Onithochiton lyellii (Sowerby in Broderip & Sowerby, 1832 )

Onithochiton lyellii (Sowerby in Broderip & Sowerby, 1832) View in CoL

( Figs 8 View FIG ; 9; 10B View FIG )

Chiton lyellii Sowerby View in CoL in Broderip & Sowerby, 1832: 26. — Sowerby 1833: fig. 7, p. 5 species n° 60. — Reeve 1847: pl. 3, fig. and species 12.

Onithochiton lyelli View in CoL [sic] – Nierstrasz 1905a: 106. — Ashby 1922: 582. — Dautzenberg & Bouge 1933: 419. — Bergenhayn 1933: 34. — Leloup 1974: 7.

Onithochiton (Chiton) lyelli View in CoL – Ashby 1924: 333 (partly).

Anisochiton View in CoL ( Enoplochiton View in CoL [section Ornithochiton (sic)]) lyelli View in CoL – Fischer 1885: 882.

Onithochiton cf. lyellii View in CoL – Salvat & Rives 1975: 187, text fig.

Onithochiton lyellii View in CoL – Adams & Angas 1858: 476. — Tryon 1883: 345, pl. 86, fig. 82. — Pilsbry 1893: 247, 248 (partly), pl. 55, figs 1-7 View FIG View FIG View FIG View FIG View FIG View FIG View FIG . — Cox 1894: 710. — Iredale 1911: 105. — Leloup 1981: 4 (partly). — Kaas & Van Belle 1980: 78; 1998: 115. — Strack 1993: 22, pl. 5, fig. 3 View FIG (lectotype designated and figured). — Slieker 2000: 96, pl. 36, fig. 25 (lectotype figured). — Gowlett-Holmes 2001: 41. — Schwabe 2004: 3160.

Chiton (Onithochiton) lyelli (sic) – Chenu 1859: 382, fig. 2874 (7-valved). — Clessin 1903: 48, pl. 17, fig. 7.

Chiton View in CoL ( Onitochiton View in CoL [sic]) lyelli (sic) – Couturier 1907: 175.

? Onithochiton cf. societatis View in CoL – Preece 1995: 351.

TYPE MATERIAL. — Lectotype (designated by Strack 1993: 22), 3 paralectotypes, dried ( NHM 1985064 ).

TYPE LOCALITY. — Polynesia , Pitcairn Island; in small round hollows formed by sea urchins in exposed situations at low water mark.

MATERIAL EXAMINED. — A total of 21 specimens (5 lots).

French Polynesia. Rapa Island, Pointe Autea (station not numbered), 27°35.2’S, 144°18.4’W, RAPA 2002, intertidal zone, on rocks, 4 spms. — N of Pointe Pukitarava, 27°35.8’S, 144°18.5’W, RAPA 2002, stn 58, 2- 3 m, sand bottom with boulders and corals, 1 spm. — Baie Ahurei (Tauna islet), 27°36.5’S, 144°18.2’W, RAPA 2002, stn 80, intertidal zone, on rocks, 7 spms. — Baie Akatanui, 27°35.9’S, 144°18.5’W, RAPA 2002, stn 81, intertidal zone, on rocks, 3 spms. — Baie Anarua, 27°36.4’S, 144°22.6’W, RAPA 2002, stn 87, intertidal zone, on rocks, 6 spms.

DISTRIBUTION. — Rapa ( Salvat & Rives 1975); Rikitea, Makapou (both Gambier Islands), Pitcairn Island ( Couturier 1907).

HABITAT. — Onithochiton lyellii is an intertidal species, inhabiting crevices and holes on solid rocks.

DESCRIPTION

Animal moderately large (largest specimen examined is 33 mm long), elongate-oval and surrounded by a rather thick and wide perinotum, which appears velvety.Tegmentum coloration variable,ranging from blackish olive to bright green. Uneroded animals with a pattern of longitudinal undulated lines, and with fine brownish lines along the growth ridges. Jugal area wedge-shaped, generally white with a smaller, darker triangle within. Dorsally the animal is slightly subcarinated. Dorsal elevation of valve iv 0.38 (5.2 mm high/ 13.6 mm wide) in a specimen (29.7 × 15.4 mm) from Pointe Autea.

