Intertidal Chitons (Mollusca: Polyplacophora) from Southern Madagascar

ABSTRACT This paper focuses on five species of Polyplacophora (Mollusca) collected from Lavanono, southern Madagascar. Ischnochiton sirenkoi is described as a new species and is compared with the two Ischnochiton species known from Madagascar, I. yerburyi (E.A. Smith, 1891) and I. sansibarensis Thiele, 1909, and with all known Indian Ocean Ischnochiton species. Findings for the other species here discussed provide a significant extension of previously known geographical range. Chaetopleura chelazziana Ferreira, 1983 is reported for the first time in the chiton fauna of Madagascar. The taxonomic status of Callistochiton ashbyi (Barnard, 1963), previously considered a synonym of C. crosslandi Sykes, 1907, is discussed. This taxon, which was described from a unique intermediate valve from South Africa, is re-evaluated and recognized as a valid species, differing from C. crosslandi in several significant characters. We also present an unusual feature detected in almost all the specimens of Cryptoplax dupuisi Ashby, 1931, which are characterized by the absence of dorsal girdle spicules on the first four valves, a feature never reported in other chiton species. Type material of Callistochiton madagassicus Thiele, 1909, Chiton ashbyi (Barnard, 1963) is figured.

gate-oval, moderately elevated (dorsal elevation 0.34), subcarinated, valves not beaked. spots of various colours. Girdle colour resembles tegmentum, uniform or in neatly alternating bands. 2A). Intermediate valves broadly rectangular, front margin slightly concave in wide in lateral parts, side margins rounded, hind margin almost straight, apices inconspicuous, strongly concave in wide central part between apophyses, mucro not elevated, submedian Tegmentum of head valve, lateral areas of intermediate valves and postmucronal area of tail valve sculptured with large, rather raised and very irregular pustules, arranged in segments of various size and shape, i.e. triangular, rectangular, rounded or sinuous, slightly overlapping each other, giving a wavy aspect (Figs 2D,2O). Segments in pleural areas (Fig. 2I) become longitudinally elongate, always very irregular, sinuose and  intersecting each other, giving the appearance of a network of irregular and elongate pits. This sculpture becomes less evident close to the jugal area, which is practically Articulamentum well developed, whitish, apophyses evenly rounded, jugal sinus wide, slit formula of insertion plates 9-10/1/8-10, slits inequidistant, slit rays distinctly punctured, teeth sharp, smooth, eaves solid (Fig. 2E). Jugal tract of intermediate and tail valves with numerous transverse slits.
Girdle dorsally clothed with small, round-topped, strongly curved, imbricating scales, riblets much less pronounced on upper part of scales, interstices slightly wider than ribs. Ventral side covered with radiating rows of elongate, rectangular, smooth scales, ca covered with small holes (Fig. 2T).
tooth equally narrow, with outwardly curved blade, major lateral with bicuspid head, den-Ctenidia arranged holobranchially and abanally.
from the other two species of Ischnochiton I. yerburyi (E.A. Smith, 1891) and I. sansibarensis Thiele, 1909(reported as I. rufopunctatus Odhner, 1919by Odhner 1919Dautzenberg 1923Dautzenberg , 1929. The colour of the new species is very variable (Fig. 4B), both glossy and dull, mostly also evenly white, or reddish (from brownish to red to violet) with various spots of colour, which are rarely blackish. Also the pleural areas sculpturing is variable, lighter in some specimens, and more evident near a band up the lateral areas or near the side margins of the intermediate valves, leaving a large central part of the valve smooth. The sculpture of I. sirenkoi, is very different to that of I. yerburi and I. sansibarensis. I. yerburyi the type of I. sansibarensis of preservation, but the thimble-like sculpture is still clearly visible. In this species the sculpture is much weaker on the central areas, with the head valve, lateral areas of intermediate valves and postmucronal area of the tail valve evenly quincuncially granulated. Ferreira (1983) considered I. sansibarensis to be a synonym of I. yerburyi, Ocean Ischnochiton species with a thimble-like sculpture, that the two species can be (I. yerburyi I. sansibarensis 10×5.5 mm; I. sirenkoi 17.6×8.2 mm), their girdle dorsal scales (24 or 25 riblets in I. yerburyi, 13-15 in I. sansibarensis; 27 or 28 in I. sirenkoi), and the ornamentation of the upper surface of the cusps of the major lateral teeth of the radula that in I. sirenkoi shows a characteristic granulation. This The new species differs from other Ischnochiton species living in the Indian Ocean. I. winckworthi a stronger, more granulose sculpture, forming irregular, radiating granulose riblets in the head valve, the lateral areas of intermediate valves, and the postmucronal area of tail valve.

