Ischnochiton sirenkoi, Dell’Angelo & Prelle & Sosso & Bonfitto, 2011

Dell’Angelo, Bruno, Prelle, Giovanni, Sosso, Maurizio & Bonfitto, Antonio, 2011, Intertidal chitons (Mollusca: Polyplacophora) from southern Madagascar, African Invertebrates 52 (1), pp. 21-21 : 23-26

Ischnochiton sirenkoi sp. n.

Figs 2 , 4A, 4B

Etymology: The specific name honours our friend Dr Boris Sirenko (ZISP), for his contribution to the study of Recent and fossil chitons.

Diagnosis: Animal elongate-oval, of medium size, moderately elevated, subcarinated, valves not beaked, lateral areas not raised. Colour highly variable. Tegmentum sculptured with large irregular pustules, arranged in segments of various sizes and shapes, becoming longitudinally elongate in pleural areas. Mucro not elevated, submedian. Slit formula 9-10/1/8-10. Dorsal girdle scales imbricated, sculptured with 27 or 28 narrow riblets. Ventrally with radiating rows of elongate, rectangular, smooth scales. Radula with bicuspid head of major lateral tooth, minute granulations on upper surface of cusps.

Description:

Animal of medium size, holotype 12.2× 6.1 mm, maximum size 17.6× 8.2 mm, elongate-oval, moderately elevated (dorsal elevation 0.34), subcarinated, valves not beaked. Colour of tegmentum very variable, whitish with pale brown and black flecks, greenish with darker flecks mainly on jugal areas, evenly reddish brown, or marbled with irregular spots of various colours. Girdle colour resembles tegmentum, uniform or in neatly alternating bands.

Head valve semicircular, front slope straight, posterior margin widely V-shaped ( Fig. 2A ). Intermediate valves broadly rectangular, front margin slightly concave in wide central part between apophyses (slightly convex in valve ii), straight or slightly concave in lateral parts, side margins rounded, hind margin almost straight, apices inconspicuous, lateral areas not raised, poorly defined ( Fig. 2F ). Tail valve semicircular, front margin strongly concave in wide central part between apophyses, mucro not elevated, submedian ( Fig. 2K ), antemucronal slope convex, postmucronal slope concave directly behind mucro ( Fig. 2L ).

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 smooth. Many aesthetes irregularly present on pustules’ surface ( Figs 2J, 2P ).

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, 103 μm long, 143 μm wide ( Fig. 2Q ), sculptured with 27 or 28 narrow riblets ( Fig. 2R ), 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 58–70×10–13 μm ( Fig. 2S ). At high magnification, surface of these small scales covered with small holes ( Fig. 2T ).

Central tooth of radula narrow, bearing roundish, outwardly curved blade, first lateral tooth equally narrow, with outwardly curved blade, major lateral with bicuspid head, denticles pointed of the same size, minute granulations on upper surface of cusps ( Fig. 2W ).

Ctenidia arranged holobranchially and abanally.

Comparison and remarks: This is a very variable species but clearly distinguishable from the other two species of Ischnochiton currently known from Madagascar (Kaas & Van Belle 1990): I. yerburyi (E.A. Smith, 1891) and I. sansibarensis Thiele, 1909 (reported as I. rufopunctatus Odhner, 1919 by Odhner 1919; Dautzenberg 1923, 1929). The colour of the new species is very variable ( Fig. 4B ), both glossy and dull, mostly greenish and more or less uniform, or with colour flecks mainly on the jugal area, but 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 has a reticulated, thimble-like sculpture pattern on all valves. We examined the type of I. sansibarensis ( Fig. 4C ) at the ZMB ( Kilias 1995); it is not in a good state 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 , but we agree with Kaas and Van Belle (1990), who examined the types of all Indian Ocean Ischnochiton species with a thimble-like sculpture, that the two species can be distinguished. In addition to the different sculpture in these taxa, they also differ in size ( I. yerburyi maximum 15× 8 mm; I. sansibarensis 10× 5.5 mm; I. sirenkoi 17.6× 8.2mm), 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 granulation on the radula is rarely seen in chitons, although this may reflect a lack of detailed examination in other chiton species.

The new species differs from other Ischnochiton species living in the Indian Ocean.

I. winckworthi Leloup, 1936 , known from the Arabian Sea and the Bay of Bengal, has 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 Leloup, 1981 known from the Mozambique Channel and KwaZulu-Natal has a mainly smooth tegmentum, with only a few fine longitudinal grooves near the front margin of the latero-pleural parts of the intermediate valves.

