals, 67 spms. — E of Baie Tupuaki, 27°34.2’S, 144°20.6’W, RAPA 2002 , stn 21, 5 m, sand bottom associated with dead corals, 11 spms. — Baie Anatakuri Nako (N of Rapa Iti islet), 27°38.4’S, 144°18.9’W, RAPA 2002 , stn 25, 3 m, 42 spms. — SW of Pointe Gotenaonao, 27°38.7’S, 144°19.2’W, RAPA 2002 , stn 27, 6 m, boulders covered by algae, 1 spm. — PointeTaekateke , 27°38.4’S, 144°20.6’W, RAPA 2002 , stn 28, 30 m, boulders covered by algae, 3 spms. — N of Aturapa islet, 27°34.3’S, 144°21.0’W, RAPA 2002 , stn 29, 2- 4 m, dead corals, 30 spms. — Pointe Mei , 27°38.2’S, 144°18.2’W, RAPA 2002 , stn 30, 16- 20 m, dead corals, 7 spms. — Entrance of submarine cave, SE of PointeTematatu, 27°34.8’S, 144°19.0’W, RAPA 2002 , stn 35, 2 m, stones, 2 spms. — West of Area , 27°36.8’S, 144°19.9’W, RAPA 2002 , stn 52, 23- 25 m, mud bottom associated with corals, 2 spms. — Far end of Baie de Haurei , 27°36.6’S, 144°20.5’W, RAPA 2002 , stn 62, 20 m, mud bottom associated with corals, 2 spms. — BaieTupuaki , Pointe Kotuaie, 27°34.6’S, 144°20.6’W, RAPA 2002 , stn 93, intertidal zone, 2 spms. — Pointe Komire, 27°34.8’S ,

144°22.8’W, RAPA 2002, stn 98, 16- 18 m, 3 spms. HABITAT. — The species was found on stones from the intertidal zone down to 30 m depth, with a preference for the upper zone (2-5 m).

DIAGNOSIS. — Animal elongate oval, moderately highly elevated, with a subcarinated back. Coloration highly variable.Tegmentum surface smooth, appearing pitted to the naked eye. Pleural and antemucronal areas with very faint longitudinal grooves, which occur from diagonal ridge towards the anterior valve margin. Growth marks present, distinctly elevated. Mucro situated centrally.

Dorsal girdle scales imbricated, sculptured with 11-14 rather coarse longitudinal ribs.Ventrally with rectangular, smooth scales. Radula with a bidentate head of the major lateral tooth.

Ctenidia arranged holobranchially and adanally.

DESCRIPTION

Holotype (pd) 4.7 × 3.2 mm, elongate oval. Head valve semicircular,posterior margin widely V-shaped, unnotched in centre.Intermediate valves broadly rectangular,posterior margin straight,on either side of the slightly protruding apex.Lateral areas clearly elevated. Jugal area not defined.Tail valve semicircular, anterior margin straight, mucro central, slightly elevated. Postmucronal area straight, moderately steep.

Tegmentum smooth,with microscopic perforations. Comarginal growth marks on terminal valves and lateral areas.Pleural areas with four or five fine, longitudinal ribs, reaching from diagonal ridges to anterior valve margin. Antemucronal area with only two of these ribs. Lateral areas, head valve and postmucronal area sculpturless, except for growth marks.

Articulamentum rather thin, with short triangular apophyses on valve ii and large trapezoid on tail valve. Insertion plates short, with deep slits. Slit formula: 9 (2 undeveloped close to posterior margin)/1-1/11. Slit rays present in all valves. Insertion teeth broad, with pectinate sharpened outer edges.

Perinotum dorsally covered with imbricate, elongate, convex scales, each measuring 84.2-100 × 42-52.6 µm ( Fig. 5 E ). They are sculptured with 10-12 convex, rather flat, radial ribs, the distal end of which is less distinctly ribbed then the proximal end. Ventral girdle scales elongate-rectangular, 30.4- 39 × 13 µm in size ( Fig. 5 F ). Girdle margin with short, straight needles, which measure 47.4-52.6 × 15.8 µm, and are distorted distally.

Radula of holotype ( Fig. 6 ) c. 1.5 mm long, with

Figures

FIG. 5. — Chiton (Rhyssoplax) rapaitiensis n. sp., holotype, Rapa Island, RAPA 2002, stn 10, 4.7 mm: A, head valve, dorsal view; B, valve ii, dorsal view; C, tail valve,dorsal view; D, tail valve, lateral view;E, dorsal girdle scales and marginal elements; F, ventral girdle scales. Scale bars: A-D, 400 µm; E, F, 80 µm.

FIG. 6. — Chiton (Rhyssoplax)rapaitiensis n.sp.,holotype,RapaIsland, RAPA 2002, stn 10, radula, anterior part. Scale bar: 500 µm.

FIG. 7. — Chiton (Rhyssoplax) rapaitiensis n. sp., paratype, Rapa Island, RAPA 2002, stn 25, 2.7 mm: A, cluster of eggs, HMDS dried, from the right mantle cavity side; B, dorsal surface of an egg (marked by an arrow in A). Scale bars: A, 100 µm; B, 10 µm.