Chiton (Rhyssoplax) rapaitiensis, Schwabe & Lozouet, 2006

Chiton (Rhyssoplax) rapaitiensis View in CoL n. sp.

( Figs 5-7 View FIG View FIG View FIG ; 10C, D View FIG )

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

TYPE LOCALITY. — French Polynesia, archipel des Australes   GoogleMaps , Rapa   GoogleMaps , Pointe Komiré   GoogleMaps , RAPA 2002, stn 10   GoogleMaps , 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   GoogleMaps , Baie de Hiri   GoogleMaps , 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 View FIG ; 6 View FIG ). — 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   GoogleMaps (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   GoogleMaps , 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   GoogleMaps , 27°38.2’S, 144°18.2’W, RAPA 2002 , stn 30, 16- 20 m, dead corals, 7 spms. — Entrance   GoogleMaps 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   GoogleMaps , 27°36.8’S, 144°19.9’W, RAPA 2002 , stn 52, 23- 25 m, mud bottom associated with corals, 2 spms. — Far   GoogleMaps end of Baie de Haurei   GoogleMaps , 27°36.6’S, 144°20.5’W, RAPA 2002 , stn 62, 20 m, mud bottom associated with corals, 2 spms. — BaieTupuaki   GoogleMaps , 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 View FIG ). 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 View FIG ). Girdle margin with short, straight needles, which measure 47.4-52.6 × 15.8 µm, and are distorted distally.

Radula of holotype ( Fig. 6 View FIG ) c. 1.5 mm long, with 42 teeth rows, of which 33 are mineralized. The central tooth is relatively short (214.3 µm), tulipshaped, with a single, slightly inward-directed blade. The first lateral tooth has a slender upper half and a roundish wide extension on the basal half. The second lateral has a long (643 µm), slender, and slightly keeled shaft. The head of the second lateral is nearly square and has two strong denticles, of which the inner is the longer. Major uncinal tooth spoon-like and 476 µm long.

Ctenidia of holotype arranged holobranchially and adanally, 18 on the left and 17 on the right side.

REMARKS

In one specimen from stn 25, eggs were found in the mantle cavity ( Fig. 10D View FIG ). According to Strack (1987: 184, 185), the only other Pacific species of this subgenus that broods its eggs is C. (R.) linsleyi (Burghardt, 1973) (as C. (R.) perviridis Carpenter, 1865 – which Bullock 1988 [pp. 688-689] has shown is a distinct species). The larger size (up to 16 mm) and the distinctive tegmentum sculpture, namely the large number (up to 15) of longitudinal grooves in the central areas, the grooved lateral areas, head valve and tail valve, and smooth girdle scales differentiate C. (R.) linsleyi from Chiton (Rhyssoplax) rapaitiensis n. sp.

The new species is close to C. (R.) rhynchotus (de Rochebrune, 1884) from New Caledonia. The latter is up to 16 mm long, has up to 12 ribs in the pleural areas, has dorsal girdle scales with up to 21 broad, close-packed ribs, and has insertion plates that appear smooth. The Indian Ocean species C. (R.) maldivensis ( E. A. Smith in Gardiner, 1903) is also similar, but is 3-4 times larger, the pleural areas are sculptured with up to 12 rather wide and deep grooves, which do not reach the anterior valve margin, and has dorsal girdle scales with up to 20 mostly very faint grooves.