I. indianus
front margin of the latero-pleural parts of the intermediate valves.
I Of the Ischnochiton species living along the South African coast, two can be compared with I. sirenkoi, i.e. I. oniscus (Krauss, 1848) and I. elizabethensis Pilsbry, 1894). These species were recently studied by Sirenko and Schwabe (2002), and differ from I. sirenkoi mainly in having a different, more granulated, sculpture and in the shape of the major lateral tooth of the radula (tricuspid in I. oniscus, unicuspid in I. elizabethensis, bicuspid in I. sirenkoi).  Tail valve with mucro almost central, small, sharp (Fig. 3C), postmucronal slope concave (Fig. 3D). Antemucronal area sculptured like central areas, postmucronal area sharply differentiated with 12-16 strong, nodulose, radial ribs; ribs of differing widths, some ribs splitting, bearing a few tubercles near outer margin.
Callistochiton species were previously recognized from C. crosslandi Sykes, 1907 ollowing valves with spaces in between. In some specimens also fourth valves seem in contact, but this depends on dissimilar stretching of girdle in dried specimens. Distance between valves vi and vii generally widest, and distance between valves v and vi wider than between other valves (iv-v and vii-viii), while distance between valves iii and iv is margin to posterior margin of valve iv, showing underlying, blackish skin. Colour of girdle buffy brown, valves darkish brown in periphery, more clearin central part.
Tegmentum of head valve semi-elliptical, anterior margin rounded, posteriorly straight (Fig. 4Q). Second valve of rhomboidal shape, more rounded in anterior part In intermediate valves, granules more rounded, less elongated, irregularly arranged, irregular towards anterior valve margin, arranged following growth marks, jugal area smooth (Fig. 4N). Antemucronal area of tail valve sculptured like intermediate valves.
Growth marks present on all valves.
Articulamentum white, strongly developed, forming large insertion plates. Intermediate and tail valves with large triangular apophyses, which always form a jugal lamina (Fig. Perinotum wide, appears velvety without tuft pores, dorsally covered by dense, almost straight, conical spicules, ca thick cuticle, sculptured longitudinally with 8 or 9 striae (Fig. 3S). Dorsal spicules do not wholly cover girdle surface, but only from posterior margin of valve iv, showing fringe shows smooth, obtusely pointed, straight spicules of ca (Fig. 3T). Ventrally, short, straight, elongate spicules, smooth or with some faint sign of dorsal ribs, ca (Fig. 3U), tending to be more elongate, ca face in upper part; second lateral tooth with broadly rectangular head, with three obtusely pointed, large denticles, almost equal-sized, outer one slightly shorter than others (Fig. 3X).
of length ca 11 mm). from the anterior margin to the posterior margin of valve iv, showing the underlying, blackish skin. The same can be assumed for the two original specimens described by have a wholly spiculose girdle. alchol (ca (31×7 mm), and the valves iv-vii are disconnected (Fig. 4K). Also the smaller of the ca (1979) not belong to Cryptoplax dupuisi, but are probably to C. sykesi Thiele, 1909. Cryptoplax species live in a hole in corals, or in deep crevices in coralline rocks; Ang Cryptoplax planus collect because they occupy very deep crevices with very narrow openings which are C. planus crawling from its burrow, only the anterior body emerges (mainly at night). already noted by Thiele (1909) and Ferreira (1983), who proposed the name C. sykesi Dinapoli (2004) reported two specimens (12 and 15 mm long) of a Cryptoplax sp. from Socotra Island, comparing them with C. sykesi and C. dupuisi, but it is possible that the two specimens belong to a Choneplax species (E. Schwabe, pers. comm.).
Other specimens were found but some of them are not considered in the present work Stenoplax (Stenoradsia) madagassica (Thiele, 1917), Lucilina carnosa (Kaas, 1979) Species Specimens