Large ocelli present in head valve, lateral areas and postmucronal area, with a diameter of 38 µm, surrounded by small apical pores with caps 14 µm in diameter. Subsidiary pores occur randomly around the other pores, with highest concentration around ocelli. Head valve semicircular with posterior margin straight and unnotched. Intermediate valves (excluding the second) wider than long with protruring apex. Second valve largest. Lateral areas slightly elevated and clearly separated from central areas by sculpture. Fine, comarginally directed, wavy growth ridges also occur along part of the diagonal ridge in the pleural areas. General tegmentum surface smooth, except for the growth ridges and pits in the jugal-pleural areas. Sculpture of lateral areas homologous with that of head valve and postmucronal area.Tail valve triangular, width more than twice length; mucro terminal, slightly elevated, forward-directed. Postmucronal slope steep and very short.

Articulamentum thick, shiny, with dark brown coloration in central area and white margins.Apophyses wide, long, triangular in valve ii to trapezoidal in tail valve, widely separated but connected by a short lamina, which shows minute teeth. Slits between them correspond to rows of transverse holes. Insertion plates rather short, with long, sharp, sawed teeth, roughly pectinate on outside. Slit formula: 8/1-1/0 (callus). Slitrays present in all valves, except for tail valve.They consist of transverse to round (innermost) holes. Callus with series of large transverse holes on its inner side. A very conspicuous thickening of the articulamentum results in two diagonal bridges.

Cuticle of perinotum thick, dorsally beset with very fine spicules (121-147.4 µm × 47-53 µm), giving the whole girdle a velvety appearance. Spicules sculptured with about nine longitudinal ribs, which are less distinct in the basal part. Among the spicules, smooth, sharp-pointed needles were found, 95 × 16 µm in size. Ventrally there are radial rows of squarish to rectangular scales, measuring 24-30 µm. They are obtusely pointed and a little concave.There are about 7-10 shallow, longitudinal grooves on their visible side.

Radula of pd specimen (17.4 × 10.2 mm) consists of 48 teeth rows, 40 of them already mineralized. Central tooth slender, 80 × 40 µm, with a single inward-curved blade, which is slightly wider in the upper part. First lateral tooth (142 µm long) has a thickened head with a simple edge.

Shaft of second lateral tooth slightly keeled, with nearly square head (125 × 100 µm). Blade simple, without denticles in the anterior part of the radula. A few rows later, cutting edges have four finger-like denticles. Innermost extremely short,

outermost triangular with a wide edge. Major uncinal tooth, 192 µm long, spoon-like, upper part 58 µm wide.

Thirty-seven holobranchial, adanal ctenidia both sides of the foot in the pd specimen.

REMARKS

Although often cited in the literature our knowledge of this species is very limited. A summary of probable misidentifications is given herein. Iredale (1911: 104) noted that the records of Onithochiton lyellii View in CoL from Australian waters most probably refer to O. quercinus (Gould, 1846) , as he showed for the report of Pilsbry (1893: 248 partly). Following Iredale it seems logical that the records from Australia of Nierstrasz (1905b: 156, pl. 10, fig. 19), Ashby (1924: 333) and Smith (1960: 167, figs 42, 2a-c) may also refer to this species. Specimens cited from the Indonesian, Philippine and Papuan region by Nierstrasz (1905b: 156, pl. 10, fig. 19), Ang (1967: 410, pl. 10, figs 1-4 View FIG View FIG View FIG View FIG ), Way & Purchon (1981: 313) and Leloup (1981: 4) are probably misidentifications of a new species, which is currently under study by Hermann L. Strack ( The Netherlands, pers. comm.). Ashby (1931: 50, as O. lyalli [sic]) pointed out that Odhner (1919: 40) misused O. lyellii for O. literatus (Krauss, 1848) . After Strack (1993: 21), the specimens cited by Leloup (1960: 42, figs 7, 9-13) belong to several species and Leloup (1980: 12) misinterpreted O. lyellii for O. erythraeus Thiele, 1910 . As no detailed study of its congeneric species is available, a restricted Polynesian distribution is assumed for O. lyellii View in CoL . It would be interesting to know if the species reported as O. quercinus from the Fiji Islands by Cernohorsky (1978) is correctly identified, or if it also belongs to O. lyellii .