I. feliduensis E.A. Smith, 1903 from the Maldive Islands, I. bouryi Dupuis, 1917 from the Andaman Islands, Pakistan, Sri Lanka and Indonesia, I. gallensis von Knorre, 1925 from Sri Lanka and Krusadai Island, I. crassus Kaas, 1985 from the Mozambique Channel, I. bigranosus Kaas & Van Belle, 1990 from the Andaman Islands, I. yemenensis Van Belle & Wranik, 1994 from Yemen and I. goudi Kaas, 1994 from the Seychelles Islands are different species, not comparable with I. sirenkoi .

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 ).

Holotype: MZB 45693, length 12.2 mm ( Fig. 4A ). MADAGASCAR: Lavanono.

Paratypes: same data as holotype, 2 specimens without soft parts, largest 13.8 mm, smallest 11.7 mm ( MZB 45694 ) ; 1 specimen coated and dismounted for SEM analysis ( Figs 2A–W ) ( MZB 45695 ) ; 1 specimen, length 15.5 mm ( MNHN) ; 1 specimen, length 14.5 mm ( BMNH) ; 1 specimen, length 12.7 mm ( NMSA) ; 1 specimen, length 16.2 mm ( ZISP 61115 ) ; 1 specimen, length 16.7 mm ( ZSM Mol 20100376) ; 2 specimens, length 14.8 and 15.5 mm ( GP) ; 2 specimens, length 14 and 15 mm ( BD) .

Other material examined: 10 specimens ( GP) , 4 specimens ( BD) , 1 specimen ( ZSM Mol 20100377), all from the type locality .

Distribution: Madagascar, Lavanono.

Family Callistoplacidae Pilsbry, 1893

Figures

Fig. 2. Ischnochiton sirenkoi sp. n., paratype, MZB 45695: (A–E) head valve, (A–C) dorsal, lateral and ventral views, (D) detail of the sculpture, (E) detail of teeth and slit rays of the articulamentum; (F–J) intermediate valve: (F–H) dorsal, frontal and ventral views, (I) detail of the sculpture of the tegmentum, (J) pustules with aesthetes, in pleural area; (K–P) tail valve: (K–M) dorsal, lateral and ventral views, (N) detail of the sculpture of the tegmentum, at the contact between postmucronal (at the left) and antemucronal areas, (O) detail of the sculpture of the tegmentum of postmucronal area, (P) pustules with aesthetes, in postmucronal area; (Q–T) girdle elements: (Q, R) dorsal scales, (S, T) ventral scales (note the small holes on the surface); (U–W) radula: (U) complete view, (V) detail of an half row of teeth, (W) bicuspid head of the major lateral tooth (note the minute granulations present on the upper surface of the cusps). Scale bars 1 mm (A–C, F–H, K–M), 500 μm (I), 100 μm (D, E, N, O, U), 50 μm (J, P–S) and 10 μm (T).

Fig.4.(A, B) Ischnochiton sirenkoi sp. n., Lavanono,Madagascar:(A) holotype MZB 45693, 12.2 ×6.1 mm; (B) a lot ofspecimens showing colour variations,length,from left to right, respectively 17.7,11.3, 11.2, 12, 10 and 9.5 mm; (C) Ischnochiton sansibarensis Thiele, 1909, holotype ZMB 102069, Zanzibar, ca 4 mm; (D–F) Callistochiton ashbyi (Barnard, 1963): (D) Lavanono, Madagascar, specimen n. 2 (Table 1), 23.3 mm; (E, F) holotype SAMC-A9337, an intermediate valve from Cape Infanta,SouthAfrica,dorsal and ventral views,9×3.3 mm; (G)Callistochiton madagassicus Thiele,1909, holotype MNHN,Madagascar,21×14 mm (from original description),some detached valves at left; (H) Chaetopleura chelazziana Ferreira, 1983, Lavanono, Madagascar, 47×31 mm; (I–X) Cryptoplax dupuisiAshby, 1931, Lavanono, Madagascar:(I–K) specimens with the girdle deprived of dorsal spicules on the first four valves; (I, J) adult specimen, 51×12.5mm; (K) the smallerspecimen,28.5×4 mm;(L) specimen with the girdle completely coveredof dorsal spicules, 44×10 mm; (M, N) detail of sculpture of valves i–ii, in situ; (O, P) tail valve, lateral and ventral views, length 6.8 mm; (Q–X) valves i–viii (same scale), width (plates, without apophyses), from left to right, 4.4 mm (i), 3.9 mm (ii), 1.45 mm (iii), 2.65 mm (iv), 1.45 mm (v), 1.45 mm (vi), 2.4 mm (vii), 2.75 mm (viii).